Here I go with a stumper! Seems this amp has two banks of PS's and only a single TL494 and an output array which is mildly impressive. At first look from like 5' away I thought it was an Arc Audio / Clarion design. Half the output transistors are GREEN which means it must be good amp and needs to work once again. But then I saw an oddball black output transistor so someone might have thrown a monkey wrench into this thing at one point.
Seems this amp is stuck in protect. But I have no way to verify as there are absolutely ZERO indicators. I have not pulled the board out of the synd. Its drawing 0.00A but the PS does blip up to 0.50A sometimes.
TL494
1. 0.010
2. 3.450
3. 0.091
4. 4.95 <-- Protect?
5. 1.473
6. 3.680
7. 0
8. 13.4
9. 0
10. 0
11. 13.4
12. 13.4
13. 4.061
14. 4.96
15. 4.031
16. 0
Found D507 cracked but a new 1N4148 didn't help. See it middle of board.
Seems this amp is stuck in protect. But I have no way to verify as there are absolutely ZERO indicators. I have not pulled the board out of the synd. Its drawing 0.00A but the PS does blip up to 0.50A sometimes.
TL494
1. 0.010
2. 3.450
3. 0.091
4. 4.95 <-- Protect?
5. 1.473
6. 3.680
7. 0
8. 13.4
9. 0
10. 0
11. 13.4
12. 13.4
13. 4.061
14. 4.96
15. 4.031
16. 0
Found D507 cracked but a new 1N4148 didn't help. See it middle of board.
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Remove whatever is driving pin 4 high to disable the protection circuit to see if there is a problem.
I removed Q502, 2SA1266 which is connected collector to pin 4 of the 494 and the amp powers up. Seems like everything is checking OK. Secondary ground is good. Rail and regulated voltage good. Amp produced output under load.
Q502 A1266
Emitter: Connected to LM318N pin 8 (5.03vDC)
Collector: Connected to 494 pin 4
Base: Connected to Q501 emitter (2.3vDC) and a few other components
Q501 A1266
emitter: connected to Q502's Base.
Collector: Connected to B- ground
Base: Bunch of components
Q502 A1266
Emitter: Connected to LM318N pin 8 (5.03vDC)
Collector: Connected to 494 pin 4
Base: Connected to Q501 emitter (2.3vDC) and a few other components
Q501 A1266
emitter: connected to Q502's Base.
Collector: Connected to B- ground
Base: Bunch of components
Is LM318 the right part number?
You may have to divide and conquer for the components that are driving the base. It likely should be pulled up to 5v.
Did you find a diagram that was close to it?
You may have to divide and conquer for the components that are driving the base. It likely should be pulled up to 5v.
Did you find a diagram that was close to it?
Tracing back one component at a time, I eventually found a group of 4x 2SC3198 NPN which had failed in the middle of the audio section. I replaced these with new MPSA06 which I twisted swapped collector/base legs. I believe the MPSA06 are rated about 20% higher across the board for the most part.
Im thinking this amp originally failed in the output section seeing as there is one mis-matched output bjt. Perhaps the previous repair missed this group in the center of the board.
Amp seems to be working with all components installed now.
Im thinking this amp originally failed in the output section seeing as there is one mis-matched output bjt. Perhaps the previous repair missed this group in the center of the board.
Amp seems to be working with all components installed now.