Earth Module 440 Bass "Producer" Head (i.e Peavey Musician 400) Problem

[Enzo]

It indicates what you already know: there is +44 on the output. The emitter is tied to the negative 45v, and the collector to the output. So if there is 45v on the output, it will be on that collector.

I asked earlier and don't recall an answer, is ther +15 and -15 respectively on pins 8 and 4 of the IC U2? Or if you prefer, the power pins of the TL074?

[JerkyPudding]

It indicates what you already know: there is +44 on the output. The emitter is tied to the negative 45v, and the collector to the output. So if there is 45v on the output, it will be on that collector.

I asked earlier and don't recall an answer, is ther +15 and -15 respectively on pins 8 and 4 of the IC U2? Or if you prefer, the power pins of the TL074?

There are no IC's or 15v. This is an older amp.

[Enzo]

I was looking at the drawing you provided in post #7.

So maybe it is closer to a 400 module instead of a 400BH.

In which case, look for a diode clamped to the heat sink or even clamped to one of the output transistors. Got one? Measure voltage at one end of it. Do you see about 45v there? If so, then all the power stuff is probaly OK.

Will this drawing open for you?

[JerkyPudding]

The drawing opened up, thank you for that! I measured +44v at the diode mentioned.

[Enzo]

SO that means in a working amp that everything to the right of that diode will also sit around +44v.

Oh, does that circuit look more like your board?

Using my drawing then, look below that dual diode on the heat sink. it is within the dotted line rectangle. Below it is a 430 transistor. On your board that is more likely a TIP29 or TIP31. A TO220 of some sort (the tab top type).

If that transistor is open, or if it is not being driven at its base, then the dual diode and the whole rest of things will snap up to V+, your +44v.

Check it.

[JerkyPudding]

The drawing you supplied matches much closer. The heat sink mounted diode does not appear to be a double, but I did trace it to a TO220 transistor, which measures 44v at base, collector and emitter.

[Enzo]

It might not be a double, but the double one looks like a normal diode too.

Look at the schematic. Only way that transistor could have +44 on the emitter is if the 10 ohm resistors there were open, one or both. The transistor could also be shorted - check for that - but if so it would burn out the resistor too.

[JerkyPudding]

I told you the wrong transistor, I traced the circuit more thoroughly to find a PNP tab transistor measuring 43.3v at base, -44.5 at collector, and 43.5 at emitter. Also, the heat sinked diode is measuring open (it's been isolated from the circuit via the socket connection). The positive side of the diode reads 44v, the negative reads -43.3v. Also, when I replaced the bad 1ohm 10W resistor and the diode (circled in green), 44v went back to the output (voltage at the output dropped to .003v after the fuse blew).

[Enzo]

The heat sink diode in the PV, and maybe the Earth, is a dual, so it will have twice the voltage drop, that may make it seem open. If you have +40v at the base of that transistor, the only place it can come from is that diode.

And that points me back at the transistor I mentioned previously, the 430 just left of your circled one.

[JerkyPudding]

So I got the 44VDC off the output, turns out a bad solder joint on an emitter resistor was to blame (this amp's past has been in rough hands, many bad solder "repairs"). Now I'm reading -0.5DC on the output.

Also, is it possible for this amp to function at all with a reversed heat sink-mounted dual diode? I checked and rechecked, and based on the marking (which looks like the typical base marking for any diode, but it is rather difficult to tell), and it seems to be reverse to what the schematic says (the schematic has been very accurate so far). The amp has been through some rough hands, so I have suspicion mixed with naivety.

[Enzo]

Probably not reversed. otherwise likely blows fuses.

[JerkyPudding]

I measured the 430 transistor at the base of the heat sink diode, readings are: base= -42.9v, collector= =-1v, emitter= -43.7. Reading -.53v at output now.

[spy]

Did you fix it JerkyPudding? Do you still have the amp?

I'm working on the same amp but some parts are fried and can't read the values. Also, I can't find them on the Peavey schematic. Maybe you can help me out.



They are the two resistors at the bottom right, one left to the coil and one next to the trimmer plus the 1W next to it. The first three seem to be the same value, maybe 360R? I have no idea about the other two. Maybe another 22R 1W? I don't think the circuit is exactly the same with the Peavey schematic.

[Enzo]

Oh, I bet the circuit doesn't differ by much.

[spy]

I can't find any 36xx ohm resistor or a trimmer at the Peavey schematic. The driver stage is definitely a little bit different.