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Re: 7704 power supply

 

Raymond,
I have repared my 7704A two times. Be sure to have the right manual. Most manuals are about the 7704 and that has no switching power supply.
The powersupply will not start without a load but it keeps trying.
I removed the psu and then connected it outside the mainframe. If there is. Problem on the rails most chances are bad caps. I had to replace to electrolitics and two tantaliums. Both rather small filtercaps. Problem is tht it is hard to get acces fr measuring while powered on.
My problem was trace compression just after starting it up.

The second problem was an easy fix, again trace compresion but this time permanent. The problem was a dirty pot on the X amp on top of the CRT, that distrorted the sawtooth. They are very nice scopes but also complex.

A hiss and penetrant smell are typical for a cooking electrolyte. I becomes hot and builds up pressure in side. Modern caps have a top that pops open, so they are easy to spot, old caps blow there guts to the underside.

Connect your psu outside the mainframe but in such a way it is connected. I can not remember if i used a sort of extension lead, sorry bit my memory is very bad.
Then measure ripple and voltages. The psu is comlicated but explained in the manual. Much rils depends on each other, so if one rail is wrong, search in that rail, if more are wrong the problem is probably one common level under that. Your scope worked so the psu can not be terrible bad. If it is not used fr many years and fired up after that the caps have. Hard time. If you are lucky they reform. If not they start leaking DC and cook dry.

If the rails are OK, check the HV testpoint. If there is a problem there you have most times problems with intensity and focus.

Good luck, hope you can fix it, it is a great scope.

Fred pa4tim

--- In TekScopes@..., jerry massengale <j_massengale@...> wrote:

You must be talking about the 7704A which is different from the 7704. The power supply needs the display unit to have minimum load. I usually measure what I can at the horizontal input connector.


Jerry Massengale





-----Original Message-----
From: raymonddompfrank <r.domp.frank@...>
To: TekScopes <TekScopes@...>
Sent: Mon, May 21, 2012 8:20 pm
Subject: [TekScopes] 7704 power supply





I am a newbie Tek collector, having used my own 465 until some 20 years ago. I am reasonably knowledgeable in electronics.

The other day, I purchased a 7704 after only giving it a very quick test. I agree that wasn't very clever. Unfortunately I noticed some brightness and timebase issues and I decided to have a look inside the instrument. That wasn't very clever either...
I checked the +54.2 +- 0.25V without removing the power supply. It turned out OK (54.4). I decided to check some other voltages on the top regulator board and decided to remove the display unit (D7704) to reach in. I realized that many switching power supplies don't like very light loads but decided it was easier and the location of the description of how to remove the D7704 in the maintenance manual was such that I concluded it was safe. Also, I haven't found any warning re. this procedure.
Anyway, since I removed the D7704 and switched the PS on, there's a hissing sound from it and from what I measured, all preregulated voltages are much lower than normal: +54V now is 30V, -54V is -30V, 7V is 2V etc. Also, the front lights remain off (not surprising).
I'm anxious leaving the PS on for more than a few seconds.

My Question: Did I indeed blow up the PS and if so, is there a component that usually blows up and is the repair doable or even easy? There's no nasty burning smell or anything.
Or is this just a coincidence, and have I just witnessed another electrolytic capacitor dying?

Thanks for any ideas!

Raymond
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