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Magic Smoke

Hmm, just gone to the listening room where the amp has been on all night and there is now a buzz coming from that channel, reminiscent of the buzz you get when the feed back circuit or cap has become faulty. Does anyone think this might tie with my earlier finding about bias not trimming? 8'd rather learn somthing here than go changing components Willy nilly.

Stefan
 
Which is precisely why, in post 2, I recommended checking that the trimmer alters the voltage across TR5 (the two points labeled 'bias test') before re-installing the output transistors.

It's easier, and less expensive in terms of output transistors, to make sure that everything is working before putting the MJ15003s back.

malcolm
 
Curiously I did check, it wad around 1.8mV which is lower than the eventual value once the output transistors are in. I didn't check I could adjust it though. I must admit I never even thought of that, simply that it was in the correct range.

Where should I go from now?

Thanks for all your help Malcolm, and everyone else :)
 
If it were me I'd go back to post 2, and remove the output transistors, check them and then fault find the board without the risk of damaging the MJ15003s again,

malcolm
 
misterc6 said:
If it were me I'd go back to post 2, and remove the output transistors, check them and then fault find the board without the risk of damaging the MJ15003s again,

malcolm
Click to expand...

I meant to say in my above post that I did the check across the bias test points without the output transistors in. When I got a reading that was in the right ball park I put them in. Although the reading with them in wouldn't adjust the reading across the bias test points still read the same as it did without the output transistors in. In fact it made beautiful music for two nights and impressed the guests I had both evenings, both of whom have far more expensive set ups (well done LesW).

Should I take the output transistors out again Malcolm? and given that the drivers are fine and the diodes that end of the board all measured fine where should I start, are there any measurements that would help me trouble shoot the board? I've got enough spares to re-populate the entire board but would much rather learn what measurements were meaningful and what action to take. When you and John Luckins helped me out last time I knew nothing and had to pass that project on for repair. This time I have learned much and have been able to trouble shoot much of it myself, but would like to carry on learning.

Stefan
 
With apologies to Les, I'll talk in terms of bias across the emitter resistors because those are the figures to hand.

If you cannot get the bias above 2.7mV the circuit is not operating correctly. It may sound OK, but it's not right.

If you have the parts you could try replacing TR4, TR5, TR6 and the two capacitors (a BC128 and a small film) that are across TR5 and see if that makes a difference. If you want to leave the output transistors in place I would set the bias to minimum before making any changes and then try to wind it up afterwards,

malcolm
 
I'll take them out again first, best to be safe. Typically the only components I don't have are those three transistors. I'll get them and report back.

Might also be worth mentioning the buzz I reported goes after the amp has been off and cooled/discharged for a couple of minutes and only returns after quite extended listening once fully warmed up. Once there it stays constant until switched of regardless of input level or lack thereof. Might be a usefully clues to anyone in the know.

Thanks again.
 
orangeart said:
I'll take them out again first, best to be safe. Typically the only components I don't have are those three transistors. I'll get them and report back.

Might also be worth mentioning the buzz I reported goes after the amp has been off and cooled/discharged for a couple of minutes and only returns after quite extended listening once fully warmed up. Once there it stays constant until switched of regardless of input level or lack thereof. Might be a usefully clues to anyone in the know.

Thanks again.
Click to expand...


Put it in a box and send it back to daddy.........

Les
 
LesW said:
Put it in a box and send it back to daddy.........

Les
Click to expand...

If i'd got spare cash i would, but i'm skint. (mainly because my wife has made me take her and the three little ones to Florida for xmas, but that is another story)

I'd also like to carry on learning which was the point of setting off down this route in the first place.

When i get home tonight i'll measure the DC offset as James asked. What is the correct, certified by Avondale way to do this Les? I also jotted down waht the bias measurement was doing it your suggested way but don't have it with me at the moment, i'll put it in here later.

What other measurements could I take that would lend further clues and aid you guys to learn more about the problem.

Thanks again for all you help everyone, and Les these boards sounds great, very holographic indeed :)

Stefan
 
Les's charges for a fix could well be less than a few sets of output transistors. I have had to use the service and I got charged very little. It's easy to take the board out and send just that back to base.

I would guess that you might need a oscilloscope to trace the signal through the amp comparing performance with the good channel. The down side is you could have a probe slip and then you've got two damaged channels....
 
It turns out Les' prices are even more reasonable than I thought so it's going back to Daddy for some TLC with a promise I won't Jimmy it up again!!

Thanks for all your help, I've learnt a bit more, I had no idea how to trouble check weather transistors were good or not a few days ago, now I do :)

Stefan
 
Well after the board getting lost in the post, Les finally got to have a look at it. Before he tested it he took out the output devices and tested them, then put them back in and fired her up. All tested fine!! All I can think of is it mast have been a bad solder joint on one of the output devices :(

Very embarrassed.

Anyway, back in stereo now thanks goodness. However listening in mono for a few weeks showed just how good these boards are, there was always a sense of depth to the decent recordings.

Stefan
 
In the B4 troubleshooting post it says to test voltage at the "Junction" of various components. I am really thick and can't work out what constitues a junction when looking at the various test points (1.2.3.4) ?
 
It means, literally, where those two components are joined together on the pcb. So find the components that PD cites on the PCB, find the place where they are connected via the tracks on the board and measure there. Its best, of course, to clip the probe to the appropriate leg of one of the components on the top side of the board if poss. Hope this helps.
 
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