Fender Sidekick Switcher Repair

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
Hey folks, new here. Working on an amplifier that belongs to my dad. The story goes that in probably 1993, when I was 2 years old, I got access to a philips screwdriver and took this amp apart while no one was watching and it hasnt worked since. Now, in the 29 years since, I have slowly built up a rudimentary understanding of electronics and have decided to tackle the problem. However, its proving to be less simple a fix than I first expected.

The main problem that seems to have persisted throughout this entire repair process is power supply. The schematic I have sourced (which I will attach) shows clearly that we have a basic bipolar power supply coming off a full bridge rectifier that provides +/-27V around the chassis ground. Beyond that a zener based regulator circuit provides us with an additional +/-15V rail. As far as I can tell, the 27v rails are used primarily for powering the main power amplifier (LM1875T) here the 15V rails power the 6 M5218 opamps that do all the pre-amps, tone modification, and reverb. The 15V rail is currently reading +17V with reference to ground and the -15V rail reads +2V with reference to ground. I have pulled the transformer, tested it by itself and with 110V out of the wall it produces +/- 17V RMS out of its secondary. Its my understanding that bridge rectifiers, especially when treated with filter caps (3300uF, 35V in this case), bring the DC voltage up around the peak to peak voltage of their input. The secondary puts out 34V RMS Peak to Peak which I think I calculated to be 48-50V, which adds up given that the transformer is wired for 120V (meaning my peak to peak or RMS would be roughly 8% lower than the transformer and schematic call for).

ANYWAY heres what I've done:

When I got the amp the power amp was just outright MISSING. Bought a pair online just so I had a spare. Installed but later found that it wasnt that great a solder job on my part, couldnt quite get pin 5 through the hole, so I tossed in the other which fit better.

Fond that someone, presumably my dad, replaced IC1, an M5218, with an RC4558. Also found that the D8 and D9, which Fender lists generically as DSF10C diodes, were (presumably) replaced with 1N4002 diodes. These bias(?) the output of the power amp to ground.

Replaced filter caps C54 and C53, replaced ripple caps C51 and C52, I suspect that the zener diodes D17 and D18 (listed as ZD-15V, 1W) were also replaced based on the solder job I found, but tested those completely removed from the circuit and they produced a forward voltage of .76V on my meter and a Zener voltage right where it should be at like 15V.

Also found that both C38 (330uF electro cap) and R51 (2.2K) were missing and looked like they had never been installed from factory. These are both in the reverb circuit. One of the holes for the capacitor instead had a wire that went straight to ground instead, it looked like it was something my dad mighta done as I doubt this is how it would have been done as a running change from the factory.


In studying the schematic, I cant find C12 or C13 ANYWHERE on the PCB. Only the first of the 6 M5218s appear to have their power rails shown to be connected in the schematic and in that area of the schematic, above IC1B, you can see two 0.1uF capacitors tying the +/-15V rails to ground.

The ground wire that goes to the PCB that has the two input jacks on it gets VERY hot. the wire coming off of the main PCB to the jack PCB that comes from R63 also shows that it has about 4V on it, and I think that is the only wire through which any voltage potential exists for the yellow wire to get hot with. (PS, I replaced these wires because the ones that were on the board looked non stock and too short).

Neither channel LED light up at all but the main power LED does light up.

C50, a 0.1uF mylar capacitor, had been replaced before I got this, and one of its solder traces had been ripped off and a bypass wire soldered in its place, presume that to be done well enough.

Q2 and Q3, which I believe were supposed to be 2SK30A FETs have been replaced by SST112 (as best I can tell), again, I think by my dad getting generic parts at a 90s RadioShack.

I bought enough capacitors to completely replace all of the electrolytic types, and I'm prepared to buy enough transistors and op amps to replace all of those as well, but I want to make sure that if I do that its because the existing ones are broken and that Ive fixed whatever broke them so I'm not just burning through more and more parts.

