It's possible you have an issue with a connection, bad solder joint, component that is sensitive to vibration etc... Is the chassis out of the cabinet? Can you tap around the chassis and jacks with a plastic tool while listening to the speaker output?
Sounds like a dodgy 'insert jack' Are there contacts on the line out jack? maybe they are dirty or one of the soldered joints is fractured.
Ron(UK)
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I think that the key here is that he says it works ok from the headphone jack, when the speaker level has gone low. On a small amp like this, the headphone jack is almost certainly straight across the output, with the internal speaker switched by the socket's contacts, so that would leave only bad contacts on the jack, bad wiring to the speaker, or of course, the speaker itself. My next move would be to shove a good squib of switch cleaner / lubricant straight into the headphone jack entry, and then work a plug in and out vigourously for 10 seconds, then try again. If it now stays on, then you either need to get at the contacts and clean them properly, or better, replace the jack.
As far as Jim doing custom work goes, I really don't know if he did a lot. This is going back like 35 years, when Marshall was a very small company. I met him as a result of going there a number of times with the friend who knew him well. At that time, mobile discos were a very new thing, and there was perhaps 4 or 5 only in my town and surrounding area, which is quite large. Jim built a custom semi-circular shaped console for my friend, with a preamp / mixer set in the middle between the two decks. This preamp had such innovative features ( for the time ) as slider level controls, separate mic input and gain control, headphone socket with pre fade listen and so on. The decks were a pair of Garrard semi autos, with the auto mech wedged, such that they would unload at the end of a record, when you could just lift off the current record, and drop the next one on. The deck would carry on to self cue, at which point you flicked the deck power off, then used the PFL to set your 'ready to roll' cue point. It took quite a bit of skill to run it, but it was a very smooth operation when you got used to doing it.
The amp that Jim built him was a pretty meaty thing - 100 watts as I recall. It used four output tubes in parallel push pull. I seem to recall that they were KT66's or maybe even 88's. They are the true beam tetrodes over the cross substitutable EL34's, which are normally accepted as being pentodes, but are sometimes actually beam tetrodes inside.
The amp definitely had the standard gold front, and Marshall logo on it. The preamp face plate was also gold, but I can't remember whether or not it had the logo. I would guess that they were probably built through the factory, at a time when any business was good business, and custom designs were taken on and built in that little factory. That console was mighty heavy I can tell you, and many was the time that I cursed it at two in the morning in the rain, as we loaded it back on that old 15cwt Ford Thames van ...
Arfa
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