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Leak Stereo 20 transformer overheating

bdbell

bdbell

Active Member
I have a Leak Stereo 20 on the bench, client brought it a few weeks ago, was not sure the transformer was OK - the 100 Ohm 3 watt resistor had gone up in smoke. I did not have a suitable replacement for the 100 ohm - ended up using 3 x 33ohm 1 watt as the client was in a hurry. I tested it, seemed to be working fine so I recapped the rest of the unit, coupling caps were very leaky, ran it for a few hours test and it was working fine. The client picked up the unit and said after 3 hours running it had shut down, the 3 x 33 ohm had broken connection.

I also got more of the backstory the second time - the seller of the amp claimed that "Amp needs rewinding or replacement of the power transformer, it overheats 10 minutes after turning on" - also when I received it the transformer was set to 110v, which I changed to 117v.

I just ran the amp for 90 minutes - with meters on all 3 transformer outputs, monitoring it closely - the outputs started out perfect - 600v, 6.25v, 5v - 0.88amps draw with 117-118 volts coming in, after about a half hour the output was down to 596v...not much of a drop, and current draw at 0.9amps. But then at around 90 minutes in I started to see the output voltage drop below 590v and current draw going up fairly quickly - shut it down at around 583v and 1.06amps draw - transformer temperature never went above 40C during my test though!

Does this mean the transformer does have a problem and needs rewinding/replacement?
 
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bdbell said:
I have a Leak Stereo 20 on the bench, client brought it a few weeks ago, was not sure the transformer was OK - the 100 Ohm 3 watt resistor had gone up in smoke. I did not have a suitable replacement for the 100 ohm - ended up using 3 x 33ohm 1 watt as the client was in a hurry. I tested it, seemed to be working fine so I recapped the rest of the unit, coupling caps were very leaky, ran it for a few hours test and it was working fine. The client picked up the unit and said after 3 hours running it had shut down, the 3 x 33 ohm had broken connection.

I also got more of the backstory the second time - the seller of the amp claimed that "Amp needs rewinding or replacement of the power transformer, it overheats 10 minutes after turning on" - also when I received it the transformer was set to 110v, which I changed to 117v.

I just ran the amp for 90 minutes - with meters on all 3 transformer outputs, monitoring it closely - the outputs started out perfect - 600v, 6.25v, 5v - 0.88amps draw with 117-118 volts coming in, after about a half hour the output was down to 596v...not much of a drop, and current draw at 0.9amps. But then at around 90 minutes in I started to see the output voltage drop below 590v and current draw going up fairly quickly - shut it down at around 583v and 1.06amps draw - transformer temperature never went above 40C during my test though!

Does this mean the transformer does have a problem and needs rewinding/replacement?
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Not necessarily. I would check the rectifier and output tubes and their sockets (for good grip) before looking at the power transformer.
 
Pop all of the tubes out and power it. A good transformer should draw very little current and not get warm. If it starts hogging power and getting hot with no load on the secondary its not a happy camper.


If that passes, put everything but the rectifier in and see what that does.

If thats happy, pop the rectifier back in and measure across the string of resistors subbing for the 100 ohm.

The fact that the 100 ohm smoked makes me think either the second cap is bad, or there was excessive current draw for some other reason. Bad coupling caps are a lazy guess but bad tubes, bad cathode bypass, or some previous accident are all possible. Voltage drop across that would tell you. Schematic shows 334 on the rectifier side and 318 on the other side, so 16 volts drop across 100 ohms = 160 milliamps. If it starts out there and starts running upward, something is not happy and its probably not the transformer.

also 16*16 / 100 = 2.5 watts dissipation so the original 3 watt part is pretty marginal. 3 33 ohm in series will each get about 0.85 watts, so ideally they should be at least a 2 watt part. if you replace the 100 ohm I'd go at least 5 watts and I would be more comfortable with 7-10. I fully admit to going overkill with parts when I have the option, and especially when I'm spending other people's money for them :)
 
Thanks everyone for the replies! I had run the amp last night without tubes for quite a while - almost no current draw and barely got warm after ~90 minutes, and voltages were all stable. I also have already replaced the 100ohm with a proper single 3 watt - and I went through and replaced any out of tolerance resistors. I do have a 5 watt resistor, but I was under the impression that the 3 watt was part of the safety mechanism with this as was the client so they wanted to keep it at 3 watts - is that not correct?

The power supply caps were changed about 10 years ago, but that was also when it overheated so I wasn't sure about them - all other caps were replaced.

I did clean all the tube sockets when I first worked on it, but I looked at them and for some reason thought it wasn't really possible to 'tighten' them...but I took another look after @ramjet 's reply - and took my tweezers to them and tightened them all - then cleaned again...have been running it for close to 4 hours now and no problems. I also have been checking the PS caps to make sure they are not heating. The first thing I noticed when I powered back up with the sockets tightened was that the 3 output voltages seemed more stable.

Thanks again - much appreciated!
 
My understanding is it needs to be left as a 3 watt resistor. It is designed to fall out if it overheats, a kind of fuse function. Have restored two of these and that is what I did.
 
