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Discussion Starter · #1 ·
This amp belongs to my buddies uncle and it's been sitting in his shed for years collecting dust and cigarette smoke. It's a Fender Sidekick Reverb 20 made in 1985. I can't find much info about it with regards to troubleshooting or schematics so any help would be great. When I lightly pluck a string the sound is ok but anything harder and the amp just crackles (I'll have to figure out how to add the video). Another problem it has is if I connect headphones to it, as soon as I power it up my headphones scream a high pitched sound I was worried it wrecked them but they are still ok. I have tested the speaker with another amp and it is fine.

Any general help on amp repair is great. I have only fixed pots and switches in amps before but have a good working knowledge of electric circuits. All I can seem to find is how to troubleshoot tube amps, and sadly, all I have are solid state.



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Discussion Starter · #3 ·
So I've gotten a little further along figuring this thing out. And I finally acquired the proper schematic which is helping a ton. Before that I was tracing the circuit and marking what I
found. I made up an amp listening tool so I can listen to the circuit through a good amp. The white lines are good clean signal coming from the input side of the circuit. The blue lines on the output side are bad distorted signal. And the magenta coloured line is where I should have a guitar signal but don't. It gives me a thud like when I probe a power section. I still need to keep tracing that failed section. It's a little easier to see on the schematic.
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Discussion Starter · #4 ·
Here is the schematic for the amp. I marked the OK sections which is the good guitar signal. Then from the middle leg of the Master pot is where I get the messed up power thud. Then there is a good clean area in the reverb section. Towards the output side anywhere I have marked an X is where I get the distorted guitar signal. I will edit these schematic images as I go. Any help is of course greatly appreciated.
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Discussion Starter · #6 ·
Cool thanks!

Also someone just advised me to unplug the speaker from the bad amp for my testing. I had read the opposite but I guess that's just for tubes. So now I can probe more places. I am getting good signal with my probe in the section I marked as BAD. So hoping this will help pinpoint the problem.
 

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Discussion Starter · #13 ·
First thing to check is the health of the power supplies. Make sure that the power supply is supplying:±15VDC and ±27VDC, both no-load and loaded conditions.
A electronic circuit's function is dependent on a healthy power supply.
Cool I will test that out. Getting ready to head out of town for the weekend so might be Monday.
I guess I could just pop the fuses out for the no-load condition?
 

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Discussion Starter · #17 ·
Right the -27V. (Vee).
Try disconnecting the drain leg of Q11 from the circuit to see if it changes the output.
Sorry I was referring to the voltage after the resistors but ya it starts as -27.

I'll try disconnecting it when I get back after the weekend and see what happens. I'm on a facebook amp repair group too and one of the guys thinks that Q4 is bad based on it's voltages. E: -24.8v, C: -.7v, B: -25.5 Since these parts are all just a few cents I may just order anything questionable once I finish troubleshooting.

I really appreciate everones help. I really don't know much about amps. I've only just learned about building effects circuits.
 

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Sorry I was referring to the voltage after the resistors but ya it starts as -27.

I'll try disconnecting it when I get back after the weekend and see what happens. I'm on a facebook amp repair group too and one of the guys thinks that Q4 is bad based on it's voltages. E: -24.8v, C: -.7v, B: -25.5 Since these parts are all just a few cents I may just order anything questionable once I finish troubleshooting.

I really appreciate everones help. I really don't know much about amps. I've only just learned about building effects circuits.
Possibly or Q7 as well if it's leaking as it could be sending the -25.5 to the base of Q4
 

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Q11 top right of first schematic) slowly loses power when I put my meter on it.
That's a high-impedance test-point; the meter's input impedance, is in parallel with R37 (3.3MΩ), to ground...if your meter has a 1MΩ input-impedance, R37's effective resistance will drop to 770KΩ and this will shift the bias-point of Q11. To measure accurately, the meter's input-impedance should be much greater than the circuit impedance that you are measuring, to prevent changes in circuit loading.
 

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Discussion Starter · #20 ·
Right the -27V. (Vee).
Try disconnecting the drain leg of Q11 from the circuit to see if it changes the output.
I disconnected the drain leg but still an losing power there. I thought maybe it was leaking through Q11 so I removed it but the power still drops. Between R38 and R37 the voltage goes from -15.4v to -13.7v after about 20 seconds. But then I remembered what @Paul Running said about the input impedance of my meter. I have no idea what it is on this meter, it's just a cheapy amazon special Kaiweets Kw100 so it could be messing with my readings I guess. But I'll be replacing Q11 anyway since I splurged on the $0.67 part. I also got some of the other questionable parts just in case like the 2SC1815's and some caps.
 
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