Hello guys, a friend of mine asked me to fix this amp, so the problem is basically when you turn it on it will immediately sound/produce/hum that sounds exactly when you have a output transformer bad, so the reason to this been someone changed the 750ohm resistors with 150ohm. I checked the schematic and the amp and i don't see to many other suspects, but since its a model im not familiar with, i wanted to see what you guys think? Thanks a lot
Frequency of the hum would point you in the right direction.
50/60HZ mans hum, usually poor/bad ground. Input sockets.
100/120HZ main smoothing/balance issue. C60/61
Does the hum increase/decrease with volume settings?
Is there DC on the loudspeaker?
750 Ohm which 750 Ohm?
50/60HZ mans hum, usually poor/bad ground. Input sockets.
100/120HZ main smoothing/balance issue. C60/61
Does the hum increase/decrease with volume settings?
Is there DC on the loudspeaker?
750 Ohm which 750 Ohm?
Here its how it goes so when you turn on the on switch immediately you hear the loud hum (so you have to turn immediately off since it sounds hurtful to the amp like a lot of current going in other terms shorted, speaker immediately sticks to bottom), its the type of hum that you would get if a bad output transformer on a tube amp or at least its the only time before i have seen something similar, due this being the problem you don’t have any chance to read nothing, amp was good to my knowledge after someone miss change this resistors what 750? They are labeled as R85 and R86, so before anything just by watching the schematic you can see it is indeed a 750ohm NOT a 150ohm so I would assume the diodes (d21/d22) are gone too, but my question i guess its more oriented in the case of , in a tube amp this immediately means shorted output so you basically know for sure that you have to change that (op) and then hunt the problem that caused this, but now in this case i know for sure that the only thing different are those resistors, i know for fact that the last person” couldn’t “ find the schematic so he went by the color he saw, so im the guy correcting this mistake and since normally i dont do SS i wanted to be sure by asking here since fender shares a lot of mistakes and problems in this specific area in a lot of model amps, also speakers work fine.
so what do you think? It’s possible that this resistors took the output transistors/drivers?
so what do you think? It’s possible that this resistors took the output transistors/drivers?
is it the KRX or the Sidekick 60?
there's a few versions so unless we know for sure it's hard to follow along...i just had a look at a KRX schematic and R85 and R86 are 10K's so you need to share a few more details.
loud buzzing noise on the output, could be a supply diode, could be an output transistor, could a bad cap....at this point there's not enough known to make any sort of determination of what could be wrong.
there's a few versions so unless we know for sure it's hard to follow along...i just had a look at a KRX schematic and R85 and R86 are 10K's so you need to share a few more details.
loud buzzing noise on the output, could be a supply diode, could be an output transistor, could a bad cap....at this point there's not enough known to make any sort of determination of what could be wrong.
750= purple green brown 150=brown green brown so in bad light and likely dirt i guess it's possible the first band was mistaken in color!
ah just found a Sidekick schematic, and yes a failure here is going to be big noise it's the preamp supply!
don't hesitate to change the caps as well.
don't hesitate to change the caps as well.
Hey thanks to reply, its one version that only says “keyboard 60” but looking at the schematic of the sidekick matches, thats the one im following, im planning changing since i got all parts handy and ill desolder anyway, let me know what you guys think
D21
D22
D23 which is the bridge rectifier (its also different labeled on board)
R85/86
R49
R87
R44/45
C37
C51/52
the caps you suggest are the 3300ufs ?
C49/50
D21
D22
D23 which is the bridge rectifier (its also different labeled on board)
R85/86
R49
R87
R44/45
C37
C51/52
the caps you suggest are the 3300ufs ?
C49/50
ahkk i thought i had the schematic it turns out it's only a partial snippet.
any chance you can PM yours?
with the pc board out check it thoroughly for bad solders.
any chance you can PM yours?
with the pc board out check it thoroughly for bad solders.
My teacher, an older radio technician teached me, almost 55 years ago, to eliminate. To look at the different parts of an electronic circuit, in your case a guitar amp with a power supply, a preamp (plus affects etc) and a power amp.
It is a very stupid idea to start replacing components if you even do not know if they are either bad or the cause of the problem.
Look at the schematic.
Then try to break it up into different pieces.
Disconnect the speaker because the loud hum can damage it.
Check each part, starting with the most important; the power supply.
If thats ok, go on with another part.
Connect the effects out to another amp and listen: do you have that hum?
If not;
Go on with the power amp.
Is there a dc voltage on the speaker terminals?
Check the components on that part.
And then, only then, if you find a bad component, replace it.
After going over all the components involved, use a lightbulb between the AC socket and your amp.
Just in case if there is stil a problem. It will save you precious components and a lot of time.
It is a very stupid idea to start replacing components if you even do not know if they are either bad or the cause of the problem.
Look at the schematic.
Then try to break it up into different pieces.
Disconnect the speaker because the loud hum can damage it.
Check each part, starting with the most important; the power supply.
If thats ok, go on with another part.
Connect the effects out to another amp and listen: do you have that hum?
If not;
Go on with the power amp.
Is there a dc voltage on the speaker terminals?
Check the components on that part.
And then, only then, if you find a bad component, replace it.
After going over all the components involved, use a lightbulb between the AC socket and your amp.
