7934 troubleshooting


Karin Johnson
 

All: The previous posting about 7934 giveaways, created almost a dozen private responses. Many with encouragement to try to restore at least one of the units. I was quite surprised. So in light of those responses, I've decided to give it the old "college try", and see if I can get at least one of the units working.
I'm not going to obsess on this process, so depending upon how frustrated I get these units may still go away to another home. I haven't made up my mind
yet. Yes, I know, I will probably be accused of being fickle. So be it.
Anyway, I'm off topic. Rant mode OFF.
Looking at the 7934 that is still in one piece, I mentioned before that I detected a burning odor when I powered up this unit that had been sitting a closet for a couple of years. This unit had the rebuilt/rewound HV transformer in it, and my first thought was the HV transformer had gone bad. When I rewound it I doubled up on insulation internally, so I a fair degree of confidence in it hanging in. When I opened up the covers I could really the detect the strong odor, but to my surprise it wasn't coming from the HV section. I was coming from the Power Supply module. I had no idea what was burning. No visible smoke.
Today while using this power supply as a test bench to take some readings to get the other supply running, I noticed that capacitor network C5 had blugged and the coating was melted on it. This part is really a "snubber" network consisting of a 22 ohm resistor in series with a 0.1uFd cap.
It is connected directly across the AC line before the common mode inductor, feeding the switcher. To my surprise the company that made this part for Tektronix is still in business and the part number as listed in the 7934 service manual is still good. There are a few US vendors that have stock.
So, for all you Sherlock Holmes fans, "The Game Is On".
Thanks for indulging me.
Karin


ken chalfant
 

Karen,

I have been where you now stand. I suspect many of us have. I have worked on a unit, been overcome with frustration and given up. After calming down, which sometimes meant going out to the trash can to retrieve it, I have revisited the offending unit and ultimately effected a repair.

I don’t think you at all fickle - just human and frustrated.

You stand a good chance of fixing it this time.

Best of luck and a little dash of skill.

Warm regards,

Ken

On 12May, 2021, at 1:25 PM, Karin Johnson <karinann@tampabay.rr.com> wrote:

All: The previous posting about 7934 giveaways, created almost a dozen private responses. Many with encouragement to try to restore at least one of the units. I was quite surprised. So in light of those responses, I've decided to give it the old "college try", and see if I can get at least one of the units working.
I'm not going to obsess on this process, so depending upon how frustrated I get these units may still go away to another home. I haven't made up my mind
yet. Yes, I know, I will probably be accused of being fickle. So be it.
Anyway, I'm off topic. Rant mode OFF.
Looking at the 7934 that is still in one piece, I mentioned before that I detected a burning odor when I powered up this unit that had been sitting a closet for a couple of years. This unit had the rebuilt/rewound HV transformer in it, and my first thought was the HV transformer had gone bad. When I rewound it I doubled up on insulation internally, so I a fair degree of confidence in it hanging in. When I opened up the covers I could really the detect the strong odor, but to my surprise it wasn't coming from the HV section. I was coming from the Power Supply module. I had no idea what was burning. No visible smoke.
Today while using this power supply as a test bench to take some readings to get the other supply running, I noticed that capacitor network C5 had blugged and the coating was melted on it. This part is really a "snubber" network consisting of a 22 ohm resistor in series with a 0.1uFd cap.
It is connected directly across the AC line before the common mode inductor, feeding the switcher. To my surprise the company that made this part for Tektronix is still in business and the part number as listed in the 7934 service manual is still good. There are a few US vendors that have stock.
So, for all you Sherlock Holmes fans, "The Game Is On".
Thanks for indulging me.
Karin





 

Karin,

I have a 7623A half open on the bench that I am having to pretend not to notice until I have calmed down enough to do a proper diagnosis of its power supply. I've been "calming down" for a few weeks. This is why it's good to have other projects to work on: it gives you time to clear your head.

-- Jeff Dutky


David Slipper
 

Is this series of devices particularly hard to access or not well documented or very complex ??

I've been tempted to get one but if they are that difficult I may have to think again.

Regards,
Dave

On 12/05/2021 22:00, Jeff Dutky wrote:
Karin,

I have a 7623A half open on the bench that I am having to pretend not to notice until I have calmed down enough to do a proper diagnosis of its power supply. I've been "calming down" for a few weeks. This is why it's good to have other projects to work on: it gives you time to clear your head.

-- Jeff Dutky




Dave Daniel
 

To what devices do you refer in your post?

