Marshall Major Blowing Power Fuse

[cluster]

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,

Thanks for that tip. Just to confirm what you said: lift the hockey pucks off the chassis and measure resistance to ground from each pin 3 ??? what should I be looking for as far as resistance? If I read 0 from pin 3 to ground, then there is a short in the OT?? Is that what you mean? thanks

[Leo_Gnardo]

Thanks for that tip. Just to confirm what you said: lift the hockey pucks off the chassis and measure resistance to ground from each pin 3 ??? what should I be looking for as far as resistance? If I read 0 from pin 3 to ground, then there is a short in the OT?? Is that what you mean? thanks

There are taps on your OT primary, each goes to a pair of those hockey puck resistors. So - if a short developed thru that resistor to chassis, you would see the resistance to ground readings you are getting. It's an easy exercise to unbolt the suspect resistors & pop them off the chassis. If your short or near-short reading changes to open then you know where the problem is.

If that doesn't show any difference there's still the possibility of a short thru a tube socket. To test this you'll have to unsolder the OT primary leads from the suspect sockets, just a little more work than unbolting those resistors.

If you still have a short to ground after having done these, there's no choice left, the short must be internal to the OT, and that will be a costly pain in the neck to replace. It would be a double disaster if you replaced the transformer only to find the short was thru a socket or SG resistor.

[cluster]

There are taps on your OT primary, each goes to a pair of those hockey puck resistors. So - if a short developed thru that resistor to chassis, you would see the resistance to ground readings you are getting. It's an easy exercise to unbolt the suspect resistors & pop them off the chassis. If your short or near-short reading changes to open then you know where the problem is.

If that doesn't show any difference there's still the possibility of a short thru a tube socket. To test this you'll have to unsolder the OT primary leads from the suspect sockets, just a little more work than unbolting those resistors.

If you still have a short to ground after having done these, there's no choice left, the short must be internal to the OT, and that will be a costly pain in the neck to replace. It would be a double disaster if you replaced the transformer only to find the short was thru a socket or SG resistor.

I see what you mean. Looking at the schematic it seems that the OT primary also feeds the screens via those hockey puck resistors. I'll preform this and report back. So once I lift these hockey pucks off the chassis and read the resistance to ground at pin 3 and see "OPEN" then that means that there is a short to ground at one of these pucks? Is that correct? Thanks again for your generous help :-)

[Enzo]

Why lift anything? You have about 30 ohms through the winding. One end is grounded, and it makes sense the other end then shows 30 ohms. You have screen taps? FIne, measure resistance to pins 4 of each socket. I bet you find the value of the resistor plus the transformer resistance. Lift the zero ohms end wire from the socket. Still shorted?

Also a good possibility instead of shorting directly to ground, your primary may be shorted to secondary. Secondary resistance is usually VERY low normally.

[Leo_Gnardo]

So once I lift these hockey pucks off the chassis and read the resistance to ground at pin 3 and see "OPEN" then that means that there is a short to ground at one of these pucks? Is that correct? Thanks again for your generous help :-)

Yes. Let's hope that's the case, OR possibly the short is at one of the tube sockets.

FWIW in the Major I worked on a couple weeks ago, there was already one output tube socket replaced. I retubed it, sent it out - with a 6 amp fuse that ought to be enough. Sure thing the fuse blew when the owner started pushing his amp hard. One of the new output tubes shorted and its socket was covered with arc debris so I replaced that too. Was it the socket or the tube? Chicken & egg question. I wasn't about to risk another tube so replaced its crusty burnt socket too.

[cluster]

Raised the hockey pucks off the chassis and measured the resistance on the leads of the ultra-linear taps. Left side 9.2 ohms to ground. Right side 20.7 ohms to ground. Left pin 3 is still 0 ohms and the right pin 3 is 29.5 ohms to ground. Not getting a good feeling here. Next is to disconnect the the primaries at pin 3 to see if there is a short in the tube socket.

On a side note, I'm sourcing an OT just in case. Here are 3 choices;

TAD at 279 euros ( $311) plus shipping: Output transf. for Marshall Major 200 W Ultralinear - Output Transf.

