Kustom KG100HFX audio gating (with video). Help

[guidoilieff]

https://youtu.be/RpffCWgKAqk

I hope you can hear the problem on this compressed youtube video. I have a guitar connected and when I strum lightly it makes like a gated effect or the signal starts to break up.


A friend gave his broken amp to a guy that fixed it but said it had nothing wrong. The problem reappeared after a few days.

I replaced the transistors 2sc5200, 2sa1943, 2sc5174, the voltage regulators and other transistors on the board because the previous guy who "fixed it" made a mess resoldering or replacing them.

Everything in the preamp is ok. When the signal enters the amp pcb goes through some resistors and capacitors ok, then it makes this gated effect. I replaced some of the components on the vicinity using a probe but its a double sided pcb and results very hard for me to keep track since there is no schematic online.



Thanks

[pinkjimiphoton]

contact kustom directly, the new folks are very cool, generous and helpful and will send you
the schematic for the amp if you ask.

check for bad solder or an inadvertant bridge. also check all the coupling caps in the problem area of the circuit for dc leakage. if you see ANY dc on the "low" side of the cap, its likely bad and needs replacement.  caps should be open gates for ac <audio> and a closed door to dc <noise>... leaking coupling caps can cause all kinds of weird sounding noises and distortions, in particular like a weird almost muted fuzztone that may be in the background of the sound, or in really bad cases, be the main component. my bud mike cleary has an old kustom combo that has bad caps but they sound so good to him he won't let me fix it.

its also possible one of the replaced transistors is not biasing correctly or damaged.

[guidoilieff]

contact kustom directly, the new folks are very cool, generous and helpful and will send you
the schematic for the amp if you ask.

check for bad solder or an inadvertant bridge. also check all the coupling caps in the problem area of the circuit for dc leakage. if you see ANY dc on the "low" side of the cap, its likely bad and needs replacement.  caps should be open gates for ac <audio> and a closed door to dc <noise>... leaking coupling caps can cause all kinds of weird sounding noises and distortions, in particular like a weird almost muted fuzztone that may be in the background of the sound, or in really bad cases, be the main component. my bud mike cleary has an old kustom combo that has bad caps but they sound so good to him he won't let me fix it.

its also possible one of the replaced transistors is not biasing correctly or damaged.

Hi. Thanks for the reply, and yes, kustom sent me the schematic! That's the amp board schematic





I went ahead and replaced the 3300uf caps and had no luck.



I can't figure out which are the coupling capacitors here. Could someone give me a hand in identifying em?


Thanks

[duck_arse]

except for C36, it appears to be DC coupled throughout. post your voltages for Q8, 9 and 10, for a start. and the supply voltages. test C14, maybe replace it. the Q11 jfet seems to be a shunt connected to the power supply - try lifting the drain connection from R33 [I can barely make out the numbers .....] and see if that clears it.

[guidoilieff]

except for C36, it appears to be DC coupled throughout. post your voltages for Q8, 9 and 10, for a start. and the supply voltages. test C14, maybe replace it. the Q11 jfet seems to be a shunt connected to the power supply - try lifting the drain connection from R33 [I can barely make out the numbers .....] and see if that clears it.

I replaced c36 with a 1uf polyester cap. No luck. Same for c36 but with an electrolytic.


Voltages:

Q8

B 0v
C -28v
E 0.5v



Q9

B 0v
C -29v
E 0.5v


Q10

B -28
vC 0v
E -29v



Q11

S 0v
G 0.5v
D 0v


Haven't tried to lift the drain of Q11 but seems that it isn't working.


(Do you speak spanish btw?)

[duck_arse]

spanish? not a word, except pato.

check/post the voltages around C27, C22, R44. again, I can barely see the part numbers. those two caps might need replacement.

you'll probably need someone that knows what's going on in the circuit to happen along to help soon, all that muting and power protection is above my pay grade [which is only "gronk"].

[guidoilieff]

spanish? not a word, except pato.

check/post the voltages around C27, C22, R44. again, I can barely see the part numbers. those two caps might need replacement.

you'll probably need someone that knows what's going on in the circuit to happen along to help soon, all that muting and power protection is above my pay grade [which is only "gronk"].

Heres a biiger version of the schem


On the negative side:
C22 -24v
C27 -27v

I cant read any other voltage.



I dont know if this helps but when I put my finger on the input of the preamp there is 0.42dcv on the speaker output.

[guidoilieff]

witch keyword anyone recommend to look on the internet for a similar problem in an amp?