HH Scott Model A510 Stereo Integrated Amplifier/ Graphic EQ - Blowing fuses in the LEFT speaker channel only. Any Help???

[Sik_6ty_Sixx]

So like the title says, I have an old Stereo Receiver/Amplifier that I picked up from the goodwill, it's a HH Scott Model A510. When I first got it home it had zero output. Opened it up, notice the fuses are missing. Pop in 2 new fuses (AGC4A 250V) and try again, within 5 seconds the L channel fuse POP! I don't see any visible indications of damage or arc burns on the PCB, no popped resistors or diodes, no burnt traces or blown out caps. So I decide to just try another new fuse, POP. Swapped around my R and L speaker wires, and checked the integrity of all wires and connections. Everything looks solid. But I try again, and same thing, POP! Cleaned out the dust on the PCB, Still POP! (I've gone thru a brand new 10 pack of fuses, just today. I've currently got 1 left, lol.)

So now I've reached the point where my (albeit limited) troubleshooting knowledge has run out, I have come here hoping someone may be able to point me in a direction. I have searched for the schematics for this unit online, and came up empty handed after a good 2 hour deep dive Google search.

I can take pictures of the internals? If that would be helpful please just let me know and I'll get on it. If there is anything else I'm forgetting to add, please let me know and I'll handle it ASAP.

I really want to get this thing fixed, and soon! A month or so ago my old receiver took a shit and so I currently can't listen to any of my records until I get this thing up and running!! I'm too broke to go and buy another receiver!!

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P.S. I went ahead and added a few pictures just for reference. If close-ups on any particular point would help, just say the word. On the last image the Left side one of the 2 fuses visible is the one that keeps blowing. Thanks In advance!!!

[frankusb]

The datasheet for the STK4191 would be a good reference. Does the left fuse connect to any of the pins on the STK4191?

[stj]

in front of the stk module it looks like something burned or some corrosive agent got dripped onto the board.
lets see that area better
i bet it lines up with vents on the case cover!

[redwire]

Sorry, not a quick fix I think.
There's no fuses for just one channel that I can see - it's one fuse for +ve rail, another for the -ve rail?
I would say the STK module is shorted if the fuses are blowing. Or the rectifier diodes.

You can test a few transistors in-circuit (inside) with multimeter (Ohms and Diode-test) measurements.
Measure between Pin 11 (+VCC) and output L (Pin 10) and (-rail) Pin 9. Measure between Pin 11 (+VCC) and output R (Pin 13) and (-rail) Pin 14. Look for shorts.

in front of the stk module it looks like something burned or some corrosive agent got dripped onto the board.
lets see that area better
i bet it lines up with vents on the case cover!

Once I got a quadraphonic Sansui receiver come into the repair shop, somebody had put their drink on it and splashed the insides. That was a tough repair, 4 channels and a dozen transistors gone I would say.

[Sik_6ty_Sixx]

in front of the stk module it looks like something burned or some corrosive agent got dripped onto the board.
lets see that area better
i bet it lines up with vents on the case cover!

This the area you are referring to? I went in with a toothpick and small brush and cleaned off what I think you were talking about, it did look like the legs of R412 and R416 had some corrosion on them. Might try and unscrew the board and check out how the underside is looking.

[Sik_6ty_Sixx]

Sorry, not a quick fix I think.
There's no fuses for just one channel that I can see - it's one fuse for +ve rail, another for the -ve rail?
I would say the STK module is shorted if the fuses are blowing. Or the rectifier diodes.

You can test a few transistors in-circuit (inside) with multimeter (Ohms and Diode-test) measurements.
Measure between Pin 11 (+VCC) and output L (Pin 10) and (-rail) Pin 9. Measure between Pin 11 (+VCC) and output R (Pin 13) and (-rail) Pin 14. Look for shorts.

I'm gonna grab my meter and try to test those, and I'll post back with my results.

[Sik_6ty_Sixx]

Sorry, not a quick fix I think.
There's no fuses for just one channel that I can see - it's one fuse for +ve rail, another for the -ve rail?
I would say the STK module is shorted if the fuses are blowing. Or the rectifier diodes.

You can test a few transistors in-circuit (inside) with multimeter (Ohms and Diode-test) measurements.
Measure between Pin 11 (+VCC) and output L (Pin 10) and (-rail) Pin 9. Measure between Pin 11 (+VCC) and output R (Pin 13) and (-rail) Pin 14. Look for shorts.



Once I got a quadraphonic Sansui receiver come into the repair shop, somebody had put their drink on it and splashed the insides. That was a tough repair, 4 channels and a dozen transistors gone I would say.

So I finally had some free time and got to checking those pins with my meter. Here's what I got:

Diode test setting
Red probe pin 11 / Black probe pin 10 = 1.5
Red 11 / Black 9 = 2.1
Red 11 / Black 13 = 0.017
Red 11 / Black 14 = 2.1
Then
Black 11 / Red 10 = 0.55
Black 11 / Red 9 = 1.00
Black 11 / Red 13 = 0.017
Black 11 / Red 14 = 1.01

So it looks like 11/13 is shorted, correct??

(When I changed it to the Ohms setting 11/13 was the only pair that beeped.)

Seems pretty straightforward...?

So, If I purchase a replacement STK4191 and swap it out, is there anything else I should check before attempting to fire it up??

[redwire]

Yeah the STK4191 has a shorted output transistor inside. It's pooched, needs to be replaced. The corrosion around R412 clean up with a q-tip and IPA.
I notice this amp does not have a speaker protect relay? so the 4A fuse blowing thump might have been through your loudspeaker! WHOMP!
I would replace the STK, need some heatsink compound too. I used a solder sucker or wick to desolder the old part. Then bolt in the new part, make it straight then solder it in.
Back in the 80's I must have replaced 100's of STK's. You can crack the old one apart with a hammer and peep inside for lolz.

You can power it up with a series light bulb or just smaller fuses. Look for 0VDC idling at the STK speaker outputs if it's OK.
Careful the smaller caps don't have a safety vent, which can be pretty loud lol. They shouldn't pop at all but I was looking at the tops to see if they got stressed and bulged.