Peavey 400 series amp board - Output xistors

[Mars Amp Repair]

Say,
I replaced all 8 of the output xistors in this power amp section of the Peavey Musician's amp as they actually were open (at least the 2 T03's that drive the other 6 outputs). I decided to use the MJ15024G transistors as they are the most powerful of the T03's. The originals were some RCA's I suppose are not stock.

I'm thinking they are not the best choice as I'm getting the typical oscillation as a result of the higher frequency capability of the newer xistors (Epitaxial vs whatever the others are). That I was able to resolve with a couple of 200pf snubber disc caps connected between the emitters & collectors of the 2 driver T03's. (going between any other elements did not help).

However the weirdest thing is that NONE of the power transistors has 0.5V B-E but they all show they are drawing current when voltage measured across the emitter resistors. Idle current between E & C of + & - driver xistors is about 0.5mv across the 1ohm 10w resistors.

I can get nearly 60V P-P swing out of the amp into 4 ohms that calculates out to about 170Watts which sounds about right into 8ohms. (I do not have the facility go make 2ohms at high wattage which is what I believe this amp is rated at).

They are not sharing current nicely in the power transistor sandbox, either. As I might expect, the 2 driver power xistors are getting the hottest (not too hot to touch, but plenty hot) & then the 2 right after those in the circuit are about 1/2 as warm & the heat diminishes as you go outward from there.

So, I cannot really determine if all the power transistors are contributing to the job of wattage production except by heat.

Any ideas on a better & available transistor for this application? Source from Peavey themselves?

Thanx all, Glen Whatley

[Enzo]

15024 is a fine sub for the originals. HAve you checked all the ballast resistors and other resistors around the output stages? WHen outputs are bad, often they took the resistors with them.

[Mars Amp Repair]

Hey Enzo,
Hope all's going well with you. Yep, all emitter & equivalent ballast resistors good. The original issue was an intermittent driver xistor that would open up intermittently & set the output fully to the positive rail. Nice to find that while running without a load (;- ]
Just ran the amp again and it does not seem the other power xistors are tending to be about the same temp, but those two initial power xistors are getting noticeably very hot to the point of concern.
Running on load at about 40watts (just for consistent level) the inner 2 'Drive' xistors collectively across the two-1ohm resistors is reading about 600mv. The other xistors at same level are reading about 80mv across the two-0.33 ohm resistors. Even if you take into consideration the difference in resistances, the 600mv reading is stil;l quite a bit higher, hence the hotter xistors and all the finals are connected essentially in parallel.
Perhaps that's just how these things run.
I have swapped the driver xistors with others just to see if they were being current hogs to really no avail. I think if those two weren't getting so hot, I'd feel comfortable the amp was repaired.
Any experience or memory of this type situation?
Thanx Enzo...glen
Also weird thing is with no signal, the 4 non-driver T03's show 58mv from emitter to base...I find that really weird unless this amp has some really crazy design that I don't understand. Of course at idle those xistors show drawing almost no idle current. Weird.

[Mars Amp Repair]

Attached basicallly the same power amp...

[Enzo]

With bad outputs I always replace the drivers regardless of how they check. And frankly the predrivers are usually cheap and might as well.

[g1]

Also weird thing is with no signal, the 4 non-driver T03's show 58mv from emitter to base...I find that really weird unless this amp has some really crazy design that I don't understand. Of course at idle those xistors show drawing almost no idle current. Weird.

I see 6 output devices (non-driver). Are you saying 2 of them show good base voltage and 4 do not? All 3 bases on top are connected together. Likewise for the 3 on bottom.

[Mars Amp Repair]

peavey-musician-400-amplifier-schematic.pdf

[Enzo]

The six on the right are outputs, 60142 type. The next two to the left are also 60142, but are the drivers. Note the slightly different wiring to them Their emitters drive the bases of the remaining ones. The 430 and 431 are the predrivers.

The RCA 60142 were indeed stock, per the schematic. Peavey has not had those in stock for decades, well, at least two decades. They originally crossed them to 2N3772, which was superseded by their 70484140. And that these days is filled by MJ15003. But in my experience a MJ15024 works just as well.

[Mars Amp Repair]

I see 6 output devices (non-driver). Are you saying 2 of them show good base voltage and 4 do not? All 3 bases on top are connected together. Likewise for the 3 on bottom.

Thanx for the response g1 & Enzo. I know it's a bit difficult to point out which xistors I'm referring to as there are no reference numbers in the schemo. When I say 'drivers', in this case I'm referring to the left most pair of 60142 outputs with the 1ohm ballast resistors, not the predrivers marked '430'.
Yes, the 6 right most 60142 marked power xistors at idle only have like 60 millivolts of b-e voltage as in not turned on. When you apply signal those 6 right most 60142's will then begin to conduct as evidenced by drop across their respective ballast resistors & heat they generate.
Driving the power stage with a sinewave shows no crossover distortion and the amp will drive 160watts into 8 ohm loads. It's really a bit crazy what's going on.
As for what I'll refer to as the predrivers marked '430', would you agree that the TIP31,32 variety would be adequate for those? I haven't done this for a while but it seems those T0220's work for well for this type of application. The offending driver that started the whole issue was the single 430 that drives the driver 430 xistors. It would open B-E intermittently causing the output to go full + rail. I replaced that one with TIP32 or 31...don't recall right off which one is npn or pnp.
Anyway, I'm beginning to wonder if those driver 60142's getting much hotter than the 60142's that follow isn't perhaps normal...
Thanx, glen

[Enzo]

TIP31C is the recommended cross for EP430

Here Glen, download the official Peavey cross reference.

http://www.talkbass.com/attachments/...rence-list-pdf

[Helmholtz]

Anyway, I'm beginning to wonder if those driver 60142's getting much hotter than the 60142's that follow isn't perhaps normal...
Thanx, glen

What are the voltage drops across the 1R emitter resistors?

The driver 60142s run in class A, which makes them get hotter at idle than the outputs (class B).
They also contribute some output current thus filling the crossover gap.

[Jazz P Bass]

Peavey had more than a few 400 series amplifiers.
Here are two that have the board layout.
Maybe one is close to the one that you have.

[Mars Amp Repair]

What are the voltage drops across the 1R emitter resistors?

The driver 60142s run in class A, which makes them get hotter at idle than the outputs (class B).
They also contribute some output current thus filling the crossover gap.

I see where the schemo indicates those drivers have 0.1V across their ballast resistors. That's about what they're measuring. Guess I don't see where those drivers would be running class A, but I'm not a designer and am open to any suggestions or ideas.

[Mars Amp Repair]

TIP31C is the recommended cross for EP430

Here Glen, download the official Peavey cross reference.

http://www.talkbass.com/attachments/...rence-list-pdf

Thanx, Enzo. That link is incomplete. I'll get my login to TalkBass & check it out. I have one cross reference but it doesn't have the Motorola or japanese subs on it. Guess I'm just getting lazy in my old age (;- ]

[Mars Amp Repair]

Say this is completely unrelated, but I cannot figure out why y-all's replies are not going to my email. I've checked settings yet, no emails...any ideas?