HP 3440a digital voltmeter repair/troubleshooting: moving on to PSU ripple

[valley001]

Now I have an intermittent fault on the -35v rail.  It will sometimes read -20v. 

Also, when the -35 rail is malfunctioning to -20v the count neon stops flashing. 

[Armadillo]

For the A9 board, Check if F2, C2 [500uF], is OK,
then if fault is not found,
measure Q1, Q1, Q2 BCE voltages as last resort.

Edit: this kind of intermittent you should also careful for cold joints, contacts etc...

EDIT: [PARTICULARLY R12 WIPER CONTACT - CONTACT CLEAN, CHECK OR REPLACE]

[valley001]

Tapping on C2 causes the voltage to come back intermittently. 

On my component tester I get:

Vloss 2v
499uf
ESR .32 ohms

Change C2 anyway?

[Armadillo]

Yes, I would replace C2.

[EDIT: NOT ANYWAY. IS A NECESSITY. - 499uF when cold, intermittent when hot caused by raise ESR. When cap out of spec, is a necessity to change it. Don't let it pass.] While your soldering iron is on, look for more cold joints.

anyway?

[valley001]

OK, understood.  This is very educational. 

Now to find a reasonable source for the replacement, my stock on had is for repairing tube radios so I dont have any 500uf caps but I will check through my parts bins for used.  New seem pricy.  I assume I have some leeway on ratings this being an electrolytic right?  510uf 80v OK for example?  Voltage needs to be at rating or higher and capacity +/- 20% or so is OK?

[Kleinstein]

The voltage rating can be higher, even much higher, though this is more expensive.

The capacitance has quite a lot of tolerance, and these can be asymmetric:  like +50/-20%.  So for an original 500 µF the standard 470 µF can be OK. depending on the use even more than +-20% can be OK. If in doubt (and high tolerance) one might want to measure the new caps, after they have been exposed to the new working voltage for some time.

510 µF 80 V sound like an odd, old cap, that might be already half dead.

[valley001]

The original is 500uf -10 +50% 75v, so probably better to go higher right?  I do see that 470uf 100v is common and cheap but with its 20%  tolerance could fall well below the rating of the original. 

[Armadillo]

100V type is OK. If it can fit in, I think will be bigger? or old type is even bigger than modern ones? possible.

[Edit : 1000uF, 680uF all should be find, 470uF....well yeah...it will still work]

[valley001]

OK, no C2 sub in my parts bins but new part arriving Friday.

Q3 is here for the A3 board, I plan to remove old Q3 and test the associated cap.    Is there anywhere else I need to look so I dont fry the new Q3?

[Armadillo]

The last time did you already check the coupling capacitor?
If the coupling capacitor is OK, then Q3 will not be fried.
When you probe, just make sure no slipping. Don't probe when power is on. Best is do hooking only when power is OFF.

EDIT: As a precaution, when the Q3 is removed, measure each empty pads with respect to power ground, non of them should be shorted to ground, particularly at the collector, since is routed to other boards. The other thing that can fried Q3, is installing it wrong way around.. ... joking.

[valley001]

Ill measure that cap today.  You mean to measure resistance between pad and power ground correct? 

I did institute a policy of switching off between probing points, after my earlier mishaps.

[Armadillo]

Yes, the empty B, C, E pads to Ground using nearby associated leads for hooking, with the board in circuits to other boards, just as a precaution check for double assurance.

You sure you didn't buy 2 or 3 spares, since usually delivery and packing is more expensive than the transistor?

[valley001]

I have two Q3 available, original HP part numbers.  Also bought a bunch of other original semiconductor parts along with these. 

[Armadillo]

Since you got a spare, then I say that from the previous BCE reading of Q3 is unlikely Q3 is shorted to ground. 
It must have died due to ageing. 

[valley001]

Ill recheck BCE on it before starting, just to be sure.  I can now also save my waveforms, so I now plan to make a file I can go back and reference rather than remeasuring every time. 

[valley001]

Remeasure Q3:

C: -11.2 to -10v
E:  -11.2 to -10v
B: -23, to -20v

No wave forms that I can discern.

Maybe I mixed up emitter and base on the initial measurements.  Im trying to slow down so I dont make those sorts of mistakes. 

[valley001]

C2 is:

Vloss 1.2
24.09uf
ESR 1.6 ohms

[Armadillo]

Edit :This re-measurement indicate that Q3 may not be defective but C2 could be leaked.
Take out C2 and reconfirm measurement again.

Edit:

Q3 is still defective with current flow thru despite it should be cut off. Are you re-measuring with the faulty original transistor?

**Double Check R13 making sure it is 68K ohm.

Remeasure Q3:

C: -11.2 to -10v
E:  -11.2 to -10v
B: -23, to -20v

No wave forms that I can discern.

Maybe I mixed up emitter and base on the initial measurements.  Im trying to slow down so I dont make those sorts of mistakes.

[Armadillo]

marginal but usable.

C2 is:

Vloss 1.2
24.09uf
ESR 1.6 ohms

[valley001]

OK, ill change c2 if I have a suitable replacement. Those measurements were with one leg lifted.

Q3, with the diode function on my meter gives .350ish volts minus lead on base, and .160ish volts with positive lead on base.  The transistor tester says its two resistors.

[Armadillo]

You are talking about the orignial spoilt Q3 yes /No?

[valley001]

Yes, original.

Testing some of my new electrolytic caps (22uf 160v) and most are reading:

Vloss 1%
21uf
ESR 1.6 ohms

Thats pretty high ESR right? Same as the old cap. Maybe I should just leave the original?

[Armadillo]

Yes, please.
You can always dressed it up later.   

[valley001]

OK.

Resistance between pads and pin 15 with card inserted:

E 4880 ohms
B 1500 ohms
C 743 ohms

Hmm..

[Armadillo]

OK you are safe to install the new Q3.