PS, I know this amp probably isnt worth all the money I'm throwing at it, even though these parts havent been terribly expensive at all, but the worth of it all comes from the journey, and I'm trying to expand my knowledge of electronics through this project.

Any help is greatly appreciated.
 

Attachments

  • Sidekick Switcher, Bass, Keyboard.PDF
    953.8 KB · Views: 14

Higgins1980

Senior Stratmaster
Feb 11, 2013
2,729
North Carolina
Hey folks, new here. Working on an amplifier that belongs to my dad. The story goes that in probably 1993, when I was 2 years old, I got access to a philips screwdriver and took this amp apart while no one was watching and it hasnt worked since. Now, in the 29 years since, I have slowly built up a rudimentary understanding of electronics and have decided to tackle the problem. However, its proving to be less simple a fix than I first expected.

The main problem that seems to have persisted throughout this entire repair process is power supply. The schematic I have sourced (which I will attach) shows clearly that we have a basic bipolar power supply coming off a full bridge rectifier that provides +/-27V around the chassis ground. Beyond that a zener based regulator circuit provides us with an additional +/-15V rail. As far as I can tell, the 27v rails are used primarily for powering the main power amplifier (LM1875T) here the 15V rails power the 6 M5218 opamps that do all the pre-amps, tone modification, and reverb. The 15V rail is currently reading +17V with reference to ground and the -15V rail reads +2V with reference to ground. I have pulled the transformer, tested it by itself and with 110V out of the wall it produces +/- 17V RMS out of its secondary. Its my understanding that bridge rectifiers, especially when treated with filter caps (3300uF, 35V in this case), bring the DC voltage up around the peak to peak voltage of their input. The secondary puts out 34V RMS Peak to Peak which I think I calculated to be 48-50V, which adds up given that the transformer is wired for 120V (meaning my peak to peak or RMS would be roughly 8% lower than the transformer and schematic call for).

ANYWAY heres what I've done:

When I got the amp the power amp was just outright MISSING. Bought a pair online just so I had a spare. Installed but later found that it wasnt that great a solder job on my part, couldnt quite get pin 5 through the hole, so I tossed in the other which fit better.

Fond that someone, presumably my dad, replaced IC1, an M5218, with an RC4558. Also found that the D8 and D9, which Fender lists generically as DSF10C diodes, were (presumably) replaced with 1N4002 diodes. These bias(?) the output of the power amp to ground.

Replaced filter caps C54 and C53, replaced ripple caps C51 and C52, I suspect that the zener diodes D17 and D18 (listed as ZD-15V, 1W) were also replaced based on the solder job I found, but tested those completely removed from the circuit and they produced a forward voltage of .76V on my meter and a Zener voltage right where it should be at like 15V.

Also found that both C38 (330uF electro cap) and R51 (2.2K) were missing and looked like they had never been installed from factory. These are both in the reverb circuit. One of the holes for the capacitor instead had a wire that went straight to ground instead, it looked like it was something my dad mighta done as I doubt this is how it would have been done as a running change from the factory.


In studying the schematic, I cant find C12 or C13 ANYWHERE on the PCB. Only the first of the 6 M5218s appear to have their power rails shown to be connected in the schematic and in that area of the schematic, above IC1B, you can see two 0.1uF capacitors tying the +/-15V rails to ground.

The ground wire that goes to the PCB that has the two input jacks on it gets VERY hot. the wire coming off of the main PCB to the jack PCB that comes from R63 also shows that it has about 4V on it, and I think that is the only wire through which any voltage potential exists for the yellow wire to get hot with. (PS, I replaced these wires because the ones that were on the board looked non stock and too short).

Neither channel LED light up at all but the main power LED does light up.

C50, a 0.1uF mylar capacitor, had been replaced before I got this, and one of its solder traces had been ripped off and a bypass wire soldered in its place, presume that to be done well enough.

