Trouble with unstable bias setting.

[ppfred]

Hi y'all,

I reworked a Dynaco ST150 a while back and in the end was unable to stablize the output bias current. It was all over the place. After blowing numerous fuses & a few transistors in trying to find the problem, I shelved the project. (2 years ago)

I have now calmed down and would like to get the amp up and running. So the question (for now) is:
Where and what should I look for to try to stablize the bias current? Is there a specific problem area or should I be worried about the whole circuit?

Thanx for any help and/or suggestions.
Fred p.

 

[lineup]

Usually there is some temp sensing component
that senses heat of heatsink.
If get hot, then reduces bias,
if get colder, then increase bias current.

This balances and stabilizes current.

This sensor can be a little resistor that changes resistance by temperature.
Can also be a transistor near to or mounted at heatsink.

If such a sensor, for some reason, is not enough close to heatsink
this control may not work properly.

 

[Eva]

If you have done too many changes or component substitutions, the result may be an instable amplifier. Note that when an amplifier is oscillating it's almost impossible to adjust bias properly.

Do you have an oscilloscope in order to check stability?

 

[anatech]

Hi Fred,
Eva's right. There are a few reasons why bias may be unstable. Everything from oscillation to intermittent connections. You need to rule out oscillation, for that you need an oscilloscope.

-Chris

 

[jblmar]

You need to know where the Bias is produced, then check the associated components. Since you have 1 working channel, use it as a reference. Don't overlook the bias VR (if your amp uses one). It may be intermittent. If so, replace it or clean with Deoxit. Try tapping the VR while monitoring the the DC offset with the DMM at the speaker terminals. An intermittent will show up that way.

Ron

 

[anatech]

Hi Fred,
P1 is your DC offset (near Q1 & 2), P2 is the bias control. Transistors Q6 & 7 are related to the temperature compensation (bias). Any broken connections or intermittent contact in thse areas (except P1) will give you grief.

If you replaced some parts, can you post the numbers you put in and their positions?

I still think you need to check for oscillation with a 'scope.

-Chris

 

[ppfred]

Hi people,
Thanks for the quick replies. Your responses were exactly what I was looking for, i.e., where do I start?

Well, for the matter of component changes, I stripped the board completely and replaced all the parts including substituting transistors - probably my biggest downfall. So I can now safely presume that the amplifier is unstable, besides all else.

I do have an oscilloscope that I picked up on E-Bay (cheap!). Now, how do I check for oscillation? Any and all pointers would be appreciated.

This whole re-build project was started as a learning experience and experiment. I guess I'm well on my way.

Thanks again for any and all help.
Fred p.

 

[anatech]

Hi Fred,
Connect the 'scope probe to the output of the amp. If the trace becomes thick (0.5 mS / div, 5V / div) the amp is probably oscillating. I would expect the heatsinks to get warm to hot in that case. Some amps will not oscillate until the bias is increased beyond a certian point.

-Chris