blown output transistors

I have little idea what you know and what you dont... npn/pnp pair is your standard output pair, normally in more or less the same configuration, but for any more details you'd need to either trace the circuit yourself or get a cct diagram.

Little chance of needing matched pairs for anything. Transistors distort so much they get lots of nfb applied to them, so it doesnt matter whether theyre matched.

not really.

NT

Well, typically these output transistors are usually arranged in a push pull type circuit. One transistor amplifies the positive side of the waveform and the other does the negative side.

But I have to ask why. Emerson is junk. Not worth spending time and money on. Buy a new one.

DaveL

With only 3 amp channels, it probably just has a low pass filter to drive the subwoofer.

My guess, TIP41/42 are outputs, 2SC945/2SA733 are drivers connected as a darlington configuration with the outputs, 2SC945 is bias regulator connected between the bases of the drivers. 2SC2245/

What you have drawn is just the current amplifying stages, there are more transistors or IC's upstream that may or may not be damaged. And there's a good chance you have some open emitter resistors on the outputs.

Well I'd like to hook it up to my PC at work. I've got a pair of Polk Audio satellites but they've got no bass, I mean NONE. I don't expect high fidelity out of it, but it was entirely free and, given the fact that I'll be listening at low volume, it's entirely adequate for my humble needs. So far my investment is two 5A fuses, four power transistors and one driver transistor, maybe $10 all together. Given that this "system" likely cost, oh, maybe $80, if you'd like to chip in the extra $70 I'd be real happy to "buy a new one".

That's why.

Dave

How can you output 2 x rear, 2 x front, 1 x ctr, 1 x subwoofer with ony 3 channels? I don't see how it could be done unless the two input channels are amplified and sent to both front and rear outputs, PLUS are combined and sent to center channel, leaving one amp channel left over to drive the subwoofer on its' own...

There are only 1 or 2 other transistors on the board but several IC's, a couple of 14- or 16-pin and one bad-lookin' dog on the underside which looks to be SMT, many pins. If the outputs were shorted, would this not smoke the outputs, then the drivers, and so on? I tested the resistors to the bases of the output transistors and all were ok, will look at the emitter/collectors next.

Thanks a lot for your help, your reply as to the operation is exactly what I was hoping for.

Dave

The closest thing I can find locally as a replacement is a 2SA817.

The original 2SA733 has I(c) of 150mA, power dissipation 0.25W and hfe of between 90 and 600 (200 typical).

The 2SA817 has an I(c) of 500mA, power dissipation of 650mW and hfe of 70 to

Is this close enough?

Sorry, the 2SA817 has an I(c) of 300mA, power dissipation of 600mW

Okay, I rec'd my TIP41/42 replacement pairs yesterday. Before I put them in I figured I'd power up the unit with no output devices installed. Checked the bias voltage on the base of each output transistor position.

On the two pairs that are driven with C945/A733 pairs, there is 300-400mV DC at the base. At the 2SC2245/2SA965 driven pair the I read a whopping

I can see what you all mean about troubleshooting a DC coupled amp. I've made a schematic from the outputs back, but I don't have anything to compare my values to because it's a completely different amp channel than the two other channels which are identical.

Going backwards from outputs, I see outputs -> drivers -> dual op-amp IC. Behind the op-amps are more op-amps (quad op-amp IC TL074CN), another half-dozen NPN's (all c945's), two diode half-wave bridges, all kinds of good stuff. Is there any hope at all, given that I don't have an identical working channel to reference, of finding the source of this dc voltage? Or should I just bring it to the dump?

I don't mind troubleshooting this piece of plastic junk as a learning exercise if there is a potential positive outcome (i.e. light at the end of the tunnel). But, maybe I'm just asking for punishment...

Any and all responses appreciated

Dave

If its all dc coupled youre most definitely asking for punishment. I'd dump it and get some other chuck out to work one, one youve got a decent chance of fixing.

NT

Unfortunately, testing with no output transistors in place will give erroneous readings. I even had an HK 570i receiver flame out on me when I tried this back in the '80s. The problem is that the base-emitter junction of each output transistor is effectively a diode to the diving circuit, clamping at around .6 volt, unless the outputs are Darlintons, of course...

