Currently the amplifier now damps its burst of oscillation seemingly triggered by the power-on sequence and it does come out of protection.
However, it seems easy to push back in to a burst of oscillation causing the protection relay to operate and then after a period recovering. Three causes , albeit not consistent, are:
1. When taking voltage measurements touching the DVM probe on many points in the first stage of the power amp.
2. Operating the tone direct button.
3. The presence of an input signal.
I still have the output stage transistors out of the board and am running via a DBT. Is there any possibility that without those transistors in-circuit the reduction in gain results in the quantity of negative feedback being reduced and causing more instability or does the gain/feedback ratio stay the same?
I'm contemplating fitting the new output transistors to see if that helps or worsens the situation.
However, it seems easy to push back in to a burst of oscillation causing the protection relay to operate and then after a period recovering. Three causes , albeit not consistent, are:
1. When taking voltage measurements touching the DVM probe on many points in the first stage of the power amp.
2. Operating the tone direct button.
3. The presence of an input signal.
I still have the output stage transistors out of the board and am running via a DBT. Is there any possibility that without those transistors in-circuit the reduction in gain results in the quantity of negative feedback being reduced and causing more instability or does the gain/feedback ratio stay the same?
I'm contemplating fitting the new output transistors to see if that helps or worsens the situation.
The gain/feedback ratio remains the same.
Don't do that...in fact, I'm thinking how to instruct you to "short" the amplifier to the output from the collector of the second differential if nothing helps.
In the meantime put 10-39pF/100V from the base of Q415(Q416) to the collector of Q419(Q420), do the same from the base of Q417(Q418) to the collector of Q421(Q422) and see if the compensation killed the oscillations.
When I look at how the compensation was applied in the entire amplifier, it can be concluded that the principle remained the same from the first versions with BJT inputs until the new variants with j-fet inputs, Audiolab 8200A for example, which is a bit strange to me...
I still have the suspicion there is something gone amiss - I understand the amp worked before it blew an output transistor so has something changed internally to cause this problem but on both channels?
Environmentally, increased RFI is one suggestion but if not that then what could it be that appears to affect both channels and renders it unusable. The power rails look OK. Running on the DBT is another common factor but perhaps not relevant as Audiolab themselves specify running at 32v for testing.
The feedback loops all go via relay contacts so I was wondering about doing something to exclude them. They read OK on the ohmeter but whether they're "clean" is hard to tell. From what I've read they have been the source of problems but affecting both channels seems less likely.
I'm getting flummoxed now!
Update: I'd been running until with no output transistors, no load and with a 60W lamp (UK 230v AC) in the DBT and was seeing instability when applying an input signal and sometimes after coming out of protection.
It looked like the power rails were sagging a bit even when not oscillating so I suspect that the current limit of the 60W lamp might be too great. I had read about current limits causing this so decided to take a chance.
Having got a 100W lamp the instability has gone away - the amp comes out of protection and is stable. Any input signals no longer upset it. I wonder if I've been chasing my tail for a bit!
Something I did notice that intrigued me - if I apply an input signal of about 8kHz upwards the current draw seems to increase as indicated by an increasing glow on the DBT lamp. Why is this and where is that being dissipated? Is it the Zobel?
That aside I've a 200 lamp to try and then direct mains if all OK. After that I'll put the flyback diodes and new output transistors in and start on the 100W lamp.
It looked like the power rails were sagging a bit even when not oscillating so I suspect that the current limit of the 60W lamp might be too great. I had read about current limits causing this so decided to take a chance.
Having got a 100W lamp the instability has gone away - the amp comes out of protection and is stable. Any input signals no longer upset it. I wonder if I've been chasing my tail for a bit!
Something I did notice that intrigued me - if I apply an input signal of about 8kHz upwards the current draw seems to increase as indicated by an increasing glow on the DBT lamp. Why is this and where is that being dissipated? Is it the Zobel?
That aside I've a 200 lamp to try and then direct mains if all OK. After that I'll put the flyback diodes and new output transistors in and start on the 100W lamp.
New output transistors now fitted and all seems OK. No signs yet of any instability but the bias needs setting followed by a good soak test.
For the output transistors I used MJW21193 for the 2SA1216 and the MJW21194 for the 2SC2922. This meant drilling the heatsink but they're pin-compatible so no big deal.
Would anyone recommend adding more ventilation holes to the case? These amps are known for running very warm as there's no ventilation other than for the main heatsink.
For the output transistors I used MJW21193 for the 2SA1216 and the MJW21194 for the 2SC2922. This meant drilling the heatsink but they're pin-compatible so no big deal.
Would anyone recommend adding more ventilation holes to the case? These amps are known for running very warm as there's no ventilation other than for the main heatsink.
If you are ever going to sell it, do not modify the case or front panel.
In winter, when I heat my house to 65 F or below, insects tend to crawl in holes to get warm. I would not drill holes not covered by a screen and fiber filter. My PA equipment that does have fan entrances, I cover those finger guards with tack coated filters I cut out of rolls from mcmaster.com