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Old 15th July 2005, 08:04 AM   #1
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Default Help with Melos SHA repair

I have an old Melos SHA-1 headphone amp. In general it is a pretty nice sounding amp, but it has developed a weird problem.

After it has been on a while, it develops a very high pitched squeal. I haven't measured it, but I would guess it is about the same frequency as the high pitch squeal that a CRT TV makes. It is only in one side (the left), it is unaffected by changing the volume, and it can be made to go away by thumping the top (or by flicking the input selector) -- essentially by causing some vibration. I tried to isolate the transformer thinking it might be that, but it did not seem to help. Another thing is that it only happens with good tubes. With the stock sovtechs there is nothing, but with my amperex 7308s or some other JAN tubes (don't recall much about them other than they were cheaper than the amperexs) the squeal is back. Swapping tubes doe snot help (and does not change the side of the squeal.)

Looking at the PCB, the solder joints are starting to corrode. My guess is that this thing needs some serious work.

Anyhow, does anyone have a suggestion on where to start looking for repairs? I am game for changing any and all parts, so long as I have some idea what part to use.

Thanks,

-d
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Old 15th July 2005, 05:38 PM   #2
Gilbert is offline Gilbert  United States
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Microphonic tubes come to mind, which means you could spend some time and money pre-selecting tubes, perhaps doing a 'thump' test on them in circuit.

In the same case, your Sovteks may be a little lower (worn?) gain so that the oscillation doesn't start up as easy?

Just shot-gunning parts replacement probably won't help all that much, while reflowing solder connection may/may not help but doesn't hurt and cost you anything. But if you do this, concentrate on the parts associated with the tubes.

Using a small cap to bypass a plate resistor might help too if you can identify, like with an o-scope, which stage is oscillating.
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Old 15th July 2005, 07:37 PM   #3
anatech is offline anatech  Canada
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Also look at the supply decoupling caps. You can try to bypass the caps with film units to see if the oscillation goes away. You may want to consider new tubes if they are old.
-Chris
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