Marshall Major Blowing Power Fuse

[cluster]

Ok, I managed to peel away some of that dried out cloth insulation and expose the break. can anyone tell me what that red coating is on the wire? Is it a type of insulation? How would I strip it? Sanding paper? Would it be ok to solder them together and shrink wrap them? That seems to be the simplest way to go about this. thanks

[Leo_Gnardo]

Thanks so much Leo. Could you please post links to the insulation you are referring to. It would be a great help.

Also, I was taking resistance measurements across the secondaries at the impedance selector and all my readings gave me 0 ohms. Is this normal? I would expect some resistance across some of these windings. Could this mean that there is a short in the secondary as well?

Resistance of secondary (speaker) windings, a fraction of an ohm normally so don't worry if you read nearly short circuit.

GE RTV silicone cement sold in nearly every hardware & home-center store. Handy 3.5 oz tube about the size of a large tube of toothpaste I expect will run you about CD$5. I used to use it in a hi-vacuum hi-voltage lab and it insulated well up to 25000 volts.

Heat shrink tubing, if you still have any open Radio Shack stores, they sell it. If any other electronic supply stores, they should have it too, in several diameters. Worst case you have to send away for it, Mouser has a nice selection, I prefer 3M polyolefin which they provide in a wide selection of diameters & colors in handy 4-foot or 6-inch cuts:

http://www.mouser.com/3M/Wire-Cable/...z0zp4dZ1z0vrwt

275 choices of diameters & colors, I'm sure you'll find something to suit your needs.

Red coating on your transformer wire is insulating lacquer. You can scrape away near the ends you need to splice, twist & solder then cover with a layer or two of heat shrink. For extra insulation you may want to coat that with a layer of RTV, let it dry then reassemble your transformer. Hopefully you'll have no further trouble from it. Whew, what an adventure.

[cluster]

Resistance of secondary (speaker) windings, a fraction of an ohm normally so don't worry if you read nearly short circuit.

GE RTV silicone cement sold in nearly every hardware & home-center store. Handy 3.5 oz tube about the size of a large tube of toothpaste I expect will run you about CD$5. I used to use it in a hi-vacuum hi-voltage lab and it insulated well up to 25000 volts.

Heat shrink tubing, if you still have any open Radio Shack stores, they sell it. If any other electronic supply stores, they should have it too, in several diameters. Worst case you have to send away for it, Mouser has a nice selection, I prefer 3M polyolefin which they provide in a wide selection of diameters & colors in handy 4-foot or 6-inch cuts:

http://www.mouser.com/3M/Wire-Cable/...z0zp4dZ1z0vrwt

275 choices of diameters & colors, I'm sure you'll find something to suit your needs.

Red coating on your transformer wire is insulating lacquer. You can scrape away near the ends you need to splice, twist & solder then cover with a layer or two of heat shrink. For extra insulation you may want to coat that with a layer of RTV, let it dry then reassemble your transformer. Hopefully you'll have no further trouble from it. Whew, what an adventure.

Thanks so much. Couldn't have done it without everyone's help. Please tell me how and where I should apply this RVT cement? Thx

[Leo_Gnardo]

Thanks so much. Couldn't have done it without everyone's help. Please tell me how and where I should apply this RVT cement? Thx

First get those wires spliced. Easiest way is scrape the lacquer off, twist the ends & solder. You could wrap that with electrical tape or use a wire nut, but my way would be to cover that splice with heat shrink, heat & shrink that first layer. Then add a second & maybe a third layer. It's probably overkill at this point but I'd coat the splice with RTV, let it harden for a day, then reassemble. Here comes some smart guys to tell me that's going too far . . . just wait they'll show up any time now. So the RTV is optional. Use the remainder to seal any leaks in your bath or shower or roof or gutters, that's what it's really good for.

[cluster]

First get those wires spliced. Easiest way is scrape the lacquer off, twist the ends & solder. You could wrap that with electrical tape or use a wire nut, but my way would be to cover that splice with heat shrink, heat & shrink that first layer. Then add a second & maybe a third layer. It's probably overkill at this point but I'd coat the splice with RTV, let it harden for a day, then reassemble. Here comes some smart guys to tell me that's going too far . . . just wait they'll show up any time now. So the RTV is optional. Use the remainder to seal any leaks in your bath or shower or roof or gutters, that's what it's really good for.

Thanks so much. Could you comment on this and would it be useful to put in my major? :

OUTPUT SPIKE PROTECTION

This modification involves taking three 1N4007 silicon diodes in series with the banded end connecting to pin three of your output tubes and the other end to ground. You use one on each side of a push-pull output. So for example, on a 100 watt Marshall the two tubes on the left, either pin three of either one of those tubes can connect to this diode to ground. And on the other side either pin three can connect to the other set of diodes to ground, as the tubes on either side are in parallel.

