Peavey XM6 300EH output transistor replacements

[DrGonz78]

So I dug into this one and found 1 shorted output transistors, 2 completed shorted drivers, 2 more shorted pre-drivers, 2 shorted CR15/16 diodes, and 4 completely burnt open 1N5402 rectifier diodes. Ouch!!!

Anyway I have a quick question about what types of output transistors I can use for replacements. The parts numbers on the actual transistors in the amp are 70484140 (NPN 140v) and 70474140 (PNP 140v). 70484140 is equal to field replacement MJ-15052 and 70474140 is equal to MJ-15053. On the schematic it labels the 70484140 as 8306 and the 70474140 as 8307. In Peavey's cross-reference list it shows 8306 = MJ-15024 (NPN 200v) and 8307 = MJ-15025 (PNP 200v). My question is if I can use the MJ-15024 & MJ-15025 as replacement transistors for this amp? I am pretty sure the answer is yes but hearing from the brain trust here always makes me feel more comfortable going forward. Thanks.

[drewl]

Yes, but don't trust my brain.

You can use higher power rated parts, it won't put out any more power but they will handle it easier.

[g1]

Yes. There was a time when the 024/025 were a more expensive part, that is probably why they would have used the lower voltage part there.

[DrGonz78]

I was almost certain these are the recommended replacements. Now if someone searches online the answer will be found on MEF.

Well I placed an order for all the parts except the output transistors. Peavey wanted $10 each for them. I can save a few bucks on mouser and order an isolation tranny for the Silvertone amp at the same time.

[Enzo]

Yes, use 024 types. Just don't mix. Make the entire output that type.

[DrGonz78]

Yes, use 024 types. Just don't mix. Make the entire output that type.

Yup don't just replace the one blown transistor but get all new 024 and 025. Plus I am sure the three working transistors got a good jolt in the failure. There are two pnp and two npn for the outputs.

[DrGonz78]

So I have been a bit at a stand still with this repair. I have gone through the amp and replaced all the faulty components. I found that all 4 of the rectifiers CR16-19 had burned up, 1 output transistor was shorted, both pre-driver & driver transistors shorted, CR15, CR11 & CR16 shorted, and I found 1 of the 1N4148 diodes shorted. So All of Q1-Q8 and all the other components have been replaced. Also 2 traces around the rectifiers burnt up, most notably the trace from CT on the transformer secondary that leads to the negative pads of the filter caps. I have tested out the filter caps and do not believe they are bad.

So now when I turn on the amp with a 60 watt bulb in the current limiter it lights up bright as if I have a short. I also tried a 100 watt bulb to see if the brightness went down, which it really doesn't. All of this is without a load on the amplifier. I have to admit that I have not had to rebuild the output circuit on a "Flying Rail" amp before so I have no experience with these. I went to measure the voltage at the filter caps and it is -/+6vdc on each cap relative to ground. Then thinking about it more I guess I should not be measuring the filter cap voltage relative to ground but relative to the C.T. connection point. So I need to remeasure that again.

So I found this file from Fender about Flying Rail designed amps. It mentions...

To take advantage of this characteristic, a power supply exclusive to the power amp is
used. The positive and negative rails are derived from each end, respectively, of a
transformer secondary winding which has a center tap. All current drawn by the supply
(for the positive and negative rails) flows through the center tap. If we now remove the
load resistor, insert the speaker in series with the center tap (positive lead connected to
the center tap, and negative lead connected to ground), we have accomplished two
things:

1. The load (speaker) has been moved from the power amp to the power
supply, and will now produce sound (driven by the current the power amp is
drawing.)
2. The power supply center tap is electrically (DC) near-ground.

[B][U]Note: This is the reason that the speaker (or an appropriate load) must be
connected when servicing these amps.[/U][/B]

So I guess I need to put a load on the amp right? Then power up via bulb and variac where at that point I am hoping to see the bulb go dim. Does that all sound right? Another thing to point out is that the output jacks are definitely not isolated from the chassis on this model.

https://music-electronics-forum.com/...7&d=1553354748

[TomCarlos]

Dr.... please look for a Private Message. I have a stash of older Peavey Parts, including the 70474140 and 70484140 transistors from old circuit boards.