On May 12, 2021, at 17:09, David Slipper <softfoot@hotmail.com> wrote:

Is this series of devices particularly hard to access or not well documented or very complex ??

I've been tempted to get one but if they are that difficult I may have to think again.

Regards,
Dave



On 12/05/2021 22:00, Jeff Dutky wrote:
Karin,

I have a 7623A half open on the bench that I am having to pretend not to notice until I have calmed down enough to do a proper diagnosis of its power supply. I've been "calming down" for a few weeks. This is why it's good to have other projects to work on: it gives you time to clear your head.

-- Jeff Dutky







David Slipper
 

As per the subject the 7934 ...

On 12/05/2021 22:23, Dave Daniel wrote:
To what devices do you refer in your post?

On May 12, 2021, at 17:09, David Slipper <softfoot@hotmail.com> wrote:

Is this series of devices particularly hard to access or not well documented or very complex ??

I've been tempted to get one but if they are that difficult I may have to think again.

Regards,
Dave



On 12/05/2021 22:00, Jeff Dutky wrote:
Karin,

I have a 7623A half open on the bench that I am having to pretend not to notice until I have calmed down enough to do a proper diagnosis of its power supply. I've been "calming down" for a few weeks. This is why it's good to have other projects to work on: it gives you time to clear your head.

-- Jeff Dutky







Dave Daniel
 

The 7934s are the fastest (500 MHz) of the Tektronix 7xxx storage ‘scopes and they are not common. The more common 7834s have a 400 MHz bandwidth.

I don’t know if 7934s are more or less difficult to repair compared to other 7xxx ‘scopes. I would guess that having the storage feature makes repairs a little more complicated than, say, a 7904A, but I don’t know for sure.

DaveD

On May 12, 2021, at 17:59, David Slipper <softfoot@hotmail.com> wrote:

As per the subject the 7934 ...

On 12/05/2021 22:23, Dave Daniel wrote:
To what devices do you refer in your post?

On May 12, 2021, at 17:09, David Slipper <softfoot@hotmail.com> wrote:
Is this series of devices particularly hard to access or not well documented or very complex ??

I've been tempted to get one but if they are that difficult I may have to think again.

Regards,
Dave



On 12/05/2021 22:00, Jeff Dutky wrote:
Karin,

I have a 7623A half open on the bench that I am having to pretend not to notice until I have calmed down enough to do a proper diagnosis of its power supply. I've been "calming down" for a few weeks. This is why it's good to have other projects to work on: it gives you time to clear your head.

-- Jeff Dutky










Dave Peterson
 

Karin,

I'll be very interested to learn what you find. I currently have a 7934 power supply set aside from my bench as I await a proper curve tracer. I cycled many times through the troubleshooting guide in the service manual (online - what a pain), and still don't quite know what's wrong.

What was interesting was that removing Q54 from the A14A1 Control Rectifier Board recovered all the LV test points. The 108V is over voltage though, and continuously climbs. The PS makes a nasty buzzing sound. I only tested it in short bursts. I think the issue is in either the Over Voltage Stop circuit and/or the inverter Q34 and Q40 themselves.

I have all the suggested transistors dismounted from the PS. That's part of the troubleshooting procedure in the SM. I've tested them with a little breadboarded curve tracer and they look quite flat. But my breadboard curve tracer is very limited. Traces 2N3904's just fine. But I suspect it doesn't have the oompf to drive these power transistors. So I'm working on getting my hands on a proper curve tracer. Fingers crossed. I will or I won't by this time tomorrow. The SM troubleshooting seems to suggest it's the inverter that's the problem. But it seems the only test methodology is to remove and test the components individually. Not very enlightening.

Still on the switched power supply learning curve.

WRT to the devices that have this power supply setup: I also have a 7854 on hand for the time being. I'd take it's PS out and compare signals, but I don't want to break it! But the point is: it's obvious that they both have the same power supply. There are minor difference I can find when I look them up in their respective service manuals. But these two scopes do seem to have the same unit. And it is a self contained box that slides out the back of the scope. It is possible to disconnect the line cable from the switch by unplugging it and undoing some ground screws. At least in the scope I have at hand. I find it a pretty nice design, construction wise.

I'm curious to find out what the common failure modes of this PS are. Might help to figure out what's going on with this one.

Dave

On Wednesday, May 12, 2021, 03:31:56 PM PDT, Dave Daniel <kc0wjn@gmail.com> wrote:

The 7934s are the fastest (500 MHz) of the Tektronix 7xxx storage ‘scopes and they are not common. The more common 7834s have a 400 MHz bandwidth.

