HP 6038A DOA, troubleshooting, FIXED

[HighPrecision]

Can CR1-4 and C2-4 be related to this issue?

No, this part of circuit is protected by fuse F1.

Am I correct that F2 blowing is related to T3 and then the associated rectifier U1 and filters C20/21?

Also possibly related to diodes from CR6 to CR9, connected to another secondary of T3

But I'll try to remove  C26  X-rated film capacitor, often these capacitors explode without a dedicated fuse as F2, especially the old RIFA ones   

[valley001]

Also possibly related to diodes from CR6 to CR9, connected to another secondary of T3

But I'll try to remove  C26  X-rated film capacitor, often these capacitors explode without a dedicated fuse as F2, especially the old RIFA ones   

Right, those test fine, CR6-9.

C26, is there an accurate way to test this cap or do I simply remove from circuit and test without it in place? 

[HighPrecision]

C26, is there an accurate way to test this cap or do I simply remove from circuit and test without it in place? 

Testing the capacitor removed from circuit with high voltage, I suggest to replace this capacitor anyway.

[valley001]

I have some .047uf film caps ill swap one in and see if that makes a difference. 

[bdunham7]

The trouble is I do not get any dead shorts in U1 measuring it in place. Is this a flawed methodology?

T3 is soldered directly to the board, so it is not trivial removing leads from the circuit.  Would be easier to remove U1 I think.

OK, a many-pinned xformer soldered to the board is no fun, and it probably isn't bad.  Do you have any vacuum desoldering equipment?  Even a hand-held solder sucker?  Looking at the diagram, I'm guessing the easiest path is to remove the rectifiers. 

As for testing, you want to use the diode check function and test both ways for each diode--you should have 0.6V one way and a completely open circuit the other way. 

[valley001]

Thank you all for your assistance. 

I will:

1) Replace C26.  I only have .1uf X cap in my inventory. Will this work in a pinch?  I have .047uf film caps but they are not X rated type. 

2) Remove and check U1 rectifier diodes out of circuit. 

[valley001]

U1 tests fine, I have reinstalled it. 

I have a substitute in the C26 position, now waiting on fuses to arrive so I can test.

The cooling fan is "B1" on the schematic, on the 120v primary.  It is also on my suspect list.  The resistance of its coil reads 211 ohms, not sure if that is reasonable or not.  It does say it should draw between 150-200 milliamps. 

[bdunham7]

U1 tests fine, I have reinstalled it. 

I have a substitute in the C26 position, now waiting on fuses to arrive so I can test.

The cooling fan is "B1" on the schematic, on the 120v primary.  It is also on my suspect list.  The resistance of its coil reads 211 ohms, not sure if that is reasonable or not.  It does say it should draw between 150-200 milliamps.

I would suggest taking the rectifiers all out (take a photo first) because that isolates the transformer.  Then put them in one bank at a time.  Even if the rectifiers themselves aren't the problem, you can identify which branch of the circuit is bad.

Is the fan a square muffin-style fan and does it say 'impedance protected' on it?  211 ohms seems OK, but can you disconnect it just to be sure?

[valley001]

It is, and it does say impedance protected. 

I have tested everything down stream from T3, I lifted legs of C20 and C21 and they test fine.  CR6-9 seem OK.  That leaves me with C26 and the fan (which will remain disconnected for now).

Ill test again when I have fuses.  I've made up a dim bulb tester also.  If it continues to fail I will remove U1 and CR6-9.  That would narrow it down to T3, or the fan.

[bdunham7]

It is, and it does say impedance protected. 

I have tested everything down stream from T3, I lifted legs of C20 and C21 and they test fine.  CR6-9 seem OK.  That leaves me with C26 and the fan (which will remain disconnected for now).

Ill test again when I have fuses.  I've made up a dim bulb tester also.  If it continues to fail I will remove U1 and CR6-9.  That would narrow it down to T3, or the fan.

What do the 14/15/16 windings measure disconnected? 

[Jay_Diddy_B]

Hi,

For those playing along at home, here is the part of the schematic with T3:




You can disconnect the parts that I have circled in Red. The unit will work without the fan or the X cap C26.

You can isolate the transformer by removing the full-bridge rectifier U1 and disconnecting one end of the rectifiers CR6-9.

If the fuse still blows with only the transformer connected, it is possible the transformer has a defect called a shorted-turn. This means the resistance of the windings will read correctly or close to correctly, but the shorted turn will cause excessive current to flow in the primary, blowing the fuse F2.

Regards,
Jay_Diddy_B

[valley001]

Thank you Jay, yes that is what we are looking at. 

