Peavey Classic Chorus 130 - No Sound From Left Speaker

[mrkeetz]

Hi folks. New to the forum here. I have just acquired a Peavey Classic Chorus that has no sound from the left speaker. I tested the speaker by swapping the wires for the right speaker. The speaker itself works fine. I have searched online for a schematic for the amp, but so far have had no luck. Does anyone have any tips for finding schematics, or even how to troubleshoot the problem? I have the board out of the cabinet, and there is no sign of damage on any of the components. Any advice is greatly appreciated. Thanks.

[The Dude]

That amp has an "amp in 1" and "amp in 2" on the rear. Plug a signal into each of those jacks and see if both speakers work respective to each jack. Also, try cleaning them and running a plug in and out of the jack a couple times. It could be a simple as a bad switching jack.

[glebert]

Here's the schematic I have.

[mrkeetz]

Thanks for the replies guys. And thanks for the schematic Glebert. That is such a big help. I was going over the board, and saw that there has been some previous work done. Resistors snipped, and I found a transistor (Q1) with its middle leg snipped and removed. I reconnected the leg for troubleshooting, and got sound from both speakers. But the quality was really bad and heavily clipped. I clipped the middle leg again and got normal sound. Upon reviewing the schematic though, I see that Q1 has been inserted backwards. It's definitely all a bit of a puzzle. To make matters worse, there were some broken solder joints on a couple of caps, including one of the filter caps that needed to be repaired. It's a can of worms, lol.
Thanks for the help though. Much appreciated.

[Jazz P Bass]

Q1 is a 2N5641 jfet.
It is used to mute the #1 preamp.
The pinout and placement is the same as Q3 (pre 2)
SDG.

[mrkeetz]

Thanks for the reply Jazz. I actually ordered a batch of them yesterday. P Channel 40v. Hopefully they'll arrive by the weekend.

[mrkeetz]

So I finally received my order of 2N5641 JFETS. I popped one in and powered the amp up. There is sound coming from both speakers but the volume is very low even with the amp cranked up, and there seems to be some gain. I noticed that U1 seems to have been replaced at some stage with an RC4558N opamp, while the originals seem to be RC4558P. Could this be a problem? I also noticed when the Lead Normal button is engaged, I lose all sound.
I appreciate any tips or help you guys can give. I'm using this project to learn as much as possible.

[The Dude]

The RC4558N won't matter in this application.

[Enzo]

A 4558 is a 4558. There are guys who worry about the brand in Tube Screamer pedals, but in the amp? Nah.

You said the original was in backwards. But was it the EXACT same part number? JFETs do not all have the same leg arrangement. One must be VERY carful when subsituting. Most any JFET will work, but you may have to turn them around or sideways.

2N5641 may well be a perfect transistor here, but it may have legs in different order than the original part.

[mrkeetz]

The JFET I replaced was actually a 2N5642. I guess they're both similar from what I've seen on data sheets. I believe 2N5641 is the original part according to the schematic. But I see a lot of 5642's in their place. Stumped now. Thanks for the quick replies and help though!

[The Dude]

According to the Peavey cross reference document, the 2N5461 is a P -CH FET. The 2N5462 is an N-CH FET. They are not interchangeable. You need to use what the schematic calls for or a correct substitute.

Edit: There is a discrepency in the Peavey crossreference document. In one place it says the 2N5462 is N channel and in the next line it says P channel, so I'm not sure if the above info is correct. Sorry to add to the confusion.

[mrkeetz]

Ahh. Lol. I just woke up and read the auto generated email and your original post woke me up pretty quick! Yeah, I was initially confused by the N Channel reference in the Peavy crossreference but finally figured it out. I checked some data sheets and basically they listed them as the same. I'm new to this kind of repair. I've some electronic training from the AF, and have spent the last few years collecting a lot of repair equipment, making some small repairs along the way. I'm hoping to do a degree in electrical engineering in the next year or two so I can get a better understanding on all that is going on.
I guess for troubleshooting I should start looking to the schematic and see what ties both sides together. I know if I remove the drain leg from the 2461, the right side returns to normal.

[Helmholtz]

Confused.

Schematic shows 5461 p-channel JFet. But you are speaking of a 5641 ??

Symbol for p-channel has gate arrow pointing outwards.

https://www.mouser.de/datasheet/2/68...5462-19415.pdf

[mrkeetz]

Confused.

Schematic shows 5461 p-channel JFet. But you are speaking of a 5641 ??

Symbol for p-channel has gate arrow pointing outwards.

https://www.mouser.de/datasheet/2/68...5462-19415.pdf

Sorry Helmholz. That was a typo from earlier on that got continued. It is definitely 5461.

[g1]

If disconnecting the drain or source legs gets things going, and new Fet's didn't help, are you sure the gate pins are getting the correct 'command' signal?

Also, I think you mentioned that legs were disconnected when you received the unit. This means someone has been in there before, and you can't trust that the Fet's are installed with correct orientation. If the board is not marked, you should verify that G,S, & D pins are going to correct components as shown on schematic.