Output transistors keep blowing on SX-2500

R45 is 15Kohm on the nose, R47 is 14.97Kohm, and C21 is 0.1035uf. Would R11 play a role? I ask, but now I'm going to check it before getting an answer. lol
 
So far, everything is checking out okay. Is this possibly a weak transistor problem. When I say that all the transistors test okay, it's not like I'm putting them on a curve tracer or anything. I'm just testing them on a cheap component tester. It just seems strange that I am getting so much lower signal on the left channel.
 
Check to see that the input signals to the Main Amp (W23-004) input pins 2 and 11 are identical, and pins 3 and 10 are connected to the chassis ground. If they are the same amplitude the problem is on the Main Amp Unit, if not it may be before the Main Amp. (Pre and Main Selector set to Inter-Coupled)
 
You probably thought about swapping them?
I have a Marantz 250 that is checking out but will not let me bias it. I'm sure its a transistor that's causing it. but they all check out.
What the heck?
 
Okay, input is the same amplitude on both pin 2 and 11. Pin 2 keeps drifting downward in voltage though, as though the center point is changing.
 
When I go though the preamp though, I still get that 60Hz harmonic. I'm going to try a couple things here, but I think I might have gotten this amplifier board figured out. I think I found a loose connection. Then I'll need to go on to the preamp apparently.
 
Does anyone know what this board is that pins 2 and 11 connect to labeled M-004? I don't find it anywhere labeled that, and no board in the service manual looks like it. It's got about 7 transistors, 2 diodes, 6 electrolytics, and some various resistors and capacitors on it. It also has 14 connector pins on one edge of it, and none anywhere else.
 
I was just trying to trace the inputs back. Strange how that board only has the M on it and not the AW.

Anyway, the electrolytics on this board are way out of tolerance, and even though I haven't diagnosed it to those caps, I'm going to replace them really quick. Knowing that it's the muting circuit, I kind of doubt they are the problem.
 
Does anyone know what this board is that pins 2 and 11 connect to labeled M-004? I don't find it anywhere labeled that, and no board in the service manual looks like it. It's got about 7 transistors, 2 diodes, 6 electrolytics, and some various resistors and capacitors on it. It also has 14 connector pins on one edge of it, and none anywhere else.

On my schematic pins 2 and 11 of the Main Amp board do not connect to the AWM-004 board. They come in through the Pre-Main Selector switch. Depending on where it is set the signal to 2 and 11 comes from the Main In jacks or the Switch Unit (W26-007) pins 3 and 4. NOTE: the 'ground' side connection to pins 3 and 11 must have a good connection to chassis ground or the Main Amp input will misbehave. The same goes for Main Amp pins 5 and 8, on the output side of the Main Amp, if they do not have a good chassis ground the Output circuit will misbehave. Check all 4 with your ohmmeter (power off). There is no 'Common Ground' point on the Main Amp pcb.
 
yep the grounds have to be good . cant remember if it was same amp i had fun with but the amp board would not play nicely on extension jumpers to work on easily . was very similar circuits .
 
On this radio, there is a wire going from pins 2/3 and 10/11 on the amp board to pins 9/10 and 11/12 of the AWM-004 board respectively. Also, pins 4 and 9 on the amp board go to that board at pins 2 and 4 respectively. They also go to the large C33 and C34 caps on the chassis.

I thought you might have had something there for a second with the ground thing though. There are two wires going from those ground pins to the chassis on each side. One of the two wires on the left channel that seems to be giving the trouble was broken off from the chassis side. I reconnected it though, and no change. All four test out 0 Ohms to ground.
 
I am still getting horrible signal from the preamp though, and the left channel has cleared up in the way it looks.
 
On this radio, there is a wire going from pins 2/3 and 10/11 on the amp board to pins 9/10 and 11/12 of the AWM-004 board respectively. Also, pins 4 and 9 on the amp board go to that board at pins 2 and 4 respectively. They also go to the large C33 and C34 caps on the chassis.
I am working from schematics and service manuals from three different sources, none of which seems totally complete. This is a typical difficulty with vintage equipment. Just part of the challenge (fun). What you are describing matches one of the AWM-004 schematics i have, and appears to be a muting circuit. I think we should just let that ride for the moment. Maybe somebody else on AK can come up with some better documentation.

I am still getting horrible signal from the preamp though, and the left channel has cleared up in the way it looks.

OK, that sounds like some improvement. Maybe there are/were 2 problems. Do you have another unit that is a preamp, or has pre-out main-in jacks? With the Pre-Main Selector in Separate try feeding another source pre-out to the SX2500 Main In and see if you get a good sounding output from the speakers. Or something similar like a smart phone or computer Line level output. Start with the new input volume low. This will test just the Main Amp output stage . . .

Also, with your sine wave input going in, look at the Pre-outs with your scope, is one channel ok, the other bad?
 
I have all sorts of test equipment. I've been using my tone generator at about 1Khz, 0.2V amplitude, and right now the waveforms look fine when I connect to main in, although the left channel is about 1/10 the signal strength of the right channel. No distortion or harmonics though.

I have pretty close to every piece of test equipment you could need for this job, so you don't have to think of ways to make things workable for the average person. Oscilloscope, tone generator, radio communication test set that does AM, FM, and FM Stereo, dummy load, speakers, variac, isolation transformer, DBT, large DC power supply, soldering/desoldering station, wire wrap tools, enough electrolytic caps, transistors, resistors, ceramic caps, etc., both through hole and SMT to choke an elephant. I even have a microscope, tweezers, and hot air if the need arises. lol

No distortion meter or curve tracer though. Also I never have enough cables, but I have a wall full of them.
 
Well dang, all that and hot air too :rflmao:, I'm jealous. OK, looks like you have a bad power amp channel and a bad (oscillating?) pre-amp channel. If they are both on the same side maybe the bad pre killed the power amp. The Main Amp has fewer parts. Check every power supply pin on the Main Amp for correct dc voltage. If OK then let's compare left and right signals at the same time. Feed both the Main In jacks with your 1kHz 0.2 v signal and scope the jacks inside, that they are the same. If so, then check pin 2 and pin 11 on Main Amp pcb (input pins). Still the same? Scope the collectors of Q1 and Q2. They may not be the same because you are now inside the negative feedback loop. It comes back from the output thru the two 15k resistors to the emitter of Q1 (Q2). If different power off and check transistors and diodes for open short or leaky, then for leaky or shorted electrolytic caps.
Enough for me for now, back at it tomorrow, good luck, this is a good challenge!
 
Right channel 92 volt rail reads 93.2V, left channel 93.0V. Pin 12 85V rail is 87.1V.

Both left and right channel are same amplitude on the inputs, and on pins 2 and 11 when power is off, but when I turn the power on, they move all over the place for about 2 minutes before they settle on a voltage level they want to be at. The collectors of both Q1 and Q2 show me a signal that is so bad my scope won't lock on to them, but they look roughly the same from what I can see.

I did notice that one of my channels on my scope is acting strange, but I switched my probes to different channels, and things are still messed up. Time to call tequipment and get it replaced I think.

Anyway, time for me to go to bed too. Thanks again for the help.
 
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