Just in case if there is stil a problem. It will save you precious components and a lot of time.
no the ones i'm talking about are C51,52
and yes Tarzan's correct in suggesting a home brew current limiter!
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Hey, yeah Tarzan no “stupid” gunshooting components, turk ill change all components involved as i listed, i just pulled c49/50 and one of them is completely dry, if you shake it you kinda feel the mass banging like a ball inside, the other one dry too not as bad but dry, im almost done in replacing bad components, then the readings and then hope to be well and working again, ill keep informed
Hello, so i finally put my hands in this and so far this is how its going, so i did change all the components listed filter caps were completely dry you can even feel the mass banging inside, anyway
i disconnected the PT and tested, it’s reading 32/33 VAC each blue cable so the PT its ok, however i do have the same problem when i turn on the amp, i did change all the output transistors and drivers also, i have two questions 1/ if i disconnect the speaker can i measure inside without anything going wrong or burning i mean i don’t want to get into more trouble that i already have i took this amp and a HDR as a favor but never again taking someone’s else half job because i do feel hesitant since i really don't know what this amps have seen, anyway as i said the hum its exactly as when you have a bad output transformer, also at this point, do you guys suspect from anything else or am i missing something ? thanks for the help again.
i disconnected the PT and tested, it’s reading 32/33 VAC each blue cable so the PT its ok, however i do have the same problem when i turn on the amp, i did change all the output transistors and drivers also, i have two questions 1/ if i disconnect the speaker can i measure inside without anything going wrong or burning i mean i don’t want to get into more trouble that i already have i took this amp and a HDR as a favor but never again taking someone’s else half job because i do feel hesitant since i really don't know what this amps have seen, anyway as i said the hum its exactly as when you have a bad output transformer, also at this point, do you guys suspect from anything else or am i missing something ? thanks for the help again.
You have blown power transistors.
You replaced them but they must have carried other parts the grave
1) connect amp through lamp bulb limiter until it tests good.
Use a 40 to 60W filament bulb, no LEDs, CFL, etc. You need that red hot tungsten wire in series.
2) disconnect speaker now.
3) measure DC across speaker out terminals.
4) post results, then we go on.
You replaced them but they must have carried other parts the grave
1) connect amp through lamp bulb limiter until it tests good.
Use a 40 to 60W filament bulb, no LEDs, CFL, etc. You need that red hot tungsten wire in series.
2) disconnect speaker now.
3) measure DC across speaker out terminals.
4) post results, then we go on.
Im really glad to find an active forum, My issue is with The same amp, Fender keyboard 60 Resistors 85 and 86 are overheating and The limiter goes in so it doesnt produce sound, The ones that it had when i bought it were a pair coiled together i changed them for The ones that The schemathic says, i checked with a teacher of My school and he suggested me to change The diodes, i did and The problem stills, any help is well recieved
it appears some components have been replaced already in order to repair it but until the preamp which this supply section is feeding is gone through and cleared of faults overheating may reoccur.
do you have a current limiter?
do you have a current limiter?
Let's start with the easiest part: The power supply.
Disconnect the speaker.
Try to remove the power transistors (Q9, Q11)
Remove one leg of R89 (100 Ohm - 2W). The +30V is only needed for the reverb.
Power on and check the voltages:
+/- 33V (for the power amp)
+/- 15V (for the preamp)
If the +/- 15V is not ok, check the zener diodes (D21, D22), then the cap (C51, C52)
Probably one or more of these are bad.
Use a higher wattage for R85, R86.
If that's ok, reinsert R89 and check the +30V.
If that's ok also, insert a signal in tot the inputjack and check at the Effect Send if the preamp is ok.
If not, start searching, stage by stage.
Now that the output transistors are out, check them if they are faulty.
If all is ok, reinsert them and power up by using a bulb current limiter.
Check the voltages. They should be about the values as in the schematic.
Switch of.
Connect the speaker and power on with the current limiter.
Check the voltages. They should be about the values as in the schematic.
Check for sound coming from the speaker (Low volume...)
If all is wel, redo the test without the current limiter.
Check the voltages. They should be about the values as in the schematic.
btw, do you have a adjustable power supply of +/- 30V, 1A or more with current limiter?
Disconnect the speaker.
Try to remove the power transistors (Q9, Q11)
Remove one leg of R89 (100 Ohm - 2W). The +30V is only needed for the reverb.
Power on and check the voltages:
+/- 33V (for the power amp)
+/- 15V (for the preamp)
If the +/- 15V is not ok, check the zener diodes (D21, D22), then the cap (C51, C52)
Probably one or more of these are bad.
Use a higher wattage for R85, R86.
If that's ok, reinsert R89 and check the +30V.
If that's ok also, insert a signal in tot the inputjack and check at the Effect Send if the preamp is ok.
If not, start searching, stage by stage.
Now that the output transistors are out, check them if they are faulty.
If all is ok, reinsert them and power up by using a bulb current limiter.
Check the voltages. They should be about the values as in the schematic.
Switch of.
Connect the speaker and power on with the current limiter.
Check the voltages. They should be about the values as in the schematic.
Check for sound coming from the speaker (Low volume...)
If all is wel, redo the test without the current limiter.
Check the voltages. They should be about the values as in the schematic.
btw, do you have a adjustable power supply of +/- 30V, 1A or more with current limiter?
