Date   
Re: 7904 - Very Dim Trace

 

On 23 Feb 2016 20:08:08 -0800, you wrote:

Small update:

I went through the Service Manual's Z-axis DC Levels Procedure (pertaining to R1757, R1810, and R1817). All went well. Or at least I had no trouble adjusting it to spec. The dim display persists. Focus is good. Deflection is not affected. (all as noted earlier in this thread)

Again, I measured the cathode at -3001V, and the control grid at -3001 to -3050, depending on the intensity controls.

I don't see how this can have anything to do with the DC restoration circuits. Though if someone can explain how that's possible, I have the caps and diodes in-hand to rebuild it...
With these measurements I do not see how either.

One tiny clue, perhaps. On the fateful night when I last switched off the scope while it was working (with a bright display), I noticed a pretty bright glow immediately afterwards on the CRT face. I don't recall any more details, but it seemed a little odd, although I hadn't been using this scope long, so didn't think too much about it. Still... Hmm...
CRTs generate an afterglow even from picking up ambient room lighting.

I wonder if the CRT was damaged in some way. The control grid is not
like the grid of a normal tube so I cannot see how it could be
damaged.

What about voltages to the other CRT elements which go through P1720
including the -50 volt and ground connections? Are they correct?

Maybe there is enough variation in CRT performance with different PDA
voltages to explain it. My 7904 using a 12 kilovolt PDA may be
atypical.

Since the only diagnosed problem that can be fixed so far is the PDA,
that may be the best option.

Re: PG506 mod

 

It might be worthwhile to go through the parts list and mark on the
schematic exactly which parts apply to the first version of the early
PG506 and which parts apply to the last version of the early PG506 to
get two different schematics instead of one merged one.

That may reveal the reason for any performance differences. As the
schematic is now, it is difficult to tell what is even going on since
it is effectively wrong for all versions.

On 23 Feb 2016 03:12:43 -0800, you wrote:

Thanks, guys. I now understand that this is stock and only depends on serial number.
Will post mine later.

Nonetheless I'm wondering why the risetime of the standard pulse was noticeably better on the unit with the large heat sink (don't know whether it has sth. to do with it, though)

Re: Need Help - Tektronix 7844 Dual Beam Scope

 

On Tue, 23 Feb 2016 15:34:34 -0500, you wrote:

So both beams have the same intensity problems?
It appears so. I feed the Calibrator to both inputs of a 7A26 Vertical
module (left slot). BEAM 1 and 2 are set to LEFT. Using the Vertical
positioning on the vertical module, I can moving either beam on and off
screen. They both appear to be the correct display (Vertical Gain and Time
Base working normally) but the brightness is very high and I have no control
over either BEAM INTENSITY or FOCUS ON EITHER CHANNEL.
The beams have completely separate intensity and focus circuits so I
would look for a problem which could affect both.

Excessive beam current should cause the power supply to reset instead
of just affecting the 17 volt output.
I was just monitoring the 17 volt line - others may change as well. This
effects seems intermittent as I just tried to monitor the other supply
voltages under the same conditions and the power supply was not affected.
I'm cautious about leaving the display on very long - don't want to burn the
CRT or possibly fry the power supply.
Are all of the regulated power supply outputs correct?

Those amplifier outputs control the focus and not the intensity.
TP1186 and TP2086 shown on schematic 14 are the z-axis outputs which
control intensity.
INTENSITY controls seem to connect to GRID 2 - I'm missing something here -
can you give me an idea how this circuit works or where I might look it up?
I thought I remembered an old Tektronix book on CRT design but can't find
it.
There is one intensity control element and one focus control element
per beam. The signal from each intensity control potentiometer splits
to drive both the z-axis amplifier and the focus amplifier for one
beam.

I checked TP 1186 BEAM 1 (Card A21) - adjusting BEAM INTENSITY - voltage is
+9.75 CCW to + 68.5 Volts CW. So far I have not been able to get as BEAM 2
card for TP2086.
So the focus amplifier output voltages are correct and at least one of
the z-axis amplifier output voltages is correct.

Check that connectors P2390 and P2490 shown at the bottom left corner
of schematic 15 are good.

Are the grid bias controls R1180 and R2080 working correctly? Do they
do anything when the intensity is turned fully CCW? They are shown at
the bottom left corner of schematic 15.

