Date   

Re: 7904 - Very Dim Trace

 

On 14 Feb 2016 16:10:25 -0800, you wrote:

Now I'm confused even more than usual. As I understand from the messages to date, some conflicting info is apparent, so I'm trying to get it straight:

1. The problem scope seems to have the right sized display, just very dim, working at 14 kV instead of 21.
That is my understanding.

2. I thought the PDA made the deflection sensitivity relatively immune to variations in anode voltage.
It does make it relatively immune compared to a change in cathode
voltage and I think this depends on the design of the CRT. Going from
full PDA to zero PDA only halves the deflection sensitivity of the
7904 and 7603 but I remember in a past discussion thread here that in
a 465, it has very little effect.

3. Jochens disagrees with that, and David's actual experiments seem to show that the deflection sensitivity is affected, the same on both a 7904 (which I believe has PDA) and a 7603 (which I believe does not).
The 7603 has a cathode voltage of -2975 volts and a post deflection
acceleration voltage of +12 kilovolts.

See the problem here? Statements 1 and 2 can fit together, but not 2 and 3, so therefore not 1 and 3. am I missing something again?

If 1 and 3 are true, one possibility is that someone adjusted the gains to compensate for the reduced anode voltage. BUT, I thought this just happened recently, and the scope was working normally before.
I suspect such a minor change in PDA voltage from +21 kilovolts to +14
kilovolts is not enough to affect the deflection sensitivity
significantly. That is why I think another explanation is needed for
the very dim display. Maybe the low PDA voltage was a long standing
but unknown issue with this particular 7904.

Ed


Re: 7904 - Very Dim Trace

Brian Bloom
 

I am new here and fairly new to EE, but I have an idea regarding your problem. The expected voltage of 21kv and measured voltage of 14kv are multiples of 7... so if the high voltage multiplier is a 3 stage one, is it possible that you have a faulty stage in your voltage multiplier?


Introduction of myself!

Brian Bloom
 

Thank you for allowing me to join this dedicated group!

My name is Brian. 
I am a 29 year old in the East Coast area of the USA.
I have always been a technically inclined person, learning trades with ease - even teaching myself some, including residential wiring.  I thought that my entrance into the EE world would go as well..... (HA!!)    After attempting to " learn as I go", I chose to buckle down and buy some books to read/study/reference.  Such books as the highly recommended set - Electronics for inventors, Engineering Mathematics, Art of Electronics - and later books dedicated to subjects such as semiconductors, logic, etc.

I currently own several 400 series scopes, one 7613, and plenty of calibration signal generators.
I also broke down and bought a modern scope to supplement all the analog gear, and especially to help me learn/troubleshoot/build.

I am aware of a bit of Tektronix history, and seeing the historical value, real value, and high quality of vintage Tektronix gear has gotten me hooked -  I want to learn and have my own little piece of that history.  The kind of quality that Tektronix offered back in it's glory days is unprecedented and will likely never be seen again, which makes me more anxious to obtain and restore these pieces of history while they are still obtainable and before people tear them to shreds for parts/scrap money.

My first and second projects will be the finishing up of the repair and calibration of a 468 and the calibration of my 465.  I just need to get my newly repaired 106 square wave gen's fast rise output section calibrated somehow....


Re: 7904 - Very Dim Trace

Ed Breya
 

Now I'm confused even more than usual. As I understand from the messages to date, some conflicting info is apparent, so I'm trying to get it straight:

1. The problem scope seems to have the right sized display, just very dim, working at 14 kV instead of 21.

2. I thought the PDA made the deflection sensitivity relatively immune to variations in anode voltage.

3. Jochens disagrees with that, and David's actual experiments seem to show that the deflection sensitivity is affected, the same on both a 7904 (which I believe has PDA) and a 7603 (which I believe does not).

See the problem here? Statements 1 and 2 can fit together, but not 2 and 3, so therefore not 1 and 3. am I missing something again?

If 1 and 3 are true, one possibility is that someone adjusted the gains to compensate for the reduced anode voltage. BUT, I thought this just happened recently, and the scope was working normally before.

Ed


Re: PG502 power lamp off

 

44h11?


