The brand new Bourns pot is very smooth from near zero to close to 100ohms (it's a 100ohms trimpot). I fear to take any measurements cause as soon as i turn the amp on, the Bias on the right channel increases form say 60 to 150MV!!!. I can't adjust it, it just runs-away in over-bias!. I did check the trimpot, to make sure i turn the amp on with the minimum resistance.
OK, what I'm going to suggest then is that you turn the bias down on the good channel as well for now and that you move over to using a bulb tester in the primary circuit for the amps safety.
150 mv sounds bad (and obviously is way to much) but if that value is across one of the 0.47 ohms then it is only 300 milliamps or so per pair of outputs and that is non destructive although it would cause a lot of heat generation over a fairly short space of time.
We need to know whether both STV's are generating the same voltage in order to determine where the problem lies.
You also say the good channel was overbiased at the start... so that points to something common or a flaky design somewhere.
Are the rails correct? Nothing like a voltage selector set incorrectly or your mains being high?
150 mv sounds bad (and obviously is way to much) but if that value is across one of the 0.47 ohms then it is only 300 milliamps or so per pair of outputs and that is non destructive although it would cause a lot of heat generation over a fairly short space of time.
We need to know whether both STV's are generating the same voltage in order to determine where the problem lies.
You also say the good channel was overbiased at the start... so that points to something common or a flaky design somewhere.
Are the rails correct? Nothing like a voltage selector set incorrectly or your mains being high?
Just to add that one other option might be to make a Vbe multiplier using a TO126 outline transistor fixed to the heatsink. One transistor and one preset mounted on the transistor (for the economy version) and it should be good.
Mooly, did as you told me, checked my mains; 118VAC and there are no provisions for other voltages on this amp as it's a north American one.
Tried a first shot with a 40watt bulb, on my DBT, then 60, 100 and finally, 200watt bulb, amp bias is PERFECT on both channels!!!. Don't ask me what happened yesterday, i have no clue and not sure i want to know either! 🙂 .
I can bias it to 17MV both sides, after 15mins, pots full out ( for max bias) but, i know it will be ok with all the covers on and let it cook for 10mins.
I hope i don't have an intermittant bug, i will check and re-check again but, THANK YOU SOO MUCH!!!!!!!!!!!!.
I will report back in a day or 2.
Tried a first shot with a 40watt bulb, on my DBT, then 60, 100 and finally, 200watt bulb, amp bias is PERFECT on both channels!!!. Don't ask me what happened yesterday, i have no clue and not sure i want to know either! 🙂 .
I can bias it to 17MV both sides, after 15mins, pots full out ( for max bias) but, i know it will be ok with all the covers on and let it cook for 10mins.
I hope i don't have an intermittant bug, i will check and re-check again but, THANK YOU SOO MUCH!!!!!!!!!!!!.
I will report back in a day or 2.
Not so fast 🙂 The bulb is to provide protection as much as anything else while we try and diagnose the problem.
If the bias happens to now set correctly then that might be because the rails are just that little bit lower because of the bulb, and so the current in the STV might be a little bit lower (and so less bias voltage across the STV is available).
If the bias happens to now set correctly then that might be because the rails are just that little bit lower because of the bulb, and so the current in the STV might be a little bit lower (and so less bias voltage across the STV is available).
Yeah, i did not mention that after all the DBT tests, i did connect to mains. Tried again today and what happens is; when first turned-on, the right channel bias goes all the way past 60MV and when the relay clicks, the bias starts to go down moderately fast to, lower then 18MV. Then it starts to go up again, reaching 18MV before the Left channel does.
I re-did the DC offset and the Bias. Decided to try to amplify tuner music and: a loud 60HZ buzz in both channels now..................
First time it does that to me. This amp, is driving me nuts!.
I re-did the DC offset and the Bias. Decided to try to amplify tuner music and: a loud 60HZ buzz in both channels now..................
First time it does that to me. This amp, is driving me nuts!.
When i got the amp, it had moved around the packaging, surprisingly... I opened it up to see if everything was allright and noticed that a grounding ceramic capacitor, under the amp, was broken off the board. I could never find where it's broken from, not on the schematics, nor on the board drawings. The amp did NOT buzz tho when i first tried it...
It sounds like the amp has a bit of history then 😱
The bias sounds like it could be OK then and just needs a few moments for the diodes and output stage to begin to reach some kind of thermal tracking together.
The buzzing should be easy to find with a scope. Check all rails first and then just trace a known signal through the amp. Seeing if the buzz responds to the volume control sort of splits the amp in half to front end or output stages.
The bias sounds like it could be OK then and just needs a few moments for the diodes and output stage to begin to reach some kind of thermal tracking together.
The buzzing should be easy to find with a scope. Check all rails first and then just trace a known signal through the amp. Seeing if the buzz responds to the volume control sort of splits the amp in half to front end or output stages.
Ok, found where that broken wire ground capacitor was going; lucky pin 13 on control board!. Once this fixed, gone is the buzzing!. Why it did not buzz when i first got the amp? search me!. The Biasing is fine now but just a tad low, on both channels. I'm ordering a 22ohms and a 24ohms parallel resistors and i should be in the trimpots ranges. Now, they are pegged, and i reach around 16MV. Keep my fingers crossed, to be done with that biasing problem!.
Again, THANKS Mooly!!!
Again, THANKS Mooly!!!
You're welcome 🙂
(with the bias and I would say always aim a little on the low side rather than high. Let the amp warm up and recheck. Anything over a milliamp or two will remove any audible distortion)
(with the bias and I would say always aim a little on the low side rather than high. Let the amp warm up and recheck. Anything over a milliamp or two will remove any audible distortion)