Melos TM90 ST schematic?

[Giaime]

Hello everybody,

I'm repairing this amp:

http://www.hi-figuide.com/apparecchi..._90/index.html

Anybody has schematics?

It has been through years of service centers, because no one was able to fix it, so it has been donated to me.

It does blow the fuse on the B+, I tried with the series-lamp trick on the mains line and the lamp stays bright.

The big coke-cans 1400uF 200V caps seem to be right (but, strangely, I found one of them disconnected from the PCB. Strange.), and otherwise when removing the 1A fuse that's in series with the B+ (but not heaters, bias PSU, etc etc) the lamp flashes and then dims out.

Thanks anyone in advance.

[Giaime]

Ok, I managed to remove the short (don't know what it was, probably a mechanical thing, not a shorted component).

One channel seems to bias correctly (I haven't tried to connect it to speakers), all the bias leds are working but the other channel makes strange crackling noise from the 6550s, and the bias leds go crazy. Also the cathode current jumps all the way in time with the crakcs, up to 200mA.

What could be this? I can't see any arcing inside the tube. Maybe the socket?

[Giaime]

I removed the tubes and monitored the bias supply of the faulty channel: it has the voltage that jumps all the way from -20 to -70V, randomly. I think that's trimmer - related: I will replace all the bias trimmers.

Of course, of the many ways to connect a bias pot in an amp, they managed to use the worst way: this way if the trimmer opens (not rare: they oxidate) the grid of the 6550 goes to ground, 0Vgk

Very very strange input stage: I'm still trying to figure it out. Schematic would be much appreciated.

It has a 6AC10, that's 2 medium mu triodes plus an high mu triode all in a bottle, and a 6LU8, that is a beam power pentode and a triode in a bottle.

[Original Burnedfingers]

As Mr. fdegrove has told me on more than one occasion..."Any competent technician can draw out the schematic in 10 minutes"

Draw it out and it will save you in the long run.

How are the coupling caps anyway?

[Giaime]

I'm no competent technician

However, I will follow your suggestion and draw a schematic.

Coupling caps are those metal cans, branded "Melos". The Sprague orange caps are there only for bypassing PSU electrolytics, it seems.

[Giaime]

Ok I replaced all the trimpots but the problem is still there

I'm looking at the 6550 grids of the left channel, without input signal there isn't just plain grid bias DC, but random waveforms, like sparks or arcing.

What could be? Coupling capacitors? But it's suspect that they fail both on the same channel...

[Original Burnedfingers]

Use your meter and check the coupling caps.

Where is that schematic that you drew. You did draw it didn't you?

[Giaime]

The coupling caps all test good on the meter! At least I checked for DC resistance, and it does show "charging up" effects, althougt not the same of some good other capacitor of the same value. They're suspect.

I will test by removing the bias supply (tubes unplugged) and see if there is still something on the grids.

(no schematic yet, sorry, I'm a bit busy in those days)

[Original Burnedfingers]

Check for DC leakage.

[Giaime]

I disconnected the capacitors, and on 6550 grids there was nothing other than pure clean bias DC.

So I will replace the coupling caps.

However, their marking is strange to me: they're "customized" (branded Melos), 2.5uF and then 250V. Very very strange since this is a 550V B+ amp. Near the "250V" printing there is a little drawing, representing a sine wave. Does it mean that the 250V is an AC rating? If so, I guess they're good for 350VDC.

I can't find 2.2uF caps more than 100V rating, can I replace them with 1uF caps (Philips MKT, the orange ones) without upsetting too much the pole splitting of the amp? They don't look to be connected to a particulary low impedance (I measure 100k between grids and ground), so I guess 1uF is fine.

I will try, let's hope it does not oscillate or things like that