Tom

[DrGonz78]

Hey Tom! You really are so kind with your offer to help with the parts. I already have new parts in place and the amp might already be fixed. My issue seems to be the bulb limiter lights up bright without a load attached. Now that normally indicates that there is still a short present in the amp and you should never connect a load in that circumstance. However, the Fender service bulletin on Flying Rail designs states that a load needs to be present for the power supply Center Tap to be electrically near ground. Seems strange to me but they say that because of this type of design a load needs to be present when servicing these amps.

Edit: I am 99.9% sure that there are no shorts or errors that I made replacing parts. I have gone over the board so many times now. I am pretty sure the amp needs a load to function correctly. But I was hoping for some confirmation that I should put a load on the amp.

[TomCarlos]

I wonder what would happen if you used a Kill A Watt meter to monitor the line current....

[g1]

I think you should try with the limiter bulb with load. The bulb should save you whether you have a load on or not.

[J M Fahey]

As a compromise:
* start with bulb but no load, "just to let it start" , if fully repaired it "should".
* now connect load, without turning it OFF, it might pop or thump but nothing really bad.
* measure voltages, specially that speaker out sits halfway between +V and -V
* apply a little signal, so you have , say, 1 W RMS at speaker, just to check it works, amplifies, both halves of output sinewave are present, etc.
* if everything looks fine, apply some music, and rise level enough to see lightbulb blinking in rhythm with music.
* if fine, short across lightbulb and check amp works
* if fine, rise level to clipping (preferrably into a resistive load so you donīt go deaf) , check amp really holds it.

After that, amp should be fully repaired and tested

[DrGonz78]

Okay I went ahead with some testing of this amp tonight. I plugged in a 4 ohm load while the amp was turned on via a 60 watt and then 100 watt bulb. The bulb was still shining too bright to feel comfortable that the amp is repaired. I ran it up on the variac in tandem with a 100 watt bulb in place and could see only 300ma current draw. So obviously the bulb was soaking up excess current but to what extent? I decided that I needed to observe the current draw without the bulb present and to throw caution to the wind I left the load plugged in. So just barely applying up to 30-40vac it was very obvious that the amp is definitely pulling 1-2 amps current. It shoots up way too fast and definitely I got a problem with something on the amp still. Only thing is that I can't find anything else wrong to repair on the amp and truly this one is being very elusive.

Now I did test the main filter caps for ESR and they passed. I even charged them with a 9v battery to see if they held a charge, which they held 9v perfectly. Unplugging the HV secondary from the board makes the light bulb turn off. I only measure -/+7vdc on the main filter caps. The caps definitely got a jolt when the amp failed originally. I am now starting to wonder about the state of these filter caps and even the power transformer. The main fuse did not blow even when the amp failed.

Anyone have an idea what I should do with this damn amp? 😉

[Jazz P Bass]

Have you taken any voltage measurements around the driver circuit with the lamp limiter on?

Any severe oddity should show what is a miss.

XM 6 and XM 4 schematic[1].zip

[DrGonz78]

I know that I had my -/+15v rails present before the repair because I measured them while running signal through the preamp observing it on a scope. All that was done right before realizing the 4 main rectifiers for the -/+45v rails were burnt open. That was when I realized something awful had taken out the power amp side of the amp. Then I replaced everything that was bad, or so I thought.

Here is some preliminary voltage checks and yes it really sheds some light on the problem at hand, thanks Jazz!! Later tonight I will check some more of the voltages and try to figure out what is going on with this amp. I have 0vdc on the output of the amp. It is looking like where the power amp and preamp voltages meet up there is something amiss. I had changed out the RC4558 op amp at one point and it made no difference. I know that I checked all the small diodes in the circuit and the only one that was bad was CR8 1N4148 which I replaced, in addition to CR11, CR15 & CR16. Something will pan out here I am sure of it.