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