An unexpected and unwanted road trip has come up and preparing for it has been consuming my thoughts and my time. I must leave in 48 hours and will be gone for a week. Consequently I cannot pursue things with this amplifier untill I return.
At present, all 8 output transistors are removed - 4 from the left channel and 4 from the right. With absolutely no output transistors in place, the DBT glows steadily when the variac reaches about 50 volts.
Jumper between C and E of Q404 is still in place.
If it will be of any benefit, I am prepared to reinstall whatever output transistors I am advised to and resume voltage checks with variac at full power and DBT in place. I suspect this will cause the DBT to glow at it's maximum brightness. And perhaps blow the fuses.
Regarding normal background current draw.
Before the bias pot let go and caused whatever damage has occurred, the DBT went dark immediately after amplifier power was switched on. At idle, there did not seem to be enough current draw to make the DBT produce any visible glow. At least not under normal room lighting. In a dark room, there may have been a perceptible, feeble glow but I did not observe it under that condition.
If there are any further suggestions, I will gratefully accept them but cannot act on any until I get back home. I appreciate the patience.
At present, all 8 output transistors are removed - 4 from the left channel and 4 from the right. With absolutely no output transistors in place, the DBT glows steadily when the variac reaches about 50 volts.
Jumper between C and E of Q404 is still in place.
If it will be of any benefit, I am prepared to reinstall whatever output transistors I am advised to and resume voltage checks with variac at full power and DBT in place. I suspect this will cause the DBT to glow at it's maximum brightness. And perhaps blow the fuses.
Regarding normal background current draw.
Before the bias pot let go and caused whatever damage has occurred, the DBT went dark immediately after amplifier power was switched on. At idle, there did not seem to be enough current draw to make the DBT produce any visible glow. At least not under normal room lighting. In a dark room, there may have been a perceptible, feeble glow but I did not observe it under that condition.
If there are any further suggestions, I will gratefully accept them but cannot act on any until I get back home. I appreciate the patience.
It sounds like something else is amiss to me.
I would look at the voltage on each main rail, the positive and negative rails which would go to the collector of the output transistors. Each rail should be equal and opposite (polarity wise). See what voltage each comes up to when the bulb is bright.
Do not fit the outputs, no point at this stage and it might confuse things.
Perhaps the original faults have taken a diode out in a bridge rectifier
That suggests something has happened beside a normal output stage issue. Check the rail voltages first with the bulb lit and report back what you measure.