Yes I wasn't sure if you had done it, that is fine, it was the failed channel just to be sure?
Yep. The four traces are from each rail so you can compare good channel + and - to bad + and -. They're are all, obviously, pretty much identical. (For some reason the server changed the + symbol in the file name--probably a reserved character or something-- so it looks like they are all from the - side.)
So we are agreed the issue cannot be the PS and it can be eliminated.
Yes, just keep in mind that if you slip with a tool and hit a supply rail or whatever as far as an accident goes you can play it safe and try the PS again. It is easy to do, you don't need to load it given the symptom.
Well, well. I pulled one leg of T1 and the oscillation stopped. I was able to put about 65v on the rails without issue.
This was with the amp in the same state as yesterday:
Q1, Q2 pulled
Q17-19 out of circuit
Q7-C to + rail
So that brings us back to Q4, right? Which I believe I have to test out of circuit. Are there any other components I should suspect and test?
We've gotta be getting close now...
This was with the amp in the same state as yesterday:
Q1, Q2 pulled
Q17-19 out of circuit
Q7-C to + rail
So that brings us back to Q4, right? Which I believe I have to test out of circuit. Are there any other components I should suspect and test?
We've gotta be getting close now...
And Q4 s base voltage reference is provided by....yes, did you guess?..
Really amazing...thank you, i had a big laugh..
Last edited:
This is not making sense but at least it is progress. I would suggest jumping across P1 to be absolutely sure that the bias is zero and also jumper across R7 to stop any possible oscillation in the VAS. It would be good if you can just tack T1 back in to turn on Q4 and test for oscillation. If it still oscillates open T1 again and continue below, leave the jumpers in for now.
It may seem like Q4 but remember that the circuit is not set up as it was designed so I would not worry about Q4 at this point - you did replace it as I recall. I would test the resistors around Q4 while you have it out and one lead is free on the resistors. Have your replaced or tested Q3 and Q6? I'd like to see it go to 75 V, then tell us the output emitter resistor voltages (across them) as you put the output stage back together - if any of them are non-zero.
My suggestion would be to put the output stage back together testing it along the way. Keep the bias pot at min resistance until we are ready to bias up the output stage. The DC tests would be to see if it can swing to the rails without leakage, then to bias it up and see if the transistors share current reasonably. Next would be to see if it can drive a low Z load and if it continues to share current reasonably, what do you have for power resistors to use for a dummy load?
Last edited:
There's a schematic at post #176.
However, here's an off-topic question. For a fee ($20), Adcom sent me a pdf version of basically everything they have on the 555, including data on variants of the amp. Since I paid for it, I'm not sure if it would be okay to re-post all the files here. I really have no idea how copyright works on this kind of stuff. Does anyone here know?
In anticipation of replacement, can anyone suggest a good substitute for 2SA1207 (Q6)? Can't find them at any of the usual places.
Amazing, I wonder how many pennies they were trying to save ... it is a 160V part. Your rails were at 84 for a total of 168 exceeding the spec, it is a 70 mA part but will only take 4mA at 160V without temp derating so that is on a cool day. Service manual indicates grades with beta from 100 to 280 so it is not critical at all. The same transistor as the VAS should work fine, or the Fairchild replacement that I suggested. Certainly a part with a 180 to 200V rating to provide some margin for line transients.
Q6 is a TO-92, the VAS (Q7, right?) is a TO-126, as is the Fairchild KSA1142. How about this SC51?
Sanyo Transistor Sc-51 -200v -.1a 1W ECB | Distributed By MCM | SANYO
Removed and tested a bunch of parts today that are near Q4. None were bad.
R14
R15
R16
D5
D6 (okay, wahab?)
Pulled Q3, it has 220hfe and no leakage.
I tested P1, it is 0 ohms when turned all the way CCW, 998 ohm full CW. I'll jump it for further testing if need be.
Ordering parts and shotgunning the remaining components is starting to look pretty tempting...
R14
R15
R16
D5
D6 (okay, wahab?)
Pulled Q3, it has 220hfe and no leakage.
I tested P1, it is 0 ohms when turned all the way CCW, 998 ohm full CW. I'll jump it for further testing if need be.
Ordering parts and shotgunning the remaining components is starting to look pretty tempting...
2sa1207?
Q6 is a 2SA1270..according to the pdf you have referenced. Don't know if the 2SA1207 noted is a typo or that's what you were looking for.
In their printed catalog, B&D Enterprises says 'call for sub'.
www.bdent.com
I've purchased a good number of parts from them I couldn't find elsewhere.
This is a most educational thread. Thanks, Fred, for your tenacity..and to all those with continuing helpful input.
Q6 is a 2SA1270..according to the pdf you have referenced. Don't know if the 2SA1207 noted is a typo or that's what you were looking for.
In their printed catalog, B&D Enterprises says 'call for sub'.
www.bdent.com
I've purchased a good number of parts from them I couldn't find elsewhere.
This is a most educational thread. Thanks, Fred, for your tenacity..and to all those with continuing helpful input.
Last edited:
Yes that part is fine as long as you get E or F grade the others are much lower beta. Excellent 300V, 100 mA, 7mA at 150V, rest of the specs are very similar Ft, Cob, etc. Perhaps someone will take a closer look but it seems completely fine.
You mention docs for different revisions, I'm starting to wonder if your unit matches the schematic that we are looking at.
The work will be worth it if you find that you don't have to replace the output devices. Off the top of my head I would say that I'd be surprised for the outputs to fail given that they are fused at 6A, but others report failed outputs so it does seem to happen. Really the work is worth it since it is a good learning experience, but I have to say I thought we'd be done by now, lol! It is not you, this is a very strange problem. I have to mention that we all make mistakes so carefully check the orientation of any new devices that you installed. There are some crazy things like not all TO-92 devices having the same pin out - the same part from the same manufacturer will be the same of course. It is usually something subtle when things don't make sense.
Last edited:
As best I can tell, there are two revisions of the board, but the schematic appears to be the same. The only differences I have found are the upgrade of C3 to 100v from 10v and the orientation of Q4. Admittedly, I have not gone through part-by-part, or traced the whole circuit.
Other than the drivers, no parts have been subbed (i.e. same p/n for all semis), and I've checked and re-checked my orientation, even double-checking against photos I took of the original board before work started (a safety measure I try to take when I'm about to work on something I don't entirely understand!). It's always possible I keep missing the same thing, I suppose.
Here is a site with that transistor.
Part Number Search Results
Also, check out findchips.com. I like how the site searches 20 other sites when looking for parts.
Hmm, on the schematic it's a 1270, but on the parts list it's 1207. The installed part (both channels) is a 1207. I'm guessing a typo on the schematic.
You're welcome, it's certainly been educational for me. The wife uses a different word than "tenacity"... 😉
Without reading every post, let me see if I understand this
situation. The repaired amp oscillates on the positive half
without a load when the rail voltage gets up to 50V
or until the rail gets to 50V.
I'm betting that it's when the rail gets high.
You should check it with an 8 ohm load to see if it still
oscillates. If so, then it's probably time to adjust the frequency
compensation upward.
😎
situation. The repaired amp oscillates on the positive half
without a load when the rail voltage gets up to 50V
or until the rail gets to 50V.
I'm betting that it's when the rail gets high.
You should check it with an 8 ohm load to see if it still
oscillates. If so, then it's probably time to adjust the frequency
compensation upward.
😎