Re: Packing a scope for shipping

S. Truong <sammy.truong@...>
 

Hi all, to add to some of the sound advice shared so far, I thought I'd
pass along these words directly from the horse, transcribed from my 2465
Service and Instruction manual:

REPACKAGING FOR SHIPMENT

If this instrument is to be shipped by commercial transportation, it is
recommended that it be packaged in the original manner. The carton and
packaging material in which your instrument was shipped to you should be
saved and used for this purpose.

If the original packaging is unfit for use or is not available, repackage
the instrument as follows:

1. Obtain a corrugated cardboard shipping carton having inside dimensions
at least six inches greater than the instrument dimensions and having a
carton test strength of at least 275 pounds.

2. If the instrument is to be shipped to a Tektronix Service Center for
service or repair, attach a tag to the instrument showing the following:
owner of the instrument (with address), the name of the person at your firm
who can be contacted, complete instrument type and serial number, and a
description of the service required.

3. Wrap the instrument with polyethylene sheeting or equivalent to protect
the outside finish and prevent entry of packing materials into the
instrument.

4. Cushion the instrument on all sides by tightly packing dunnage or
urethane foam between the carton and the instrument, allowing three inches
on each side.

5. Seal the carton with shipping tape or with an industrial stapler.

6. Mark the address of the Tektronix Service Center and your return address
on the carton in one or more prominent locations.


Best,
Sammy

On Wed, Feb 24, 2016 at 6:18 AM, Tom Gardner tggzzz@... [TekScopes] <
TekScopes@...> wrote:



Thanks to everybody for their thoughtful and practical tips.



--
--
sammy.truong@...

Re: Reinserting CRT into a 465 chassis

Tom Gardner
 

On 24/02/16 13:24, Malcolm Hunter malcolm.r.hunter@... [TekScopes] wrote:

On 24 February 2016 at 13:13, Tom Gardner tggzzz@... [TekScopes] <
TekScopes@...> wrote:

Did you manage to find a replacement CRT then?
Hopefully yes, and from the source you most helpfully suggested. But since
it
isn't yet in my possession, I'm not "counting my CRTs until they are
hatched" :)
​Here's hoping it won't be hatched in transit :)
A major advantage of this source is that I'm picking it up in person (date TBA).
If it hatches, it'll be my fault!

Re: Reinserting CRT into a 465 chassis

Malcolm Hunter
 

On 24 February 2016 at 13:13, Tom Gardner tggzzz@... [TekScopes] <
TekScopes@...> wrote:

Did you manage to find a replacement CRT then?
Hopefully yes, and from the source you most helpfully suggested. But since
it
isn't yet in my possession, I'm not "counting my CRTs until they are
hatched" :)
​Here's hoping it won't be hatched in transit :)

Malcolm​


[Non-text portions of this message have been removed]

Re: Reinserting CRT into a 465 chassis

Tom Gardner
 

On 24/02/16 12:53, Malcolm Hunter malcolm.r.hunter@... [TekScopes] wrote:

On 24 February 2016 at 12:32, Tom Gardner tggzzz@... [TekScopes] <
TekScopes@...> wrote:

​I just used my Lindstrom needle-nose pliers when I took the base board
off
a few times.​ I've been told forceps are quite good for this too:

http://www.amazon.com/5-5-KELLY-Hemostat-FORCEPS-Straight/dp/B000QWK174
I'm hoping it won't be necessary to remove the interface board; too may
connections to get wrong. (Me a pessimist? Shurely shome mishtake!)

It is hopeful that those forceps are similar in concept to my "tweezers"
like:

http://www.amazon.com/TWEEZER-CROSS-TWEEZERS-CLOSING-NOVELTOOLS/dp/B011ABLEE8
​If you can get a decent grip with those, you should be okay. You need a
firm grip so they don't slip when you're pushing the pins back on.
Yes; the grip is unclear at the moment. Forceps would be better in that regard.

No, you won't need to remove the board if you're changing the CRT - I did
so to get to the bridge rectifier connections, though even that wasn't
really necessary. Did you manage to find a replacement CRT then?
Hopefully yes, and from the source you most helpfully suggested. But since it
isn't yet in my possession, I'm not "counting my CRTs until they are hatched" :)

Thanks.



