Date   

Re: Value of Tek 465/DM44

Bob Albert
 

I don't think much more than $100.
Bob

On Saturday, October 16, 2021, 07:28:05 AM PDT, Oz-in-DFW <lists@ozindfw.net> wrote:

I'm trying to set a price for selling on behalf of an estate.

It's in
fair shape and passes very cursory checks (lights up, sweeps, both channels show activity on finger contact with probe, timebase changes have visible effect) and has three probes (2 X P6202B/1 clip and ground lead, 1 X P6006)
--
Oz (in DFW, Texas USA) N1OZ


Re: Series 71 Switch Repiar

David Holland
 

On Sat, Oct 16, 2021 at 2:15 AM Jeff Dutky <jeff.dutky@gmail.com> wrote:
<Snip>

. I may try flushing it with IPA and see if that makes a difference. It's already been flushed with WD-40, which hasn't made as much difference as I would have expected.
I'd recommend that anyway. If it's the stereotypical Blue/Yellow WD40
here in the states (https://en.wikipedia.org/wiki/WD-40) I've never
heard anything good about leaving it in place. It turns sticky/gooey
over time.

Might not be a problem if you're using it to get a nut off your car's
exhaust. Somewhat more so in a piece of electronic equipment.

David


Price of Tek 465/DM44

Oz-in-DFW
 

I'm trying to set a price for sale on behalf of an estate. I know what I'd ask were it mine, but I'm looking for some more objective data.

It's:
- in fair shape with a cover
- rear feet have disintegrated
- power cord is functional but needs replaced (cracked in places, aftermarket plug)
- has three scope probes 2 X 6062B w/ one ground lead and one clip an a P6006 with a modified ~2 ft ground lead)
- has a DMM probe set
- passes very cursory checks (lights up, both channels show expected changes on finger contact with probe, timebase change effects display, etc,)

--
Oz (in DFW, Texas USA) N1OZ


Re: 7B92 with no repeating sweep

Dave Peterson
 

Good morning Jeff,

I'm chiming in because I've had somewhat similar experience with a 465 which suffered problems with the socketing of the https://w140.com/tekwiki/wiki/155-0049-00 sweep control IC. I found that reseating brought the sweep back - usually. Meaning that the chip was ok and it was the classic old Tek socket issue. I ordered some backups from the auction site and I recall the sellers listing saying that they were recovered from a 7B92. I see in the TekWiki doc that U310 is either a 155-0049-00 or 155-0049-01 based on SN. So your time base has that and is likely socketed.

Also I found https://w140.com/tekwiki/images/5/5d/068-0315-00.pdf very helpful in understanding sweep circuits and diagnosis.

Good luck with it!


Re: Continuing Problems with 7704A Display

n4buq
 

Hi John,

I plan to recheck the voltages in this entire circuit. Hopefully something will become apparent. As you say, with 3178V (presuming I didn't measure something incorrectly), it doesn't make sense that I can get -2960V (or a lot more) at TP4221. I'll follow up with anything I find.

Sorry I didn't remember who had asked about that. It's a pretty long thread now...

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "John Kolb" <jlkolb@jlkolb.digitalspacemail17.net>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 16, 2021 12:14:53 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,

That was me. The purpose was to make sure the voltage from T4201 and the
voltage doubler wasn't to low to be able to reach the -2960V at the test
point. Instead, 3178V looks to high. The Collectors of Q4105/4115 can
only range from 0 to 150V (maybe 154 or 156) and 150 subtracted from
3178 is still -3028. The Voltage at the top of of C4208 should be
between 0 and + 150 with respect to ground, and the bottom should be the
same reading as at TP4221. About the only thing that could make them
different would be R4221, 100 ohms, turning into a very high resistance.

John KK6IL

On 10/15/2021 3:14 PM, n4buq wrote:
Hi Ozan,

I checked the differential voltage between -54V AND +54V and it was 108.5V. I'm
not sure why the schematic shows 109.1V but I presume that happens to be what
someone measured during development.

BTW, I don't remember who asked, but I just measured the voltage across C4208
and, unless I did something wrong, that shows to be 3178V.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 1:27:12 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Ozan,

I'm getting 153.5V at C4201. If 156V is expected, then that might be a symptom
of other issues.

