Acoustic 270 problem

[xjbear]

http://music-electronics-forum.com/t26639/

[J M Fahey]

The row of transistors directly off the 85v rail get hot but the 4 on the other side do not.

Well, thatīs impossible.

That amp is single rail capacitive output so *all* current which passes through the top ones, *must* pass the lower ones.

Whatever the top ones dissipate, the lower ones will too.

[Enzo]

Unless the output cap is shorted or VERY leaky, shunting current to ground around the lower side transistors.

[xjbear]

Ive got several output caps. Ill pull it and see. Thanks guys
Theres no dc on the outputs, the drivers are still cool after 5 minutes of playing through it but yes the first four trans, control panel side, are verywarm/hot. The other four are cool.

[xjbear]

Yeah somethings dragging the 47v rail down to 4 volts. I get 47v between r416 and c407. On the other side of r417 its down to 4volts only. Diodes are all good, caps are replaced and transistors replaced and double checked. Arrggg

[Enzo]

There is no 47v rail unless you mean the output bus. If that sits at 4v then it sure is dragged down.


I don't know which side the transistors are on, so measure the voltage on the collectors of the output transistors. One row should have about 90v. The other side about 45. WHich one is getting hot? If i read you, one row has the 90v but the other row only 4v? Wwhich row is the one getting hot?


R417 open?

[J M Fahey]

Please post which is the valid schematic link you are all using.

[xjbear]

This schematic seems a bit different. My power board has a 120k resistor on the backside of it and no sign of that on the schematic.

Q406 gets hot but no others do. Its quieter until I hit a chord then blam distorted but not splatty loud volume. R416 and r417 get hot too. Im gonna try and find a schematic for a 370-320-220 whatever and see if any show this 120k resistor

[Enzo]

If the part is on the solder side of the board, chances are it is not on the schematic, that sort of thing tends to be either a factory fix or update, or perhaps something a tech did at some point to tweak something. If that is the only difference, don;t worry about it. Look at where that resistor is connected, each end connects to something that as far as we know IS on the schematic, so knowing where the two ends go we can look at the schematic and determine what it is for.

How about the questions I asked in post 21?

Check those 10w resistors.

Assuming there is only 4v on the low end of R417, then there is about 90v across R416,417, and I don't doubt they get hot.

[xjbear]

R417 isnt open. I went through and checked all the output trans again and all read 500k but on was 73k replaced that one and swapped .1ohm 10watt out with others from a salvage board. Something interesting just happened as I was probing around q405 a 40409npn. I shorted the base to the emitter accidentally with my probe and it kicked on and the voltages started coming down in spec. BUT it started smoking the 100ohm base resistor.

[xjbear]

I meant 5 megaohm on the transistors. Hold it on one pin for a few seconds the readings will drop from 33megs down to 5meg then switch to the other pin and itll read 100k. Thats on the output transistors. Im assuming thats good. Is an MJ15003 a good sub for these? And ibhave a couple nte 128 and 129 ill try for drivers tomorrow

[xjbear]

Also are these little metal can 40409/10 and such pretty delicate when soldering? I pulled em off a good board and put them on this one and maybe I got em hot. I was quick but I have heard that some die instantly without a heatsink

[xjbear]

Much cleaner hi res schematic found

https://www.dropbox.com/s/9ujlx06zeq...ice-manual.pdf

[xjbear]

Looks like I got it whipped today. The 170k bias resistor off the wiper and leg of the bias pot was open. I had checked it twice but maybe was intermittent. And this is why SS circuits drive me insane.thanks everyone for helping me out and your suggestions.

[xjbear]

Now off on the hunt for a 375mh inductor/chokd for the geq. Fun.