stereo amp cleaning

I use a very similar device and have for years. It was instrumental in making me a halfway decent tech. Without having this available I might have given up on being a tech and moved on to something else before I ever "got a clue".

Once a person has learned how to use this thing it really speeds up the process of doing quick checks on components in-circuit. A leaky transistor junction becomes obvious. Open capacitor, no problem. You can even connect it to a phototransistor and fire a remote at the transistor and observe it turning on and off on your 'scope screen (though this doesn't tell how well the device works in-circuit...)

Mark Z.

I must say I am intrigued... sort of an all-in-one tester which costs less than $5 to make. Doesn't get much better than that. What is the value or the variable resistor ? 1K? 100K? Or would one use the guess and check method for an acceptable output on the scope?

Dave

"Dave" wrote in news:VZm0i.38249$KN6.1602@edtnps89:

Yep. You want to have a good horizontal deflection. Depends on the gain on your horizontal input.

I built my into a wallwart, brought the scope leads for the resistors directly out through some slits in the case. I just clip the probes from my scope to the leads.

Glued a diode to the case to use as a reference.

Used a few feet of light duty speaker wire and a couple of E-Z Hook clips for the circuit board test end.

Seems like some cheap scopes came with a component tester. probably about the same. Reminded me I somehow aquired a commercial unit, so i am looking at it, never used it. It has a 555 oscillator and some circuitry, but I don'r have a schematic. Can't find it on the web but will look some more. i should use this usefull tool.

greg

If the circuit is anything like the one in the link bz posted, it's a few resistors and a transformer, shouldn't be hard to figure out schematic.

Something is still failing in my amp. As I noted, I put the original caps (C401, 403, 405, 407) back in and the problem went away. For awhile. It's back... I measured voltages after the right channel had cut out.

The collector of Q407 was 0V. So was the collector and base of Q409, and the base of Q413. Makes sense as they're all tied together. The collector of Q405 was also 0V. the collectors of Q411 and Q413 were ALSO 0V. I noted that although R437 tested fine, it looks like it's been run hot. As in it's dark brown and you can't really read the color bands any more.

I don't understand how Q405 works as far as the turn-on delay, but if it were bad could it be causing no voltage where I've indicated?

I tested the other, working channel and all voltages were correct.

Thanks

Dave

I need the base and emitter voltages on Q405 versus Q406. I wasn't thinking clearly before, but the basic principle is the same. As the power supply comes up, a positive voltage is passed through D401 and D402 to turn ON Q405 and Q406. In the case of Q405, you can see that there should be a negative voltage at the emitter, and a somewhat LESS negative voltage at the base. This turns ON the transistor and should pass a negative voltage through the transistor, so that the negative 7 volts or so appears at the collector. If there's no voltage at the collector of Q405, it's likely there is no negative voltage at the emitter either. Possibly R429 or R429 are opening up, or there could be bad solder connections on one or both of them.

Let me know what you find.

Mark Z.

Oops I guess I omitted information... had typed it then deleted it as insignificant. I did test the negative power rail at R417 where the schemo shows -14.2V. It was, I think around -14.9V. In the zone anyways which would rule out R427 or R429 as culprits. If one of these had opened up would I not likely see a positive voltage at the base of Q405?

Base Collector Emitter Q405 -14.2 -0.4 -14.7 Q406 -12.7 -6.9 -13.5

As you can see the base vs. emitter of the two transistors has a comparable delta... say a half-volt lower at the base which is what you'd expect I think.

So, if I have the correct voltages at the base and emitter of Q405, but the wrong collector voltage, I should be looking hard at Q405? I would think that the -0.4V seen at the collector would be derived from the 2 x -0.6V coming out of Q401 and Q403 attenuated by R413/R415/R483 with no contribution from Q405.

Dave

I do think I'd replace Q405 at this point. You say the B-E voltage is comparable but I don't think so. There's a big difference between 0.5 volts and 0.8 volts when it comes to turning on a transistor. If the transistor isn't the problem I'd still say it's not turning on sufficiently. Could be a resistor is not open but the value has changed, for example.

Mark Z.

Mark Z.

Well, I'll lift the legs of R427 and R429... and, I suppose, R407, R417 and R405 just to rule out the resistors as I likely have replacements kicking around for them whereas the transistor will need to be procured. Something has got to be causing the ~1.5V differential between the R and L amp channels in the first place.

I could add a small ~40-ohm resistor to the base, too and see if Q405 turns on with a 0.8V B-E drop... FYI the spec drop is only 0.6V and I doubt the designer would cut it so close that a 0.1V difference causes failure... but them I'm often surprised.

Will post results, might be awhile before I get my hands on the transistor.