[Non-text portions of this message have been removed]

Re: Reinserting CRT into a 465 chassis

Malcolm Hunter
 

On 24 February 2016 at 12:32, Tom Gardner tggzzz@... [TekScopes] <
TekScopes@...> wrote:

​I just used my Lindstrom needle-nose pliers when I took the base board
off
a few times.​ I've been told forceps are quite good for this too:

http://www.amazon.com/5-5-KELLY-Hemostat-FORCEPS-Straight/dp/B000QWK174
I'm hoping it won't be necessary to remove the interface board; too may
connections to get wrong. (Me a pessimist? Shurely shome mishtake!)

It is hopeful that those forceps are similar in concept to my "tweezers"
like:

http://www.amazon.com/TWEEZER-CROSS-TWEEZERS-CLOSING-NOVELTOOLS/dp/B011ABLEE8
​If you can get a decent grip with those, you should be okay. You need a
firm grip so they don't slip when you're pushing the pins back on.

No, you won't need to remove the board if you're changing the CRT - I did
so to get to the bridge rectifier connections, though even that wasn't
really necessary. Did you manage to find a replacement CRT then?

Malcolm​

Re: Reinserting CRT into a 465 chassis

Tom Gardner
 

On 24/02/16 11:46, Malcolm Hunter malcolm.r.hunter@... [TekScopes] wrote:

On 24 February 2016 at 11:38, tggzzz@... [TekScopes] <
TekScopes@...> wrote:

Of particular concern are the wires to the horizontal deflection plates on
the CRT's neck. Removing the wires to the horizontal deflection plates on
the CRT's neck is probably not problem. If nothing else, simply pulling on
the wires to the interface board will probably slip the wires off the CRT.
Replacing them looks trickier, since they are about 5cm down inside a 1.5cm
hole, and my fingers aren't that small!

So, are there any practical techniques that aren't mentioned in the
manual?
Are long tweezers sufficient?
Is it sensible/possible to desolder the wires from the PCB, attach them to
new CRT before it is reinserted in the chassis, tie a thread to the wires,
feed the thread through the chassis and PCB holes, reinsert the CRT in the
chassis using the thread to guide the wires, and finally resolder the wires
to the PCB.
​I just used my Lindstrom needle-nose pliers when I took the base board off
a few times.​ I've been told forceps are quite good for this too:

http://www.amazon.com/5-5-KELLY-Hemostat-FORCEPS-Straight/dp/B000QWK174
I'm hoping it won't be necessary to remove the interface board; too may
connections to get wrong. (Me a pessimist? Shurely shome mishtake!)

It is hopeful that those forceps are similar in concept to my "tweezers" like:
http://www.amazon.com/TWEEZER-CROSS-TWEEZERS-CLOSING-NOVELTOOLS/dp/B011ABLEE8

Thanks.


[Non-text portions of this message have been removed]

Re: Reinserting CRT into a 465 chassis

Malcolm Hunter
 

On 24 February 2016 at 11:38, tggzzz@... [TekScopes] <
TekScopes@...> wrote:

Of particular concern are the wires to the horizontal deflection plates on
the CRT's neck. Removing the wires to the horizontal deflection plates on
the CRT's neck is probably not problem. If nothing else, simply pulling on
the wires to the interface board will probably slip the wires off the CRT.
Replacing them looks trickier, since they are about 5cm down inside a 1.5cm
hole, and my fingers aren't that small!

So, are there any practical techniques that aren't mentioned in the
manual?
Are long tweezers sufficient?
Is it sensible/possible to desolder the wires from the PCB, attach them to
new CRT before it is reinserted in the chassis, tie a thread to the wires,
feed the thread through the chassis and PCB holes, reinsert the CRT in the
chassis using the thread to guide the wires, and finally resolder the wires
to the PCB.
​I just used my Lindstrom needle-nose pliers when I took the base board off
a few times.​ I've been told forceps are quite good for this too:

http://www.amazon.com/5-5-KELLY-Hemostat-FORCEPS-Straight/dp/B000QWK174

Malcolm

Re: Reinserting CRT into a 465 chassis

Brian Bloom
 

I use a pair of hemastats to get a firm grip, then gently and very slightly twist if it doesn't initially want to go on. Just pushing it on is usually sufficient, though. Needle nose pliers would work, too. You just have to make sure you don't accidentally stab the glass!

