Presto D7 guidance needed

[Lightfoot]

New here, but not new to working on electronics. BUT I am new to lathes, and could use some help troubleshooting this one.

The owner says that there's no sound from the speaker. Before I pull out the amp, I want to check what I can to confirm that either the speaker, the output transformer, or the connections to them aren't the culprit. It doesn't seem like there's a schematic or service manual out there for this, so I'm flying a little blind as to what I should expect to see resistance wise. That said, based on what I'm measuring at the pins for the output plug and the speaker itself, nothing appears to be open. Feedback appreciated!

Output plug:

Speaker Plug.jpg

Wires going to the front voice coil:

Speaker 2.jpg

Wires going to the rear magnet:

Speaker 1.jpg

Someone spliced in these wires:

Speaker 4.jpg

The model of speaker:

Speaker 3.jpg

[markrob]

Hi,

That speaker has a field coil rather than a permanent magnet. The field coil was usually used in dual duty as a choke in the B+ supply and as an electro-magnet for the speaker. I'm guessing that the speaker is fine, but the B+ supply is dead (bad rectifier tube or shorted filter cap, etc.). Note that the speaker connector was also an interlock for the B+ supply so that if the speaker is unplugged, no B+. This prevents from running no load on the power amp. I see some funky taped wiring, which might be an issue. You should be able to trace out that part of the circuit if you use an RCA tube manual for the rectifier tube basing diagram. It would be a good idea to check the power transformer B+ secondary winding with the tube pulled to see if it produces 300 or so VAC.

Mark

[Lightfoot]

Hi Mark,

Thanks for the detailed reply and advice. Very helpful. I had forgotten about field coils. The last (and only!) time I dealt with one was during the repair of a Zenith 5S237.

All the tube filaments are lighting up and all caps were supposedly replaced, but I'll pull the amp to check that the B+ is everywhere it should be and at the correct voltages. Speaking of pulling this out, it appears that the entire top needs to be removed, not just swung up and back, in order to get the amp to clear the case. But maybe the opening toward the front only seems slightly too small. . . .

Those taped wires look dubious to me too. It appears that someone spliced them inline with two of the four wires running from the plug and then taped them up. I assume this was for testing.

Matt

[Lightfoot]

Side issue question involving the female three pin Cannon connection in on the top of this lathe. The original wire is broken and needs to be replaced. The center conductor ties in with that of the wire connecting to the anode cap for a 6C6. The shielding of the Cannon wire is tied to ground via what looks like a green resistor, but may be a capacitor (not measuring anything in the ohms setting). Since I believe the audio signal is passing through the center conductor but it's also tied in to the anode of the 6C6, which is carrying voltage, I'm wondering if something like RG-58 would make sense here.

Cannon 1.jpg

Cannon 2.jpg

Cannon 3.jpg

[markrob]

Hi,

The cap of the 6C6 is the signal grid, not the plate as is the case with higher power tubes like you might find in the horizontal output on a TV. So, there are no high voltages or currents present. Just low level, high impedance. Mic grade shielded cable should be fine here.

Mark

[Lightfoot]

Hi Mark,

Thanks for clearing that up for me.

Matt

[Lightfoot]

Hi,

That speaker has a field coil rather than a permanent magnet. The field coil was usually used in dual duty as a choke in the B+ supply and as an electro-magnet for the speaker. I'm guessing that the speaker is fine, but the B+ supply is dead (bad rectifier tube or shorted filter cap, etc.). Note that the speaker connector was also an interlock for the B+ supply so that if the speaker is unplugged, no B+. This prevents from running no load on the power amp. I see some funky taped wiring, which might be an issue. You should be able to trace out that part of the circuit if you use an RCA tube manual for the rectifier tube basing diagram. It would be a good idea to check the power transformer B+ secondary winding with the tube pulled to see if it produces 300 or so VAC.

Mark

Hi Mark,

Check me on something. I don't know what the DC resistance spec would be for this Jensen field coil, but what I'm measuring--around 0.3 to 0.5 ohms--doesn't seem right.

Speaker 1.jpg

Matt

[markrob]

That does not sound right. Are you sure you have the field coil isolated from the rest of the circuit? If so, then I suspect its shorted. If so, you might get lucky and find its due to pinched wire insulation at the entry points.

If you do an internet search on Jensen field coil speakers, you will find quite a bit of info. I see values listed from 250 ohms to 5K depending on how it was powered. They seem need about 12W of power to energize properly.

Mark

[Lightfoot]

I measured it again, this time with the tan lead wire disconnected from where it was soldered to the terminal strip, i.e. free floating. The black wire is still connected to the lug that also ties into the lead that goes to the main speaker coil. I'd think that would be sufficient to get a true reading. Same result: .03 or so ohms.

[Lightfoot]

Never mind. Apparently, I was measuring the wires for the hum bucking coil. The field coil wires are tucked further back. There I measure about 1,000 ohms.

[Lightfoot]

Hi,

The cap of the 6C6 is the signal grid, not the plate as is the case with higher power tubes like you might find in the horizontal output on a TV. So, there are no high voltages or currents present. Just low level, high impedance. Mic grade shielded cable should be fine here.

Mark

Hi Mark (or anyone else who's worked on a D7),

So I believe the green component is a resistor, though if it is, it's measuring open. Any idea what value would make sense here? Also, for the three pin connections, I'm thinking the signal conductor goes to one and the shield is shared with the other two. Correct?

Cannon 4.jpg

Jack 2.jpg

[markrob]

Hi,

I would think this was a cap. 85pf seems like a reasonable value for a high impedance circuit. See if you can fully expose the part and look for markings.

Mark

[Lightfoot]

Hi Mark,

I removed it completely but no markings. I then "dissected" it.

Cap 1.jpg

Coiled lead windings at either end with a core of black material. Infinite resistance measured when probing across a millimeter of the black core, so it looks like it is a cap.

It would seem I can swap in anything around 100pF.

Any thoughts on the pin connections?

Matt