Tektronix DM501 repair

[dan3460]

I bought a lot of 4 DM501s, 3 of them work fine but one of them displays just a # 1 to the left of the screen. Poking around I have no -12V to the circuitry. I have attached the part of the circuit that deals with the -12V. The -12V is reference from the 5V line, which is fine. I ohmed from the right side of R435 looking to see if I had a short there, but got 1.2k which I think is fine. I looked at the voltage at the junction of R439,Q438 and Q432, I was expecting -10 to -12 Volts there but only got near 0V. So I suspected Q432, being a 2N3906 which I had a bunch, I replaced it but did not improve anything. It was a tantalum cap that looked weird so I replace that one too.
Does anyone have any experience repairing this things?

[mij59]

No experience in repairing this one.
The -12V is powered from the -33.5V rail, so that is the first thing you should check.

[dan3460]

Yes, that was the first thing I did, -33V is fine that comes from the mainframe.
This is a frustrating repair, I know the problem is with the -12V line but the circuitry is simple enough that should be a breeze to find the problem. I'm starting to suspect that there is a broken connection somewhere.

[voltz]

Hi,
Also no experience but generally; Check output transistor marked 'located on mainframe'. And its wiring back to board.
Would also test R444 1.2 R. Does the collector have -33v on it? if so, test at the emitter pin for -12.

[mij59]

Assuming mainframe is OK,  maybe it's  the connector with the main frame, or the transistors Q438, Q440.
Since you got several units compare the measurements  of the transistors using a multimeter set to diode test.

[Richard Crowley]

Does it have the same symptoms in OTHER slots?
That "differential diagnosis" would isolate either the module or the rack as the problem.

[dan3460]

Thanks for everyone's input.
I have two other units that are working ok. I tried on of this units in the extender that i'm using and works fine. I try the bad unit on the slot previously occupied with the good unit that I took out and didn't work, so definitively I have a bad unit.
I started to make measurements between the good unit and the bad unit, I discovered that the 4.6k resistor is low (measuring around 3.5k). I made sure that I switched the position of the probes after each measurement, to make sure that was not a diode or transistor screwing things up. Waiting for resistors to come, so I can replace and keep comparative testing.

[mij59]

Thanks for everyone's input.
I have two other units that are working ok. I tried on of this units in the extender that i'm using and works fine. I try the bad unit on the slot previously occupied with the good unit that I took out and didn't work, so definitively I have a bad unit.
I started to make measurements between the good unit and the bad unit, I discovered that the 4.6k resistor is low (measuring around 3.5k). I made sure that I switched the position of the probes after each measurement, to make sure that was not a diode or transistor screwing things up. Waiting for resistors to come, so I can replace and keep comparative testing.

That is probably no the problem, to make sure remove the resistor and measure again, compare the active components first.
Resistors tend to go open instead of shorting out.

Since you got the extender, post the voltages on the transistors of the -12V power supply part.

[MrSlack]

I've noticed resistor values drift terribly over time rather than fail completely particularly in hot Tek power supplies. Value goes up and down. These are dicks to find as you have desolder one end to measure it properly. Then the issue goes away. Must have spent days on this sort of problem over the years. Good luck.

Also if it's got any silver can axial tants in it, replace them. They seem to like exploding after about 25 years.

[dan3460]

That is probably no the problem, to make sure remove the resistor and measure again, compare the active components first.
Resistors tend to go open instead of shorting out.

Since you got the extender, post the voltages on the transistors of the -12V power supply part.

I agree, that is not probably the whole problem but in the good DMM that resistor shows very close the correct value (forwards and backwards).
I will post the voltages around the -12v.

[dan3460]

I've noticed resistor values drift terribly over time rather than fail completely particularly in hot Tek power supplies. Value goes up and down. These are dicks to find as you have desolder one end to measure it properly. Then the issue goes away. Must have spent days on this sort of problem over the years. Good luck.

Also if it's got any silver can axial tants in it, replace them. They seem to like exploding after about 25 years.

Don't remember seeing any axial tantalums, but there are a few of the other type. I have not replace them yet as they are in other parts of the circuit. I'm trying not to desolder resistors, comparing the reading with the good DMM. But at the end I may have to resort to that.

Thanks.

[cncjerry]

I've repaired a few of these.  It is not unusual to pop one or two of the transistors in that 12v circuit.  All you have to do is pull it out of the slot and push it back in under power.  You should be able to find the culprit pretty quickly by diode testing the transistors in-circuit.

Many times the latch is broken so when you pull the leads out the plug-in pulls off the back power connector and then you automatically push it back in.  Bad idea as the pull-out isn't as damaging as the pushing back in.

I have a bunch of the DM's all models for the TM500 series as I was using them to monitor voltages from the unmetered power supplies.  I am thinking about selling them all as I went to metered supplier on the TM5000 series.