Mark Z.

then youve still got as set of output trannies yu can use in a project. Not often I'd say forget it, but with dc amps I wouldnt bother.

NT

Yeah, that's about where I'm at with it. Given it was a cheap unit to start with, it's not worth it... I just have a hard time letting go once I've sunk my teeth into a project. However the board does have LOTS of useful regulators and diodes on it, plus I recover a nice 5A CT transformer.

Thanks for the pragmatic advice.

Dave

Well, the good news is the sooner you get onto another piece of kit the sooner you're going to have a result. Plus you can use your TIPs to make a nice big power amp. Coincidentally, you've even got the psu, case & preamp all already there.

Mark Z wrote:

I reckon this one has horrors instead of darlings for the output

NT

The pre-amp/amp are *heavily* integrated, same PSU. I'd have to look pretty hard to find a point to break into the circuit at pre-out. I think I'll just build myself a 20-150Hz variable low-pass filter and 50W push/pull amp for the subwoofer with the TIPs. I can cut the board off (the PSU is well isolated off by itself) and use the relatively large empty space for my new amp. Conveniently, the 4-channel audio processor chip which provides input switching btween three sets of input jacks (even a high-gain set!) are all on the PSU end of the board.

I'm disappointed I can't get it working fully as it *potentially* does exactly what I want.

Oh well, success is knowing when to cut your losses.

Dave

IIUC all you've lost is your power amps, the rest works. I'm not clear what you mean by pre and main being integrated, I've not seen anything like that since the valve days. Same pcb sure, same psu of course, but circuit intertwined? I cant think how it would be.

Simple no frills power amp modules are quite easy to make. Use a fast opamp and you can do away with output bias circuits altogether. Forget about all the nonessential points and you can get some very simple power amps done. If you cant get them into the case any way, they can always sit outside it on their own, using the main units psu.

Anyway, It ounds like you'll get something out of it one way or another.

NT

I'm not that clear on where the amp begins and where the pre-amp stops in this case. It's not like an amp with discrete components where everything after the tone/balance pots is amp and everything before is pre. I'd expect at least four of the 12 op-amps on the board (2 x dual & 2 x quad) to be a buffer/integrator for the subwoofer signal... I guess that's the equivilant of a "pre-amp" for the subwoofer. Maybe four more (or less) to combine the signal for the center channel output? I really know very little about multi-channel (I mean more than two) systems and this one is a real deviant because I think it's basically just some sort of delay circuit to simulate surround sound from using only two analog R & L inputs.

If the amp is just the drivers and output transistors, and I've replaced the outputs and a good portion of the drivers, and I've still got DC bias problems, that would lead me to think that the source of the problem may well be in the preamp UNLESS there is a coupling cap between the pre-amp and amp stages. Hmm, if I can figure out where to make the split, maybe I can add such a cap which would at least tell me on which side of the fence (pre or main amp) the problem lies... ah, but I was going to dismantle it and use the components... silly me.

right, it sounds like the problem is one of circuit tracing then. When you use a capacitor coupled power amp(s), the dc conditions on its input dont matter. So you can follow the signal back from the (now blown) output tr pair using a probe thats a single opamp amplifier and a 2" speaker until you get a nice clean souding signal, and take it from there. Or, if it uses a pot for volume, you could tap 2 channel putput straight off the volume pot.

Preamps routinely have dc bias on them, its not an issue.

If you probe around with something very low power, you wont scare your ears or damage anything, and you should find where its all good. The signal quality may not be good within some parts of the old dead power amp, because the dead output trs will have made the feedback loop open there. Just keep going back till its good and clean.

NT

Well, turns out it IS as simple as discrete components: the pre-amp ends just past the tone/balance *IC*, and the power amp starts there. I made a probe out of the inputs to a disposable powered computer speaker.

Thanks for your insight.

Dave

Practically all the amps I work on are dc coupled. Not a problem in most cases. Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970 or so are DC coupled. Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in topology. Certain components tend to go out along with the outputs (resistors and driver transistors for example) and symptoms such as premature clipping, DC offset etc have predictable causes. As to the OP, it does seem his piece is quite possibly a "piece" all right, but I'm not at all sure I'd classify it as such a difficult repair simply because it may be a "DC" amp. As I say, I fix such amps all the time. No problem.

Mark Z.