I disconnected P4212 and P4213 and measured the voltage across the inner
conductors. That shows 118.2VAC. I measured that with my Fluke 27 which
_should_ be okay at that frequency (the meter is spec'd up to 30kHz).

I haven't measured the differential voltage between P313's pin 1 and pin 2 but
I'm pretty sure the individual voltages are correct with respect to ground.

I'm not sure what that all might indicate but it's what I measured.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@erdogan.us>
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, October 11, 2021 10:55:26 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,
There is a possibility that T4201 is not generating high enough voltage. As
others pointed out even when collector of Q4115 is as low as it can go you are
not able to get low enough (or high in absolute sense) cathode voltage.

What voltage do you measure across C4201 at +150V terminal? +156V is expected.

T4201 voltage depends on what comes from J4212 and J4213. If you have access to
those terminals each should show about ~ 100Vpp 20kHz signal according to the
schematic,

54V setting that matters for T4201 is in the lower unit, sheet <8>, it is the
+54V adjustment R3143. Differential voltage at connector P313 pins 1 and 3
should be 109.1V according to the schematic.

741 could still be bad since you are measuring 0.8V at pin2. If R4120 measures
470k (i.e. it is not faulty) there is too much current coming out of pin2.
However, replacing the 741 may not fix the real problem.
Ozan


On Mon, Oct 11, 2021 at 08:29 AM, n4buq wrote:


+54V was about +53V. I set it properly and tweaked -50V (was -50.1V).
Virtually no change with the ability to set -2960V and the voltage at
Q4105/Q4115 is still at about 0.5V.

I presume that since the HV multiplier is a X14 and the anode voltage is
supposed to be 21kV, then the input to the multiplier (from
T4201) should be 1500V RMS @~20kHz. So far, I haven't found this stated
empirically so would like confirmation on it. Furthermore, from there, the
doubler circuit should yield 3kV. Is that correct?

Until I get the replacement 741, I'm not sure what else I can check.

Thanks,
Barry - N4BUQ






7B92 with no repeating sweep

 

After freeing the EXT÷10 trigger source switch, replacing the broken rear plate (with one from a poorly treated 7B53A), cleaning the trigger, readout, and sweep boards with IPA to try to remove some kind of white residue (with limited success), and generally giving the entire instrument a close visual inspection, I decided that it looked good enough to try it in my 7633. I did verify that the white residue was not shorting anything out, though I have not figured out what the residue is or where it came from.

The good news is that it powers up and lights most of the lights. The bad news is that I can't get a repetitive sweep or trigger. I can get one sweep to happen when I switch the TIME/DIV knob between different positions, but that's it. I've tried in AUTO, NORM, HF SYNC and SINGLE SWEEP modes, with AC and DC coupling, using INT trigger source as well as LINE. Nothing gives me a repeating sweep. In SINGLE SWEEP mode the RESET button has no apparent effect.

I've read through the circuit description in the service manual, and did not find it particularly enlightening. I'm in the process of reading the Circuit Description Supplement, which is much more detailed (if not more enlightening for me). I'm tempted to just start testing circuit continuity and diode drops across diodes and transistors, in the hope of stumbling on the culprit (or eliminating a bunch of possible culprits), but I'd like to know if anyone has any wisdom or experience that might guide me. My plan is just to work my way through the internal trigger path and hope that I find a dead transistor or diode (regular diode, not a tunnel diode, which I don't understand how to check with a multimeter, though from reading old posts in this group, it sounds like I basically can't check tunnel diodes with a multimeter).

I would be very happy (and unsurprised) to discover that I have been setting the controls incorrectly (it's certainly happened before), but I have a feeling that's not what's going on here.

I'd really like to be able to get this 7B92 working, both because I'd like to have ALT horizontal mode on my 7603 and 7633, and because I just like the lighted delayed trigger controls.

-- Jeff Dutky


492BP fuzzy trace on left side of crt - any ideas on what to look for?

Edward Prest
 

The left side is a bit out of focus compared to the center and right side. I have read most of v1 of the service manuals and I don't see anything on this.

Any ideas? CRT maybe? It's a well used SA with minor phosphor burn in.


Re: 492 fan replacement - speed control - update

Edward Prest
 

FYI With a room temp of 22C and almost an hour warm up I measure the back power supply and sides of the instrument temperature around 31C. The bottom where the air exits is 37C. This was measured with a hand held IR meter.