Q2 and Q3, which I believe were supposed to be 2SK30A FETs have been replaced by SST112 (as best I can tell), again, I think by my dad getting generic parts at a 90s RadioShack.

I bought enough capacitors to completely replace all of the electrolytic types, and I'm prepared to buy enough transistors and op amps to replace all of those as well, but I want to make sure that if I do that its because the existing ones are broken and that Ive fixed whatever broke them so I'm not just burning through more and more parts.

PS, I know this amp probably isnt worth all the money I'm throwing at it, even though these parts havent been terribly expensive at all, but the worth of it all comes from the journey, and I'm trying to expand my knowledge of electronics through this project.

Any help is greatly appreciated.
Based on this post, I don’t think rudimentary means what you think it means.

V=IR is rudimentary. What you’ve done with your amp is way beyond. I wish I really understood electronics enough to help but I was lost after about the first paragraph.

Also very impressive that at 2 years old you took a screwdriver to an amp and didn’t electrocute yourself.

Welcome to the forum and somebody who knows a thing or two will be by shortly to offer advice.
 

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
Based on this post, I don’t think rudimentary means what you think it means.

V=IR is rudimentary. What you’ve done with your amp is way beyond. I wish I really understood electronics enough to help but I was lost after about the first paragraph.

Also very impressive that at 2 years old you took a screwdriver to an amp and didn’t electrocute yourself.

Welcome to the forum and somebody who knows a thing or two will be by shortly to offer advice.

I'll take this as a compliment, thanks. I can tell you what many components do in isolation and their basic example circuits but when it all starts coming together I just wonder "WHY SO MANY COMPONENTS" and cant begin to piece together how any of it is supposed to work.


I too am surprised I did not die.
 

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
I will get back to you about this later.
I know it worked before I got my hands on it, my dad claims that it actually sounded great, and thats part of why I'm so invested in this, so that both he and I can see whether its actually as good as he remembers. If it was plugged in and turned on I am even more lucky to be alive than I thought. I arc'd one of the two filter caps while working on it and it scared me, and that was with a screwdriver and when I MEANT to arc it.

Funny thing, only one of the filter caps is charging up, lending further evidence to something being wrong with how power is being supplied to the op amps. If something across the negative rail is shorting directly to ground, I would assume that the filter cap wouldnt charge up at all either.

I HAVE replaced the LM1875T twice. I ordered 2, soldered one in then later found that leg 5, the positive voltage supply, was not properly soldered in and came undone, so I tried my other one just because the leg on the first one was having a hard time fitting in the PCB hole, and now its far more secure. Both were screwed down to the sizeable heatsink and had thermal compound applied.

The only jacks I havent played with are those on the rear, the headphone and line out jacks. I replaced the two front jacks just because one had stripped threads and I wanted them to be identical, both are the 9 pin Fender variety, as are, I believe, the ones in the rear. If I get a chance tonight I might see about hooking up my oscilloscope to the output to see if that lends me any more insight.
 

Higgins1980

Senior Stratmaster
Feb 11, 2013
2,729
North Carolina
I'll take this as a compliment, thanks. I can tell you what many components do in isolation and their basic example circuits but when it all starts coming together I just wonder "WHY SO MANY COMPONENTS" and cant begin to piece together how any of it is supposed to work.


I too am surprised I did not die.
It was indeed a complement. I’m just starting on the road of electronics and wiring. Having fun building simple circuits and wiring guitars but I have no real clue how it all actually works together.
 

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
It was indeed a complement. I’m just starting on the road of electronics and wiring. Having fun building simple circuits and wiring guitars but I have no real clue how it all actually works together.
Ive found that electrosmash.com has some great explanations and analyses of pedals and amps that really help me start wrapping my head around the next level of concepts relevant to guitar stuff
 

Stevem

Senior Stratmaster
Sep 3, 2014
1,525
NY
After reading what you have posted many times I have to say that this amp has too many potential problems to sort thru for someone with your electronics knowledge level right now.