Jerry Massengale

-----Original Message-----
From: christophkukulies@... [TekScopes] <TekScopes@...>
To: TekScopes <TekScopes@...>
Sent: Sun, Feb 14, 2016 3:43 pm
Subject: [TekScopes] PG502 power lamp off






With the TM5006 frame I recently acquired came a couple of modules.
After I luckily got the PG506 repaired fairly easily I also have one
PG502 needing care.

The power lamp is off when seated in a powered up TM frame.

I suspect Q650 (X44C282). Anyone knowing a replacement type for this ?


--
Christoph




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









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


Re: OT - Centronics printer cables

 

In the past I have been guilty of using line printer cables to make extender cables.


Jerry Massengale

-----Original Message-----
From: dan-meeks@... [TekScopes] <TekScopes@...>
To: TekScopes <TekScopes@...>
Sent: Sun, Feb 14, 2016 2:00 pm
Subject: [TekScopes] OT - Centronics printer cables






My neighbor has a "box full" of old-school Centronics printer cables. These have the big D-Sub connector on one end (for the "parallel port").
I am sorry for the BW here but I thought maybe there might be some old piece of gear that used this type of connection. I think there is some Tek equipment that used this connector.
He also has an Oki laser printer with a box full of toner, says it still works. Probably not worth shipping but if you're in or near Austin and want it, we can figure something out.
Thanks

Dan



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









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


Re: OT - Centronics printer cables

 

Jerry Massengale

-----Original Message-----
From: dan-meeks@... [TekScopes] <TekScopes@...>
To: TekScopes <TekScopes@...>
Sent: Sun, Feb 14, 2016 2:00 pm
Subject: [TekScopes] OT - Centronics printer cables






My neighbor has a "box full" of old-school Centronics printer cables. These have the big D-Sub connector on one end (for the "parallel port").
I am sorry for the BW here but I thought maybe there might be some old piece of gear that used this type of connection. I think there is some Tek equipment that used this connector.
He also has an Oki laser printer with a box full of toner, says it still works. Probably not worth shipping but if you're in or near Austin and want it, we can figure something out.
Thanks

Dan



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









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


Re: 7904 - Very Dim Trace

Jim McIntyre
 

Thanks for your report David. I appreciate your taking the time to run the experiment. Yes - I too found there's a right and wrong way to pull the PDA connector apart...

On my scope, after the dimming event occurred, at first I saw no trace at all. It was only after I tweaked the grid bias that I was able to see a dim trace. Maybe it was mis-adjusted in the first place, but I have lingering doubt, that dropping from 21 down to 14 kV would cause the dramatic dimming I am seeing. I'll re-check the deflection sensitivity, but as I recall. it was spot-on.



Comment about carbon resistor in the heater circuit noted - thanks.


Re: 7904 - Very Dim Trace

 

I do not think so. I tested my 7904 with the PDA disconnected and
while the display was dimmer, it is not what I would consider "very
dim" and the focus was reasonably sharp.

On 14 Feb 2016 13:12:01 -0800, you wrote:

Could the drop from 21 to 14 kV be the sole cause for the very dim trace?


Re: PG502 power lamp off

 

Hi Christoph,
Wouldn't it be more likely that the bulb burned out than a transistor went
bad?
The TM500 plugins all used ordinary incandescent bulbs and they all, in my
opinion, had relatively short lifetimes compared to how long the plugins
have been in use. Of course when most of the plugins were designed LEDs were
not common or as cheap as these bulbs were. The power bulbs in most TM500
plugins I own are dead by now.
For a while I was buying exact replacements for the bulbs when I could find
them but now I just replace them with LEDs whenever a dead one becomes
annoying enough.

Dennis

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Sunday, February 14, 2016 1:43 PM
To: TekScopes@...
Subject: [TekScopes] PG502 power lamp off

With the TM5006 frame I recently acquired came a couple of modules.
After I luckily got the PG506 repaired fairly easily I also have one
PG502 needing care.

The power lamp is off when seated in a powered up TM frame.

