Amp repair, hopefully. B&K

SKsolutions

Got this in the FM. 1 channel was down due to input cable being unplugged during operation. On initial testing, another channel partially let go (No HFreq), but still passes with gain. I'm just opening this up, and am going to give it a go. I've forgotten most of what I once knew, but still remember some of the basics. As far as diag goes, I'm going to begin by measuring what I can against the known good channel (1 good, 2 bad). Fuses check out OK. From there, I'm going to follow the input circuit. Anyone have component level experience with this amp type, or have any knowledge of known issues. . . I'm sure this must be common. Haven't googled yet.

SKsolutions

Pic added, can't edit:mad:

ben62670

Take an ohm meter and read resistance across the legs of the output devices. If they are all the same check the sand cast resistors. They also should be the same.

SKsolutions

Each ch. has 2 pair of output trannys. I've found one that is stuck/bad/closed. It's Channel 3 which is the completely dead ch. Board reads 1997. Trannys have two different markings, so I'm assuming that they are complementary pairs (2) for each. One reads J162, Japan, the other K1-6H. Look like the old Toshibas.

The output resistors measure OK within tolerance. Now to find a tranny. I hope Channel 2 isn't a complete ghost. If 1, and 3 get working, I'll be OK with it. If I can't find the issue, I'm going to gift it as a 2ch to a client/friend who's using a POS receiver to power some RS6's.

Thanks Ben.

ben62670

Good luck on your parts search.
Ben

SKsolutions

Dig key, 7$. New tranny has gate protection diode built in. I believe there is one already on the board, but I don't think it will matter. . .
Maybe I should order 2

ben62670

Might not be a bad idea in case there was something that caused the failure.
Ben

disneyjoe7

An amp could be damaged by not using an input, I heard this before but never believed it can anyone elaborate?

ben62670

A real simple test you should do is an ohm test from the center of the inputs to the ground of the inputs. Both should be the same.
Ben

disneyjoe7

Come on Ben answer my question above please!

A Co-worker brought that Parasound 6 channel amp in the FM to power some A9's and going to use the 5 channel for the center this leaves channel 6 free. Safe to leave open? At least this is what I told him, am I steering him wrong?

MillerLiteScott

I am only using 4 channels of a 5 channel amp for about 9 months now. I can't see how it would be a problem or hurt the amp. I am all ears.

Systems

disneyjoe7 wrote: »

An amp could be damaged by not using an input, I heard this before but never believed it can anyone elaborate?

Not likely,but if an amp has fets in its input stage it might be damaged if an input is plugged or unplugged into it while its powered up.

Systems

SKsolutions wrote: »

New tranny has gate protection diode built in.

Thats nice, less chance of nuking it with static while handling.:cool:

disneyjoe7

Thanks for the info

Systems

I tried to add to previous post but it was beyond 10min.:mad:

SK if you happen to get 2 new trannys I would replace both.This way they have a better chance of being reaonably matched as they will likely be from the same production batch.If they are not matched they will not share current equally.Also the bias will probably need some tweaking.

ben62670

I'd give you an answer but you already got it on all accounts.
Ben

SKsolutions

Thanks for the extra info Ben, & GV. There is a pot on each channel. It's a fairly simple circuit, but the board is double sided, so it doesn't take much to get lost at this point. I hope the tranny will fix this channel but maybe I should have ordered three of them. The trouble will be finding why the HF is rolled off on the other problem channel. Time to get the schem.:D

ben62670

With the "gain" or "trim" turned all the way up did you get a reading on the input impedance?
Ben

Systems

GV#27 wrote: »

Not likely,but if an amp has fets in its input stage it might be damaged if an input is plugged or unplugged into it while its powered up.

Common sense tells me to always power down, unplug the cord and go get a snack. Come back and THEN start **** with the connections. My Pioneer 880 in my truck recommends unplugging the battery before moving RCA's from the headunit or the amps. If not you can blow the Pico Fuses/resistors within the unit. I failed to read this part and smoked the fuses. You can't replace them as in just snapping them out and replacing. They are actually on the board as resistors and a **** to get at. Since then I always power down and unplug all my gear before **** with the connections.

Chuck