I would take someone like me or other SS capable tech’s on this board a whole crap load of time to make a many very long post to steer you thru repairing this amp, an I am not willing to do that.
I applaud you for wanting to learn and don’t loose that drive, but I would ditch this amp.
If you brought this amp to me to fix and told me all about it like you posted here I would refuse to accept it for repair.

Find something SS that is blowing fuses to fix that has not been butured and learn on, as that likely only needs a output pack , filters , recto bridge or a regulator.
 

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
After reading what you have posted many times I have to say that this amp has too many potential problems to sort thru for someone with your electronics knowledge level right now.

I would take someone like me or other SS capable tech’s on this board a whole crap load of time to make a many very long post to steer you thru repairing this amp, an I am not willing to do that.
I applaud you for wanting to learn and don’t loose that drive, but I would ditch this amp.
If you brought this amp to me to fix and told me all about it like you posted here I would refuse to accept it for repair.

Find something SS that is blowing fuses to fix that has not been butured and learn on, as that likely only needs a output pack , filters , recto bridge or a regulator.
Thats all fair and good, especially when as far as I can tell, just like automotive, professional diagnosis seeks to find the most efficient way to fix something by replacing as few things as possible. However, I already have a good amount of replacement ICs and transistors on the way. In theory, if I swapped out EVERY part on this board with known good, brand new parts, one would presume that the amp would work right?

The thing is that what you're describing for a good first amp to fix is stuff I've pretty much already replaced on this amp and was my initial response. I already know how to decently diagnose a rectifier, filter caps (which TBH I was willing to replace no matter what, given the amps age), etc.

PS, and I'm gonna edit my post above, forgot to mention, it HAD a blown fuse when I got it, or blew it almost immediately. It calls for a 1.5A Normal-Blo fuse, and I replaced it with what I believe to be a Normal-Blo fuse (in that it wasnt a time delay or a fast blow) and it hasnt blown since I put in the power amp, which was the first thing I installed.
 

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
Furthermore, presuming the PCB is good, couldnt I just desolder every component from the board, and slowly put them in, one by one, in a fashion that sorta resembles how the signal path will flow from in to out? Like, build up the power supply first, test to make sure its outputting the right voltages on the rails, then slowly start building up each op amp stage one by one, until an op amp or other component causes the voltage rails to sink to ground, as is happening on my -15V rail? Yes its time consuming but that WOULD identify a problem right?
 

Stevem

Senior Stratmaster
Sep 3, 2014
1,525
NY
Ok.
A event I was going to this weekend got canceled because the darn place burned down.
It’s jus as well because they have horrible food, lol!
Anyway, now I have some free time to try and help you.

To that end please post up a bunch of close up pictures of the whole circuit board are of the amp, and both top and bottom please.
Maybe 3 to 4 shots to make up one total view.
Thanks in advance!
 

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
Ok.
A event I was going to this weekend got canceled because the darn place burned down.
It’s jus as well because they have horrible food, lol!
Anyway, now I have some free time to try and help you.

To that end please post up a bunch of close up pictures of the whole circuit board are of the amp, and both top and bottom please.
Maybe 3 to 4 shots to make up one total view.
Thanks in advance!
Will do, I WAS working on putting together a rough composite, high res image of the PCB top and bottom, but will see about starting over and making sure I get all the details I missed on my first attempt
 

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
Here are some quick shots I rifled off, I'll see if I cant composite them and label the components on the bottom view
 

Attachments

  • _VWJ8120.jpg
    _VWJ8120.jpg
    197.2 KB · Views: 9
  • _VWJ8121.jpg
    _VWJ8121.jpg
    177.7 KB · Views: 9
  • _VWJ8124.jpg
    _VWJ8124.jpg
    215.5 KB · Views: 8
  • _VWJ8127.jpg
    _VWJ8127.jpg
    419.1 KB · Views: 8
  • _VWJ8128.jpg
    _VWJ8128.jpg
    411.6 KB · Views: 8
  • _VWJ8129.jpg
    _VWJ8129.jpg
    226.7 KB · Views: 7
  • _VWJ8131.jpg
    _VWJ8131.jpg
    269.3 KB · Views: 10

Stevem

Senior Stratmaster
Sep 3, 2014
1,525
NY
Ok, here we go.
First off thank God this amp is as old as it is and does not have circuit traces on both sides, just components and jumpers on the top side.
You have no idea how much easier this makes things in terms of trouble shooting this amp!!!!