I suspect Q650 (X44C282). Anyone knowing a replacement type for this ?
Christoph









------------------------------------
Posted by: christophkukulies@...
------------------------------------


------------------------------------

Yahoo Groups Links


Re: 7904 - Very Dim Trace

 

That is the way it works for the most part. The post deflection
acceleration can have *some* effect on the deflection sensitivity but
it is no significant. On a 7603 for instance, removing the PDA
reduces the deflection sensitivity to about 1/2.

I tested my 7904 and got the same results; the display was dim and
slightly out of focus but readable and the deflection was exactly 1/2.

On 14 Feb 2016 10:10:56 -0800, you wrote:

The vert and horiz calibration are still spot-on. Odd, and that threw me off-track for a while, but I'm now pretty sure that's the way Post Deflection Acceleration works. (pretty sure, that is, until someone straightens me out :) )


Re: 7904 - Very Dim Trace

 

On 14 Feb 2016 11:48:50 -0800, you wrote:

Oops - one more additional thing too. This is more of a something-to-ponder item, not a possible cause or recommendation.

The beam current that leaves the cathode partly has to be absorbed in the dome mesh circuit. The rest has to pass on through to the anode. The mesh effectively shields the gun and deflectors from the anode potential, so whatever happens at the gun end should happen regardless of the anode condition. Not a hundred percent, but perhaps mostly.

So, what happens when the CRT anode voltage is greatly reduced? What if it's zero?
I tested it on my 7904. With the PDA lead disconnected from the high
voltage multiplier and shorted to the chassis, the CRT display was dim
but readable, clearly focused, and had exactly half of the deflection
sensitivity so full scale vertical deflection was 4 divisions instead
of 8 divisions. This matches what happens to a 7603 with the PDA
disconnected.

Note that when adjusting the PDA connector on the 7904, it is easy to
accidentally disconnect the large connector going to the z-axis board
producing no display. Also the CRT can maintain considerable charge
on its PDA lead which can deal a nasty shock to the oscilloscope or
yourself so carefully ground the lead into the chassis; I stick the
pin of the PDA lead into the perforated shield below it.

Ed


Re: HP 3325A synthesizer generator cold start

Max Frister
 

Sorry, folks. I sent this to the wrong group.

(please don't reply here so we don't bother people that don't want to hear about non-tek equipment).


PG502 power lamp off

Christoph
 

With the TM5006 frame I recently acquired came a couple of modules.
After I luckily got the PG506 repaired fairly easily I also have one
PG502 needing care.

The power lamp is off when seated in a powered up TM frame.

I suspect Q650 (X44C282). Anyone knowing a replacement type for this ?


--
Christoph


Re: 7904 - Very Dim Trace

 

On 14 Feb 2016 10:36:50 -0800, you wrote:

Whoa, let's back up a few lines here. If I understand properly, in a previous post, the actual voltages were measured with a good HV probe. If these measurements are valid, then the anode accelerating voltage is way too low, while the cathode supply seems right, pointing to a multiplier problem.

I don't have the 7904 manual here, but as I recall, all the HV supplies and the DC restorer come from the same place, namely a single HV winding built into the main power transformer, and are thus regulated by virtue of the proper voltages appearing on all the windings, including the various "preregulated" supply voltages. The low voltages are then further linearly regulated to their final values, while the HVs depend on the prereg value, which is sufficiently good for proper operation. If the cathode voltage is right, and the DC restorer is functioning properly to supply the right G1-K voltage range, then the lack of anode voltage must be due to a multiplier or related circuit problem. The DC restorers should actually be able to work even if the HV winding is way low, since those circuits depend on current sources, which are high resistances (multi-megohm) from the HV winding, and need only modest power compared to the beam supplies.
The cathode supply is further independently regulated on the secondary
side by U1635 although it depends on the regulated +50 volt supply as
a reference. The peak-to-peak output from the high voltage winding
needs to be within about 80 volts for this to work which is apparently
the case here.

It would be worthwhile to also look at the waveforms on any of the low voltage prereg supply windings to confirm that a clean, symmetric squarish wave is present, indicating proper operation of the main supply.