I suppose its possible they went with a marginal resistor as a 'safety' but its been my experience with resistors that they will happily dissipate way beyond their ratings without burning open for a whole lot longer than you'd want. A fuse would be a better choice as a protection device.
 
gadget73 said:
I suppose its possible they went with a marginal resistor as a 'safety' but its been my experience with resistors that they will happily dissipate way beyond their ratings without burning open for a whole lot longer than you'd want. A fuse would be a better choice as a protection device.
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This 3 watt resistor rests in two V shaped tag strips and when soldered in and no mechanical connection is made to the Vs prior to soldering the resistor leads. Then when in operation the 3 watt resistor is held in via solder only. In theory if it gets too hot the solder will melt and the resistor falls out of the V solder points by gravity. At least that is the story I was told here back in the early 2010s when restoring a Leak 20 amp.

EDIT: In the first Leak 20 I restored this 100 ohm 3 watt resistor had done its job as designed. It had also burned up as it fell out of hte Vs.
 
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hm, working like an old style thermal overload. I guess that works but still seems a little unpredictable. Different solders melt at different points, and it depends a bit on how much solder is put in there.
 
I've never seen one of these amplifiers in person but according to the "Installation, Operation and Maintenance Manual" I have seen, the US model calls for a 2 amp fuse and for the British model a 1 amp slow blow. The fuse holder is shown located next to the "A.C. Power" cable on the chassis diagram.
 
bdbell said:
Thanks everyone for the replies! I had run the amp last night without tubes for quite a while - almost no current draw and barely got warm after ~90 minutes, and voltages were all stable. I also have already replaced the 100ohm with a proper single 3 watt - and I went through and replaced any out of tolerance resistors. I do have a 5 watt resistor, but I was under the impression that the 3 watt was part of the safety mechanism with this as was the client so they wanted to keep it at 3 watts - is that not correct?

The power supply caps were changed about 10 years ago, but that was also when it overheated so I wasn't sure about them - all other caps were replaced.

I did clean all the tube sockets when I first worked on it, but I looked at them and for some reason thought it wasn't really possible to 'tighten' them...but I took another look after @ramjet 's reply - and took my tweezers to them and tightened them all - then cleaned again...have been running it for close to 4 hours now and no problems. I also have been checking the PS caps to make sure they are not heating. The first thing I noticed when I powered back up with the sockets tightened was that the 3 output voltages seemed more stable.

Thanks again - much appreciated!
Click to expand...
If it were on my bench the PS caps would be replaced because I was in it.
 
+1. Absolute bare minimum they would get checked for leakage at rated voltage. Depending on the quality of the caps though, they could be dead at 10 years old.
 
When I redid my Fisher 70s, the first set of PS filter caps I used cracked open on one of the amps, with some irritating results. They can definitely cause things like overheating PTs.
 
One possibility is that power tubes ( i have no schematic at hand) is unstable and needs replacement. Is there some
point to measure DC current in the power tubes ?
 
gadget73 said:
Pop all of the tubes out and power it. A good transformer should draw very little current and not get warm. If it starts hogging power and getting hot with no load on the secondary its not a happy camper.


If that passes, put everything but the rectifier in and see what that does.

If thats happy, pop the rectifier back in and measure across the string of resistors subbing for the 100 ohm.

The fact that the 100 ohm smoked makes me think either the second cap is bad, or there was excessive current draw for some other reason. Bad coupling caps are a lazy guess but bad tubes, bad cathode bypass, or some previous accident are all possible. Voltage drop across that would tell you. Schematic shows 334 on the rectifier side and 318 on the other side, so 16 volts drop across 100 ohms = 160 milliamps. If it starts out there and starts running upward, something is not happy and its probably not the transformer.

also 16*16 / 100 = 2.5 watts dissipation so the original 3 watt part is pretty marginal. 3 33 ohm in series will each get about 0.85 watts, so ideally they should be at least a 2 watt part. if you replace the 100 ohm I'd go at least 5 watts and I would be more comfortable with 7-10. I fully admit to going overkill with parts when I have the option, and especially when I'm spending other people's money for them :)
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Agree. That a good troubleshooting approach.
 
ramjet said:
I've never seen one of these amplifiers in person but according to the "Installation, Operation and Maintenance Manual" I have seen, the US model calls for a 2 amp fuse and for the British model a 1 amp slow blow. The fuse holder is shown located next to the "A.C. Power" cable on the chassis diagram.
Click to expand...
Those 1 or 2 amp fuses are right after the AC power plug and before the power transformer. The 100 ohm 3 watt resistor being discussed here is after the rectifier tube.
 
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BmWr75 said:
Those 1 or 2 amp fuses are right after the AC power plug and before the power transformer. The 100 ohm 3 watt resistor being discussed here is after the rectifier tube.
Click to expand...
gadget73 said:
I suppose its possible they went with a marginal resistor as a 'safety' but its been my experience with resistors that they will happily dissipate way beyond their ratings without burning open for a whole lot longer than you'd want. A fuse would be a better choice as a protection device.
Click to expand...
Looks like a fuse is also being discussed as well, hence my comment.
 
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