We use 1N4007 silicon power diodes which are effective in suppressing spikes at certain frequencies. These will not necessarily prevent spiking on all amps! There is a fast recovery or high speed diode which will suppress higher frequency arcing however these diodes have a very distinct disadvantage as they alter the tone of the amp! They make the amp sound muddy as they bleed off high frequencies from the output transformer! Trainwreck does not use these kinds of diodes because of their effect on the tone.

While the Trainwreck method does not eliminate spiking in every amp, it does not effect the tone at all. Every other method which will totally eliminate spiking will have a negative effect on the tone of your amp.
Another point to remember concerning spiking is that hot biased amps tend to generate more high voltage spiking than properly biased amps.

[Leo_Gnardo]

OUTPUT SPIKE PROTECTION

This modification involves taking three 1N4007 silicon diodes in series with the banded end connecting to pin three of your output tubes and the other end to ground. . . . .

I'm a fan of Ken Fisher & I used to add this to practically every amp I worked on. Now, only if I find there's a recurring problem that may benefit from this technique. In an amp with extra hi voltage like yours I'd series 4 x 1N4007 instead of just 3. On rare occasions I've seen the rectifier string short then you have to hope a fuse blows before the skinny wire in an OT primary burns or the power transformer fails. Not a good thing when your "safety circuit" causes unexpected wreckage.

[cluster]

So I spliced the primary wire and double wrapped it in shrink wrap. Put the OT back together and took some readings:

Primary winding:

end to end = 29.1 ohms
CT to left = 14.8 ohms
CT to right = 16.4 ohms
UL leads end to end = 11.5 ohms
CT to left UL = 7.4 ohms
CT to right UL = 6.7 ohms

All leads to ground = open (no resistance)

Does this sound right? Are there any other tests I should do before testing it with the PT. Would it be worth testing the OT with the battery method on the secondary?

Thx,

Joseph

[cluster]

Also, I'm considering replacing the can caps in this major but the 200uF cancaps are oversized. Modern cans are smaller. Does anyone know of way to modify the large clamps to accommodate the smaller cans? BTW, I know these cans are 47 years old. ESR meter was giving me a reading of .67 which should be good? Is it worth replacing these caps?

[cluster]

Just any update. I used a small transformer to deliver 7.4 VAC to the secondaries to test the windings and I believe it to be working well. Please check my work. 7.4VAC on the 16 ohm tap of the secondary resulted in 90.6 VAC across the primaries with an even 45.3/45.3 VAC from CT to each end. Seems like it's working. Might be time to wire it all up and put the amp on the Variac.

[cluster]

Just follow up... I did the math to calculate the OT's impedance:

90.6VAC/7.4VAC = 12.24 turns ratio

12.24 squared (impedance ratio) x 16 ohms = 2,398.4 ohms or 2.4 KOhms, which is dead on spec.

Next step wire it all back up and put it on the variac (no tubes) and hope she purrs....

[olddawg]

Also, I'm considering replacing the can caps in this major but the 200uF cancaps are oversized. Modern cans are smaller. Does anyone know of way to modify the large clamps to accommodate the smaller cans? BTW, I know these cans are 47 years old. ESR meter was giving me a reading of .67 which should be good? Is it worth replacing these caps?

I haven't done it on a Marshall Major but in several repair/restorations I have been able to either clean out and stuff the old can with smaller, modern caps or mount new ones under the can with the can cap disconnected. As far as old caps are concerned... there's a lot of debate. The consensus would say replace anything to do with your bias. Filter caps? I have a 1963 Temolux that is bone stock except the output tubes. Works great even with modern line voltage. No hum at all and has a strong tremelo channel. I haven't even changed the power cord and death cap. BUT.. it's not a Marshall Major.

[Leo_Gnardo]

Also, I'm considering replacing the can caps in this major but the 200uF cancaps are oversized. - - - - - Is it worth replacing these caps?

The one I worked on had replacement main filter caps, same diameter as the old ones. Replaced I guess 10-15 years ago. I'm not sure what the diameter was. FWIW JJ has some caps in 1 3/4 inch diameter. Other than that you can pore thru online catalogs, see what the big supply houses like Mouser and DigiKey have to offer. Downside - prices bound to be high. Good side - they have warehouses in Canada, should be able to ship without getting hit with customs & cross-border shipping charges.

[cluster]

Question: I don't have a speaker cab here to use as a load but I do have a 16 ohm THD hotplate with a load option on it. I don't intend to test it with tubes in. Would the hotplate be a good enough load to test the OT w/o tubes or should I wait to have the proper cab? If all goes well and I want to test/bias the amp with tubes in (no input signal - just idle) would I need a cab or could I use the 16 ohm THD hotplate load? Thx

Joseph

[The Dude]

If you're going to test it without tubes installed, you don't need a load at all.

[cluster]

If you're going to test it without tubes installed, you don't need a load at all.

Thanks. Yes, the no tubes test is just to see if no fuses blow and to measure voltages at pin 3 (plates), the screens and bias. Hopefully if all goes well i'll put in the tubes and bring the amp up slowly with a variac.