Marshall Major Blowing Power Fuse

[cluster]

Hello everyone,

Got me a Marshall Major. I was playing it loud (9 or so) the other day and all of a sudden it blew a power fuse. Replaced the fuse, turned on Ok but as soon as I took it off standby, the power fuse blew right away. Thought it was the tubes, so I pull the tubes to see but it blew the power fuse again immediately as I took it off standby. Pulled the chassis out, looked around. No smoke or burn marks. Unsoldered the PT from the standby switch and hooked up a variac. Turned on ok. Tuned up the variac to 117 VAC with the standby off no problem. PT seemed fine. Wired back the standby switch to the PT and brought the variac from 0 very slowly (still no tubes). As soon as I engage the 1 volt on the variac I get a low hum on the speakers. Turn the variac to 5 volts and the hum is loud. Its a loud girthy hum and gets louder as I turn up the variac. Can't get the variac over 30 volts AC. The hum is super loud. Tuned it all off and scratched my head. Could it be the diodes, filter caps, OT?? Put the fluke on the diodes in place and they seem to read fine. Used my blue ESR meter on the filter caps and all read around .68, which seems in range for the ESR. Don't know what else to try. Hope I didn't blow my OT. I'm sure if I didn't use the variac and just took the amp off standby I would blow the power fuse again, so I didn't try. There is a power surge coming from somewhere. Any suggestions? Thanks in advance.

joseph

[Leo_Gnardo]

Next troubleshooting step I'd take, is to pull each output tube from its socket, test the tubes individually, also inspect the tube bases & sockets for signs of arcing. Have you used this amp successfully in the past, ran it up to 9 & let 'er rip?

There are several types of Majors, which is yours?

[cluster]

Fuse blows when tubes are OUT, so it's not the tubes. Marshall is a 1970 standard model (not the bass or the pig).

[cluster]

I'm thinking either bad diode or bad filter cap. loud hum points to filter cap issues. can anyone tell me how to test for a bad filter cap? Thx.

[Justin Thomas]

I'm sorry, I have to ask - when you said Volume on 9 or so, did you mean BOTH or just one or the other? I mean, 200W isn't <THAT> loud... most people think 20W on 9 is too loud.

Justin

Edit: okay, I should probably contribute something... you can check a suspect filter cap by placing a known good one across the One you think is bad - in parallel. If the hum gets better, the initial cap is bad.

DON'T FORGET TO CHECK THE BIAS CAPS!!!

[g1]

you can check a suspect filter cap by placing a known good one across the One you think is bad - in parallel. If the hum gets better, the initial cap is bad.

This is true for an 'open' cap, but not for one that is breaking down and shorting causing the fuse to blow. You will need to disconnect the suspect cap.

[Justin Thomas]

Thanks, nobody ever mentioned that nugget before...

Justin

[Enzo]

Not for fuse blowing. If a cap is shorted, a parallel cap won't cure that.

Rectifiers are just diodes, a few seconds with an ohm meter tells you if they are shorted. You can always measure across a cap to see if it tests shorted. Of course your meter runs on a volt or two, and some caps are real leaky at working voltages, but not down at 2v.

Pin 3 of the power tubes is the plate, measure resistance to ground at each. If low resistance, you have an issue on the B+. Then disconnect the output transformer center tap wire. Does the short go away?

Think about this, if your speakers make a hum with no tubes installed, SOMETHING is drawing current through the output transformer. And sorry to say that may be a short to frame or to secondary.

I don't recall these having flyback diodes. A schematic would sure help.

[Leo_Gnardo]

I worked on one like this recently. Although the following won't lead you to what's wrong, the bias adjustment pot was intermittent in the one I worked on. Additionally it's in series with the supply. You may want to alter your bias supply so that won't be a potential problem before you finally button yours up.

Something worth checking, there's four 250 ohm screen grid resistors, hockey puck shape & bolted to the chassis. If you developed a leak to ground thru one of these you could be having the symptoms you describe. I eightysixed all of 'em, and replaced them with 1000 ohm 10W wirewounds suspended on tie strips. There was a small loss of power, about 10%, but I reckoned this change would make the output tubes a tad more comfortable & expect longer life out of them. KT88's with about 600V on the plates, and nearly that much on the screen grids with the output transformer primary having ultralinear taps, for those unacquainted with this model. It is a beast!

[cluster]

This all good stuff. I'll have another look in the morning. In the meantime here are some schematics

[cluster]

BTW, the hum I'm hearing is clearly more like 120 cycle vs the more common 60 cycle. Let me ask this: Could a bad filter cap (or diode) cause 120 cycle hum through the speaker even though tubes are OUT?? Looking at the schematics the filter caps do connect to the OT primary. Thx again.

[Leo_Gnardo]

BTW, the hum I'm hearing is clearly more like 120 cycle vs the more common 60 cycle. Let me ask this: Could a bad filter cap (or diode) cause 120 cycle hum through the speaker even though tubes are OUT?? Looking at the schematics the filter caps do connect to the OT primary. Thx again.

The fundamental frequency of rectified 60 cycle power IS 120 Hz (100 in 50 Hz power areas) so not at all unusual to be hearing it. And if there's a short as I described, thru a screen grid resistor, that's what you would be hearing. Could be another fault too. We'll pick up the search after some shuteye.

[Enzo]

If it hums with no power tubes, then as I said, something else is pulling current through the OT. There are no flyback diodes, so that leaves us mostly with a transformer failure. SO please do read resistance to ground from pin 3 of each socket/

120Hz hum is from power supply ripple currents. It may be lack of filter cap, but also quite common if the supply is loaded down even with good caps.

[cluster]

Thanks for the help everybody :-)

I took resistance readings at the primary of the OT via pin 3 and the centre tap and here is what I got:

pin 3 (left side) to pin 3 (right side) = ~28.7 ohms.
pin 3 (left side) to CT = 14.9 ohms.
pin 3 (right side) to CT = 13.6 ohms.
pin 3 (left side) to ground = 0
pin 3 (right side) to ground = 30 ohms

it seems the left side winding has a short to ground. now could this be a fail or something else, since all the other readings seem normal??

[Leo_Gnardo]

something else

It's looking bad for the OT but still worth looking for a short to ground thru a tube socket, OR thru one of those screen grid hockey puck resistors. The resistors are held to chassis with a nut & bolt, just loosen them, pull up the resistor, remeasure your primary-to-ground. If no joy there, remove the primary leads to pins 3 of output tube sockets & re measure. If after all that you still show resistance to ground, the OT has an internal short to ground,