Help fixing a very vintage Peavey

[Leo_Gnardo]

Ouch. That's gonna be fun to find.

Get a photocell intended for Morley pedals, Antique/tubesandmore has 'em. I use them for fixin' Fender vibratos all the time.

[fastmerc]

The blue electrolytic cap goes into the center of the board with the positive end connecting to the transistor Collector, so in other words towards the center of the board. At this point unless you can test the old cap, just replace it with a new one.

The other missing part at the edge of the board is a cds photocell for the tremolo. It should be sleeved to the small lamp that is to the right of it in the photo. The photocell does not need to be replaced in order for the circuit to pass signal. The tremolo will not work unless it has been replaced, but you can do that at a later time if you want.

Forgive my ignorance but when you say "sleeved" what does that mean?

I have soldered that capacitor back in and nothing has changed so I will find a new one for that. I suppose until I get the new cap in there I can't really tell much more?

[fastmerc]

You may not understand the circuit, but tracing the circuit out gives us a schematic, and we can explain any circuit we have a drawing of.

Peavey made a bunch of amps with transistor preamps, like the ARtist and Classic. I bet their circuits are real similar to this one.

I will attempt to map out what I can. It will take me a good while as I will probably have to look up each individual component, I have found an interesting free program that will let me build the schematic though.

[fastmerc]

Another question, since I have no electronics stores in town I will have to online order any parts. In the near term for diagnostic purposes would this cap be usable in place of the one in question? The value of the one in question is 25uf 25v...I have a 22uf 50v new one here?

[Justin Thomas]

That'll work just fine. You could just leave it in permanently if you want.

Justin

[fastmerc]

OK...Thanks to Justin I tried replacing the cap with the new one I had....no fix sadly, but I have done a bunch more trouble shooting...It would seem the issue is somewhere in the circuit between the input Jack and the Volume pot. I tried swapping the known working jack and volume...so it doesn't appear to be either of those components. If I touch my finger to the volume pot lugs you get the normal hum you would get if say you touched the end of your instrument cable, so everthing after the volume pot is working properly as far as the power amp is concerned.

[fastmerc]

More information: Whomever was in this amp before me replaced the first transistor in the circuit on the reverb channel. The original working board has a transistor marked F 2N5163 741, the replacement on the other board is marked MPF102. I tried looking at the spec sheets of each of them, but Im way out of my league trying to read that. Are these parts interchangeable or could this be causing the problem?

[52 Bill]

More information: Whomever was in this amp before me replaced the first transistor in the circuit on the reverb channel. The original working board has a transistor marked F 2N5163 741, the replacement on the other board is marked MPF102. I tried looking at the spec sheets of each of them, but Im way out of my league trying to read that. Are these parts interchangeable or could this be causing the problem?

They are both n-channel jfet transistors and probably should work interchangeably. That doesn't mean that the MPF102 is working.

By sleeving, I mean that the photo cell and the light bulb need to be facing each other and encased in a sleeve of some sort to keep the two parts aligned and shielded from stray light.

Do you own and know how to use a multimeter to safely read voltages in the amp? Do you know how to test a transistor? The best way to find out what is happening with the dead channel is to read and compare the voltages between the two channels.

[fastmerc]

They are both n-channel jfet transistors and probably should work interchangeably. That doesn't mean that the MPF102 is working.

By sleeving, I mean that the photo cell and the light bulb need to be facing each other and encased in a sleeve of some sort to keep the two parts aligned and shielded from stray light.

Do you own and know how to use a multimeter to safely read voltages in the amp? Do you know how to test a transistor? The best way to find out what is happening with the dead channel is to read and compare the voltages between the two channels.

I do own a digital mulitmeter. I don't particularly know how to test a transistor but I can certainly compare readings if I know what to set the meter to. If any would like to give me some pointers that will be great. Meanwhile I will hit up youtube an see what I can learn.

[SpareRibs]

I do own a digital mulitmeter. I don't particularly know how to test a transistor but I can certainly compare readings if I know what to set the meter to. If any would like to give me some pointers that will be great. Meanwhile I will hit up youtube an see what I can learn.

Hello,
You can find out more about your amp. Go to this website (samuelbarker.net/t60 mafiaforum). It is the T60 mafia forum. You will have to register to become a member to ask a question. Some of the members there were actual Peavey employees from times gone by. Those people may have copies of the original schematics. If not they will be the ones most knowledgeable about that particular amp.