Banzai Music at 154 euros ($171) plus: https://www.banzaimusic.com/Transformer-T-OP-M200A.html

Or Mercury at a wapping $446: http://www.mercurymagnetics.com/imag...TC-MMAJ-OA.pdf

Any experience with TAD or Banzai?

[Enzo]

Those readings simply verify the zero ohm end is shorted to ground or secondary. the taps are mid-resistances that fit right into the scenario.

[Enzo]

Since it looks like the transformer might be bad, we are at the what is there to lose stage. Note carefully where they all go, and disconnect all the secondary wires. Is pin 3 still shorted to ground? If so, the primary is shorted to frame (most likely). If the short went away, then the short was to secondary.

So pull the transformer, and CAREFULLY cut into the fish paper or whatever surrounds the windings. Look for the points where the winding wires are bonded to the insulated wires that leave the transformer. Is it possible somehow that joint is touching and could be corrected?

[cluster]

Bad news. De-soldered the primaries at pin 3 (left and right) as well as the CT. Tubes sockets are fine (no short to ground at pin 3). One side of the primary is still 0 ohms to ground, so it seems like there is a short from one side of the OT to the frame/chassis. Next step is to do what Enzo said. This will be a pain since I will need to de-solder the secondaries at the Christmas tree stand-off array at the top of the chassis. Keep you posted. This amp was made in 1970 and it took 47 years to blow this OT. Hopefully, i will able to fix it and keep it original otherwise a new OT will be put in. Either way, it was sad prying apart those primaries that were stuck together with some kind of wax.

[Enzo]

take notes take notes take notes... never rely on memory or "I'll figure it out" to get wires back where they belong.

[nsubulysses]

Raised the hockey pucks off the chassis and measured the resistance on the leads of the ultra-linear taps. Left side 9.2 ohms to ground. Right side 20.7 ohms to ground. Left pin 3 is still 0 ohms and the right pin 3 is 29.5 ohms to ground. Not getting a good feeling here. Next is to disconnect the the primaries at pin 3 to see if there is a short in the tube socket.

On a side note, I'm sourcing an OT just in case. Here are 3 choices;

TAD at 279 euros ( $311) plus shipping: Output transf. for Marshall Major 200 W Ultralinear - Output Transf.

Banzai Music at 154 euros ($171) plus: https://www.banzaimusic.com/Transformer-T-OP-M200A.html

Or Mercury at a wapping $446: http://www.mercurymagnetics.com/imag...TC-MMAJ-OA.pdf

Any experience with TAD or Banzai?

There are two places that would probalby do it cheaper or same price as banzai. Magnetic Components (chicago) and Heyboer (Michigain). They will make whatever you want. Heyboer has no product line and only does custom transformers. Magnetic Components product line is Classic Tone but they will also make whatever you want.

Mercury is a ripoff IMO and banzai is far away so no need to have it shipped overseas.

[cluster]

so here are a couple of photos from the christmas tree array on the secondary OT. For sake of what Enzo said to do would it be better to unsolder from the top (white wires - shorter time) or the bottom (more work but if the OT is shot then it's already to pull out of the chassis). Thoughts?

[cluster]

There are two places that would probalby do it cheaper or same price as banzai. Magnetic Components (chicago) and Heyboer (Michigain). They will make whatever you want. Heyboer has no product line and only does custom transformers. Magnetic Components product line is Classic Tone but they will also make whatever you want.

Mercury is a ripoff IMO and banzai is far away so no need to have it shipped overseas.

Sent classic tone an email but no luck. They don't make those. Will check with Heyboer. MM is a the biggest ripoff around but for resale the sheep would probably pay more for it than a no-name. I hope Heyboer can come through. Maybe, if i'm lucky, like Enzo said, it might be a simple fix. Lets see...

[cluster]

take notes take notes take notes... never rely on memory or "I'll figure it out" to get wires back where they belong.

Thanks Enzo. Yes, I am using my iPhone to photo document the work and I'm labeling each lead with different colour tape and writing down what it is.

[cluster]

Question: Why does each tap coming off the secondary have two wires: 2 reds wired together, 2 yellows wired together, 2 greens wired together, 2 blacks wired together and a single green to ground??