Magnatone M10A

[stokes]

Got this amp in a couple days ago, the rotary power switch was loose and spinning freely when I tried to turn it on.There are 4 tabs on the outside perimiter of the switch, 3 of the four connections were broke off only the ground connection was still intact. So I quickly figured out where the ground wire from the power tube cathodes connected for the standby switch. So that leaves 2 connections. One is the .047 that is connected to the back of the switch at one end leaving the other end of it hanging loose. The other loose connection is a wire coming from pin 1 of that 12 connector plug which is the other side of the ac line. When I turn the switch on I get 117vac between the loose end of the cap and the loose wire. So that part of the switch is working, but I'm having a hard time wrapping my head around whats going on inside that switch.Heres a sch em and photo. I assume the cap goes to one of the tabs and the wire goes to the other tab on the outside of the switch. I tried them connected to each and neither gives me any juice to the transformer. Bad switch? Any idea where those 2 loose ends should go?

[The Dude]

The schematic shows the switch as a 4 position switch with 3 separate switches enclosed. Off, Standby, and 2 different positions for switching 2 death caps . I would remove death caps, install a grounded AC cord, and ignore that part of the switch. Then use your meter to see which pins of the switch to use for AC power and standby (which grounds the cathodes through that 100 ohm resistor). The switch contacts for mains power will be connected in all positions except Off (probably the switch at the back of the assembly). Standby contacts will short in the next 2 switch positions. Note 2 pins of the standby switch are externally jumpered. Hopefully that makes sense.

[stokes]

Somebody installed a 3 prong cord already. It looks like a lot of things have been changed at some point. Whats throwing me is those two .047 caps that are in series with each side of the ac main. I'm confused by the numbered connection points in the switch, 1, 2, 3, 4. There are 4 tabs on the outer part of the switch and 2 on the back of the switch. How do I determine the numbers on the tabs around the outside? I'm not familiar at all with this type switch, obviously. What I have now is the wire from pin one in the plug, which is the fused leg of the ac line, supposed to be connected to #3 on the switch, is disconnected. Where is that #3 on the switch? And one leg of that .047 cap on the other side of the ac line hanging loose. Its getting late, I'm tired, had a busy day, I'll get back at it tomorrow.

[The Dude]

Those .047 caps tied from mains and switched to ground are the death caps. As I said, remove them. As far as pin numbers on the switch, you don't really need to be familiar with the switch. Just use your meter to test continuity of the switch contacts in different switch positions and use that information to wire it up.

[stokes]

This is what was puzzling me, on the schem it looks like one of those .047's is in series with the ac line, but in reality I dont even need the wire going to pin one of the 12 pin plug and then on to the switch. So my only concern with the switch is that it breaks the one side of the ac line and then grounds the power tube cathodes when its taken out of standby position,which it does seem to do. To sum it up, the switch seems to be wired correctly but I'm not getting ac to the PT.The cap and wire that are not connected to the switch that I thought were broken off from the loose switch spinning dont need to be connected to anything. It seems who ever did the 3 prong conversion left both those caps intact. The power tranny is in a separate chassis with about 2 ft of umbilical cord with 11 wires connecting them.I am now assuming the problem is the juice getting back to the PT via that wire bundle. Thanks Dude. I'll get at it later and let you know what I find.

[stokes]

Thanks, Dude. I got the switch sorted out, that .047 cap looked like it was in series with the ac line, that was screwin with my head . Got that sorted out. It seems the problem is with that "interlock" switch between pin nine and the power switch. It looks like some kind of circuit breaker. I get continuity when the button is held in but it wont keep contact when I release it. Looks like its just a second "fuse" in the other ac line. I think I'll bypass it.

[g1]

Interlock would be some physical thing that held that switch depressed when unit assembled. Prevents electrocution when unauthorized types are in the chassis, or sometimes protects OT when load not connected. Another example would be the Ampeg's with the 4 pin XLR speaker connector, where 2 of the pins were used as interlock so unit wouldn't work without speaker connected.
Probably the actuator got broken or lost in this unit.

edit: apparently actuated by one of the chassis mount screws: https://el34world.com/Forum/index.ph...9926#msg209926

[stokes]

That interlock switch thing was the problem. Voltages all look good, now I can turn it on to see what it might need. I already changed the electros, fired it up with the current limiter and no shorts. Its funny how problems that are simple seem complicated til you get a different view, I was looking at this issue from the wrong angle til Dude pointed me in the other direction.

[g1]

If you missed it, see edit/link in post #7

[stokes]

Interlock would be some physical thing that held that switch depressed when unit assembled. Prevents electrocution when unauthorized types are in the chassis, or sometimes protects OT when load not connected. Another example would be the Ampeg's with the 4 pin XLR speaker connector, where 2 of the pins were used as interlock so unit wouldn't work without speaker connected.
Probably the actuator got broken or lost in this unit.

Ah, I see now. When the amp is assembled one of the retaining bolts will hold that button in. I had forgotten about the Ampeg spkr arrangement.