I acquired an Alto amp last October, and it has run fine until now, apart from a loud buzz which was tracked down to the volume pot's earthing connection. Loosening the mounting nut stopped that. But now the right channel has started to run away, thermally. I have measured everything I can with a multimeter, comparing left and right channels, and have not managed to find any differences so far. This is unpowered, as I only get 20-30 seconds before the fuse on the left channel blows.
I have an infrared thermometer, so I can run it until it gets to about 30 degrees and then power off. The left channel barely warms. Looking at the instructions, I get half PSU voltage on R21/R22 as I should. I haven't yet checked across R21/R22 for 30mV.
I am fine building amps, but not so hot with fault finding, as I can follow a schematic without fully understanding exactly how it works. Hands better than brain! And I have all the transistors and diodes if I need to replace them. I haven't removed any components yet to fully check them.
What I need is help with understanding which components are the most likely to be causing the problem. I can replace any I need to, I can reflow solder joints, etc.
I have an infrared thermometer, so I can run it until it gets to about 30 degrees and then power off. The left channel barely warms. Looking at the instructions, I get half PSU voltage on R21/R22 as I should. I haven't yet checked across R21/R22 for 30mV.
I am fine building amps, but not so hot with fault finding, as I can follow a schematic without fully understanding exactly how it works. Hands better than brain! And I have all the transistors and diodes if I need to replace them. I haven't removed any components yet to fully check them.
What I need is help with understanding which components are the most likely to be causing the problem. I can replace any I need to, I can reflow solder joints, etc.
Check the TR3, it sets the bias.
Also check driver and output transistors. Make sure they are not shorted.
The circuit is just a JLH Class A with Darlington output. The bias would be finicky.
Also check driver and output transistors. Make sure they are not shorted.
The circuit is just a JLH Class A with Darlington output. The bias would be finicky.
Thanks. Testing in situ showed no shorts, but I will pull TR3 and check it properly, and work from there. It's possible VR1/2 is the culprit, of course, so I will look at that too.
Until it overheats, it plays music as normal, so I am hoping the driver and output transistors are OK, but I have spares if needed. I don't have a spare VR1/2, but that should still be available.
Until it overheats, it plays music as normal, so I am hoping the driver and output transistors are OK, but I have spares if needed. I don't have a spare VR1/2, but that should still be available.
How much bias does it run at? Based on the instruction, the voltage drop on R21/R22 should be 30mV. Maybe it simply runs bias too much that let it thermal runaway.
I haven't checked the bias yet, as I would have to be quick before the fuse blew. I hoped to narrow down the possibilities before trying it. I will do so first, as soon as I get time. I can remove the fuse and check the other channel first, and check which way to turn the trimmer for minimum bias on that one. 50 ohm trimmers are rather thin on the ground, so I hope it is re-usable.
The amp and CD player sound pretty good*, in the context of a second system, the CDP's DAC was designed by John Westlake who has (had!) a good reputation as a designer. And they look pretty; I am collecting a few chassis to put other amps or CDPs into.
* The originals, not the later Richer Sounds versions.
The amp and CD player sound pretty good*, in the context of a second system, the CDP's DAC was designed by John Westlake who has (had!) a good reputation as a designer. And they look pretty; I am collecting a few chassis to put other amps or CDPs into.
* The originals, not the later Richer Sounds versions.
I checked the bias, which jumped about, but I got it to settle down at 29.6V the same as the other channel. After it had stabilised I connected the speakers and a CD player, and it played music. But then it blew the fuse again, so I disconnected the speakers and CDP, and now it goes haywire again. The trimmer pot checks out on a multimeter, goes from zero to 50 ohms, has continuity to where it should, all the associated resistors check out, or at least match the other channel.
So back to square one. I will start pulling the transistors, (and the caps, which I can't really test in situ) and replace the trimmer in case it has intermittent contact. I now have replacements, so if in doubt, replace!
So back to square one. I will start pulling the transistors, (and the caps, which I can't really test in situ) and replace the trimmer in case it has intermittent contact. I now have replacements, so if in doubt, replace!
C5 has a wiggle! It lifts the track, so it might be the culprit, and it would explain the intermittent nature.
Tomorrow, hopefully, caps arriving from RS.
Tomorrow, hopefully, caps arriving from RS.
Running a Zapco 6X to tweeters and mids. It's 100wx6. So I tried bridging the mids at 300x2 and even though it sounded great the amp went into thermal shutdown. When I just run it normally at 100w per channel it seems to run fine, with less output but it's fine. So could overdriving the speakers cause this problem? I think the speakers are rated for 80w.I acquired an Alto amp last October, and it has run fine until now, apart from a loud buzz which was tracked down to the volume pot's earthing connection. Loosening the mounting nut stopped that. But now the right channel has started to run away, thermally. I have measured everything I can with a multimeter, comparing left and right channels, and have not managed to find any differences so far. This is unpowered, as I only get 20-30 seconds before the fuse on the left channel blows.
I have an infrared thermometer, so I can run it until it gets to about 30 degrees and then power off. The left channel barely warms. Looking at the instructions, I get half PSU voltage on R21/R22 as I should. I haven't yet checked across R21/R22 for 30mV.
I am fine building amps, but not so hot with fault finding, as I can follow a schematic without fully understanding exactly how it works. Hands better than brain! And I have all the transistors and diodes if I need to replace them. I haven't removed any components yet to fully check them.
What I need is help with understanding which components are the most likely to be causing the problem. I can replace any I need to, I can reflow solder joints, etc.
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I replaced C5, on the collector of TR3, and bypassed the lifted copper track, and that seems to have done the trick. So far. I will see if it holds up, but I think that is all it was. At least now I have lots of spare transistors that I didn't use, in case there are other faults at a later date.
So thanks jxdking for pointing me in the right direction.
So thanks jxdking for pointing me in the right direction.
It's all still working, and set up with the phono amp. I just need to make some matching speakers. 🙂