Re: Series 71 Switch Repiar

 

Well, I guess I don't need to try to clean the switch now. I noticed that the sticking switch gave a tiny amount when another switch in the group was pressed, and a tiny clicking sound could be heard, as if the latch was trying to release the stuck switch. I was then able to pull the stuck switch to the "out" position with a pair of needle nosed pliers. Repeating this cycle (press in to stuck, press other switch, pull stuck switch with pliers) several times loosened up the stuck switch enough that it would fully release when another switch is pressed.

There is still slight stickiness, and the switch will still stick if pressed in just the wrong way, but it is otherwise working, and I expect it to get better with use. I'd still like to know what is going on inside the switch, but not enough to risk destroying it in the investigation. The current difference in behavior is subtle but noticeable: when most Series 71 switches are depressed they latch firmly and have almost no outward motion once latched. When the sticky switches are depressed, however, there is noticeable outward motion of the switch as you pull your finger away.

The switch does not sit out any further than other switches in the latched position, but I think that it is actually sticking for a very short time longer in the fully depressed position than the other switches. This implies that there is something gummy inside the switch that is holding the switch in when depressed. I may try flushing it with IPA and see if that makes a difference. It's already been flushed with WD-40, which hasn't made as much difference as I would have expected.

-- Jeff Dutky


Re: Continuing Problems with 7704A Display

John Kolb
 

Hi Barry,

That was me. The purpose was to make sure the voltage from T4201 and the voltage doubler wasn't to low to be able to reach the -2960V at the test point. Instead, 3178V looks to high. The Collectors of Q4105/4115 can only range from 0 to 150V (maybe 154 or 156) and 150 subtracted from 3178 is still -3028. The Voltage at the top of of C4208 should be between 0 and + 150 with respect to ground, and the bottom should be the same reading as at TP4221. About the only thing that could make them different would be R4221, 100 ohms, turning into a very high resistance.

John KK6IL

On 10/15/2021 3:14 PM, n4buq wrote:
Hi Ozan,
I checked the differential voltage between -54V AND +54V and it was 108.5V. I'm not sure why the schematic shows 109.1V but I presume that happens to be what someone measured during development.
BTW, I don't remember who asked, but I just measured the voltage across C4208 and, unless I did something wrong, that shows to be 3178V.
Thanks,
Barry - N4BUQ
----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 1:27:12 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Ozan,

I'm getting 153.5V at C4201. If 156V is expected, then that might be a symptom
of other issues.

I disconnected P4212 and P4213 and measured the voltage across the inner
conductors. That shows 118.2VAC. I measured that with my Fluke 27 which
_should_ be okay at that frequency (the meter is spec'd up to 30kHz).

I haven't measured the differential voltage between P313's pin 1 and pin 2 but
I'm pretty sure the individual voltages are correct with respect to ground.

I'm not sure what that all might indicate but it's what I measured.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@erdogan.us>
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, October 11, 2021 10:55:26 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,
There is a possibility that T4201 is not generating high enough voltage. As
others pointed out even when collector of Q4115 is as low as it can go you are
not able to get low enough (or high in absolute sense) cathode voltage.

What voltage do you measure across C4201 at +150V terminal? +156V is expected.

T4201 voltage depends on what comes from J4212 and J4213. If you have access to
those terminals each should show about ~ 100Vpp 20kHz signal according to the
schematic,

54V setting that matters for T4201 is in the lower unit, sheet <8>, it is the
+54V adjustment R3143. Differential voltage at connector P313 pins 1 and 3
should be 109.1V according to the schematic.

741 could still be bad since you are measuring 0.8V at pin2. If R4120 measures
470k (i.e. it is not faulty) there is too much current coming out of pin2.
However, replacing the 741 may not fix the real problem.
Ozan


On Mon, Oct 11, 2021 at 08:29 AM, n4buq wrote:


+54V was about +53V. I set it properly and tweaked -50V (was -50.1V).
Virtually no change with the ability to set -2960V and the voltage at
Q4105/Q4115 is still at about 0.5V.

