Repairing super old bass guitar combo power amp... One resistor is burned and I can not make out its value. Measuring with ohm meter gives 1.223K ohm. I have quickly drawn schematics so hopefully someone can help as I am only hobbyist... Resistor in question is marked RX. Thanks
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It may be that other component failures caused the resistor to burn out. I think the schematic is incorrect, can you post a picture of the front and back of the board?
Main filter capacitors are dead for sure but I will replace all electrolytic capacitors. Transistors seems to be fine - it worked for a while, then i turned it off as soon as I noticed that burning resistor. Schematic is best I could make out of it - this thing is ancient...
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A lot of times with a good metal film/metal oxide resistor, they will keep their value even after they cook. That being the case, I would vote for the resistor being 1.2k as measured.
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Normally we draw the schematic input on the left and the output on the right, the component connections look correct except for some transistor B-C-E confusion.
I think Rx of 1.2kΩ seems reasonable. My suggestion is to replace it and use a dim bulb tester to power the amp to avoid surprises.
I think Rx of 1.2kΩ seems reasonable. My suggestion is to replace it and use a dim bulb tester to power the amp to avoid surprises.
Oh. I see. It wasn't obvious that it's RX in the schematic that we're looking to find. I didn't inspect the schematic with a magnifying glass... 🙂
Are you sure you have the pins right on the transistors? Having the input go to the collector of the right-most transistor in the schematic doesn't seem right. Here's the pinout of BC212, which I pulled from a data sheet found on Mouser.com:
Tom
I think this one makes more sense, it reminds me of the Ampeg B100.
Great. Now we just need OP to indicate the values of all the other resistors in the circuit. Then we can make some guesses about Rx.
Tom
Tom
Now it makes sense.
It must be a low power amp, 20/25 W tops.
For any significant power out, power transistors "should" be Darlingtons, but these look like regular ones.
The very low Vas load resistors partially confirm this.
Please measure and post +V supply and what you have at the + terminal of the 1000uF output capacitor, which should be around half of +V
The amp is very repairable, thanks to it's simplicity.
1k2 looks very reasonable there, as the BE resistor in the Vas transistor.
If it burnt as shown, it's because that transistor shorted, together with 1 or 2 of the power transistors.
Which I guess must be similar to TIP 31/32 or TIP42/42
Thanks for all replies. I have currently transistor tester on order. Also new caps, new power socket and few more things...
This amp has very "soviet" construction - parts used seems mismatched - output transistors for example. It looks like someone built it with what was available. It is easily 30+ years old (those capacitors definitely). But I like the case - it is very well built and also speakers are decent.
+V is 30V, I will measure capacitor when I change it. I will also test it with bulb tester... 25W is so weak for speakers that are used in cabinet... Now when I think about it - boards in preamp are different to this power amp. So maybe it burned out at some point and somebody put "new" power amp with what was laying around just to make it work...
Power transistors are KD503 and KD616. With simple multimeter I can only tell they are not shorted - same as other 3 transistors.
Regarding other resistors - those 4 I have no values for in schematic - 2 of them are simply gray, without any color code(near 22uF cap). 1 is with some russian letters but I cant see it clearly and 1 has weird color code that can be read from both sides which confuses me. I can post picture but I checked all color charts and this is really weird - no way to tell orientation. These mismatched resistors is another tell this was build from scraps probably... I can desolder them all and measure them that way...
Yes BC212 should be other way around...
I will try to post some more pictures...
This amp has very "soviet" construction - parts used seems mismatched - output transistors for example. It looks like someone built it with what was available. It is easily 30+ years old (those capacitors definitely). But I like the case - it is very well built and also speakers are decent.
+V is 30V, I will measure capacitor when I change it. I will also test it with bulb tester... 25W is so weak for speakers that are used in cabinet... Now when I think about it - boards in preamp are different to this power amp. So maybe it burned out at some point and somebody put "new" power amp with what was laying around just to make it work...
Power transistors are KD503 and KD616. With simple multimeter I can only tell they are not shorted - same as other 3 transistors.
Regarding other resistors - those 4 I have no values for in schematic - 2 of them are simply gray, without any color code(near 22uF cap). 1 is with some russian letters but I cant see it clearly and 1 has weird color code that can be read from both sides which confuses me. I can post picture but I checked all color charts and this is really weird - no way to tell orientation. These mismatched resistors is another tell this was build from scraps probably... I can desolder them all and measure them that way...
Yes BC212 should be other way around...
I will try to post some more pictures...
Power transistors - mismatched
Cabinet - note those big speakers
Power supply - definitely an art piece
Main caps - measured 110 and 200 uF instead of 5mF - note "Made in Czechoslovakia" - that federation dissolved on 31 December of 1992
Preamp board - not tested yet, I will change electrolytic caps there as well
Power amp board - note different resistors. That weird one is next to smallest blue cap in upper right. Colors: violet, orange, black, green, violet. It is either 73M or 750k. I do not know how to tell from which side are you supposed to read it as it is symmetrical...
Cabinet - note those big speakers
Power supply - definitely an art piece
Main caps - measured 110 and 200 uF instead of 5mF - note "Made in Czechoslovakia" - that federation dissolved on 31 December of 1992
Preamp board - not tested yet, I will change electrolytic caps there as well
Power amp board - note different resistors. That weird one is next to smallest blue cap in upper right. Colors: violet, orange, black, green, violet. It is either 73M or 750k. I do not know how to tell from which side are you supposed to read it as it is symmetrical...
