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Calculating Current Through a Transistor

gslikker said:
Suggest solder two wires on a track to check, run low voltage high current (1 Amp or so) through the traces, and measure remaining voltage over the tracks, I know it is time consuming doing all tracks.
Maybe even bend the board a little and see if voltage goes up (or the trace burns;)).

One day I found a problem such way. Of course, best to be done with an empty circuit card for components

(of course, only to be done if desparate....)
Click to expand...

That reminds me of a troubleshooting technique I used years ago for finding shorted traces. I would run enough current through them to warm them up, then pour Freon TF (flux cleaning solvent) on the board and follow the boiling lines of Freon right to the short. Worked great for finding copper whiskers under soldermask that were otherwise invisible. Then they banned Freon . . .
 
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Sounds like a hairline or microscopic fracture in one or more of the tracks. It gets a minute bit more separation as it warms up, changing the conductivity at that point(s).

I have an inexpensive 12:1 IR Temperature gun. the 12:1 means that it is 'sensing' from 1 square inch when held 12" from the surface. Something like this (or better) would let you identify the higher temp areas on the bad board and see how that compares to the other board. This assumes that you can get the necessary access to the boards when they are powered up.

Under 20 bucks at Walmart or Home Depot
 
Is such a flir display device already that cheap? I have a Fluke 62 thermal meter but it is not that useful for finding small spots.
 
merlynski said:
Can you post a clear high resolution picture of the foil side of the boards, please?
Click to expand...

Here you go. As you can see, I've re-worked some of the traces on the bad board that looked questionable. These traces are very heat sensitive and some were starting to lift. I also had to extend a trace to install a multi turn trimmer. None of these minor modifications made any difference.

DSC02471.JPG

Here's the good board for comparison.

DSC02473.JPG

gslikker said:
Suggest solder two wires on a track to check, run low voltage high current (1 Amp or so) through the traces, and measure remaining voltage over the tracks, I know it is time consuming doing all tracks.
Maybe even bend the board a little and see if voltage goes up (or the trace burns;)).

One day I found a problem such way. Of course, best to be done with an empty circuit card for components

(of course, only to be done if desperate....)
Click to expand...

I understand what you are saying but that would be incredibly time consuming. To speed up the process you might be able to do it with small alligator clips rather than soldering or just holding probes on the trace.

- Pete
 
Well, it's finally fixed but I cheated a little. I was able to find a replacement driver board. It's from an S-7910, a 60 WPC version of this receiver. It's the identical board but populated with lower power transistors. I've upgraded it to match the good original board. Now on cold start up the DC offset drops quickly to just a 1 or 2 mVs. Life is good.

One last question concerning bias setting. Sherwood doesn't publish a voltage or amp value. The service manual shows an elaborate procedure using a distortion analyzer. Since I don't have a distortion analyser, I measured the voltage drop across the emitter resistors (R651/R652) and set it to 30mV then checked the temperature of the output transistors with a IR thermometer. The temp seems OK.

The service manual shows an alternative procedure using a watt meter, which I do have. It says to adjust the bias until you see a "slight rise" in current draw. It also says the current draw at idle should be under 40W.

With the bias set to 30mV, at idle, the receiver is drawing around 51W. Since the manual says it should be below 40W so I started playing with the bias. I've dropped to to as low as 5mV and the current draw is slightly over 40W. I don't want to drop it any further but I still haven't found the point where the watt meter bottoms out.

I don't hear any issues with sound quality at that low of a bias setting and some of the big Sansui receivers set it at 5mV but that seems unusually low. Whether it's at 30mV or 5mV nothing seems to be getting hot and there is no noticeable difference in sound quality. What do you think?

- Pete
 
Well done! Got that monkey off your back.

Glad to hear that you finally found a solution, but I was expecting something oddball, like scratching your opposite ear with your arm behind your head (haha). However, any fix is a good fix.
 
If you have a scope and a signal generator you can hook that up to verify it doesn't have crossover distortion. If its too under-biased you'll get a little notch or flat spot at the zero crossing point.
 
Ghazzer, I'm very happy to get this "monkey" off my back but replacing the board is not a satisfying repair. I would have prefered to find that I had to scratch an opposite ear with my hand behind my head.
gadget73, good idea. I'll take a look. Thanks.

- Pete
 
I'm not seeing any blip anywhere in the sine wave. I adjusted the scope to make the pattern almost full screen and changed the bias between 5mV and 30mV. The sine wave is perfectly smooth across that range. I'm going to set it to 15mV and call it a day.

- Pete
 
A scope photo would help some of us who are curious. Sounds like a useful procedure.
Gadget is going to a Glenn Miller orchestra concert today so I cannot ask him at the AK Mid Atlantic Gathering today.
 
Yup, I found that video and a couple of others yesterday. I'll snap some pictures but they won't show anything like that.

- Pete
 
I set the receiver up with a 1k Hz sine wave into the Aux input and a dummy load on the output. Here's the trace at 50w output with the bias set to 15mV as measured across the emitter resistors. I don't see a hint of a ripple anywhere on the trace. I also monitored the temperature of the output transistors and they were all within a degree or two of each other.

- Pete

20191013_110124.jpg
 
Yes, it does make sense. And a good graphical example of why transistors need to be correctly bias.

Thanks - - -
 
Still don't see it. I set it up for 10kHz at about 1 watt and it had no no noticeable notch or ripple. Then I started cranking up the bias. Here it is set to 50mV at idle, with about 1 watt output. Good idea but it doesn't seem to show anything.

- Pete

50 mV.jpg
 
OK, here's the other extreme, 3mV bias with a 10kHz signal at 1W output. I don't see anything that even hints at a notch on the zero point. Unless I'm missing something, this doesn't seem to be a valid test to set bias current.

- Pete

3mv.jpg
 
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