I presume that since the HV multiplier is a X14 and the anode voltage is
supposed to be 21kV, then the input to the multiplier (from
T4201) should be 1500V RMS @~20kHz. So far, I haven't found this stated
empirically so would like confirmation on it. Furthermore, from there, the
doubler circuit should yield 3kV. Is that correct?

Until I get the replacement 741, I'm not sure what else I can check.

Thanks,
Barry - N4BUQ


Re: Continuing Problems with 7704A Display

n4buq
 

Hi Ozan,

Your analysis makes perfect sense to me. I'll do more checking tomorrow morning to confirm the voltage across C4208 and across those two resistors as well as the collector voltage at Q4115. I'll also check the P42-6 cable/connections.

I really appreciate this.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@erdogan.us>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 9:15:00 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,
My comments are below:

I checked the differential voltage between -54V AND +54V and it was 108.5V.
I'm not sure why the schematic shows 109.1V but I presume that happens to be
what someone measured during development.
The difference is small enough that it shouldn't cause the issue you are seeing.

BTW, I don't remember who asked, but I just measured the voltage across C4208
and, unless I did something wrong, that shows to be 3178V.
It wasn't me but this is an interesting measurement. One end of C4208 is
connected to collector of Q4115 with a 330-ohm resistor and the other end is
connected to TP1 with a 100-ohm resistor. Collector of Q4115 is almost at zero
volts so 3178V-2960V=218V is dropped somewhere on R4211, P62I-6/P41I-6
connector, or R2221. I think Albert asked the same question before.

This is how you can debug:
1) Top of C4208 (junction of C4208 and R4211) should be very close to the
voltage at Q4115 collector (less than 1V since 741 is railed). If you are
seeing more than couple of volts at the top end of C4208 either R4211 increased
in value or there is a poor connection in P42I-6 wire.

2) If the top end of C4208 is as expected, bottom end should still be ~ -3178V.
If TP1 is ~ 2960V then all the voltage (200V!) is dropping on R4221. This
points to a defective R4221.

Ozan



Re: Continuing Problems with 7704A Display

Ozan
 

Hi Barry,
My comments are below:

I checked the differential voltage between -54V AND +54V and it was 108.5V.
I'm not sure why the schematic shows 109.1V but I presume that happens to be
what someone measured during development.
The difference is small enough that it shouldn't cause the issue you are seeing.

BTW, I don't remember who asked, but I just measured the voltage across C4208
and, unless I did something wrong, that shows to be 3178V.
It wasn't me but this is an interesting measurement. One end of C4208 is connected to collector of Q4115 with a 330-ohm resistor and the other end is connected to TP1 with a 100-ohm resistor. Collector of Q4115 is almost at zero volts so 3178V-2960V=218V is dropped somewhere on R4211, P62I-6/P41I-6 connector, or R2221. I think Albert asked the same question before.

This is how you can debug:
1) Top of C4208 (junction of C4208 and R4211) should be very close to the voltage at Q4115 collector (less than 1V since 741 is railed). If you are seeing more than couple of volts at the top end of C4208 either R4211 increased in value or there is a poor connection in P42I-6 wire.

2) If the top end of C4208 is as expected, bottom end should still be ~ -3178V. If TP1 is ~ 2960V then all the voltage (200V!) is dropping on R4221. This points to a defective R4221.

Ozan


Re: Series 71 Switch Repiar

 

Since I managed to free the stuck EXT÷10 switch I am no longer planning to try to disassemble the switch, but I would like to try cleaning the switch in the hope of freeing EXT÷10 push-button (which is still stiff, and would stick again if I actuated it).

While I can live without the EXT÷10 feature, it would be nice to have the plug-in in full working order. Would it be safe to soak the switch assembly/PCB in IPA?

-- Jeff Dutky


Re: Continuing Problems with 7704A Display

n4buq
 

Ozan,

BTW, you asked about the 741 voltages. I just measured again (with the new chip installed) and Pin 2 shows 0V but Pin 3 is at 1.1V. From what I understand, that's an indication that the chip is pegged trying to increase the negative voltage as far as it can but it can't get it down to -2960V.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 1:27:12 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Ozan,

I'm getting 153.5V at C4201. If 156V is expected, then that might be a symptom
of other issues.

I disconnected P4212 and P4213 and measured the voltage across the inner
conductors. That shows 118.2VAC. I measured that with my Fluke 27 which
_should_ be okay at that frequency (the meter is spec'd up to 30kHz).