The green color on the bottom side of thd Board is called soldering mask and is used when the board is put thru the process of what’s called wave soldering which is when all the solder connections are made in one shot, and the solder will only adhere to where there is no green mask.

As we troubleshoot you will be cutting traces open, this means you will need to scrape some of the mask off to solder back on jumper wires to re-join the circuit.

Let’s start.
You will need a single edge razor blade or a Xacto knife, or a cut off disc in a Dremel tool to make the cuts that we need to do.

I want you to make a cut at the output side of c51 and c52.
Then with your meter set for dc volts, read and note down what you have for voltage.

If everything on each dc rail from these two filter caps on back to the rectifier bridge diodes is good then your negative voltage should then be what your positive dc voltage is, but they will both be higher then 15 volts since there is no load on this section of the power supply any more.

Report back with your findings and we will continue on.

Note that I am on EST, and I will mostly be posting back around this time of day, or in the late afternoon or early evening.

PS.
For your own reference take a lot of shots from many different angles, as this helps greatly when you snap wires off from moving the board and parts around and need to find out where they came from.
 

Attachments

  • D79DEC6A-7EBB-453F-8301-71F9FBCBD27E.jpeg
    D79DEC6A-7EBB-453F-8301-71F9FBCBD27E.jpeg
    104.8 KB · Views: 0
Last edited:

Stevem

Senior Stratmaster
Sep 3, 2014
1,525
NY
Hi again.
If your voltages where checking seem good ( high but equal) then let’s do a resistance test.
Hook you meter up from the other side of each circuit trace that was cut to ground .
Ground will be the common connection point of both of those filters .

What resistance do you read, is the negative side a lower resistance then the positive rail side?
 

tycrawford

Strat-Talk Member
Sep 20, 2022
17
Florissant, Missouri
Hi again.
If your voltages where checking seem good ( high but equal) then let’s do a resistance test.
Hook you meter up from the other side of each circuit trace that was cut to ground .
Ground will be the common connection point of both of those filters .

What resistance do you read, is the negative side a lower resistance then the positive rail side?
Ok so I did the first test, severed the output of those capacitors that are at the end of regulating the 15V rail and my voltages appeared the same as they have been in all my testing. That is to say, my positive 15V rail was reading about 17.6V and my -15V rail was nearing ground, usually like 0.1-0.2V+. I tested resistance anyway while I had it severed and found that my resistance was all over the place, starting negative on my meter then climbing to positive, leading me to believe that my meter might have been charging something with capacitance?


Should I start looking into my 27V rail for shorts to ground?
 

Stevem

Senior Stratmaster
Sep 3, 2014
1,525
NY
The only way you could be having no -15 volts would be if r81 was open, or if the feed side of that resistor has a bad connection, or if that circuit trace going from the negative side of that filter ( c54) back to the rectifier bridge is open, or has a bad solder joint.
I resistance test should tell you the story.

Also note that any semiconductor in the amp ( not diodes) that is in need of both a positive and negative power feed can be possibly harmed by only having one of the two needed rail voltages.

This is certainly true of the output IC pack!

Yes, when you make a resistance check across a filter cap the reading will start low and go up slowly as the cap charges and then the meter may read a open .

PS.
If you are getting -27 volts then the problem is either r81 is open, or there is a bad connection from that-27 volt feed to the input side of r81.
 
Last edited:


Top