Setting audio amplifier bias current

[tab a]

As mentioned in another thread I put this Dynaco 400 amp together about 30 years ago. I've recently replaced some bad transistors and resistors (and old electrolytic caps, Nigel ). A site I found

**broken link removed**

indicates that I now need to check/set bias current and input offset voltage.

In doing these repairs I found that a previous professional repair had replaced R32 on this schematic

https://i213.photobucket.com/albums/cc93/tharba/dynpc-28.jpg

with a significantly different value. Stock was 750 ohms, in there now is 7500. Both L and R boards are done this way--and the amp seems to have worked fine for years since that was done.

My question is, will that 7500 ohm resistor change mean I need to set the bias current, or input offset voltage, to a different value than specified?

Thanks.

 

[ronsimpson]

R28 & R33 =1K OK
R29 & R32 = 750 & 7500 ????
R30 & R31 = 300 & 3k ????

Plese measure R29 & 32 , R30 & R31.
The 10:1 is not good.

 

[tab a]

R29 and 32 are indeed 750 and 7500.

R30 and 31 are both 300. (I'd written 3k? on the schematic, as one of the repairmen had crossed out 300 and written 3k in my manual on the parts list. The numbers on the schematic are not what I've measured, but are what's spec'd.)

I'm guessing R32 was replaced in the past--it's neither the correct value or tolerance-- and looks nothing like the other resistors--per the kit's part list. I did not replace it at this time. R29, on the other hand, I DID replace a couple weeks ago when I started this. Don't remember for sure what it measured, but it looked original and and wasn't within spec.

Thanks.

 

[ronsimpson]

We need to hear from AudioGuru or Nigel but I do not like you resistor values. (on the other hand it is working)

 

[Nigel Goodwin]

None of those should require any bias changes, set it as the manual specifies.

It's quite possible that the values were changed later on?, it's common to get value changes to either improve performance, or to improve reliability.

 

[audioguru]

I don't see any resistor values on the very poor quality fuzzy schematic.

 

[tab a]

Hmmm.... I seem to have let some smoke out.

I tried adjusting the bias on the R channel. Was perhaps overly cautious: Measured, shut the amp off, moved pot maybe 10 degrees, re-measured. Had moved in wrong direction, so shut it down and moved pot other way maybe 30 degrees from original setting. Powered up, immediately smoked R35, and immediately shut it off.

Found R35 and R37 and R14 bad. All had checked ok before. Also found that R32 probably had developed a poor connection at some point.

Q209, 213, 214 and all 4 output transistors blown.

Did the adjustment of P2 really do all this, or do I likely need to look for another problem?

Found a schematic on the net, of a slightly newer version of this amp, that may be easier to read, than my scanned version.

https://www.electro-tech-online.com/custompdfs/2008/02/st410.pdf

Thanks.

 

[Nigel Goodwin]

If you adjust the bias too high you will probably blow the output devices (and quite possibly other components), in a VERY short time, could be milliseconds.

 

[tab a]

Yeah, milliseconds sounds about right.

I measure the good channel's pot's resistance at about 370 ohms. I'm guessing I set the other at somewhere between 500 and 600. Sound like enough to have done the deed?

Would using either a Variac, or a lightbulb in series, have saved the output transistors?

Expensive lesson, but I don't think I'll forget it!

Thanks again.

 

[Nigel Goodwin]

Probably not, transistor can die so fast!.

The technique is to set the bias to zero before you turn it on, then gradually turn it up whilst monitoring the current - then continue monitoring , and adjusting accordingly for the next 15 minutes or so (it will tend to increase from the cold setting).

 

[audioguru]

I think the age and quality of the bias setting pot can cause trouble when it hasn't been moved for many years then it becomes "crackly" intermittent when it is adjusted.
Carbon track skeleton trimpots are horrible.
Cermet sealed trimpots should be used.

 

[tab a]

Is 'setting the bias to zero' equivalent to turning P2 down as far as possible?

Thanks.

 

[Nigel Goodwin]

Yes, minimum resistance is minimum bias - it's a VERY poor design because of that, if the pot fails the bias goes to maximum.

 

[tab a]

<it becomes "crackly" intermittent when it is adjusted.>

I wondered about that. I just ran it through its range a couple of times while watching resistance on an analog meter. Very smooth without spikes. Would you tend to trust that??

Thanks.

 

[audioguru]

An unused pot that has had many years of current through it and is dirty might be very intermittent the first time it is turned.
An analog meter might not show brief spikes that would kill transistors.