MASCO MAP 15 - need help install 3 prong ac cord

[sig]

I brought this MASCO MAP 15 back to life with a cap job and new tubes. Yesterday, I replaced the original 2 prong power cord with a gounded 3 prong. It didn't work, it blows the fuse repeatedly. I removed the C11 cap which you can see in the schematic, that's the death cap.

I suspect I have to remove another component. If I lift the ground wire, the amp works.

Probably something very simple, I just don't see it.

Kindly request your review of the attached schematic / layout and advise please.

[trem]

In a nutshell.....
Get rid of #11.
Reverse the order of 40 and 41 so that you hit the fuse first.
Make sure fuse and switch are on the Hot Lead.
Put a bolt through the chassis for the mains ground.
White/Neutral connects straight to transformer.
Use a Slo-Blo fuse of the correct value.

[Jazz P Bass]

Something is amiss with the amp if it blows the fuse when you connect the mains ground to the chassis.
By rewiring the transformer primary , black/white, there is nothing notably different.
Unless the chassis itself is "HOT".
That is what the mains ground is for.
To protect your butt.
But I would expect it to blow the circuit breaker of the outlet that it is plugged into before the amp fuse.

[sig]

I had originally merely removed the c11 cap, replaced the cord without concern for black / white, grounded the green wire to Power Transformer lug. This caused fuse to blow. I just tried and reversed 40 / 41, and like Jazz P Bass expected, the circuit breaker blue. Does this suggest I've got a hot chassis and I need to go back over it to identify a loop of sorts? Appreciate next step recommendations! Thanks.

[Jazz P Bass]

Sounds to me like the power transformer itself.
With power off you could try some basic continuity ckecks.
See if any of the primary windings are internally shorted to the outside of the transformer.
Then work on the secondary side.
Same thing. Looking for windings that are shorted to the outside.
If nothing notable shows up, then you could disconnect the secondaries.
See if that pops the breaker.
It would be nice if you had a variac.

[sig]

No continuity on the PT Primary leads to ground

Green secondary pair at 207 oHms
Yellow secondary pair at 50 oHms

I didn't disconnect the secondary leads on this test, I suspect I may have caused the problem when installing new caps and perhaps connected something wrong somewhere. I'll have to disconnect the secondary leads and retest. More to follow.

[sig]

I didn't thing of this before but it might be a cause of my problem as there are subtle differences in the schematic from my amp because my amp has 7F7 loctal tubes in lieu of the 6SL7 tubes. Thoughts? I removed the new green ground wire just to get back to a baseline starting point and the amp works pretty good. Connect the green wire and ckt breaker blows.

[Jazz P Bass]

One more time.
Youi have a hot chassis!

[g1]

Sounds like this is not the schematic. Like Jazz said you have a hot chassis. If it is hot by design (which I usually suspect when I see any tubes that don't start with 6 or 12) you can't run a 3 prong and you need an isolation transformer.

[sig]

Aye, aye, Captain. Thanks Jazz P Bass. I've done all I can do at my level I think. I might disconnect one end of each of the caps I replaced and power up to make sure the hot chassis isn't something I caused. But, I think my next step is to take it to a pro and get it done by a pro. This has been fun, I wish I had the skill to continue safely but I don't. Appreciate the help thus far, I was hopeful someone would be able to see something on the schematic other than the C11 death cap that might reflect older design / signal ground or something to try. If there's nothing standing out, I'll throw in the towell and take the amp in for service by a pro. Thanks again, I really appreciate everyones time!

[sig]

g-one, the schematic reflects the 6SL7, but as a sidenote in the table it lists or 7F7 which is the loctal equivilent. Thank you for your reply, I believe you and Jazz P Bass are both right. I think the 7F7 tubes are using the chassis for signal ground. I have another amp like this that gave me hell about 2 years ago, and old "Fidelity 3T" that also uses the 7F7 tube and I tried to put a 3 prong cord to it, had the same problem and I never completely resolved that either. I need to learn more about the loctal tube, it's center is grounded to chassis which was my first clue this is most likely the issue.

[52 Bill]

According to the tube manual there is no internal connection on the base pin, and in any case this should be no cause for a hot chassis problem.

Start by checking your ac cord. Are the color wires correct to code? For that matter is the ac outlet that you are using correct?

If the chassis is hot to the ground wire from the ac cord there has to be something in the transformer or the wiring that is connecting the hot to the chassis. Check the resistance from the ac wiring to the chassis.

[JHow]

I'm wondering about the heater circuit here. Does it have a common return to the transformer, or does it use the chassis for a return? I can't tell from the schemo, but you can tell by looking at how it's wired.

[sig]

The last time I powered the amp up and it tripped the breaker, it blew out my 5Y3 as well. I attempted to power it up again today without tubes, it powered up while grounded and didn't trip the breaker. Next, I put in the 5Y3 and 6L6s and powered up, the breaker again, didn't trip.
Next, I tried one 7F7 at a time thinking I might be able to isolate the problem to a tube but after putting them both in and powering up, the breaker didn't trip. After realizing the 5Y3 and 7F7s weren't glowing, I powered down and pulled and tested all the tubes. The 5Y3 was blown.

Does this shed light on or isolate where my problem is likely to be?

Thanks!

Paul

[Prairie Dawg]

I'm assuming that your green wire is grounded. With the rectifier tube out you should be able to check your six volt filament windings (green), your 5v winding at the rectifier tube (yellow) and your HV winding at the rectifier tube socket (red). Remember these are all AC. You should be seeing about 6.3v. 5.0v and 325 v on either red lead to ground.

If that's all good try another 5Y3 or other fine rectifier tube, but don't install the other tubes just yet. If it blows you've localized your problem. If you're good put the power tubes in and see what happens. If the fuse blows you've localized the problem.

If you find that installing a rectifier tube precipitates the problem but you have AC on the red leads you can always bypass the rectifier tube with a couple diodes or a rectifier solid state replacement like a Sovtek SS-5U4.

Think of it as a system like the water in your house. You work from the source of the pressure outward until you identify the fault.