Meck XB-702 restoration

[jr_tech]

Perhaps try a lower value (1 to 5 meg) for R53?

jr

[bandersen]

Replacing the 6AT6 seems to have helped. However, when I put the original back in, it seems better now too. I did clean the tube sockets with DeOxit right at the beginning but maybe it wasn't thorough enough?

Or maybe an ugly possibility that there is some contamination in the tube socket. I'll eliminated every other possibility before I even think about replacing it.

Another observation (clue?) - the hum/noise slowly changes over time. Perhaps in sync with the slowly traveling wave in the horizontal. I assume that's some 60Hz signal interacting with the 29.97Hz vertical oscillator.

[old_tv_nut]

You have hit on something, maybe as simple as there are two sources of hum, the vertical rate and the AC power rate, so you may need to eliminate two problems to completely get rid of it.

Dumb question - could it have something to do with the hot chassis, like reversing the power plug, or running it with vs. without an isolation transformer?

[Notimetolooz]

Might be lead dress.

Is the volume control ganged with the ON/OFF switch?

Hum can also be coupled magnetically.

The schematic has a cap, C53, indicated from the pot case to ground, but in the photo it looks like the volume control is simply attached to the chassis.

[bandersen]

Yes, it is ganged with the power switch. C53 is located on the other side of the chassis near the ratio detector coil. It's connected to the braid off the coax going to the volume control.

[bandersen]

Hmm, maybe not. There's an error in the Sams. They list C53 as a 0.02uF cap in the parts list and parts locator diagram.

[Tom9589]

The SAMS is full of discrepancies. It shows a 5000 pF cap between the case of the volume control to ground on the schematic. The photo of the capacitors (page 11) shows no capacitor from the volume control case to ground. On the other capacitors photo (page 18) C53 is shown near the 6AL5 tube socket. Furthermore, the parts list shows C53 as a .02 uF 400 V tubular cap. Good luck with this one.

[old_coot88]

Just wondering - where is the 6AT6's heater located in relation to ground? Does one leg go to ground? Or is the heater "higher up" in the series string? If higher up, the elevated AC voltage between heater/cathode could transfer hum.

In most series string radios f'rinstance, it's common practice for the 1st audio tube's heater to be at the very bottom (ground) end of the string to minimize hum.

[bandersen]

The 6AT6 is right in the middle of the filament string. The Riders version of the schematic does not show a cap from the volume control body to ground.

Here's a close-up photo of the area. The Tele-tone service info says to not ground the volume control to the same point as the AC line. That's exactly what they've done here. It's no wonder it picks up some hum.


Here's the Tele-Tone schematic. Looks like someone forgot to bring their compass to work.


Finally, here's the Sams for the Tele-Tone showing C53 on the volume control.
That makes me wonder if Sams was working on both the Meck and Tele-Tone service info at the same time.


First I'll try adding a 5,000pF ceramic dsic cap from the pot body to the chassis. If that doesn't help, I'll reroute the power switch chassis connection to another tie point.

[Kevin Kuehn]

The 10meg resistor is being used as grid leak bias with the cathode grounded. It's a perfect recipe for high impedance/ hi gain buzz. Unless there's a lack of volume(not likely if it's buzzing) you can try an unbypassed cathode bias resistor with something more like 1meg between grid and ground. The unbypassed cathode will allow some negative feedback and tame things down a little. You'll likely end up with volume control turned up higher. Probably leave the 2 unused diodes ground.

[bandersen]

Thanks for the suggestion. I have plenty of volume - maybe too much so this may work out very well.

[Kevin Kuehn]

Actually the diode anodes should probably stay tied to the cathode, with the cathode resistor between the 3 pins to ground. Not sure if it really matters but you probably don't want the diodes biased on when the cathode goes negative. You may be able to take the volume control wiper directly to the grid without the .002 coupling cap, otherwise you may loose bass response unless you increase it's value.

[bandersen]

It seems the body of the volume/power switch is making physical contact with the chassis so the bypass cap isn't going to do anything. I did reroute the AC power to connect to a separate point on the chassis. That helped a bit.

It does seem that the really sensitive part is the side of the cap connecting to the grid. I'm thinking if I shorten that lead it may help.

Will also try a cathode resistor next.

[old_coot88]

If all else fails, I would do a bit of rewiring to put the 6AT6 heater at the ground end of the string.

[bandersen]

Interesting results with a 1M cathode resistor. The gain does drop, but the grid is far less sensitive to external noise. However, there is still a buzz present. Grounding the grid does nothing to reduce it. Grounding the cathode reduces it by about 50%.

I'll continue to experiment.