Good Gnews and Bad Gnews w/ my F5 - help?

[CBRworm]

The good news is that I got one channel almost 'together' and it sounds pretty good even though it doesn't look too hot yet.

I am having two issues that I assume are related. 1st issue is that the bias drifts on the Q3 side. The thermistor changes resistance when I put my hot finger on it. I removed R15 and R3 and measured their values - 2.2k. Q4's bias tracks very well from cold to hot. Q3 will start out cold at over .7 volts across R11 and come into range pretty quickly within a minute - there is an offset at the output during this imbalance. I also verified the resistance of both R11 and R12 - they are the same, although they measure .6 ohm instead of .47. On both sides the thermistors are glued to the transistor body.

The second issue is that when I turn up the volume - once the voltage across R3 and R4 just comes off idle (coming out of class A?) I get bad distortion that sounds like the scope shot below looks. I don't know if that is the current limiting coming into play? it seems kind of rough - it looks like what I would see from a broken amplifier. This is with an 8 ohm speaker connected.

I guess I could pull Q5 and Q6 out and see if the distortion goes away but I figured I would check for input here first since I figure both problems are probably related.

I am using 2Sk170/2SJ74's and FQA19N20c/FQA12P20 and PDaniel boards.



Any thoughts? Thanks!

[DigitalJunkie]

The scope shot looks like it's bursting into high frequency oscillation to me..I'm no pro on the F5 though,so we'll see what the others think.

[CBRworm]

Also - I have a 22K resistor at R21.

[Nelson Pass]

I think I see capacitors on that channel board.

[CBRworm]

In it's near future?

[CBRworm]

Hmmm. . . I seem to have lost the ability to edit my posts. Too many edits in one day maybe?

I will try some caps in the morning and see if I can tame the beast.

[CBRworm]

1.6mhz is the frequency it bursts at.

Putting a cap across R10 seems to quell it. Now to figure out the ideal size.

[CBRworm]

Ok, that's working. Any ideas about the cold bias on Q3? If not I will live with it until I build the next channel and see if it behaves the same way.


Thanks!

[Nelson Pass]

It seems to me

If I say "I think I see capacitors on that board"

You are supposed to respond with an explanation as to
what those capacitors are / or not are.

[rkevans]

Nelson Pass:
what those capacitors are / or not are.

From the description (Fairchild devices) and a few other clues (vertical resistor mounting, R5..R8 replaced with a pair of 50 ohm Dale 6.5W units, etc) -- I'm going to predict that you're seeing a tech-diy parts kit.

Which means the big white items at R11 and R12 are Ohmite TWW5 resistors. http://www.ohmite.com/cgi-bin/showpa...tww_twm_series

Rick