Peavey XM6 300EH output transistor replacements

[DrGonz78]

If I remove the high voltage to the power amp the -/+25v and the respective -/+15v rails are present. With the HV connected it drags down my low voltage supplies. I rechecked every single diode and pretty much every component on the board but can't find anything suspicious.

I am wondering if it's possible if the secondary HV & LV windings on the transformer have shorted together somehow. I know jumping to the transformer as the possible fault is unlikely but how would I test for this? Obviously I ohm out the meter between HV & LV connections on the transformer. Is it correct in my thinking that I should not see resistance between these separate windings?

Edit: there is no short between HV and LV on the transformer windings.

[DrGonz78]

Also to note. When I disconnect the LV supply from the board the HV is now -/+15v rather than -/+7v.

[DrGonz78]

I have to wonder at this point if these new transistors have a different bias point or something. How would I test that theory? Something is dragging the voltage down but it doesn't appear to be a gross short. Tom offered to help since he has the original transistors. But I would hate to go through all that only to find that It did not fix anything.

[Enzo]

No, the power transistors are just plain old vanilla transistors. And no, a load is not required.

You replaced CR8? OK, measure resistance across it right now where it sits. Does it act like a diode or do you get like 33 ohms resistance? You should see 33 ohms. ....or is that why you thought it was bad to start with?


Also, the CR8 in the drawing is BACKWARDS. DO NOT install it like the schematic, it has to face the same way as the rest of the diode string, CR7,10,11,12. If you installed it backwards like the drawing, then the outputs cannot bias and will all turn on. Compare to the similar 300CH amp which is drawn correct.

If your big transistors are not shorted, then the most likely thing to drag down the rails is both sides turning on at the same time.

Voltage readings taken on the bulb or at reduced mains voltage (as on the variac) are pretty meaningless.

The power supply has low voltage rails 25 and 15v. The 25v rails are used in the power amp, but the 15v supplies are not!!! (CR21,22) Those 15v rails are ONLY used for feeding the preamp. The op amp in the power amp has its own 15v supplies made by CR4,5.

Please, there is nothing wrong with the transformer.

Flying rails are really no different from "regular" amps. Most amps connect the speaker load between the power transistor emitters and the power supply common. The difference between the regular circuit and the flying rail is simply which end of the speaker gets grounded.

[DrGonz78]

Yup when I first checked CR8 I knew it was in parallel with the 33 ohm resistors. With so many components blown out I lifted a leg on diode just to be sure it wasn't a false reading and actually to my surprise it was open. So I replaced it.


Of course it's not the PT but at this point I just thought to at least test my thinking, which obviously only pondered that thought for a moment. This thing has just turned into a rabbit hole and it failed after not being used for a long while. Just turned it on to test it before selling it and it died. Just gotta keep at it. Will check all transistors and even the mica insulators. I can tell that the insulators are good but at this point I might pull the transistors to make sure. This one is just kicking my ass for some reason and I didn't expect it to be a tough repair.

[Enzo]

You can sshort across CR8 or for that matter even the dual diode. All that that would do is increase crossover distortion. But it also would reduce the bias making the amp run cold. SO that is then a technique for checking hot amps. SHort the bias diodes and see if the amp now runs at a more normal current draw.

[g1]

And no, a load is not required.

I'm not sure if you use a bulb limiter Enzo, so I'll ask if it's possible a combination of no load and the bulb limiter might cause a start up issue.
That is why I thought the Fender service note in post #7 stated that a load is required for servicing, to center the supply 0 point.

[Enzo]

I do not. Bulb is a great tool,, and I recommend them, but my shop technique was honed many years before the bulb became common. PV amps are robust and pretty basic, and I never encountered one that was picky about having a load.

The bulb is for when the amp wants to blow a fuse. Once past fuse blowing, we lose the bulb.

[DrGonz78]

So when you repair something we get a little rush of dopamine. In my case I just felt like a dope.

Turns out CR10 was open and actually one other 1N4148 was open too. So two diodes and the amp is back in business. I hate when I miss something so obvious but we're only human I guess. Thanks for all support as usual 👌

[Enzo]

With CR10 open, then both sides of the output turn on full, basically shunting the main power rails together.

[J M Fahey]

So when you repair something we get a little rush of dopamine. In my case I just felt like a dope.

Turns out CR10 was open and actually one other 1N4148 was open too. So two diodes and the amp is back in business. I hate when I miss something so obvious but we're only human I guess. Thanks for all support as usual ��

Often problems are looking straight at you, but you donīt catch them.
"Hidden in plain sight"





Happens all the time.

[g1]

And another

[DrGonz78]

Just like that owl showing only 1 eye I am legally blind in my right eye too. Very cool picture as well G1!!

I got a picture to show. So I finally got to rewire another one of these (ON | OFF | ON) power switches on the Peavey amps. In the past I just did the quick rewiring where you cut off the criss-crossed legs that flip the polarity of the mains power and wire it for (ON | OFF | OFF). This time I spent the extra time to wire the switch (ON | OFF | ON).

[nosaj]

And another


[ATTACH=CONFIG]53643[/ATTACH]

You see the mother but did you see the baby?
nosaj

[DrGonz78]

You see the mother but did you see the baby?
nosaj

Actually when I first looked at the picture I only saw the baby at first. Then a few seconds later my eyes focused on the mother lastly. Pretty cool!!