I haven't measured the differential voltage between P313's pin 1 and pin 2 but
I'm pretty sure the individual voltages are correct with respect to ground.

I'm not sure what that all might indicate but it's what I measured.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@erdogan.us>
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, October 11, 2021 10:55:26 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,
There is a possibility that T4201 is not generating high enough voltage. As
others pointed out even when collector of Q4115 is as low as it can go you are
not able to get low enough (or high in absolute sense) cathode voltage.

What voltage do you measure across C4201 at +150V terminal? +156V is expected.

T4201 voltage depends on what comes from J4212 and J4213. If you have access to
those terminals each should show about ~ 100Vpp 20kHz signal according to the
schematic,

54V setting that matters for T4201 is in the lower unit, sheet <8>, it is the
+54V adjustment R3143. Differential voltage at connector P313 pins 1 and 3
should be 109.1V according to the schematic.

741 could still be bad since you are measuring 0.8V at pin2. If R4120 measures
470k (i.e. it is not faulty) there is too much current coming out of pin2.
However, replacing the 741 may not fix the real problem.
Ozan


On Mon, Oct 11, 2021 at 08:29 AM, n4buq wrote:


+54V was about +53V. I set it properly and tweaked -50V (was -50.1V).
Virtually no change with the ability to set -2960V and the voltage at
Q4105/Q4115 is still at about 0.5V.

I presume that since the HV multiplier is a X14 and the anode voltage is
supposed to be 21kV, then the input to the multiplier (from
T4201) should be 1500V RMS @~20kHz. So far, I haven't found this stated
empirically so would like confirmation on it. Furthermore, from there, the
doubler circuit should yield 3kV. Is that correct?

Until I get the replacement 741, I'm not sure what else I can check.

Thanks,
Barry - N4BUQ


Re: Continuing Problems with 7704A Display

n4buq
 

Hi Ozan,

I checked the differential voltage between -54V AND +54V and it was 108.5V. I'm not sure why the schematic shows 109.1V but I presume that happens to be what someone measured during development.

BTW, I don't remember who asked, but I just measured the voltage across C4208 and, unless I did something wrong, that shows to be 3178V.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 1:27:12 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Ozan,

I'm getting 153.5V at C4201. If 156V is expected, then that might be a symptom
of other issues.

I disconnected P4212 and P4213 and measured the voltage across the inner
conductors. That shows 118.2VAC. I measured that with my Fluke 27 which
_should_ be okay at that frequency (the meter is spec'd up to 30kHz).

I haven't measured the differential voltage between P313's pin 1 and pin 2 but
I'm pretty sure the individual voltages are correct with respect to ground.

I'm not sure what that all might indicate but it's what I measured.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@erdogan.us>
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, October 11, 2021 10:55:26 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,
There is a possibility that T4201 is not generating high enough voltage. As
others pointed out even when collector of Q4115 is as low as it can go you are
not able to get low enough (or high in absolute sense) cathode voltage.

What voltage do you measure across C4201 at +150V terminal? +156V is expected.

T4201 voltage depends on what comes from J4212 and J4213. If you have access to
those terminals each should show about ~ 100Vpp 20kHz signal according to the
schematic,

54V setting that matters for T4201 is in the lower unit, sheet <8>, it is the
+54V adjustment R3143. Differential voltage at connector P313 pins 1 and 3
should be 109.1V according to the schematic.

741 could still be bad since you are measuring 0.8V at pin2. If R4120 measures
470k (i.e. it is not faulty) there is too much current coming out of pin2.
However, replacing the 741 may not fix the real problem.
Ozan


On Mon, Oct 11, 2021 at 08:29 AM, n4buq wrote:


+54V was about +53V. I set it properly and tweaked -50V (was -50.1V).
Virtually no change with the ability to set -2960V and the voltage at
Q4105/Q4115 is still at about 0.5V.

I presume that since the HV multiplier is a X14 and the anode voltage is
supposed to be 21kV, then the input to the multiplier (from
T4201) should be 1500V RMS @~20kHz. So far, I haven't found this stated
empirically so would like confirmation on it. Furthermore, from there, the
doubler circuit should yield 3kV. Is that correct?

