Hi All,
I've just had the primary winding in the transformer for this amp go u/s, no continuity, through white and green wires. Has anyone successfully transplanted a suitable replacement transformer and if you could you let me know what you used? Other then that has anyone has transformers rebuilt and if so any suggestions who might be able to do one like this one:
Many Thanks in advance,
Joe
I've just had the primary winding in the transformer for this amp go u/s, no continuity, through white and green wires. Has anyone successfully transplanted a suitable replacement transformer and if you could you let me know what you used? Other then that has anyone has transformers rebuilt and if so any suggestions who might be able to do one like this one:
Many Thanks in advance,
Joe
There is no schematic diagram. You could buy a replacement toroid transiformer if you know the voltage and wattage. For class AB amps with split supply the output voltage is typically half the single rail voltage. You can guess the rail voltage at 80% of that on the rail capacitor label. Or you can guess the rail voltage from the doubling the voltage calculated from the nominal wattage. V=sqrt(W*Z) where Z is speaker impedance the rating was quoted at. For 70's products you can assume speaker impedance was 8. So if amp is 25 W/ch rated V is sqrt (25*8) or 14.1 v. So twice that for class AB losses is 28 v rails. double 25 watts is 50 watts. Double again for two channels. so you need a transformer with 100 VA of two 28 v windings. Or 56 VCT in an e-core transformer. If wattage of amp was really stereo 12 watts, then you calculate for that.
Buy a mount kit with the transformer. Toroids with two separate windings, you tie the middle of the two together to make a center tap. If this amp also has lower voltage for the input stage or tone controls, vendors like antekinc.com sell transformers with +-15 v windings in addition to the rail wattage windings.
You need to check the esr & leakage of the rail caps to see if leakage burnt out your transformer. Especially if they date from the 1970's. Some products didn't have a mains fuse. If this doesn't have one, install one suitable for the VA rating of the transformer.
Safety for newbies. Don't touch metal over 25 v with two hands, voltage >25 across the heart can stop it. Use an alligator clip lead on the negative probe of your meter, not two hands. Don't touch the mains part of the amp with the power plug in the wall. Wear no jewelry on hands wrists or neck. Current at high amperage from 1 v can burn the flesh under a ring to charcoal. Wear safety glasses. Parts can explode and solder can splash in your eyes, especially when desoldering.
Buy a mount kit with the transformer. Toroids with two separate windings, you tie the middle of the two together to make a center tap. If this amp also has lower voltage for the input stage or tone controls, vendors like antekinc.com sell transformers with +-15 v windings in addition to the rail wattage windings.
You need to check the esr & leakage of the rail caps to see if leakage burnt out your transformer. Especially if they date from the 1970's. Some products didn't have a mains fuse. If this doesn't have one, install one suitable for the VA rating of the transformer.
Safety for newbies. Don't touch metal over 25 v with two hands, voltage >25 across the heart can stop it. Use an alligator clip lead on the negative probe of your meter, not two hands. Don't touch the mains part of the amp with the power plug in the wall. Wear no jewelry on hands wrists or neck. Current at high amperage from 1 v can burn the flesh under a ring to charcoal. Wear safety glasses. Parts can explode and solder can splash in your eyes, especially when desoldering.
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Hi Indianajo,
many thanks for your detailed reply - I think I may have solved it - the Voltage selector was damaged, so I removed it and had planned to set the amp up on a 240V supply - but missed the fact that the 220V and 200V supplies from the selector are connected ( at the selector!). So when I removed them I broke the connection for the whole thing. Hence Dead transformer. I checked the connections on another one and indeed there is continuity through all 'junctions', I've now replicated this on the 'duff' transformer and at the moment I have regained continuity through the primary 240V windings.
I'll replace it in the amp over the weekend and hopefully have power.
These Transformers do hum though - some more than others, so I think I should look into getting some details on them to ensure a replacement can be sourced. The Secondaries supply 26VDC and 31VDC from what I know as this is whats passed to the Power amp boards. There is a separate 31VAC on this side of the transformer output for the 2 channel audio pre-amp, but thats not totally necessary a seperate PSU could be worked up for that. The transformer does have 2 ground wires too. The Amps are rated at 25W per channel into 8 ohms or 40W into 4 ohms, the first 11 watts of which are Class A. I'm not sure how that works into your calculations above? Additionally, I did fit an IEC with fuse drawer so the Mains fuse is a 2A Slow Blow.
Kind Regards
Joe
many thanks for your detailed reply - I think I may have solved it - the Voltage selector was damaged, so I removed it and had planned to set the amp up on a 240V supply - but missed the fact that the 220V and 200V supplies from the selector are connected ( at the selector!). So when I removed them I broke the connection for the whole thing. Hence Dead transformer. I checked the connections on another one and indeed there is continuity through all 'junctions', I've now replicated this on the 'duff' transformer and at the moment I have regained continuity through the primary 240V windings.
I'll replace it in the amp over the weekend and hopefully have power.
These Transformers do hum though - some more than others, so I think I should look into getting some details on them to ensure a replacement can be sourced. The Secondaries supply 26VDC and 31VDC from what I know as this is whats passed to the Power amp boards. There is a separate 31VAC on this side of the transformer output for the 2 channel audio pre-amp, but thats not totally necessary a seperate PSU could be worked up for that. The transformer does have 2 ground wires too. The Amps are rated at 25W per channel into 8 ohms or 40W into 4 ohms, the first 11 watts of which are Class A. I'm not sure how that works into your calculations above? Additionally, I did fit an IEC with fuse drawer so the Mains fuse is a 2A Slow Blow.
Kind Regards
Joe
By way of an update:
On the subject of hum / buzz:
I did check a couple of my amps and on those a few the main 26V wires from the transformer ( big single core wire) had shown signs of actually fracturing the solder joint at the PSU pcb. Initially I just re flowed these and solved the problem to some extent.
As a more permanent fix, later I attached flexible tails of suitable wire to each of these wires and then soldered those to the board. so the mechanical vibration of the transformer is somewhat damped. This was less of a problem with later amps with screw down connectors, but in the end they all got the treatment. I also noted that the sleeves had become quite brittle with age. So I gently heated it with a heat gun before moving the wires, any sleeve that just broke away was replaced with heat shrink.
Joe
On the subject of hum / buzz:
I did check a couple of my amps and on those a few the main 26V wires from the transformer ( big single core wire) had shown signs of actually fracturing the solder joint at the PSU pcb. Initially I just re flowed these and solved the problem to some extent.
As a more permanent fix, later I attached flexible tails of suitable wire to each of these wires and then soldered those to the board. so the mechanical vibration of the transformer is somewhat damped. This was less of a problem with later amps with screw down connectors, but in the end they all got the treatment. I also noted that the sleeves had become quite brittle with age. So I gently heated it with a heat gun before moving the wires, any sleeve that just broke away was replaced with heat shrink.
Joe