Marshall Superlead Guitar Amplifier Repair

[Wicksy]

Hope this is the right forum to post in. If not, apologies.

I have had a old '77 - '78 Marshall Superlead head. A 1959 model. I bought it about 11 years ago. When i bought it, i put new tubes in and tried to bias it. The bias pot was knackered and i tried to replace it, damaging the PCB traces. I opted to throw in an earlier turret board i purchased from a kit vendor. I cut the old board out, made the new board then shelved the project for a decade or so. In the mean time, i obtained the period correct replacement from a mate and i have just recently installed it. I fired it up for the first time, standby off. Got 6.3VAC on the filaments as expected. I had doubled checked the voltage selector so i know thats good. I flicked the standby on and it immediately blew the mains fuse. I hunted around for my fuses and realised i had a few, which i tried but it blew the mains fuse once more so i left it and ordered some more fuses. As i'm running 240V mains with a 100W amp, i went for 3A slow blow for the mains and 1A fast for the HT supply. Now this is where the rust comes in... If memory serves, a blowing mains fuse is usually either an issue with the transformer and/or associated wiring or the rectifier. I am hoping i simply had the wrong fuses and adding the right ones will allow me to power it up. I've double checked the power section several times and everything is in the right place.

If i have to check the transformer, i can simply disconnect the secondary taps from the rectifierand measure each side referenced to ground for resistance. If i wanted to power it up, could i connect it to the input of my multimeter? Each of the taps as i recall puts out about 280VAC (its centre tapped so 2x 280VAC). If i lift the ground and connect the secondary to the DMM, i should see about 550V - 600V. Is this a safe thing for the DMM or transformer? Is the DMM a good enough load? I am praying the PT isnt fried as i want to keep this old amp as original as possible.

[BreakingOhmsLaw]

Hi Wicksy,

if my memory serves well, the Marshalls from that era had a pretty crude power supply. Basically mains transformer, then rectification by three diodes in series towards center tap, because the voltage rating on them was so bad. If present, check the 6 diodes directly after the transformer and check the main filter caps for shorts.
Later units had big bridge rectifiers next to the filter caps.
It would really be helpful to get the exact model number so we can find a service manual.
Also, it's best to run it from an isolation transfomer when servicing, theres 200V DC in there somewhere for the tubes.

Also visual check for loose parts on the "board" (if you can call it that). Workmanship on the puppies was always pretty awful, all the leads are uninsulated and the can bend easily touch neighboring parts.

[CaptDon]

Doesn't the main power transformer also power the filaments?? Sounds like
bad silicone rectifier diodes as the other poster mentioned. Don't keep throwing
fuses at it until you investigate for shorts. I don't personally like Marshall's as
the capacitors seem under rated as were many of the power parts. I serviced
a unit similar to yours, the main filter capacitor was dead shorted and the owner
put a 10 amp slow blow fuse in it. Burned the power transformer to a crisp.
VERY expensive repair!!! The unit had a 450v rated, 525 volt surge filter capacitor
and there was over 600vdc on the cap until the power tubes warmed up and
loaded down the high voltage. Very poor design.

[mawyatt]

I used to repair guitar amps along with other electronics to get thru undergrad school back in the 60s. One very useful "tool" with these amps is a simple mains extension cord with a 50~100W light bulb in series with the hot lead. If something is wrong in the supply instead of blowing fuses the light will shine, if things are OK then just a slight glow.

Best,

[CaptDon]

Early on in my repairing of the Carver PM-2.0T units I often used the
light bulb trick. It works great!!! Had a 100w normal looking bulb,
a 150w flood light for heavier testing and a 60w for working on old
tube type table radios with suspected shorts or failed caps. Always had
a variac driving an isolation transformer driving the unit under test.
Many of the table radios had a hot chassis and the Carvers had SMPS
and isolation was a must between UUT and test equipment!!

[Wicksy]

Thanks for all of the advice, i'll definietly use the lightbulb trick. I'll check the diodes at the weekend and see what happens. I have enough replacements i could just swap the low out. I best make sure i have everything correctly hooked up... i'd imagine if the recitifer was wired up wrong it'd trip the fuses.