Re: Need Help - Tektronix 7844 Dual Beam Scope

Martin Mehlhose
 

you Need the check of Voltage at both cathodes of the CRT.

This requires a HV Voltmeter.


greetings

Martin






Gesendet von Windows Mail





Von: TekScopes@...
Gesendet: ‎Dienstag‎, ‎23‎. ‎Februar‎ ‎2016 ‎21‎:‎34
An: TekScopes@...








So both beams have the same intensity problems?

It appears so. I feed the Calibrator to both inputs of a 7A26 Vertical
module (left slot). BEAM 1 and 2 are set to LEFT. Using the Vertical
positioning on the vertical module, I can moving either beam on and off
screen. They both appear to be the correct display (Vertical Gain and Time
Base working normally) but the brightness is very high and I have no control
over either BEAM INTENSITY or FOCUS ON EITHER CHANNEL.

Using the BEAM FINDER (with both beams positioned off screen) BEAM 1
produces a soft display - not really bright, and both INTENSITY and FOCUS
sort of work poorly. BEAM 2 Beam Finder produces a very bright display with
no control of INTENSITY or FOCUS.

What about the readout intensity?

No readout is visible.

Excessive beam current should cause the power supply to reset instead
of just affecting the 17 volt output.

I was just monitoring the 17 volt line - others may change as well. This
effects seems intermittent as I just tried to monitor the other supply
voltages under the same conditions and the power supply was not affected.
I'm cautious about leaving the display on very long - don't want to burn the
CRT or possibly fry the power supply.

Those amplifier outputs control the focus and not the intensity.

TP1186 and TP2086 shown on schematic 14 are the z-axis outputs which
control intensity.

INTENSITY controls seem to connect to GRID 2 - I'm missing something here -
can you give me an idea how this circuit works or where I might look it up?
I thought I remembered an old Tektronix book on CRT design but can't find
it.

I checked TP 1186 BEAM 1 (Card A21) - adjusting BEAM INTENSITY - voltage is
+9.75 CCW to + 68.5 Volts CW. So far I have not been able to get as BEAM 2
card for TP2086.

Any ideas are welcome - it's been over 40 years since I did service for a
living - brain cells are rusty.

Jim

From: TekScopes@... [mailto:TekScopes@...]
Sent: Monday, February 22, 2016 12:28 PM
To: TekScopes@...
Subject: Re: [TekScopes] Need Help - Tektronix 7844 Dual Beam Scope

So both beams have the same intensity problems?

What about the readout intensity?

Defocusing the beams will help prevent phosphor damage until the
intensity can be controlled.

On 22 Feb 2016 07:10:23 -0800, you wrote:

An old coot from the tube era needs some help with a Tek 7844.

Trace Intensity seems to be at maximum with no Intensity control response.

Power supply voltages seem normal so long as the Beams are positioned off
screen. Moving the beams just barely on the edge of screen to see the trace
indicates vertical and time base appear to be working.

Moving the Beams on screen momentarily results in the fan slowing down and
the 17 volt rail dropping (excessive CRT beam current?).

Excessive beam current should cause the power supply to reset instead
of just affecting the 17 volt output.

On the A23 Board TP2445 Beam 1 - Beam Intensity varies voltage from CCW 126
to CW 64 and at TP 2472 Beam 2 Intensity varies the voltage from CCW 127 to
CW 17. This suggests the Intensity control circuit is working.

Those amplifier outputs control the focus and not the intensity.

TP1186 and TP2086 shown on schematic 14 are the z-axis outputs which
control intensity.

Anyone have any suggestions as to where to go from here?

Board A23 seems virtually impossible to access - anyone know how to get to
it to replace components?

Reinserting CRT into a 465 chassis

Tom Gardner
 

I need to replace the CRT in a 465. This question is about the practicalities of re-attaching wires to the new CRT's neck.

Of particular concern are the wires to the horizontal deflection plates on the CRT's neck. Removing the wires to the horizontal deflection plates on the CRT's neck is probably not problem. If nothing else, simply pulling on the wires to the interface board will probably slip the wires off the CRT. Replacing them looks trickier, since they are about 5cm down inside a 1.5cm hole, and my fingers aren't that small!

