K7sparky
Active Member
Low gain is not your problem, however the transistor still might be bad.
1st guess is still oscillation
Time to check for heat just after the fuse blows:
1) A transistor is going into thermal runaway and will be hot
2) Oscillation with no speaker connected. The resistor in series with cap across output (10 ohm 1W) will be hot.
If fuse blows fast enough you still might not feel any heat in the 10 Ohm, but a transistor should be hot unless it has thermal failure just above ambient.
Food for thought before blowing another fuse.
Is the bulb still in series with one side of line?
A lower watt bulb will result in a lower current limit and may save the fuse. If you can stop the fuse from blowing check the 10 Ohm for heat (oscillation & may cook) then start shooting the transistors with freeze mist.
From one of your first posts I think the idle current was about 60ma at rail voltage which is about 4W. Try a 10 or 15W bulb. Remember it will take awhile for your caps to charge with that low a bulb.
I don't know if you have a lab power supply with adjustable current limit. I would start with 60 to 100ma if I was using a limiting supply in place of the internal bulk supplies.
The main filter caps still might have enough stored energy to blow fuse depending on cause. Since you are not driving speakers, put a resistor in series with one cap lead for open circuit testing only after trying a smaller bulb.
I saw a picture that had a partial of your test bench and you appear to like electronic stuff. Variable supplies abound and are cheap on eBay. You can series the outputs of multiple supplies and get higher voltages and are limited by the lowest amp range supply. If several have adjustable current limits you can use them as toppers.
Older lower bandwidth scopes are also cheap and are a good addition.
1st guess is still oscillation
Time to check for heat just after the fuse blows:
1) A transistor is going into thermal runaway and will be hot
2) Oscillation with no speaker connected. The resistor in series with cap across output (10 ohm 1W) will be hot.
If fuse blows fast enough you still might not feel any heat in the 10 Ohm, but a transistor should be hot unless it has thermal failure just above ambient.
Food for thought before blowing another fuse.
Is the bulb still in series with one side of line?
A lower watt bulb will result in a lower current limit and may save the fuse. If you can stop the fuse from blowing check the 10 Ohm for heat (oscillation & may cook) then start shooting the transistors with freeze mist.
From one of your first posts I think the idle current was about 60ma at rail voltage which is about 4W. Try a 10 or 15W bulb. Remember it will take awhile for your caps to charge with that low a bulb.
I don't know if you have a lab power supply with adjustable current limit. I would start with 60 to 100ma if I was using a limiting supply in place of the internal bulk supplies.
The main filter caps still might have enough stored energy to blow fuse depending on cause. Since you are not driving speakers, put a resistor in series with one cap lead for open circuit testing only after trying a smaller bulb.
I saw a picture that had a partial of your test bench and you appear to like electronic stuff. Variable supplies abound and are cheap on eBay. You can series the outputs of multiple supplies and get higher voltages and are limited by the lowest amp range supply. If several have adjustable current limits you can use them as toppers.
Older lower bandwidth scopes are also cheap and are a good addition.
