Epiphone CH-70 Chorus Not Working, ideas?

[Pinhole]

Recently picked up an old Epiphone EP-CH-70 chorus, late 80's model. Bought it as is with the hope I could get it running again.

Pedal turns on fine but no chorus effect is present. Also, no guitar volume once pedal is switched on.

I've resoldered some points that looked iffy, looked for damaged wires or components, checked that it wasn't grounding out anywhere, but no luck. Any ideas on what needs fixing? I can post pictures if that'll be helpful. Thanks!

[MaxPower]

An audio probe would probably be a good place to start.

[anotherjim]

Sometimes the control knobs have been stomped on and damaged the pots. This can be hard to spot visually.

[GuitarFreak302]

Definitely check the circuit with an audio probe and check the IC chips. There may also be a bad transistor, check them with a multimeter if you have one. I repair pedals on the side so feel free to ask questions.

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[Hemmel]

I repair pedals on the side

Really?
I repair them flat on the table.
When I try to repair them on the side, the pedal keeps tipping over...

[Mark Hammer]

Chorus pedals from that era are strikingly similar in design.  Apart from the things you have checked, there are going to be a few "usual suspects" to consider:

1) The bias trimpot is either mis-set or the wiper is not making contact, resulting in an inappropriate bias of the delay chip and no signal passing;

2) The single FET used to engage the effect is fried.  Scores and scores of commercial chorus pedals use a single FET to engage the effect, since lifting the delay signal cancels the effect.  It wouldn't be the first time someone had a problem with a switching FET.  You can test that issue by simply tacking on a hardwire bypass across the FET on the copper side of the board, putting it permanently in "effect" mode.  If the chorus comes on, there's your problem.  Unsolder the wire bridge and replace the FET.

3) If there is a CMOS chip somewhere on the board, it might have fried.  They are generally pretty robust, but can be the weakest link sometimes.  Most chorus pedals are going to use an MN3007/MN3101 combo, but sometimes they don't.  Look for anything with a CD4xxx number.  Incidentally, the CD4007 is often used for effect switching, such that the FET s fine but the circuit that turns it on and off, using a 4007, isn't.

I don't know that your problem lies there, but it's bit like divorce: if you hear someone is getting divorced, it's a high likelihood that a) someone is fooling around, b) someone has money/booze/temper issues, c) someone got married too young or too quickly, d) someone wants children much less than the partner, or e) someone just found out they're attracted to the sex that their partner isn't.  Those aren't the only reasons, but that list is a pretty safe bet to start from.

[GuitarFreak302]

I know you are just joking but I meant as a side job. It's not my normal day to day profession.

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[Pinhole]

Thank you for all the ideas! Just looked up how to put together an audio probe so I'll make that asap, hopefully identify the problem.

So the audio probe will be able to determine if any of components are bad, but not the FET?

[PRR]

Signal tracing *will* confirm/deny that this type FET switch passes signal.

(DC testing won't.)

Mark did give you a simpler way- jumper the FET so "it is ON!", whether it really is or not. If now you get the signal, something around the FET isn't right, and Mark says it is often the FET.

[Mark Hammer]

I wouldn't say "often", but over the years we've seen enough instances where a switching FET needed replacing in this pedal or that, that it can be treated as a "usual suspect".

Paul is certainly right about the value of signal tracing.  For instance, if there was nothing wrong with the biasing of the BBD, but something wrong with the electronic switching circuitry (whether the FET, a diode leading up to it, or a CMOS chip used to form a flip-flop circuit), then you would expect to see a reasonable AC output fro the output pin/s of the BBD.  Ideally, you'd take that reading from the point where the delay signal reaches the FET, but it's probably a simpler task to look up the datasheet on the BBD, identify the output pins and plunk your probe down there.  At least the chip will be clearly marked and you won't have to push components out of the way to see what it is.