So, are there any practical techniques that aren't mentioned in the manual?
Are long tweezers sufficient?
Is it sensible/possible to desolder the wires from the PCB, attach them to new CRT before it is reinserted in the chassis, tie a thread to the wires, feed the thread through the chassis and PCB holes, reinsert the CRT in the chassis using the thread to guide the wires, and finally resolder the wires to the PCB.

Thanks.

Re: Packing a scope for shipping

Tom Gardner
 

Thanks to everybody for their thoughtful and practical tips.

Need Help Fixing A Couple of 2445 Scopes

Michael Louie
 

I bought a 2445 (for $50, with the front cover and a pouch) as a project to repair 2-3 years ago, and have been dropping into TekScopes when I have a chance to absorb knowledge about how to fix the thing. Seeing so many messages about buying a spare scope to use for parts, I held off trying to fix the first one until I got a second 2445 to use for parts.


The first 2445 is #B0255XX (standard scope without any options) and the second 2445 (has GPIB) is #B031XXX. Although these scopes serial numbers are only 6XXX numbers apart #B031XXX is a completely different scope inside.


#B0255XX has a Siemens fan, and show a big bright blob close to the middle of the screen that I cannot control. Tonight a found out that on J119 on the main board, I have -15, -5, +5, and +10. All other positive and negative voltages are too low. So, obviously, it needs to have the LVPS repaired or replaced.


#B031XXX I got it for $40 off eBay, and the description said it didn't turn on, and had a dent (on the bottom in the back). The Photos showed it was missing one small knob, the rear feet, and had the wrong screws in the back. The dent in the bottom would keep anybody from taking the scope out of the case to inspect it.


I bought it planning on using it for parts, but I wanted to try turning it on before I started taking parts off it (there was also some faint hope that it might work). Also, it has the GPIB Option, which I figured I could sell to recoup the cost of the parts scope, since sets of GPIB parts are selling for $100-$150 on eBay.


I plugged it after getting the dented/wrecked case off (didn't want to take the chance of a short with the big dent in the bottom). The first thing I notice is that it has a boxer fan on the back and not a Siemens, The scope would not turn on when I plugged it in, so I took a better look at it under bright lights. Noticed that it looked like someone took it apart (uplugging all the cables to the inside/hidden, and just left the poor thing like that. No wonder it didn't work. I checked the voltages at J119, and they were all there and within tolerance (did not check for ripple).


Here's the problem. There are more insides, inside since this scope has the GPIB Option, the problem is that I don't know where all of the wires go. I think I got all but one cable plugged in in the right places, but I have one end of one cable left, and I can't figure out where it goes.


The four conductor cable (all white wires) from the mode light panel above the CRT, Does it plug into the four conductor terminal down low on the solder side of the GPIB board?


The cable that really has me stuck is a (seven?) single row conductor cable with all white wires. One end seems to go on the Expansion Motherboard that fits inverted on the top of the scope chassis, next to a 34 ribbon connector cable that goes to the A5 Control Board. Where does the other end of this cable go? I can't seem to see any open plugs where it would fit.


I've looked but can't seem to find a copy of the GPIB Option 10 Service Manual to try to confirm where these wires go.

Re: 7904 - Very Dim Trace

Jim McIntyre
 

Small update:

I went through the Service Manual's Z-axis DC Levels Procedure (pertaining to R1757, R1810, and R1817). All went well. Or at least I had no trouble adjusting it to spec. The dim display persists. Focus is good. Deflection is not affected. (all as noted earlier in this thread)


Again, I measured the cathode at -3001V, and the control grid at -3001 to -3050, depending on the intensity controls.


I don't see how this can have anything to do with the DC restoration circuits. Though if someone can explain how that's possible, I have the caps and diodes in-hand to rebuild it...


One tiny clue, perhaps. On the fateful night when I last switched off the scope while it was working (with a bright display), I noticed a pretty bright glow immediately afterwards on the CRT face. I don't recall any more details, but it seemed a little odd, although I hadn't been using this scope long, so didn't think too much about it. Still... Hmm...

