I would appreciate some help. I have a line stage amplifier anno 2003 based on super reg (allen wright design) and using a 7308 per channel. One channel was dead - I swapped the valves, expecting the fault to be with the valve - which it was, however, immediately after swapping and power on there was a short (spark) in the valve (now in the good channel) - now both channels are dead. I assume (a) I need new valves and (b) something in the power supply has blown. The question is what? What would an anode short to earth do - blow the top IRF710? Or part of the superreg? There was no heating, no smell, no blown fuse. The valve heaters still glow. No sound at all from either channel now.
Where should I start?
Where should I start?
Start by checking ALL the supplies. Confirm the 10, 20 and 210 volts are present. There is a also a relay voltage shown (12 volts) but that doesn't seem to be derived from what I can see.
There are some errors on the diagram to, the 220 volts magically becomes 210 and - (minus) 210 is shown on the ground connection. The 15 volt bias I assume is the 10 ? volts on the PSU.
Despite the inconsistencies, see if any are missing. The relay drive is favourite to look at too as that would kill the audio if missing.
There are some errors on the diagram to, the 220 volts magically becomes 210 and - (minus) 210 is shown on the ground connection. The 15 volt bias I assume is the 10 ? volts on the PSU.
Despite the inconsistencies, see if any are missing. The relay drive is favourite to look at too as that would kill the audio if missing.
Thanks - yes, there are errors. The circuit diagram came with the amp - yes, I assume the bias is actually 10v, shown as 15 and I don't know why there is a spurious -210.
Measurements, both channels -
222v (as marked on PCB layout) - so the superreg is working
20v
10v bias
I can see the heaters glowing, so I assume the 6.3v is OK
12v for the relays present - I can hear them clicking if I change inputs and I can hear the mute relay switch a minute after switch on, as it should.
- all OK.
So - both valves are faulty? and the fault has blown one IRF710 on each channel?
Measurements, both channels -
222v (as marked on PCB layout) - so the superreg is working
20v
10v bias
I can see the heaters glowing, so I assume the 6.3v is OK
12v for the relays present - I can hear them clicking if I change inputs and I can hear the mute relay switch a minute after switch on, as it should.
- all OK.
So - both valves are faulty? and the fault has blown one IRF710 on each channel?
Sounds more like the one faulty valve has damaged both channels. If you know which was the good valve then use that to fix both channels initially. The two FET's and the 15 volt zener are cheap enough to just replace with new. For you to see a spark in the valve means current has flowed through a low impedance path. Depending on how the valve failed internally, well there is one obscure path via the 100 ohm grid resistors and the volume control wiper to ground. Its only a minutes job to check the resistors and that the volume pot wiper has correct continuity.
I've checked the volume pot (dact stepped attenuator). Ok. Can someone explain why the fet or the Zener diode might have been damaged?
I thought we concluded it was most probably a faulty valve because you swapped valves between channels and the good channel then failed. You saw it spark l think which means an inter electrode short might have occurred. Even a brief over-volts condition on a semiconductor will cause instant failure.
OK, thanks for your help. After putting in new IRF710s, 12v zener and new tubes, both channels are working on the bench with a 20 Euro digital amp for testing, but I have a strange problem - one channel does not work when plugged into my valve amp. I have swapped leads to check it is not the lead. There is no sound at all from one channel - another sympton that was not there before is a thump when the mute relay opens (after 1 min) - that was not the case before. I checked the boards thoroughly for shorts, and all connections are as they were previously. Any thoughts?
I would look at the 4.7uf coupling cap.
With the amp on and not connected to anything there should 0.0000 volts DC across the 470 k resistor at the output.
Also check your main valve amp inputs and confirm that there is 0.000 volts DC as measured across the input sockets of the main amp.
With the amp on and not connected to anything there should 0.0000 volts DC across the 470 k resistor at the output.
Also check your main valve amp inputs and confirm that there is 0.000 volts DC as measured across the input sockets of the main amp.
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