PV Session 400 LTD...again

[Pryde]

TO3 power transistors and actually all transistors have 3 leads but one of them may not be apparent at first. The case of the transistor is the missing third lead. You need to check between the case and lead one in both directions, the case and lead 2 in both directions and from lead 1 to 2 in both directions to make sure they are not shorted.

Checking only the two leads only checks the emitter/base junction.

Ah.

I will re-check all. thanks

[Pryde]

Well after testing all output transistors correctly with the EBC they are all functioning correct. Not sure where to turn now other than flipping the boards and looking around?
any more thoughts?

[gbono]

Did you check or replace the driver transistors? They are attached to the heatsinks on the PCB - there are two flavors (schematic #'s 430C & 431C). Also check the big ceramic resistors used/connected to the output transistors. You could even CAREFULLY disconnect the output transistors and see if the amp powers up (using light bulb limiter) with/without issue.

[Jazz P Bass]

Have the output transistors been removed for that EBC test?

If not, take them out & test them again.
(actually, you can only test them so far in circuit, for the 1 ohm/10 watt resistors will throw off the readings)

And while they are out, see if the amp is still drawing hard.

If it is, then you need to check the + & - Vdc rail voltages.

One may be missing.

[Pryde]

Have the output transistors been removed for that EBC test?

If not, take them out & test them again.
(actually, you can only test them so far in circuit, for the 1 ohm/10 watt resistors will throw off the readings)

And while they are out, see if the amp is still drawing hard.

If it is, then you need to check the + & - Vdc rail voltages.

One may be missing.

Yes all of the output transistors were removed for the test and they all were fine. I did not try to power the amp when they were out but can remove them again to try.

So with transistors out, and the amp still drawing hard with a bulb limiter, then check the +/- rail voltages? Were would the best test points be for that?
Thanks

Alsio, I have not removed the 430/431c transistors to test them. Likely suspect?

[Jazz P Bass]

I am not too sure how you are testing things or what you are using to test items with.

I use a digital volt meter that has the diode check function to 'test' transistors for shorts or not.
That is about all you can look for.

Or in the case of an open resistor, that will lead you to a bad transistor.

The power rails can be checked right at the bridge diodes.
As a matter of fact, the diode check function would come in real handy to test those diodes.

[Pryde]

Yes I use a digital voltmeter with diode test function to check the transistors (and diodes of course). All the output transistors were removed and test good with correct forward voltages (between .4v and .5v and no shorts the other way (reverse bias).

With the power transistors out, I powered the amp with a 60w bulb limiter and took voltage readings of the bridge diodes (which I also diode tested previously). I got .5vDC on 2 and -.01v DC on the other 2.
Not sure if that is helpful in anyway?

[52 Bill]

Yes I use a digital voltmeter with diode test function to check the transistors (and diodes of course). All the output transistors were removed and test good with correct forward voltages (between .4v and .5v and no shorts the other way (reverse bias).

Out of curiosity, did you test the outputs from collector to emitter as well? I will assume that you tested base to collector and base to emitter.

[Pryde]

Out of curiosity, did you test the outputs from collector to emitter as well? I will assume that you tested base to collector and base to emitter.

Yes. All 6 tested fine.
Also, the amp is pulling hard with all 6 transistors removed so there is something else faulty here. I don't really know what else to do without pulling EVERY component off the boards and testing each one.

[Pryde]

OK after re-reading the thread to make sure I am following everyone's advice I missed a step:

I just disconnected the power rail wires from the pre-amp board and the amp is not pulling hard voltage. Plugged them back in and the amp is pulling hard again. SO, it appears my culprit short is somewhere on the preamp board.

Thoughts on usual suspects in preamp area? I know it is still a lot of components but...anything to try test?

[Jazz P Bass]

Well that header that goes to the preamp is a perfect place to test the + & = 50 volt rails.

As both voltages are used on the pre & power amp, I would suggest maesuring the voltages without the preamp connected to make sure that they are fit.

The fact that the preamp is pulling current indicates a short of sorts on that board.

After verifying that the power supply is good, reconnect the preamp & see what it does to those two voltages.
That may give a hint.

The next step would be to check all eleven transistors on the preamp.

[Sowhat]

OK after re-reading the thread to make sure I am following everyone's advice I missed a step:

I just disconnected the power rail wires from the pre-amp board and the amp is not pulling hard voltage. Plugged them back in and the amp is pulling hard again. SO, it appears my culprit short is somewhere on the preamp board.

Thoughts on usual suspects in preamp area? I know it is still a lot of components but...anything to try test?

Ahhh, finally did the unhook the preamp test and it had fruit hanging on it. That's a good start. You did a lot of work on that preamp and I have a feeling that you got a wire scrap short in there somewhere because of the nature of the failure... "had distortion over 5 and when banged got some snap crackle and the resultant pop!" I would pull this board and closely inspect it on both sides before trying to check all those transistors which will be a pain, not that removing the board is easy but just pulling out a piece of debris is easier than troubleshooting a preamp without the right tools. I don't think a transistor failed here but that's possible and if one did it would be near the power voltage divider circuitry because beyond that, the voltages on that board are radically reduced through voltage dividers before powering the low voltage circuitry. You client could have also popped the 1st stage transistors with his steel guitar rig but that's a long shot as far as I can see.

[Pryde]

With the preamp power leads unplugged and no bulb limiter in the circuit I am getting +51v and the red lead and -51v on the black lead. (Both leads are just coming directly from the filter caps BTW). So these voltages appear to be ok coming direct from the filter caps.

So on the preamp board really any/all transistors and diodes are suspect

[Sowhat]

One other thought is that usually when preamp transistors fail they fail open or kind of open which would not produce a high current draw. It's possible that they can fail as a short but that's more in the domain of power transistors... damn their perverted and twisted souls! Usually when a preamp stage blows you just get silence or terrible distortion, not a blown fuse but crazier thing have happened before.

[Enzo]

The -50 as far as I know is only used in th preamp for the reverb driver.

All the transistors in the preamp, mostly all have substantial series resistances, so it is hard for them to load down a rail or pop a fuse. The exception is the reverb driver. If those transistors short or their bias diode opens so both sides turn on hard, you now have 100 ohms (well, 47x2) across +50 and -50. Even the various filter caps are behind resistors. There is a 0.1uf bypass cap at each power rail as it enters the board.

Get out your ohm meter and check each rail to ground for resistance - power off of course.