Until I get the replacement 741, I'm not sure what else I can check.

Thanks,
Barry - N4BUQ


Re: Continuing Problems with 7704A Display

n4buq
 

Apparently mine is not an HVP1. That one appears to have a Lucite head and three flash guards and the tip is screwed together a bit differently. I'm thinking mine is an HVP2.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Daveolla via groups.io" <grobbins=netflash.net@groups.io>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 3:40:49 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
I see the HVP2 also , I guess I missed ever
seeing that one, thanks! I found the HVP-1 here;
https://elektrotanya.com/eico_model_hvp-1_high_voltage_probe_operating_instrucations.zip/download.html

Dave

.ziAt 11:26 AM 10/15/2021, you wrote:
Dave,

I see an "HVP 2" file. That manual mentions
using that probe with a Triplett 630NA on the 500V scale.

Unfortunately, I'm not sure what the model
number is for my probe. I've never found anything on it to indicate the model.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Daveolla via groups.io" <grobbins=netflash.net@groups.io>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 10:28:21 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
The manual for the EICO HV probe is on BAMA (boat
anchor manual archive) google it .It lists the
various resistors in the probe depending on
which input resistance VTVM you have. I think the
file is named "HVP 1"....... unless I changed my file name).
Dave

At 04:49 AM 10/15/2021, you wrote:
The odd ratio is because these probes were
designed for analog VTVMs. These meters had an
input resistance of 9 MOhm and used an 1 MOhm
series resistor inside the X1 probe, making
a total of 10 MOhm input resistance. Since
this arrangement forms a 9/10 attenuator the
meter scales are drawn taking this in account so
the measurements are ok. The HV probe has a
resistor whose value is the right one for an
instrument with 9 MOhm input resistance that is
calibrated to show 1.0 when the true input
voltage is 0.9. If you use this HV probe with a
digital meter with 10 MOhm input resistance you
need to to do the math to have the real value.

An alternative is to make an adapter box to be
inserted between the HV probe and the meter.
This adapter just consists of an attenuator to
further divide the probe output and the ratio
has to be chosen to give a total attenuation of
10000, including the probe resistor, so you can
use a lower range (which is not a problem with a
DVM) and you will get the right measurement
without doing any math other than multiplying
the value by 10000. Part of one resistor of the
attenuator should be variable so you can calibrate the ratio.

I hope this is understandable (English is not my
mother language) but if it is not clear and you are interested, jusk ask.

Regards,

Ignacio EB4APL


El 12/10/2021 a las 15:47, n4buq escribió:
John,

It appears the 90M string works quite well
and gives me an easy 10:1 ratio.

After seeing that the 1R09G was giving me odd
results at higher voltages, I removed the
resistor, cleaned it thoroughly with IPA, and
reassembled it without touching the body of the
resistor. After that, 1000V on the calibrator
showed 9.09V which is the 110:1 it should be
(actually, the Fluke waffled between 9.09V and
9.10V and reducing the calibrator voltage by
about 0.5% caused it to settle at 9.09V). I
don't know if a less-than-clean resistor was
the root cause, but it did seem to make that
measurement what it should be.

Since 10:1 is a much easier ratio to work with
and I'm not likely to need something for higher
that 10kV, I'm considering making a probe out
of some PVC pipe; however, I don't know how
safe PVC is at higher voltages and need to
research that a bit before deciding to use
it. If not PVC pipe, I'm not sure what other
means I could use to make the probe. Nylon?

Anyway, thanks for the pointers/suggestions!

Apologies to the list if this has gone too far
off topic. If so, please let me know. I
tend to get "wordy" at times. :(

Thanks,
Barry - N4BUQ



On 10/11/2021 9:32 AM, n4buq wrote:
Hi Albert,

Lacking a very precise way to measure
voltages of that magnitude, I'm using a
Triplett 630NA on the 6000V scale. I have an
old EICO HV probe but I'm unsure
what it's comprised of; however, I used it
with my voltage calibrator set to
1000V and derived a multiplication factor
to use with my Fluke 25. While
that's all a bit crude, both measurement
methods read pretty close to each
other. That said, I'm seeing a maximum
voltage of -2900V and that's with R4123
set to maximum CW rotation.

