PV500

[lowell]

This thang came in with R133 and R112 burnt up. I replaced both as well as driver 15032. The thing was fine until I pushed the RESET switches on the back just to make sure they worked. This time only R133 smoked. I'm not sure as to why this would be. Anyone have an idea?

[lowell]

Oh I failed to mention the lack of a 3rd prong on the power cord... if that clues anyone into the issue. I've since fixed the cord but have yet to re-repair.

[Enzo]

Just thinking aloud here:

follow the line from the transistor over to pin 4 of the ribbon and on to the bases of Q100, Q101 on the output. Look for cracked solder, especially on the ribbon, or otherwise any opens.

And check that 180 ohm resistor onthe output side, R101 I think?

And if you had to replace that driver, did you replace those outputs associated with it?

And though the top side seems to be the parts burning, make sure to check their counterparts on the negative half.

[lowell]

Enzo I'm gonna check all that stuff. If you have time could you please explain how a bad output transistor could cause a bad driver? As I see it if Q100 output shorts then Q100 sees the same supply voltage on it's collector and emitter... actually its emitter would be a bit higher maybe? Anyways that shouldn't cause it to short should it?

Why would R101 blow?

Good call on the negative side as they are all in series therefore the same amount of current flows through all of em.

[lowell]

Well brain fart on my part. I went back in to diagnose the issue and found that I connect the ribbon cable off by 1 pin... whoops. Found a bunch of burnt components. Replaced 'em all, but need to figure out what's fried on the power amp board as something there is smoking.

[Enzo]

Well, I get points for the open ribbon connection call even though it was operator error.


OK, Q100 shorts base to emitter, basically then the base is grounded. That is pin 4 of the ribbon. Pin 4 is also the emitter of the driver, I can't read the Q#. That poor driver transistor is now sitting dead across the V+ rail and ground with nothing more than the lonesome R133 4.7 ohm resistor to try and save it. Might as well be a chihuahua puppy trying to fight off a grizzly bear.

You didn't think those transistors could only short E-C, did you?

The V+ rail is 56vDC, so a 4.7 ohm resistor would limit current through that little transistor to 12 amps. I think the transistor would fail at a lot less than that.


WHy would R101 blow? If Q100 and Q102 both short collector to base, that leaves 180 ohm R101 across the +56 and -56 rails - 112vDC. 112v across 180 ohms, well, that resistor would have to be able to dissipate 70 watts if it doesn;t want to burn up.

If Q100 shorts base to collector and Q102 shorts base to emitter, then R101 "only" has the +56 across it. They it has only to withstand 35 watts instead of 70.

Not saying all those things will happen at once, but there are many scenarios, and we can't rule out any of them. And when it takes mere seconds to find 180 ohms across that resistor, we have no trouble ruling it out or finding it bad.

[lowell]

Great... that does make sense now, thank you.

When I look at the diagrams in the schematics of the board layouts they don't seem to match. If you look you'll see all the "200" parts on the RIGHT side of the power amp board layout. In fact, I believe that these components in the amp are actually the "100" components. I checked continuity of R101 and it goes to Q100 driver on the other board.

Let me know if these layouts seem wrong please.

[Enzo]

Backwards? Without the actual board in my hand, I have no way of knowing, however, that board looks almost the same upside down.

Look at the two ribbon connectors near each end. Are the TO220 triacs on the near side of them like the drawing, or on the far side? Or is the coil of wire for each side on the far side of the ribbbons like the drawing or on your side?

[lowell]

Enzo,
I'm not sure I follow what you're asking. What does near side and far side mean? Maybe it doesn't matter if I tell you this. The R201 on the right side of the drawing is actually connected to R112 through the ribbon cable, which connects onto the other driver board right next to all of the "100" components. Either the output board is drawn with incorrect numbering or the driver board is. This is making my head spin while trying to diagnose the thing.

[Enzo]

Sorry, I meant when the board is sitting on the bench in front of you, or if it is not, imagine that it is. SO looking at it it is either oriented exactly like the layout drawing or it is flipped end for end - "upside down" from the picture in other words. The board looks very similar both ways, so the differences are whether the triac is on the side closer to you from the ribbon connector or farther from you.

SInce both circuits are identical and the two ribbon cables are wired the same, it really ought not to matter which end is which. In fact, not this model, but on similar ones, I have gone so far as to swap driver connections for troubleshooting purposes.

[lowell]

Okay thanks. So either way though, the drawing is backwards left to right. If I flip the board, the drawing still has the "200" components on the right, however regardless of the board flip, the "right" side of the board is connected to the same driver board components. Therefore the drawing should have the "100" components on the right, not the left. Make sense?

Anyway, I've been checking the power transistors... there are some bad ones. I have a question about one though. On my meter's diode function I was getting a .2v BE reading and .2v BC reading. This seemed odd and less than the .6v readings I was getting on another side of the output. So I pulled one of them and it still reads low at .2v. Is it common to see a transistor that's compromised but not a dead short?

[lowell]

I replaced all transistors that read abnormal. All seems good w/ the transistors now. BE and BC diode test looks good on all of em. However, there is a dead short b/t +/- supply rails for one channel's PA. I've lifted a leg of CR106 and the diode is good... however still a short. When I check DC resistance of C100/C104 they seem fine at around 2k which makes sense. This leads me to the rectifiers... they all test good w/ my diode function. So I'm a bit lost here. BTW the board is disconnected from the rest of the amp.

[Enzo]

A failed transistor means one that no longer meets its specs. COUld be shorted, could be open, could have damaged silicon that is almost shorted but not quite, could pass the tests but have almost no gain, could be noisy, could be intermittant, etc. There is no exactly-this-every-time failure mode.

Dead short from V+ to V-? And the ribbons are off? Transformer leads off? Either V+/- shorted to ground? Oh silly me, if one was shorted to ground, so would the other be, since they are shorted together. SO, are they shorted to ground too? Probably not, I'm guessing.

You checked CR106, so don;t forget CR101 on the other rail.

There really is no component wired from V+ to V-, so to get a dead short, we'd have to have at least two dead shorted parts. SInce you have been working on this thing, I have to suspect you have made a solder bridge somewhere. Or you left out the insulators under some transistors on the heat sink. The parts layouts no longer include the trace art, so I cannot predict where such a solder bridge might occur, if there is one.

[lowell]

I forgot the dog garn insulators. I then found a bunch of shorted diodes throughout the driver board. The thing is basically up and running now, but I have no signal out of Channel A. Channel B works.

... oh I found a bad p-channel jfet on the input of the 1st opamp.... we'll see if that fixes it.

[lowell]

replaced the jfet and voila. Thanks for the help!!