I have a friend's Akai AM-2600 sitting on my bench. There was no sound on the left channel (it had blown before and was repaired by someone else). There was an open resistor in the channel and after replacing it it seemed to work fine again. That is with a load : when an 8R load is connected both channels put out a clean and perfectly equal sine wave. When I disconnect the load the right channel is still fine but the left channel clips the positive side of the sine wave
. I checked all the voltages on the channel and they're good. Also all the resistors tested good. In the past I had some amps that worked ok without a load but showed problems with one, never this though 😕.
. I checked all the voltages on the channel and they're good. Also all the resistors tested good. In the past I had some amps that worked ok without a load but showed problems with one, never this though 😕.
That's a funny one.
Unfortunately I don't have a very good idea what it could be, possibly an iffy connection somewhere around the output stage or SOA protection acting up. I don't think a leaky bootstrap capacitor would do something like this.
Is there really no difference in voltages w/ and w/o a load?
Unfortunately I don't have a very good idea what it could be, possibly an iffy connection somewhere around the output stage or SOA protection acting up. I don't think a leaky bootstrap capacitor would do something like this.
Is there really no difference in voltages w/ and w/o a load?
Thank you for your answers. I won't be able to work on the amp for two weeks but I'll check those things when I get back 🙂
I checked and yes DC offset is 8mv in both cases of loading (+/-3mV). Voltages seem to be all the same except for the 0.7v on the emitters of the diff pair is dropping when I turn up the volume, all the way to about -1.3v when volume is at maximum.
C2 and C5 have risen in value (1.2µF and 69µF respectively) but other than that they seem fine. I replaced them just to be sure but that doesn't make any difference. I see that I didn't mention before, without a load, until the power meter indicates about 8w the sine wave is clean, it only start clipping after that.
I did some more research and I found out that the signal starts clipping at 8w with a 60w dim bulb, no load, with a 100w dim bulb it starts clipping at 1W and without any current limiting it doesn't get any signal at all and I have -1.6v on the left channel output. Something seems to have changed because now I have the same results with or without a load.
Without the dim bulb I measure -0.7v instead of +0.7v on the emitters of the diff pair, -3v on the base of TR7, -2v on its emitter, -1v on the emitter of TR6 and -0.3v on its base. The base of TR5 is at -1.9v.
Without the dim bulb I measure -0.7v instead of +0.7v on the emitters of the diff pair, -3v on the base of TR7, -2v on its emitter, -1v on the emitter of TR6 and -0.3v on its base. The base of TR5 is at -1.9v.
I tested all the resistors out of circuit and they're all good, the diff pair has been replaced by a pair of matched KSA992, the caps have been replaced jut in the bad channel, connections tested, and transistors tested (only in diode mode though) and everything checks out fine. The drivers and output transistors have been replaced in a previous attempt and are now TIP31C/TIP32C and 2SB655/2SD675. Here are some of the voltages, without a load, without signal and on full line voltage. It seems that the more I drop the voltages inside the amp the better the channel works (that's probably why at first I said it was working fine with a load but that was with a dim bulb so the higher the volume, the brighter the bulb and the lower the voltages...)
Hi Mona, I checked R13 and it measured fine, just to be sure I replaced it but that didn't change anything. Same for TR5 (It's now a KSC1845) and that didn't change anything either.
You measure at TR5, base -1,9V and emitter -3V -> Vbe=1,1V not normal for a healty transistor.
Emitters diff pair -0,7V because of desperate attempt to get the output in positive direction .Normaly that's no drive for TR6 or not responding.
Mona
Emitters diff pair -0,7V because of desperate attempt to get the output in positive direction .Normaly that's no drive for TR6 or not responding.
Mona
I Checked TR5 several times and replaced it again just to be sure but I still get the same voltages. I added the voltages around TR3 and TR4. There seems to be a short between the negative voltage rail and the base of TR3 but R5 has been replaced and measures 4.3k as should be 
With those voltages TR3 is completely blocked.But something is still pulling the base of TR7 negative.
Possebilities;
b-c juntion of TR7 leaking
C12 leaking
something wrong on the circuit board
(bad measurements)
Mona
Possebilities;
b-c juntion of TR7 leaking
C12 leaking
something wrong on the circuit board
(bad measurements)
Mona
Found it !! I assumed the TIP41/42C drivers were ok as they were installed in a previous attempt just before it was brought to me but that was apparently my mistake. I swapped them for the original drivers on the good channel and the problem moved to the right channel. It's not the first time I had transistors test good with a multimeter but fail in circuit. Is it still possible to get the original drivers for a descent price without having to worry too much about them being fake or should I just go for some equivalents ?
the voltage drop across the emitter of t1 should be +0,6v.......but you measured -0,6v
is there something wrong with t1? ( built in wrong/reverted , wrong pin layout or sowhat)
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.........o.k. problem could be solved.....
is there something wrong with t1? ( built in wrong/reverted , wrong pin layout or sowhat)
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.........o.k. problem could be solved.....
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The diff pair has been changed for a pair of KSA992, they have the same pinout as the 2SA640. Now that I got the drivers swapped I get all the right voltages on the left channel and all the wrong ones on the right so I think getting a new pair of drivers will take care of that 🙂
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