1976 Roland JC-120 help

[Brettdvd]

Howdie everybody! Ive recently created an account but i've lurked for a while. As the title states, i've got a jc-120 that does not produce sound when my guitar is plugged in, regardless of what channel i am plugged into. I've plugged my cellphone into all channels and get about a maxed cellphones amount of volume as well as occasional popping from the speakers. Just was wondering if anyone has run in to this problem before. Transistors? Bad solder joint or cable. Thanks for your time!

[phatt]

Welcome Brett, Does it have an efx loop setup?
If so try injecting a signal from your iphone into the return socket then post results. :tu:
Phil.

[Brettdvd]

Appreciate the welcome! Unfortunately (haha) this is an older model, 1976 i believe. Vibro/chorus selector input, 2 outputs for 8ohm speakers and a line out are the only i/o on the rear of the amp.

[Brettdvd]

So now i get an initial surgenof volume, then it drops down to faint again. Transistors?

[phatt]

Ok good you have a *line out*
If you have another amplifier then Use the line out and take the signal to another amplifier.
If the signal is strong then you know that the preamp section is working and that helps to narrow down the problem.
Also there might be a broken solder pad and something has lost contact.
A good thump with your fist might even get it working again, that tells you the problem is a mechanical issue and then you unplug and go on a big search of the circuit to find the break.
Phil.

[Brettdvd]

Ok good you have a *line out*
If you have another amplifier then Use the line out and take the signal to another amplifier.
If the signal is strong then you know that the preamp section is working and that helps to narrow down the problem.
Also there might be a broken solder pad and something has lost contact.
A good thump with your fist might even get it working again, that tells you the problem is a mechanical issue and then you unplug and go on a big search of the circuit to find the break.
Phil.

No sound from secondary Amp when running from jc120 line out to input jack. I think im going to purchase some new transistors. I appreciate all of your help @phatt!

[J M Fahey]

So now i get an initial surgenof volume, then it drops down to faint again. Transistors?

No sound from secondary Amp when running from jc120 line out to input jack. I think im going to purchase some new transistors.

what makes you think you have bad transistors? 
What tests did you do to reach that conclusion?

I suggest you do not start "shotgunning", meaning replacing parts at random with no clue, hoping to solve the problem; all you´ll get is messing/destroying the PCB from repeated soldering/pulling/resoldering ... and then you´ll end with a 60 lb doorstop or night table.

Can you recognize and find physical parts based on a schematic?
Can you measure and post voltages and follow PCB traces?

You will need those skills to start troubleshooting, only valid repair path.
Plain guessing (gambling) is disappointing, to say the least, and in that case you will be better off sending it to some experienced Tech.

[Brettdvd]

So now i get an initial surgenof volume, then it drops down to faint again. Transistors?

No sound from secondary Amp when running from jc120 line out to input jack. I think im going to purchase some new transistors.

what makes you think you have bad transistors? 
What tests did you do to reach that conclusion?

I suggest you do not start "shotgunning", meaning replacing parts at random with no clue, hoping to solve the problem; all you´ll get is messing/destroying the PCB from repeated soldering/pulling/resoldering ... and then you´ll end with a 60 lb doorstop or night table.

Can you recognize and find physical parts based on a schematic?
Can you measure and post voltages and follow PCB traces?

You will need those skills to start troubleshooting, only valid repair path.
Plain guessing (gambling) is disappointing, to say the least, and in that case you will be better off sending it to some experienced Tech.

Nothing on the PCB will be touched...the output transistors 2sd425s located on the bottom outside of the chassis, simply unscrew pull out and replace. I AM recieving very low audio output from the speakers when i have my phone plugged in, but there does not seem to be any amplification. This is the one "shotgun" troubleshooting step, that should not harm the amp. No soldering,  or tampering with anything on the PCB. Identify the base of the transistor with a DMM and install. Let me know if im incorrect?

[DrGonz78]

Yeah don't replace those Output transistors now. Maybe later if they prove to be faulty but only then. You have already narrowed the problem to being on the preamp side of the amp. Perhaps there is a problem on the output side too, but there is DEFINITELY a problem that needs to be fixed on the preamp. Signal tracing and voltage readings will be your best friend right now. Hey you might end up fixing the preamp to find the amp working normal again. Don't shot gun this thing seriously.

[Brettdvd]

Yeah don't replace those Output transistors now. Maybe later if they prove to be faulty but only then. You have already narrowed the problem to being on the preamp side of the amp. Perhaps there is a problem on the output side too, but there is DEFINITELY a problem that needs to be fixed on the preamp. Signal tracing and voltage readings will be your best friend right now. Hey you might end up fixing the preamp to find the amp working normal again. Don't shot gun this thing seriously.

fixed it! After more inspecting, i found a loose wire. Resoldered and its up and running! Thanks

[J M Fahey]

 :dbtu:

Imagine what would have happened if you had replaced 100 parts with no results, your PCB would now look like:


and as Enzo always says : "bad parts" are only 50% of amp troubles", you can replace *all*  and still not solve the problem, remaining 50% is "something else" meaning bad connections, cracked solder, improperly set bias or controls, etc.

[Brettdvd]

The PCB wouldnt have been touched....... the chassis wouldnt even have to come out. But i get what you're saying.