Hi, I have this amp that is blowing the fusible resistor R632 in this Left Channel Amp Circuit. I've tried to isolate different parts of the circuit and have all of the driver transistors Q616, 617, 618, 619. 703, 707 as well as Q609 and all of the complimentary right channel transistors unsoldered and out of circuit and the resistor still blows after some time. When the resistor is out, I measure -70v on one side and like 2v on the other. When I remove right channel's complimentary resistor 682l, I still get -70v but on the other side where on the left I get 2v, on the right I get around 7v. Just looking for some guidance on how to hunt down what's causing this. I have read the variable resistors on this board can cause fits but not sure how to identify if that's the problem here...
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Hi,
I would suggest to do the following: de solder all output transistors and those "power envelope" ones - Q703 & Q707, test each of them as well as Q704 & Q708.
Test on the board trimmer R624 300 ohm - they fail after years of the usage. I'll replace these with precision trimmers, if you cannot find 300 ohm, 470 ohm can do the job. Even this trimmer measures good, I would recommend to replace it. You can check R618 200 ohm trimmer about the values as well.
Check Zener diodes D601 & D602.
If Right channel works, compare trimmer values with Left channel and tune new one about the same resistance and install it.
Left channel: Without output power transistors and Q703 & Q707, you can power it and see if you notice any changes.
BR,
bekim
I would suggest to do the following: de solder all output transistors and those "power envelope" ones - Q703 & Q707, test each of them as well as Q704 & Q708.
Test on the board trimmer R624 300 ohm - they fail after years of the usage. I'll replace these with precision trimmers, if you cannot find 300 ohm, 470 ohm can do the job. Even this trimmer measures good, I would recommend to replace it. You can check R618 200 ohm trimmer about the values as well.
Check Zener diodes D601 & D602.
If Right channel works, compare trimmer values with Left channel and tune new one about the same resistance and install it.
Left channel: Without output power transistors and Q703 & Q707, you can power it and see if you notice any changes.
BR,
bekim
Attachments
they're all currently out of the circuit and the resistor still goes open. I think I have previously tested q704 and 708 but I'll try again.
The excess current in R632 is most likely flowing into Q613 collector. From there maybe into R6502? If you can measure quickly, measure voltage across R6502. I suggest voltmeter via clip leads to R6502 so that can look during brief power-on. Failed Q613 and/or Q615 are suspects. P.S. Is output at negative rail, ahead of any protection relays?
Good luck.
Good luck.
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Thanks I'll check all of those next. Coincidentally when I was checking the aforementioned transisitors, trim pots and diodes (all tested fine) I found that trim pot R668 in the right channel was bad. It's the one right under the bar that connects the selector switch knob to the switch at the back so I don't know if I can sneak one of those taller units in there. Either way, glad I found it now instead of after the issue is fixed and I went to set everything back to spec.
Ok I'll focus more on caps. I tested q613 & 615 and they're fine. I also tested c615 and it seems to be fine as well.
Many times when i have repaired output stages in nad amplifiers i have had to change 2SB892 and 2SD1207. Weird looking, they are abit longer then the regular small transistors.
Re trimmer replacement--- I can't find the post, but @anatech recommends against muti-turn trimmers. IIRC, he argues that their wipers don't handle current through the wipers as well as single turn types. This design has a relatively low pot value, so wiper contact resistance may be a vulnerability.
A positive is that this design fails into a safe state if the wiper opens.
A positive is that this design fails into a safe state if the wiper opens.
Hi BSST,
Thanks. Yes. Not only that, but with a single turn trimmer you know where you are.
Bias current is not an exact adjustment anyway. You will find it varies with temperature and often line voltage. You do wan to set it as close as you can and follow the manual. I always let them sit at the last adjusted value to make sure it is stable there. Therefore, a multiturn control to set bias does not make any sense.
Some brands have too much range on the control. We used to call it "Yamaha syndrome" for example. That makes them sensitive to adjustment. The fix? Get the bias current close, measure the control value and adjust the series resistor and pot to make the control less sensitive while bringing the adjustment in range. Then you can set it much more easily. Under warranty we couldn't do that of course.
Thanks. Yes. Not only that, but with a single turn trimmer you know where you are.
Bias current is not an exact adjustment anyway. You will find it varies with temperature and often line voltage. You do wan to set it as close as you can and follow the manual. I always let them sit at the last adjusted value to make sure it is stable there. Therefore, a multiturn control to set bias does not make any sense.
Some brands have too much range on the control. We used to call it "Yamaha syndrome" for example. That makes them sensitive to adjustment. The fix? Get the bias current close, measure the control value and adjust the series resistor and pot to make the control less sensitive while bringing the adjustment in range. Then you can set it much more easily. Under warranty we couldn't do that of course.
Seems the issue was D602 was shorted allowing -43v to leak out. I checked it previously and it seemed fine, not sure what I did wrong. It's so awesome that you guys are willing to help neophites like me out...
D602 is a zener diode that sets up the current source with Q606 and Q608. R615 sets the current flow in this zener and also D601. Q605 is another current source using D601 as a reference voltage. If it was shorted, you must have shorted something that sent a current pulse through the zener.
To replace, match D601 and D602 for the same voltage drop to keep the currents equal.
To replace, match D601 and D602 for the same voltage drop to keep the currents equal.
Thanks. I suppose that's a good rule of thumb for anything that affects current, right? Replace v+ and v- and maybe even the L and R components to try to keep things as balanced as much as possible? The owner thinks the unit stopped working when he had a power surge in his house. I'll assume that the diode isn't the root problem and make sure I get spares in case there's still a deeper problem but hopefully using a bulb tester prevents things like that from fusing again...
Hi FatMaul,
These specific current sources are supposed to be balanced, close. They run from the same current source through the zeners. So replacing the one zener would probably unbalance them, do both, match voltage drop.
Use a variac please. Forget "bulb testers" exist.
These specific current sources are supposed to be balanced, close. They run from the same current source through the zeners. So replacing the one zener would probably unbalance them, do both, match voltage drop.
Use a variac please. Forget "bulb testers" exist.