Tek 576 possible power supply failure


 

Hi all,

I put the schematic of those 2 supplies together with the voltages (hopefully readable) in failure mode in the files section. See if anyone finds the faulty transistor from that information alone...

https://groups.io/g/TekScopes/files/2021.01.31%20-%20Tek%20576%20power%20supply%20failure%20mode.pdf
the winner is: Q837.

My first thought was of Q834, but alas no, it was Q837 with a sufficient short across CE (behaves exactly like a resistor). Its the current limiting transistor and the supply sprang to life as soon as I took it out. I think I could have guessed it by looking at the CB-voltages, which are very close.

This fault developed over time - at first the Tek went dark after an hour or so, a year later I barely could make a measurement before it happened. Thats what made me think of a capacitor rather than a transistor.

Curiously, all capacitors in this rather old unit seem to be OK...

My repairing of the step generator succeeded... it took me an afternoon understanding how it works (good manual), making tons of measurements to follow up the signal path until the counter, just to end up with a working step generator without having changed anything. The wiggling of the ICs and transistors must have done the trick.

There's an excellent review of the step generator here (in german):
https://amplifier.cd/Test_Equipment/Tektronix/Tektronix_other/576.html
The guy found out the there is an inherent design issue with the generator, as they are applying relatively slow ramping signals to the TTL logic. The old ones in the 60ies do switch at some point and thats OK, but the newer ones are faster and tend to switch forth and back during the time the signal is in the undefined area. At the very end this produces two or more clock pulses in a row, making the generator jump the corresponding steps. The best way to correct this is to replace the 7400 by a 74LS132 that has schmitt triggers at the entrance.

cheers
Martin


 

Hi all,

I fixed this one... a faulty transistor.

I noted all voltages in the -100V and +12,5V supply, as they seem to depend on each other. With a little try-and-error (easy, as transistors are all socketed) I found the culprit.

I put the schematic of those 2 supplies together with the voltages (hopefully readable) in failure mode in the files section. See if anyone finds the faulty transistor from that information alone...

https://groups.io/g/TekScopes/files/2021.01.31%20-%20Tek%20576%20power%20supply%20failure%20mode.pdf

Later on I checked the small capacitors on the regulator board, they are all fine. When the unit is running I merely get some millivolts of ripple on any of the supplies, so I guess theres not much that can be improved.

Next its the step generator that needs some attention. Some steps are missing or skipped, looks like a problem in the counter / converter area. When that is done there are at least 2 bulbs in the readout that must be replaced. And the "invert" switch array cleaned, maybe.

cheers
Martin


Michael W. Lynch
 

C829 and C848 in the 100V Supply are failure prone. Those are two of the many that I replaced in my later model unit.

--
Michael Lynch
Dardanelle, AR


Eric
 

Martin,
-75 V is the important one that is the main reference supply. In one of the 576's I have rebuild the 5V filter cap was bad and needed to be replaced. Also what version of the rectifier board do you have in one of mine it had sprauge black and red paper and foil caps on the board I had to replace ALL of them. They all measured 100+ Ohms ESR 111 to 119 if I remember correctly. There is also some small electrolytic cans on the regulatory board that needed changing as well. On this unit in question it was a VERY early one gold plated PCB's everyware. With a -00 tube if that dates it at all.

Eric

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Martin
Sent: Friday, January 29, 2021 3:44 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tek 576 possible power supply failure

Right now it seems the -75V supply is working. I disconnected all 3 loads of the 100V supply and it still does not come on, so I'll concentrate on that.

I was supplying an external voltage into the 100V leads, that seems to do the trick as the 12,5V supplies came to life. The screen looked a bit weird though (not a nice sharp trace, but a stable trace at least). Current draw off the 100V external supply was around 80-90mA.

cheers
Martin


 

Right now it seems the -75V supply is working. I disconnected all 3 loads of the 100V supply and it still does not come on, so I'll concentrate on that.

I was supplying an external voltage into the 100V leads, that seems to do the trick as the 12,5V supplies came to life. The screen looked a bit weird though (not a nice sharp trace, but a stable trace at least). Current draw off the 100V external supply was around 80-90mA.

cheers
Martin


Michael W. Lynch
 

Martin,

Well, at least you can mark those off the list of possible causes. I had a couple of bad bulk filter caps in the power supply that loaded down the supply as well. The 5V supply is especially vulnerable to this. I also found several shorted electrolytic caps scattered around that added load as well. Perhaps you could measure the load on each individual supply at the LV power supply and determine which one is drawing excess current? I had to go back and isolate each supply individually to finally locate the source of the excess load.

--
Michael Lynch
Dardanelle, AR


 

Michael,

this time it does not seem to come from the HV section. I disconnected step generator and HV, no change.

I'll disconnect the plus and minus 12,5 supplies as they can tear down the 75V suppy when shorted.
And the 75V supply as it can tear down the 12,5 supplies.
-> Thats what the service manual says. Headscratching.

cheers
Martin


Michael W. Lynch
 

Martin,

Did you disconnect the 100V to the HV section? Sounds like another shorted HV transformer.

--
Michael Lynch
Dardanelle, AR


 

Hi all,

I spent some covid-time to clear up a collection of tubes I have in my basement, untouched for 25 years or so.
Now I intent to analyse some of them with my 576 in some way...

Only the 576 isn't working anymore. It had a HV transformer replacement some years ago (see here: http://www.vintage-radio.net/forum/showthread.php?t=112106) with a full alignment.
Now when I switch it on it comes on for some time (a minute or so some months ago, now only 1-2 seconds) then the beam disappears, if it had the time to appear, with the clicking of some relays. Readout lamps go off, too. What remains is the mains indicator bulb and the graticule light.

Yesterday I opened it up and made some measurements. Power consumption is down to 20-30 Watts (instead of 80), and practically all voltages are down and stay down (measured against chassis ground as indicated by the manual).

Since all supplies seem to depend on all others in that unit: if they are all down, is it the 100V supply the one to start with?

cheers
Martin