468 sweep offset issue

Brian Bloom
 

I have been slowly working on getting this 468 fixed, back into spec, and calibrated.
Physically, it is mint. There's no damage anywhere.

I had to replace 3 of the PSU capacitors: the 55V, the 5V, and the -8V.
In the time where I hadn't the slightest clue what I was doing, I replaced lots of parts that I didn't need to - It has all new capacitors. New tantalums, new lytics, new poly HV caps... even new ceramics. I was very, very careful to make sure that I used the correct values for everything - and not to mix anything up.

There are still a couple of things left to resolve:

- There is a voltage offset between the horizontal deflection plates, which I believe is causing the vertical elements of the traces on the far right 1/4 of the CRT to have a slight bow to them. The Y axis and Geometry adjustments are not able to resolve this. I have not yet tracked down the source of the issue here.

- The B sweep and A sweep do not sync at the start of the sweep. The sync pulse is correct. I have checked all of the waveforms numerous times trying to isolate the problem, but I believe this is a problem in the timebase section (according to "Tek: troubleshooting your oscilloscope") - making it a royal pain for a beginner like me to track down.
**This is the issue that is giving me all of the trouble**

I have swapped out the trigger board and also the timebase board in an effort to at least isolate the fault to a specific block. The problem must reside on the main board. My guess is that it's going to be in the sweep logic circuitry.


I have been using the "Tektronix: troubleshooting your oscilloscope" book. I have read the theory of operation for this scope a couple of times. I have even read the circuit description section of my 465 manual to try and get a better understanding of the basic functionality of these scopes.

I'm hoping that I will have an easier time troubleshooting since I bought a new digital scope.
I'm confident that I'll be able to track down the problem with the horizontal amp , but this sweep start offset is driving me nuts and I just cannot figure it out! Will someone please point me in the right direction?



Pics of the problem:
https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1461836945/lightbox/1384398865?orderBy=ordinal&sortOrder=asc&photoFilter=ALL https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1461836945/lightbox/1384398865?orderBy=ordinal&sortOrder=asc&photoFilter=ALL

New file uploaded to TekScopes

TekScopes@...
 

Hello,


This email message is a notification to let you know that
a file has been uploaded to the Files area of the TekScopes
group.


File : /468 repair by analogaddict013/P1120759.JPG
Uploaded by : analogaddict013 <analogaddict013@...>
Description : 468 sweep offset


You can access this file at the URL:
https://groups.yahoo.com/neo/groups/TekScopes/files/468%20repair%20by%20analogaddict013/P1120759.JPG


To learn more about file sharing for your group, please visit:
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Regards,


analogaddict013 <analogaddict013@...>

Re: Packing a scope for shipping

Paul Amaranth
 

465s are particularly sensitive to getting the B trigger level control smashed since it's on a corner. I've received a couple of scopes with that shipping damage.

On Tue, Feb 23, 2016 at 04:11:18PM -0800, analogaddict013@... [TekScopes] wrote:
A month or so ago I purchased a 454A. The shipper used layers of 3/4" styrofoam to pack it, and just stacked it around the handle. This scope didn't come with the original cover.

I had requested extra padding on the front of the scope... and good thing I did. I don't think that it was intentional, but one of the layers of styrofoam ended up getting press-fit around all of the knobs and switches.

Since then, I've been thinking that it may be a good idea to do so for any future shipments where the front of the instrument is susceptible to damage.
Next time I have to ship a scope, I'm going to triple up - styrofoam, then bubble wrap, then semi-rigid foam hot glued to the box.





!DSPAM:56ccf430299081463716623!
--
Paul Amaranth, GCIH | Rochester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows

Re: Packing a scope for shipping

Brian Bloom
 

A month or so ago I purchased a 454A. The shipper used layers of 3/4" styrofoam to pack it, and just stacked it around the handle. This scope didn't come with the original cover.

I had requested extra padding on the front of the scope... and good thing I did. I don't think that it was intentional, but one of the layers of styrofoam ended up getting press-fit around all of the knobs and switches.

Since then, I've been thinking that it may be a good idea to do so for any future shipments where the front of the instrument is susceptible to damage.
Next time I have to ship a scope, I'm going to triple up - styrofoam, then bubble wrap, then semi-rigid foam hot glued to the box.