When I was experimenting with injecting
voltages where U4110's pin 6 normally
sets it, I could get the collector
voltages at Q4105/Q4115 anywhere between
0.5V and 100V but TP4221 didn't appear to
move - at least not that I could
detect on the analog meter.

I'll check the voltage at R4211/R4209/C4208 and report back later today.

Thanks again!
Barry - N4BUQ









Re: Continuing Problems with 7704A Display

Daveolla
 

I see the HVP2 also , I guess I missed ever seeing that one, thanks! I found the HVP-1 here;
https://elektrotanya.com/eico_model_hvp-1_high_voltage_probe_operating_instrucations.zip/download.html

Dave

.ziAt 11:26 AM 10/15/2021, you wrote:

Dave,

I see an "HVP 2" file. That manual mentions using that probe with a Triplett 630NA on the 500V scale.

Unfortunately, I'm not sure what the model number is for my probe. I've never found anything on it to indicate the model.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Daveolla via groups.io" <grobbins=netflash.net@groups.io>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 10:28:21 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
The manual for the EICO HV probe is on BAMA (boat
anchor manual archive) google it .It lists the
various resistors in the probe depending on
which input resistance VTVM you have. I think the
file is named "HVP 1"....... unless I changed my file name).
Dave

At 04:49 AM 10/15/2021, you wrote:
The odd ratio is because these probes were
designed for analog VTVMs. These meters had an
input resistance of 9 MOhm and used an 1 MOhm
series resistor inside the X1 probe, making
a total of 10 MOhm input resistance. Since
this arrangement forms a 9/10 attenuator the
meter scales are drawn taking this in account so
the measurements are ok. The HV probe has a
resistor whose value is the right one for an
instrument with 9 MOhm input resistance that is
calibrated to show 1.0 when the true input
voltage is 0.9. If you use this HV probe with a
digital meter with 10 MOhm input resistance you
need to to do the math to have the real value.

An alternative is to make an adapter box to be
inserted between the HV probe and the meter.
This adapter just consists of an attenuator to
further divide the probe output and the ratio
has to be chosen to give a total attenuation of
10000, including the probe resistor, so you can
use a lower range (which is not a problem with a
DVM) and you will get the right measurement
without doing any math other than multiplying
the value by 10000. Part of one resistor of the
attenuator should be variable so you can calibrate the ratio.

I hope this is understandable (English is not my
mother language) but if it is not clear and you are interested, jusk ask.

Regards,

Ignacio EB4APL


El 12/10/2021 a las 15:47, n4buq escribió:
John,

It appears the 90M string works quite well
and gives me an easy 10:1 ratio.

After seeing that the 1R09G was giving me odd
results at higher voltages, I removed the
resistor, cleaned it thoroughly with IPA, and
reassembled it without touching the body of the
resistor. After that, 1000V on the calibrator
showed 9.09V which is the 110:1 it should be
(actually, the Fluke waffled between 9.09V and
9.10V and reducing the calibrator voltage by
about 0.5% caused it to settle at 9.09V). I
don't know if a less-than-clean resistor was
the root cause, but it did seem to make that
measurement what it should be.

Since 10:1 is a much easier ratio to work with
and I'm not likely to need something for higher
that 10kV, I'm considering making a probe out
of some PVC pipe; however, I don't know how
safe PVC is at higher voltages and need to
research that a bit before deciding to use
it. If not PVC pipe, I'm not sure what other
means I could use to make the probe. Nylon?

Anyway, thanks for the pointers/suggestions!

Apologies to the list if this has gone too far
off topic. If so, please let me know. I
tend to get "wordy" at times. :(

Thanks,
Barry - N4BUQ



On 10/11/2021 9:32 AM, n4buq wrote:
Hi Albert,

Lacking a very precise way to measure
voltages of that magnitude, I'm using a
Triplett 630NA on the 6000V scale. I have an
old EICO HV probe but I'm unsure
what it's comprised of; however, I used it
with my voltage calibrator set to
1000V and derived a multiplication factor
to use with my Fluke 25. While
that's all a bit crude, both measurement
methods read pretty close to each
other. That said, I'm seeing a maximum
voltage of -2900V and that's with R4123
set to maximum CW rotation.