UPDATE- I am happy to report I have a somewhat functional A1 board.    With C26 removed it will now power on and remain running without blowing the fuse.  This eliminates T3 and associated components and the fan.  Ill be putting in a mouser order for a new X cap. 

Here's hoping the power mesh and computer bits are in working order...

[Jay_Diddy_B]

Hi,

The date code on that Rifa CV6 is June 1987. I don't know if that is consistent with other date codes in your unit?

I have been using:

B32022A3473M289‎

or

   
PHE850ED5470MD13R06L2

in this location.

You should probably change C20 and C21 while you have the board out.

These are 1000uF 50V 105C 7.5mm pitch electrolytic capacitors.

Refer to this message:

https://www.eevblog.com/forum/testgear/hp6035a-500v-5a-1kw-power-supply-repair/msg697835/#msg697835

And bad Rifa caps.

https://www.eevblog.com/forum/testgear/hp6035a-500v-5a-1kw-power-supply-repair/msg3281284/#msg3281284

Jay_Diddy_B

[valley001]

I plan to have this in more or less regular use so a recap is in the plans.  Once I have it working ill tear it down once again and replace those two.  I am afraid to see the price on the 400v caps. 

Roughly yes, 1986/1987 seems to be the general date range in this thing.

Did you replace all the Y caps in yours?

Hi,

The date code on that Rifa CV6 is June 1987. I don't know if that is consistent with other date codes in your unit?

I have been using:

B32022A3473M289‎

or

   
PHE850ED5470MD13R06L2

in this location.

You should probably change C20 and C21 while you have the board out.

These are 1000uF 50V 105C 7.5mm pitch electrolytic capacitors.

Refer to this message:

https://www.eevblog.com/forum/testgear/hp6035a-500v-5a-1kw-power-supply-repair/msg697835/#msg697835

And bad Rifa caps.

https://www.eevblog.com/forum/testgear/hp6035a-500v-5a-1kw-power-supply-repair/msg3281284/#msg3281284

Jay_Diddy_B

[valley001]

Looks like I am in for some more fun. 

 Installing the GPIB board now causes the bulb on my dim bulb tester to stay lit.  Unplugging the power lead from T3 to the GPIB, 14/15/16 on T3 resolves the issue.  Going to look at the associated diodes and cap. 

[Jay_Diddy_B]

Hi,

You can try this:





1) with the GPIB board out of the unit, measure the resistance from the 5V. You can measure between pins 8 and 5 on U31 for example. You may have a shorted deoupling capacitor.

2) Can you try connecting a bench power supply set to 8V to J10-1 and  0V to pin J10-2
and measure the 5V rail.

3) The LM309K is a 1A regulator, so I would expect the current to be less than 800mA

Jay_Diddy_B

[valley001]

Actually this example has a different GPIB board. 

I found C20 to be a dead short.  What else should I look at here?  Hoping this did not fry the digital stuff down the line.

[bdunham7]

UPDATE- I am happy to report I have a somewhat functional A1 board.    With C26 removed it will now power on and remain running without blowing the fuse.  This eliminates T3 and associated components and the fan.  Ill be putting in a mouser order for a new X cap.

So your RIFA cap:

1) measured OK (not shorted) with a DMM
2) conducted enough at 120VAC to blow the fuse but,
3) didn't explode all over the place.

That's a first, but a good result.  Replace every single cap that looks like that whenever you see them.  Get a bulk deal on them from Mouser--they have a nice price break at 100.   

[bdunham7]

Actually this example has a different GPIB board. 

I found C20 to be a dead short.  What else should I look at here?  Hoping this did not fry the digital stuff down the line.

A dead short when removed from the circuit?  What does it look like?

[valley001]

Looks totally fine.  Dead short.

[bdunham7]

Looks totally fine.  Dead short.

OK.  Order up a replacement, no need to be picky--just get the lead spacing and size correct.  Meanwhile, for testing purposes, you can bodge almost anything in there.  1000uF + and any voltage over 35 will work.  The rest of the board may well be fine.

[valley001]

Bodged in a cap (two actually, 680uf 100v in parallel).  Making progress!

[Jay_Diddy_B]

Hi,

The OT circuit starts with a thermostat on the power board:




It should be fairly easy to troubleshoot this circuit.

Regards,
Jay_Diddy_B

[valley001]

That must have been an anomaly, it has not thrown the OT warning since.

R20 and 21 on the A1 board get HOT!, 160 deg F or so.  Is this normal?

[George Edmonds]

Hi

They are 5W resistors so they will get hot.

George G6HIG