MA100H - low output volume problem

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MA100H - low output volume problem

Postby 138 Gear Repair » 13 Nov 2013, 09:18

A friend of mine has passed me their MA100H and asked me to have a look at it, I said of course, and here begineth the tale of woe...
The reported issue was that it was playing at a very low volume.

Got her home and plugged her in, annnnd no output at all, the lights came on but no sound on any channel.
Opened her up and found a very blown FS1.

Replaced fuse, and had a look for possible causes.
Spotted the following straight away:
mixture of manufacturers power tubes
a very very hot power tube (glass deformed)
a slightly discoloured power tube (blackening on the plate)
a cracked ceramic etc. etc.

Sooooo, got a nice new set of matched EHX EL-34's and 3 new JJ ECC83S.
Fitted, biased both pairs of power tubes (76mA) and put her all back together again..

Plugged her in annnd.... output (hoooray) but volume is nowhere near what it should be....
Have to have volume (clean) Volume+gain (OD) at half way to get much of anything
The channels work in as much as there is an audible difference between clean and OD

Plugged in a seperate preamp through the FX return and bingo... proper loud as I would expect from a 100w head.
So I'm thinking there's something up with the preamp side.
Have swapped in/out different preamp tubes - no effect
Have checked for dry joints - nothing
Managed to snag a wiring diagram and checked componant values - all good

Took some voltage measurements from the pre valves (all referenced to chassis)...

V1B
A (6) 189.3V
G (7) 0.3mV
K (8) 1.441V

V1A
A (1) 134.2V
G (2) -3mV
K (3) 1.03V

V2B
A (6) 260.9V
G (7) 0.5mV
K (8) 2.65V

V2A
A (1) 173.5V
G (2) -1mV
K (3) 1.034V

Would anyone here know if these values are correct/typical?
Would anyone here have information on what the secondary voltages should be in case it's poor output from the input transformer?

Any other suggestions would be greatly appreciated,

Thanks in advance,
Johnny

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Re: MA100H - low output volume problem

Postby thephantum » 13 Nov 2013, 16:43

As far as those voltages, I'm not sure. I don't know what the secondary voltages are either (if you did, everything else would be academic....you could calculate it).

However, I don't think you have a transformer issue. Both the preamp and poweramp are fed from the same set of secondaries. Not sure if this is the schematic you have, but you can see on page 3 that Sec 1 (W65 & W66) feeds the HT (red) line. That line feeds juice to the poweramp and, if you flip back to page 1, that same HT line feeds juice to the preamp.

If it's the preamp it has to be something outside of channel switching. In other words, since the whole preamp has low volume, the problem can't be between the channel switching relays. That narrows it down to a hand full of components to check...

...but I'm not convinced that the preamp is the problem. My guess is the FX Loop. Have you tried a short guitar cable across send/receive and/or trying the FX switch in different positions?

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Re: MA100H - low output volume problem

Postby 138 Gear Repair » 14 Nov 2013, 09:31

Had a look at your FX loop suggestion... tried all permutations, no difference I'm afraid.

That is indeed the same schematic.
Secondary 1 is putting out 231.4V measured between W65 and case and also between W66 and case.

This strikes me as low... I can't find anything for the MA100H input transformer but shouldn't the voltage be up around 300V+ on the tube feed?

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Re: MA100H - low output volume problem

Postby thephantum » 14 Nov 2013, 13:26

Did you have the tubes in and idling when you pulled the V? If so, with no load, those voltages would go up slightly. Each side of the rectifier should be getting half the transformers output (out of phase)...so...assuming those numbers are within spec, that would make it a ~500V transformer.

I'm actually getting ready to do some surgery on my 100C within the next week of so (99% sure I've got a bad filter cap). Once I get all the parts and open it up, I'll pull some measurements on my secondaries.

I still don't think you have a transformer issue though. The poweramp works and the preamp works. It sounds like the issue is getting the signal cleanly from pre to post. If you have access to a signal generator and a o-scope, that's the way to track it down.

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Re: MA100H - low output volume problem

Postby 138 Gear Repair » 15 Nov 2013, 13:44

Voltage on secondary 1 was measured with tubes in.

Thanks for the input phantum, am working my way from point to point along the circuit, great way to spend a friday night, rock n roll or what :p

Actually managed to get everything working at a good volume by plugging in an eq pedal with a level boost at the front end.

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Re: MA100H - low output volume problem

Postby thephantum » 18 Nov 2013, 13:37

Nothing says Rock n' Roll more than fixing a busted tube amp. :headbanger

I'll be curious to see what you find. I know mine is pretty loud at 3 when native (no pedals and/or boosts) much less half way up.

FYI - I did my surgery yesterday and measured the Sec 1 voltages afterwards (with the EL34's idling). They were almost exactly what you got: 231.7V.

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Re: MA100H - low output volume problem

Postby 138 Gear Repair » 19 Nov 2013, 08:02

Thnks for that Phantum... I kind of thought you would get the same voltage... FS1 is rated at 250V so that ties in.

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