The amp was silent, valves all lit up but no output whatsoever and no extra current draw when the Standby switch was closed. OK - so where is the damn HT fuse ? Nothing on the back so it must be inside - another first for a Marshall valve amp. On the PCB is a 630mA delay fuse - very blown - fitting a new one got the amp running.
Bit of poking about finds that both 1/4 inch QCs on the PCB used for output valve grid drive are very loose - easy fixed. But was this the cause of fuse blowing? Nope.
That 630 mA HT fuse should be 1 amp at least, since it is fitted in the secondary of the PT. On test, the fuse had to pass just over 1 amp rms when the amp is over-driven. The primary AC current draw is 2 amps rms at the same time ( ie 480VA) .
On sine wave test, there is significant crossover distortion on the CRO screen at 1/2 output and above. The bias setting seemed OK - there are two trims inside and a pair of 1 ohm resistors in each pair of output valve cathodes.
On checking the screen B+ supply, the cause is simple. There is no filter choke of course ( what are they ?) and instead a 470 ohm ,7W resistor. So the screen B+ supply falls like a rock soon as the amp is driven. Guess this helps the crappy Chinese EL34s to survive overdriving.
Besides the above bad points, there are lots more.
The AC tranny is undersized - it looks like a 250VA type and needs to be
400VA to withstand hard use in this amp. Even sitting on the bench with a 605mA idle current from the AC supply, the internal temp rise is almost 40 degrees C. A couple of hours of heavy metal on a warm night will see it off.The same * ridiculously fragile* plastic shaft pots as used on other recent Marshalls have been fitted - one light bump on the knob = nasty intermittent fault.
There is no AC voltage selector fitted and no internal option either, another first in a Marshall valve head. All the fasteners are metric too, not one single concession in sight to the brand's UK origin.
There ARE two small screw-in bulbs ( labelled 12V ) fitted inside the cabinet that illuminate the whole chassis and the "Marshall" logo through the open weave cloth. These are not ordinary 12 volt dial lamps but special dual, orange LEDs - one chip connected for each polarity when fed with AC.
Next the these is a non Accutronics ( Belton ?) reverb tank.
All the jack sockets ( 7 of them ) are very fragile, PCB mount types.
Every single part in the amp looks like it came either from China, Korea or Taiwan and then assembled in Vietnam.
Good points:
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The 4 x EL34 output valves and the 12AX7 PI are on ceramic sockets, hand wired to the two trannys and the PCB.
The main PCB looks like it is easy to get lose and invert for servicing.
The amp is pretty cheap to buy.
But a used example from the 1980s is a way better product, full of UK made transformers and other parts that have proved their ability to survive in a Marshall.
.... Phil