Thermal fuses in transformers are a pain, we used to get many such failed components, not on quality gear it must said, but a pain non the less.
Safety has to be considered and shorting it out can't be recommended. However that leaves the same problem for any generic replacement which almost certainly won't have such a fuse anyway. If the fuse is the sole means of protection then there is a safety issue. By that I mean if the secondaries are unfused (no "normal" fuses present on the output) then a short would overheat the transformer.
What I would do here is test the amp before doing anything. Just solder a bulb across the thermal fuse. Two wires soldered directly to the bulb. Put a cardboard or plastic box over the bulb for safety and test the amp.
And then take it from there.
Safety has to be considered and shorting it out can't be recommended. However that leaves the same problem for any generic replacement which almost certainly won't have such a fuse anyway. If the fuse is the sole means of protection then there is a safety issue. By that I mean if the secondaries are unfused (no "normal" fuses present on the output) then a short would overheat the transformer.
What I would do here is test the amp before doing anything. Just solder a bulb across the thermal fuse. Two wires soldered directly to the bulb. Put a cardboard or plastic box over the bulb for safety and test the amp.
And then take it from there.
I've seen capacitors in these amps that look just like that, and they're OK. So i would say they're fine.
A few things to bear in mind:
* The output MOSFETs are connected directly to the heatsinks with no insulators. So those heatsinks will have +/- 35V on them. Be careful if you do any tests while powered up.
* The frontend has a pre-regulator to produce a slightly higher voltage. This is achieved by use of a voltage doubler circuit (see page 13 in the service manual). Be aware that this can mean there are voltages of 80-100V on this part of the circuit. While there isnt enough current to kill you, it'll certainly give you a nasty zap.
I'd consider fitting some 275V rated MOV's to your amps. They should be fitted across the L and N, after the fuse. These should help absorb any spikes. If that's too much hassle, simply run your kit through one of those surge-protected socket strips (which also have MOV's inside them)
A few things to bear in mind:
* The output MOSFETs are connected directly to the heatsinks with no insulators. So those heatsinks will have +/- 35V on them. Be careful if you do any tests while powered up.
* The frontend has a pre-regulator to produce a slightly higher voltage. This is achieved by use of a voltage doubler circuit (see page 13 in the service manual). Be aware that this can mean there are voltages of 80-100V on this part of the circuit. While there isnt enough current to kill you, it'll certainly give you a nasty zap.
I'd consider fitting some 275V rated MOV's to your amps. They should be fitted across the L and N, after the fuse. These should help absorb any spikes. If that's too much hassle, simply run your kit through one of those surge-protected socket strips (which also have MOV's inside them)
Hi Mooly,
I was thinking the same when it comes to very old transformers (early 70s vintage stuff) if they had thermal fuses inside, relying all the protection on external fuses. (And then shorting it)
But, at this point I'd go for a 240VAC (or 275VAC according to jaycee's suggestion) rated new trafo and call it a day.
Just have to know if it's a 25-0-25 secondary outputs. The amp is rated 35W RMS into 8 ohms.
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Rectifiers are ok.
Measuring again for continuity across the points I realized there's continuity between the two external points where the probes are (also measuring around 12.5 ohms, variating between 11 and 13 ohms if I stay there longer).
If I "jump" the open white wire to the point where the black probe is, then I have again the primary "working", but with no thermal fuse, right?
Measuring again for continuity across the points I realized there's continuity between the two external points where the probes are (also measuring around 12.5 ohms, variating between 11 and 13 ohms if I stay there longer).
If I "jump" the open white wire to the point where the black probe is, then I have again the primary "working", but with no thermal fuse, right?
An externally hosted image should be here but it was not working when we last tested it.
Correct. I'd recommend you do this through a bulb, and carefully measure the secondary voltages.
I recommended 275V MOV's - surge absorbers. Your replacement transformer should have a 240V primary if you can get one... though see my comment about cheap EI transformers. A well made transformer should have no problem with a slightly higher primary voltage.
I recommended 275V MOV's - surge absorbers. Your replacement transformer should have a 240V primary if you can get one... though see my comment about cheap EI transformers. A well made transformer should have no problem with a slightly higher primary voltage.
Perfect. I have the MS but I didn't see any rated VAC out of secondary.
25-0-25VAC?
Edit: back panel of amp
An externally hosted image should be here but it was not working when we last tested it.
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Quick Testing...
Just wire a 100W lamp across the transformer thermal fuse.
If the amp is ok, the lamp will glow dull orange with no signal.
If all is well, try low signal levels.
If all is still well, wire a fuse or fuse wire across the thermal fuse connections.
The main caps are perfectly ok, appearance is normal.
Likely reason for thermal fuse failure is sustained overdrive with lousy ventilation.....idiot proofing.
Dan.
Just wire a 100W lamp across the transformer thermal fuse.
If the amp is ok, the lamp will glow dull orange with no signal.
If all is well, try low signal levels.
If all is still well, wire a fuse or fuse wire across the thermal fuse connections.
The main caps are perfectly ok, appearance is normal.
Likely reason for thermal fuse failure is sustained overdrive with lousy ventilation.....idiot proofing.
Dan.
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