So, if the winding waveform is right, the cathode voltage is right, and the DC restorers are working properly, but the anode is only 2/3 of normal (especially with no load), then the multiplier has a problem internally, or in its feed from the HV winding. The diagnosis depends on the validity of the measurements.
Another possibility might have been the CRT anode loading down the
multiplier output but since the high voltage measurement was made
without the multiplier output connected to the CRT, that seems
unlikely unless the high voltage probe also loaded it down. The
schematic for the 7704 shows 50 megohms in series with the output of
the high voltage multiplier and I wonder if the 152-0493-00 is built
the same way. The Fluke 80k-40 has an input resistance of 1 gigohm so
50 megohms would only increase the measurement error by 5%.

...

I don't have the 7904 manual here, but as I recall, all the HV supplies and the DC restorer come from the same place, namely a single HV winding built into the main power transformer, and are thus regulated by virtue of the proper voltages appearing on all the windings, including the various "preregulated" supply voltages.

...

I may have more to say on this later. Over the years I have lost two 7844s and a 7904 to HV transformer winding failures, and recently another 7904 just crapped out - I hope it's not heading for the same demise. As I recall more and more, I'll offer some ideas and theories on the subject.

...
It does not apply here but the 7834 and 7934 drive a separate high
voltage transformer from the 54 volt AC output of the off-line
transformer. I assume this is because they have a more complicated
high voltage secondary to support their storage and reduced scan
functions.


Re: 7904 - Very Dim Trace

Dave Hills
 

On the Z-axis board there is a resistor, R1887, in series with the CRT heater supply. It is shown in the parts list as a fixed composition type.
Have you checked it?

I ask because I have found and replaced several low ohm, 1/2W composition resistors in my 7904 power supply which were very high in value relative to spec. Could your dim display be from low CRT cathode emission due to diminished heater current?

Also note that Tek changed the value of R1887 in later S/N's from 10 to 16 ohms in a March 15,1983 Change notice.

Dave


Re: 7904 - Very Dim Trace

Jim McIntyre
 

Could the drop from 21 to 14 kV be the sole cause for the very dim trace?


Re: 7904 - Very Dim Trace

Ed Breya
 

Yes, Jochens, if the anode is only the face, there would mostly be axial acceleration, which would shrink the deflection at higher anode voltage. However, I believe most CRTs have a large portion of the jug (inside) also at anode (or gradient) potential, which makes for some kind of field pattern that would provide radial acceleration too. I think the electron optics and the shape of the acceleration field all interact to get the end result. I don't know the details, but I can picture what's going on - sort of.

What I think I recall about PDA is that the mesh dome is an electron optical element and a shield. The mesh has an image of the deflected beam, which is magnified uniformly spherically toward the screen by the anode field. I think the shielding effect keeps the anode field from interacting with the deflection plates, so they are in a more controlled environment after the gun, and before the mesh. They also don't have to deflect the beam as far to make the small image on the mesh, so sensitivity can be higher for given BW.

I could be way off on this - I haven't looked at that stuff in years, but I think I remember the purpose. The main thing is that if it didn't do these things, then why bother.

Ed


Re: HP 3325A synthesizer generator cold start

 

Well just to throw in a quick comment. It might be that an eprom may be on the edge of failing. Use some cold spray to cool down the firmware eproms and see if it happens after a running session.

I'll follow this on the HP group.


Regards,
Tom

----- Original Message -----
From: 'Dennis Tillman' @Dennis_Tillman_W7pF [TekScopes]
To: TekScopes@...
Sent: Sunday, February 14, 2016 3:31 PM
Subject: RE: [TekScopes] HP 3325A synthesizer generator cold start



Hi Max,
There is a very active HP Forum on Yahoo. You will have a better chance of
finding help there.
Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Sunday, February 14, 2016 9:56 AM
To: TekScopes@...
Subject: [TekScopes] HP 3325A synthesizer generator cold start

I have an HP 3325A function synthesizer that I bought cheap a couple of
years ago because it has a problem. When the machine is cold (literally,
below room temperature), it does not start right away. Usually, you can see
some lights on the panel light and sometimes even the "1000" on the display,
but in less than a second the display goes blank, the lights freeze and the
keyboard is locked.

However, if I leave it on for a few minutes (frozen), and then power cycle
it it always starts correctly.