[52 Bill]

I do own a digital mulitmeter. I don't particularly know how to test a transistor but I can certainly compare readings if I know what to set the meter to. If any would like to give me some pointers that will be great. Meanwhile I will hit up youtube an see what I can learn.

What meter do you own and what functions does it have? Diode test? Cap test? Testing transistors is fairly easy, but having a diode test function on the meter makes it simple to communicate the test results.

I haven't had a chance to review the photos of the front and back of the 2nd preamp board yet, but the volume control appears to be set between the first two transistors. It appears to be similar to the typical Peavey circuit where the volume control raises and lowers the gain of the first transistor stage and acts like a normal voltage divider on the output of the second transistor stage. I think that the original Classic schematic may be similar in design. I will have a look when I get a chance.

[fastmerc]

What meter do you own and what functions does it have? Diode test? Cap test? Testing transistors is fairly easy, but having a diode test function on the meter makes it simple to communicate the test results.

I haven't had a chance to review the photos of the front and back of the 2nd preamp board yet, but the volume control appears to be set between the first two transistors. It appears to be similar to the typical Peavey circuit where the volume control raises and lowers the gain of the first transistor stage and acts like a normal voltage divider on the output of the second transistor stage. I think that the original Classic schematic may be similar in design. I will have a look when I get a chance.

Its a Craftsman 82170, it does have a diode/continuity test mode and has a cap section but it looks like it maxes at 20uf, It has ohms, Vac/dc, Aac/dc, temp and frequency. I have since learned the pin out is different on the two transistors but made sure they were oriented correctly. It does seem the mpf 102 was bad, but I switched the known good 2n5163 over to the 2cd preamp board and still not functioning. I put everything back, even though I know the mpf was bad. I also started taking voltage readings from where the power comes into each preamp board across every component and compared them....everything looked very similar till I got around the bad transistor. I may try swapping the good one back over one more time tomorrow and take the voltage readings again.

I also started looking for photocells, is there certain specs I need to look for or is it just a size thing.

[52 Bill]

It does seem the mpf 102 was bad, but I switched the known good 2n5163 over to the 2cd preamp board and still not functioning. I put everything back, even though I know the mpf was bad.

How do you know that it is bad? It may just be the wrong part for that purpose or maybe the drain and source are reversed.

Have you done power off tests of the resistors and caps on the board with your meter? Again you can compare the results with the parts on the other board. If as you say there is signal (hum) from the circuit after the volume control, I'd check the two 2.2uF caps that connect to the volume control.

I also started looking for photocells, is there certain specs I need to look for or is it just a size thing.

If you use the one that Leo Gnardo suggested, it will probably work fine. If you are looking at other sources, then it has to have a fast response time and needs to have fairly low resistance when exposed to light, like maybe less than 1K-2K ohms.

[Enzo]

Drain and source can be reversed with no harm. JFETs are a low resistance path between source and drain, with a gate capable of making it higher resistance, to the point of cutoff.

A good JFET will measuer some low resistance between the source and drain legs, that is not a bad part.

[fastmerc]

How do you know that it is bad? It may just be the wrong part for that purpose or maybe the drain and source are reversed.

Have you done power off tests of the resistors and caps on the board with your meter? Again you can compare the results with the parts on the other board. If as you say there is signal (hum) from the circuit after the volume control, I'd check the two 2.2uF caps that connect to the volume control.

If you use the one that Leo Gnardo suggested, it will probably work fine. If you are looking at other sources, then it has to have a fast response time and needs to have fairly low resistance when exposed to light, like maybe less than 1K-2K ohms.

I figured the transistor is bad because A) tested in circuit against the know functioning one each corresponding leg measured as follows:
Gate: 2n5163 0.0V mpf102 12.61v Source: 2n5163 18.3V mpf102 12.91V Drain: 2n5163 3.31v mpg102 12.63

Also, out of circuit I would get readings on all legs, which if Im not mistaken from watching some videos yesterday means the gate is letting voltage through both directions when it should only be one way. I tried reversing the pins, just in case with the same result.

I have not yet done the power off tests of each resistor or caps, but will do that next, although some caps are above my meters ability.