Nice mess in a 78 JMP Superlead...need advice!

[donzoid]

Now you guys see why I called this a "nice mess" yeah?

[martin manning]

I wouldn't worry too much about those 0.22 caps if they weren't there originally. You might see open spots on the PCB for them. F&T are good for the cans, JJ's too, I think. If you order some, check the diameter to be sure they will fit your clamps.

[donzoid]

Thanks Martin. If the caps are helpful in stopping power spikes on the standby side, I could easily put them in at the turrets on the end. Is that the intended function? To smooth out a ripple when you flip that switch? If so, seems worth it for the two or three bucks.

[martin manning]

Those caps are more about noise reduction. I suspect that there are two in series to double the voltage rating (~500V p-p across the secondary), and they are wired across the bridge inputs. The standby is switching AC, so it is in good shape without any other precautions.

[donzoid]

Martin, you "do" realize that the point in your drawing where the PT secondary center tap connects is not strapped directly to ground (like the other cap), that is OK?

Since the original 1959 schematic shows those .22uF caps, I went ahead and picked up a couple (hey less noise in these things is always good, I run the gain up) and will attach some gut shots as I put this thing back together. PT is scheduled to arrive tomorrow...woohoo!

[martin manning]

Martin, you "do" realize that the point in your drawing where the PT secondary center tap connects is not strapped directly to ground (like the other cap), that is OK?

Since the original 1959 schematic shows those .22uF caps, I went ahead and picked up a couple (hey less noise in these things is always good, I run the gain up) and will attach some gut shots as I put this thing back together. PT is scheduled to arrive tomorrow...woohoo!

Re the CT, yes it goes between the two series-wired caps to balance the voltage. The 56k resistors on the screen filters (the other two 50-50 cans) serve the same purpose.

I guess you can attach those two 0.22uF in series on the standby switch, but they are pretty big, no?

Truth be told, you actually could have used the other PT you had, as I see that the PC board is set up to connect the diodes either way, give or take a jumper and a ground lead. There would be a couple of other changes required as described above, though, and with the FWB you are returning it to its stock power supply configuration and as it is shown on the schematic.

[donzoid]

Here is what I did last night with the caps--yes, they are pretty large.

Now, I can solder the transformer leads to the turrets on the outside ends of the caps.

[donzoid]

Bulb test: 25 watt comes on very dim, and the 60 watt "not at all" (but the unit is powered on and the switch glows). Normal? Seems like it is. Caps must have stayed loaded 'cause the 25w bulb never really even came on bright. At least my understanding (thanks Structo) is that it will come on brighter, then dim as the caps load.

Tried to cleanup "some" of the mess while doing the install. LarMar is in and I think, ready to move to full AC with some checks of the voltages.

[Structo]

I'm going to move this to the Marshall section as it is valuable Marshall
information that will be more helpful in that section.

[martin manning]

At least there are no direct shorts ;^)

Go up to a 100W bulb and get some measurements down the power string. Also check for negative bias voltage.

[donzoid]

OK...I will go hunt up a 100w and let you know what we got here.

[donzoid]

105v (AC) at the bias turret.... -40.2 to -48.5 (DC) at the swamp resistors.

[martin manning]

No tubes in yet I guess? That should increase the current draw enough to get a glow on. Measure the AC voltage across the PT primary, and compare to the wall. Other voltages in the amp will be reduced proportionally, more or less.

[donzoid]

No tubes in....Tom told me to start without, put in the preamp tubes, then put in the power tubes in that order...which makes perfect sense. Wow did I just say that?

OK...will measure the volts at the fuse/switch. I already know there's 121 coming out the wall because this monster power conditioner a friend gave me has a digital readout.

[donzoid]

We're only one volt shy at the fuse at 120.3