Looking around the web, I see other reports of similar problems:
http://holzleitner.com/el/hp-3325/index-en.html
http://holzleitner.com/el/hp-3325/index-en.html
http://www.eevblog.com/forum/testgear/hp-3325-function-generator/
http://www.eevblog.com/forum/testgear/hp-3325-function-generator/

I've tried several times to track this problem down. I thought it was a
great opportunity to try out my newly repaired Tek 2430a *digital*
oscilloscope. I had come to suspect the cpu was not getting the start
signal. Apparently it is.

Here's what I think I know:

1. Power supply is fine. New caps, low ripple, within spec, etc.
Holzleitner was able to fix his 3325a by tweaking voltages but I've tried
various (small) tweaking of both the +5 and -5 "back voltage" with no
apparent change.

2. The CPU is getting the start signal (yea digital capture!) and the clock
signal is present.

3. I think it is a main-board problem. When the machine starts both the
front panel and GPIB are available. When it doesn't, neither are.

4. I think the processor is starting and working. You see evidence of it
starting with the transient front-panel display (the processor runs the
display pretty directly). NB: it never starts working by itself without the
additional power cycle.

5. I think the processor is continuing to run, at least partly, even when
the machine freezes. If I use the scope on the address bus, I can see
digital data continuing.

I've tried to follow the troubleshooting map in the manual, but without the
display working and a "signature analyzer", I can't go very far with that.

This is sort of an annoying problem to trouble-shoot since after a few power
cycles it has to cool for an hour before it will misbehave again. I also
have to work on it in winter since it usually starts fine in the summer in
my unheated shop.

I'm would appreciate any advise as to where to look.



------------------------------------
Posted by: maxfrister@...
------------------------------------

------------------------------------

Yahoo Groups Links


Re: HP 3325A synthesizer generator cold start

 

Hi Max,
There is a very active HP Forum on Yahoo. You will have a better chance of
finding help there.
Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Sunday, February 14, 2016 9:56 AM
To: TekScopes@...
Subject: [TekScopes] HP 3325A synthesizer generator cold start

I have an HP 3325A function synthesizer that I bought cheap a couple of
years ago because it has a problem. When the machine is cold (literally,
below room temperature), it does not start right away. Usually, you can see
some lights on the panel light and sometimes even the "1000" on the display,
but in less than a second the display goes blank, the lights freeze and the
keyboard is locked.

However, if I leave it on for a few minutes (frozen), and then power cycle
it it always starts correctly.

Looking around the web, I see other reports of similar problems:
http://holzleitner.com/el/hp-3325/index-en.html
http://holzleitner.com/el/hp-3325/index-en.html
http://www.eevblog.com/forum/testgear/hp-3325-function-generator/
http://www.eevblog.com/forum/testgear/hp-3325-function-generator/

I've tried several times to track this problem down. I thought it was a
great opportunity to try out my newly repaired Tek 2430a *digital*
oscilloscope. I had come to suspect the cpu was not getting the start
signal. Apparently it is.



Here's what I think I know:

1. Power supply is fine. New caps, low ripple, within spec, etc.
Holzleitner was able to fix his 3325a by tweaking voltages but I've tried
various (small) tweaking of both the +5 and -5 "back voltage" with no
apparent change.

2. The CPU is getting the start signal (yea digital capture!) and the clock
signal is present.

3. I think it is a main-board problem. When the machine starts both the
front panel and GPIB are available. When it doesn't, neither are.

4. I think the processor is starting and working. You see evidence of it
starting with the transient front-panel display (the processor runs the
display pretty directly). NB: it never starts working by itself without the
additional power cycle.

5. I think the processor is continuing to run, at least partly, even when
the machine freezes. If I use the scope on the address bus, I can see
digital data continuing.

I've tried to follow the troubleshooting map in the manual, but without the
display working and a "signature analyzer", I can't go very far with that.

This is sort of an annoying problem to trouble-shoot since after a few power
cycles it has to cool for an hour before it will misbehave again. I also
have to work on it in winter since it usually starts fine in the summer in
my unheated shop.

I'm would appreciate any advise as to where to look.














------------------------------------
Posted by: maxfrister@...
------------------------------------


------------------------------------

Yahoo Groups Links