When I was experimenting with injecting
voltages where U4110's pin 6 normally
sets it, I could get the collector
voltages at Q4105/Q4115 anywhere between
0.5V and 100V but TP4221 didn't appear to
move - at least not that I could
detect on the analog meter.

I'll check the voltage at R4211/R4209/C4208 and report back later today.

Thanks again!
Barry - N4BUQ






Sphere's 2021 Holiday Stuff Season is officially open....

 

My apologies for the delay, but a lot of items had to be edited, discounted, re-worked, added and deleted, so it just wasn't possible to keep the page open during all that chaos. Hopefully you will like the new look and feel, I tried to improve the navigation, parts association and appearance wherever I could. There are some new categories, Surprises (kind of speaks for itself), Scopes (I put them all in one place for easier browsing), and SMD Reels (for those who are SMD fans). I will be updating every day or so, the top red banner will tell you when.

https://www.sphere.bc.ca/test/stuffday.html

please let me know if you want to see something specific I have left out, you can email me directly.
all the best to everyone for the upcoming holidays.

-walter & susan
(walter2 -at- sphere.bc.ca)
sphere research corp.
https://www.sphere.bc.ca/test/index.html


Re: Continuing Problems with 7704A Display

n4buq
 

Hi Ozan,

I'm getting 153.5V at C4201. If 156V is expected, then that might be a symptom of other issues.

I disconnected P4212 and P4213 and measured the voltage across the inner conductors. That shows 118.2VAC. I measured that with my Fluke 27 which _should_ be okay at that frequency (the meter is spec'd up to 30kHz).

I haven't measured the differential voltage between P313's pin 1 and pin 2 but I'm pretty sure the individual voltages are correct with respect to ground.

I'm not sure what that all might indicate but it's what I measured.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@erdogan.us>
To: "tekscopes" <TekScopes@groups.io>
Sent: Monday, October 11, 2021 10:55:26 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,
There is a possibility that T4201 is not generating high enough voltage. As
others pointed out even when collector of Q4115 is as low as it can go you are
not able to get low enough (or high in absolute sense) cathode voltage.

What voltage do you measure across C4201 at +150V terminal? +156V is expected.

T4201 voltage depends on what comes from J4212 and J4213. If you have access to
those terminals each should show about ~ 100Vpp 20kHz signal according to the
schematic,

54V setting that matters for T4201 is in the lower unit, sheet <8>, it is the
+54V adjustment R3143. Differential voltage at connector P313 pins 1 and 3
should be 109.1V according to the schematic.

741 could still be bad since you are measuring 0.8V at pin2. If R4120 measures
470k (i.e. it is not faulty) there is too much current coming out of pin2.
However, replacing the 741 may not fix the real problem.
Ozan


On Mon, Oct 11, 2021 at 08:29 AM, n4buq wrote:


+54V was about +53V. I set it properly and tweaked -50V (was -50.1V).
Virtually no change with the ability to set -2960V and the voltage at
Q4105/Q4115 is still at about 0.5V.

I presume that since the HV multiplier is a X14 and the anode voltage is
supposed to be 21kV, then the input to the multiplier (from
T4201) should be 1500V RMS @~20kHz. So far, I haven't found this stated
empirically so would like confirmation on it. Furthermore, from there, the
doubler circuit should yield 3kV. Is that correct?

Until I get the replacement 741, I'm not sure what else I can check.

Thanks,
Barry - N4BUQ


Re: 7B50 - No Sweep

n4buq
 

Hi Roger,

I tried the things you mention and I get no hint of a trace. The RESET button's light is never on.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Roger Evans via groups.io" <very_fuzzy_logic=yahoo.com@groups.io>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 15, 2021 10:26:54 AM
Subject: Re: [TekScopes] 7B50 - No Sweep
Barry,

Following from Albert's suggestion that Lockout may be preventing the sweep from
starting, can you try the following.

Select a slow sweep speed eg 0.5 seconds /div.

With a suitable trigger signal present set the trigger level so that the trigger
light is off.

Set the sweep mode to Single Sweep and press the Reset button, the reset light
should stay on. Single sweep reset 'should' reset all the stages of the
lockout / holdoff circuitry.

Turn the trigger level to the point where the sweep should trigger. Do you see
a single slow sweep?

Just for clarity, with Auto sweep enabled do you see anything at all on screen?

Regards,

Roger


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