I don’t know if 7934s are more or less difficult to repair compared to other 7xxx ‘scopes. I would guess that having the storage feature makes repairs a little more complicated than, say, a 7904A, but I don’t know for sure.

DaveD

On May 12, 2021, at 17:59, David Slipper <softfoot@hotmail.com> wrote:

As per the subject the 7934 ...

On 12/05/2021 22:23, Dave Daniel wrote:
To what devices do you refer in your post?

On May 12, 2021, at 17:09, David Slipper <softfoot@hotmail.com> wrote:
Is this series of devices particularly hard to access or not well documented or very complex ??

I've been tempted to get one but if they are that difficult I may have to think again.

Regards,
Dave



On 12/05/2021 22:00, Jeff Dutky wrote:
Karin,

I have a 7623A half open on the bench that I am having to pretend not to notice until I have calmed down enough to do a proper diagnosis of its power supply. I've been "calming down" for a few weeks. This is why it's good to have other projects to work on: it gives you time to clear your head.

-- Jeff Dutky










Karin Johnson
 

First off, an immense thank you to all of the encouragement. I'll get to all of the questions soon. Stay tuned.
On the troubleshooting front, I think I may have found what is causing the inverter to not start. It has to do with the "Start" circuit related to the programmable unijunction transisistor and VR38. I have another 7934 power supply to use as a test bench, so I can compare voltages and waveforms with this other unit. It seems that the unijunction transistor never fires because the voltage across VR38 never reaches 33 volts. I've checked all of the semiconductors on the inverter board. The testing was with a basic ohm meter and was done "in circuit". I've allowed for sneak paths and all the semis
seem OK. It is a bit difficult to in circuit test the unijunction, as one path is through VR38 in the forward direction. Also getting the inverter board out of the power supply is tedious, as it is intimately connected to the heat sink. I've ordered parts and am going to write a Spice simulation of this circuit, as well as take some photos of the waveforms and do more detailed recording of voltages from the somewhat working other unit.
I thought this missive might be of interest. Dave Peterson I will get back to you. I've been down that road before and have some suggestions.

Regards,
Karin


JJ
 

I had the power supply ticking problem on my 7904 - eventually, after tracing the problem for weeks changing one tantalum cap after another (didn't even use tantalum for temporary testing), it finally became resurrected! I then replaced the replaced caps. Ticking usually signals first change the caps, one after another, testing for power up after each change. If that doesn't fix it, then you need to get in deeper. You can get fooled easily measuring voltages around the caps. and diodes. Note that I measured the capacitance and leakage of each tantalum cap, using my beautiful Sencore LC75 after removal, and they all were within 1% with strikingly low leakage except for the problem child that was shorted. The caps that Tektronix used were very high quality indeed. Good luck! Seeing it power up will make your heart skip a beat!


-
 

I haven't worked on much Tektronix equipment but your experience agrees
with my experience on HP and some other GOOD brands of TE. I rarely find
bad caps in them so I always recommend against replacing caps wholesale.
IMO that grade of TE should always be properly troubleshot and only the
failed parts should be changed when ever possible. That's one of the
benefits of the internet, it's easy to find out if a particular model or
brand of TE has a reputation for failed caps or other common failures.

On Thu, May 13, 2021 at 6:43 PM JJ <jajustin@gmail.com> wrote:

I had the power supply ticking problem on my 7904 - eventually, after
tracing the problem for weeks changing one tantalum cap after another
(didn't even use tantalum for temporary testing), it finally became
resurrected! I then replaced the replaced caps. Ticking usually signals
first change the caps, one after another, testing for power up after each
change. If that doesn't fix it, then you need to get in deeper. You can get
fooled easily measuring voltages around the caps. and diodes. Note that I
measured the capacitance and leakage of each tantalum cap, using my
beautiful Sencore LC75 after removal, and they all were within 1% with
strikingly low leakage except for the problem child that was shorted. The
caps that Tektronix used were very high quality indeed. Good luck! Seeing
it power up will make your heart skip a beat!






JJ
 

In my 7904, I purchased some ribbon cables on eBay (UXCELL Hot 40Pin Header Rainbow Color Flat Ribbon Cable Idc Wire 1.3mm DIY for F). to use as extender cables for the power supply. The wires can be stripped off to get the number of wires you need for each cable. I needed to move the power supply out the back so that I could remove the cage and get to the circuit boards while still having the PS connected to the 7904. It wasn't that difficult. Just be careful that you don't misalign the cables. And, work slowly. Stop when you get tired. Good luck.