Out of curiousity, what does the Q5/Q6 circuit which biases Q405 do? As near as I can tell, it takes 30VAC prior to rectification, runs it through a diode to separate out the negative half of the wave, then past a 12V zener to drop the voltage. Not sure what the function of D9 is, it is oriented the reverse of D8. you end up with -1.1V at the base of Q5. The net result being -0.56V out of the collector of Q6 to bias Q405/6. Seems like a lot of work to derive a half-volt of regulated power but I guess they wanted it fully independent of the amp channel power rails.

Thanks again.

Dave

The function of Q5 and 6 is just to provide a more positive bias to Q405 after a delay (mainly determined by R24 and C22), enabling the amp channel to "un-mute". It neen't be actually "positive" just more positive than the negative rail at the emitter 0f Q401. Diode D9 is a question mark to me also, but I suspect that since the capacitor C21 is only 33uF then some ripple would still be present, and the diode might then act similar to a zener and clamp the voltage drop across R24. I have to admit that since I'm no engineer I can lose it when trying to figure the theory aspects. I just fix them when they break.

You can swap Q405 and Q406 and see if the problem moves to the other channel. Resistor R417 (560 ohm) can be checked in circuit. R405, a 68K, would probably need to be checked out-of-circuit. Experience tells me that high-value resistors can change value or go open-circuit for no good reason. Worth keeping in the back of your mind sometimes as you're troubleshooting problems such as this.

Not sure how you would use a 40 ohm added to the base of Q405.... in series with R405 (68K) would do nothing, and in parallel with the BE junction of Q405 would just keep the transistor from ever turning on in this application.

At this point I think R427 and 429 are probably OK, or you wouldn't see the voltage at the emitter of Q405. Our problem seems to involve getting 405 to turn on, which is to say, getting the base of Q405 a bit more positve (less negative) than it is right now.

Mark Z.

R417 and R418 both test around 560 in circuit

R405 and R406 both test 66k in circuit

I'll try swapping Q405 and Q406 and see if the problem follows the transistor. I found them relatively close (4 hour drive) in stock, or I can order them in to my local supplier and wait a couple of days/weeks to receive. How many do you think I'd have to order to find a pair with DC gain matched to within, say, 15%? The 2SC2603 crosses to NTE289, which is available as NTE289AMP, a matched pair for use in amplifiers...

For sure Q405 is not turning on. I watched the output of Q406 and, about 5 seconds after turning power on, it goes from -0.4 to -7.1V. Q405 just stays at -0.4V.

Dave

I meant in series with R407 to drop another 0.2V at the base of Q405 to try to get it to turn on.

If they are using a 2SC2603 then it is not critical. The 2603 is a pretty standard signal transistor - nothing special. A 2SC945 would be fine, heck, even a ECG / NTE 123AP. Just make sure the basing is correct. An American type transistor will usually go EBC rather than ECB as viewed from the front.

Mark Z.

Ah. I get it.

Mark Z.

I figured that if 3.6K dropped the negative rail voltage from -42.6 to -14.2 (through R427/R429) then that's about 125 ohms/V. 40 ohms ~= 1/3V.

OK. I have lots of 2SC945's, they're in everything. But... I swapped Q405 and Q406 and the problem DID NOT follow the transistor.

I kept thinking about C405 and C407. If either one leaked, even a little bit, that would throw off the base voltage of Q405, maybe enough to keep it from turning on. So I threw in a new C405 and, voila, Q405 now shows -5.9V at the collector. Great, I says to myself, I am so smart. (Note to self, find new source for electrolytic caps, two bad ones out of 30 so far). Set about checking other voltages in the channel, but HEY! what the ???? Something else is not right.

The voltages that were out were:

Base Collector Emitter Q409 +34.2 +34.2 Q411 +33.0 Q413 +33.9 +33.8 Q401 +0.2 Q403 +0.4

I didn't check the collector voltages of Q401/Q403, I'll do that next but... I'd be surprised if they weren't +40.5 given that all the other positive voltages check out.

The negative power rail is fine at -43.5V, and I am getting my -12.5 at the base of Q405 so it's not the negative supply.

If Q409 fails to turn on, then that would make Q413 not turn on either. There would be then be no negative voltage applied to the collector of Q411 to get down to the +1.1V we want to see. I'll check out R439.

I wouldn't have expected to see a positive voltage at the collector of either Q409 OR Q413 given that they're basically connected to the negative power rail through a small value resistor. Almost makes you think that if R439 is open, R445 would be too.

But then we're back to my basic lack of understanding of how the transistor works...

I think I'm learning something here.

Dave

My kids have a game which is a 4 x 4 grid of holes. At any given time, there is a mouse poking out of one hole. One pounds the mouse into the hole with a hammer and this forces another mouse out of another hole. Sort of how I feel here... fix one thing and it appears to break another.

What would happen if C415 were open?