troubleshoot after recap

[hebaton]

...i don't think the bias ajustment pot will allow that much range, i think I was already wel past the half on the pot...

In that case you would have to adjust the value of a resistor in the bias circuit. As already noted, someone has already modified it to convert from bias balance to bias level, and added a resistor in parallel to an existing one to adjust the range.

do you have enough info to suugest a value ?

[johnnyreece]

It may be that replacing those caps will give you more voltage to work with. No guarantee, but if your old caps are tired, it could be a possibility.

Edit: Of course, I'm not trying to contradict Martin, who absolutely has you covered. Just throwing out something that seems to have been forgotten, and, while not the most likely culprit, may still be a factor.

[Stevem]

I have not read all the way thru this string, but just to clarify, the A270 does not have a overall bias adjust, it has a bias balance and on the center of that bias balance pot the schematic calls out a -44 volts and from there you adjust the pot for the least idle hum heard out of the speaker.

[hebaton]

I have not read all the way thru this string, but just to clarify, the A270 does not have a overall bias adjust, it has a bias balance and on the center of that bias balance pot the schematic calls out a -44 volts and from there you adjust the pot for the least idle hum heard out of the speaker.

I appears this was modded to be a bias adjust at some point.

[johnnyreece]

I have not read all the way thru this string, but just to clarify, the A270 does not have a overall bias adjust, it has a bias balance and on the center of that bias balance pot the schematic calls out a -44 volts and from there you adjust the pot for the least idle hum heard out of the speaker.

It's easy to miss, but it was mentioned that the bias circuit was modified.

Edit: Looks like hebaton beat me to it, picture and all!

[martin manning]

hebaton, pull the power tubes out and see what range you can get for the bias voltage rotating the bias pot from stop to stop. Measure that at the bias pot lug with the 68k connected to it. Unless some resistor has drifted, you should have enough range there.

[hebaton]

hebaton, pull the power tubes out and see what range you can get for the bias voltage rotating the bias pot from stop to stop. Measure that at the bias pot lug with the 68k connected to it. Unless some resistor has drifted, you should have enough range there.

THe resistor on the cap side reads 3K5 the one going to the board reads 75K

[hebaton]

hebaton, pull the power tubes out and see what range you can get for the bias voltage rotating the bias pot from stop to stop. Measure that at the bias pot lug with the 68k connected to it. Unless some resistor has drifted, you should have enough range there.

You mean that I measure at that log referenced to ground ?
That gives me a range from -37 to -51v

[martin manning]

you mean that I measure at that log referenced to ground ?

Yes, you should get a range of something like -20 to -45V there depending on the pot setting.

[hebaton]

So why am-I getting -37 to -51 ?
Could this have to do with the caps on the other side. And what about that resistor that reads 75K should I replace that ?

[martin manning]

Which one reads 75k? carbon comp resistors are known to drift up in value, so that is likely what is happening.

[hebaton]

Which one reads 75k? carbon comp resistors are known to drift up in value, so that is likely what is happening.

the one on the log where you told me to measure, going to the board. SO it should be 65K 1/2 watt ?
I don't have that but I may ahve 30K and 35K I could put in series

[martin manning]

That one won't matter for the bias, and it's not that far off... 75 vs. 68 is only 10.3%, just outside of its tolerance. The ones that matter are the 1k, the 3k3, and the two that are grounded on the pot case. Do you have new caps in the bias supply?

[hebaton]

I am expecting them to be delivered today ( new caps ) but as of now, still the old ones in place.
THe resistor going to the caps side reads 3K5 the two on the pot read 11K together

Where is the 1K you are refering to

[martin manning]

On the other side of the bias supply rectifier, on the small board. To measure them accurately you should lift one end, but with the red meter lead next to the rectifier you should get a good reading for the 1k. Try it both ways and you'll see the difference.