1962 TrueTone 4DC6231-A

[Hi-Fi-Mogul]

I finished the electrolytic caps and powered up the TrueTone.

And RATS, there's a volume-dependent 120 Hz hum.
on both channels.

I've checked ground continuity on the cartridge and chassis
which appear OK.

The 60FX5 tubes tested good, but I wonder if they
could still be the source of the hum.

The amplifier output is good, and not distorted.

[Ken Doyle]

Because the hum is volume dependent, I would think it's coming from before the volume control.

[Hi-Fi-Mogul]

Because the hum is volume dependent,
I would think it's coming from before the volume control.

Yes Ken, I'm working on that assumption.

I shorted each stereo channel input wire to the changer
base, which is wired to the amplifier chassis.

The hum disappeared.

Should I re-flow the wiring connections on the Vol pots ?

AND, the SAMS schematic is incorrect indicating that
the phono cartridge is grounded to B-minus.

The phono cartridge ground is wired to the chassis
connection which goes across C5 to B-minus.

[TC Chris]

Check the connections at the cartridge pins?

Chris Campbell

[Hi-Fi-Mogul]

Check the connections at the cartridge pins? Chris Campbell

Done, all OK, and the output is clean.

[TC Chris]

In the old days the wax/paper caps had the "outside foil" end marked so they could be oriented to reduce hum, in certain applications. Now most caps aren't marked and mostly it doesn't matter. but I wonder if it could be a problem in your amp? I only see two signal caps and they are tone-limitation devices so should not be culprits. Or maybe your C5--did you use a safety cap? I've got an old Bogen DB115 amp that developed a hum until I lifted the ground cap (long, long ago--if I revive it I will replace the old disc cap with a safety cap).

Chris Campbell

[Hi-Fi-Mogul]

I re-flowed the wiring on the Vol pots, no change in hum.

I re-flowed the tonearm wires on the terminal strip mounted
under the changer, no change in hum.

I replaced the cartridge with a good P-228, good output,
but hum remained.

Tomorrow I'll recheck connections on C5.

Yes, I put in a 0.01 mF X1Y2 safety cap there, Chris.

I will solder new tonearm wires to a new terminal
strip and see if that helps.

I had to walk away from it tonight, attitude gettin' bad !

[William]

Just asking, are you powering it up with a Variac.....

Bill

[Hi-Fi-Mogul]

Just asking, are you powering it up with a Variac.....

Bill

Hi Bill,
Now WHY would you ask that ? [ ; )

Using regular old house current.

[electra225]

I may have missed this, so if I did please forgive me. I don't see where you subbed the cables from the changer to the amp. Sometimes those lose their shield connection. If it isn't a big problem to do, that is about all you haven't tried. Good luck.

[Hi-Fi-Mogul]

I may have missed this, so if I did please forgive me. I don't see where you subbed the cables from the changer to the amp. Sometimes those lose their shield connection. If it isn't a big problem to do, that is about all you haven't tried. Good luck.

If you go back to the first page, down in the photos, you can
see a red and white wire from the amplifier.

Those are the amp's stereo channels' input wires that are attached to
a terminal strip under the changer.

The tonearm wires are soldered onto that terminal strip.

This unit did not use shielded wires, which I learned was not
uncommon in these little stereos.

Since the wiring is factory, I know the problem is somewhere else.

But, WHERE ? Ha ha !

[electra225]

Maybe the hum is "factory" as well....

There isn't much to that amp, not much to cause trouble that would make you scratch your head trying to figure it out.

One of the little GE Trimline stereos I have, the one with the crystal cartridge, is reputed to have a factory hum. If it's there, I don't hear it.

[TC Chris]

No wonder you're frustrated--that amp is as simple and low-parts-count as it can get.

Any possibility that you have an out-of-spec filter cap? Maybe just tack another one across and see what happens?

Chris Campbell

[Hi-Fi-Mogul]

No wonder you're frustrated--that amp is as simple and low-parts-count as it can get.

Any possibility that you have an out-of-spec filter cap? Maybe just tack another one across and see what happens? Chris Campbell

I'll be open to trying lots of fixes.

The amp is dead quiet when the Vol pots are
turned all the way down.

Wouldn't a bad filter cap allow for some
hum with the Vol pots all the way down ?

And Bill made a suggestion about hooking up the changer/cart
to another amp/speaker source to see what happens.

When I get back to the project, I'll try that, too.

[William]

Are the red and white input wires shielded within themselves?

Bill

[electra225]

The schematic does not show shielded wires from the cartridge to the amp. I would also think hum caused by a bad filter cap would not be affected by the volume control. Have you tried a redundant ground from volume control to common negative? Go around the .1 cap to B-.

[Hi-Fi-Mogul]

I tried a separate ground from Vol control to common Neg.

No change.

BUT, I don't know if I missed this previously.

The LEFT channel is the only channel with the bad hum.

RIGHT channel has the expected low level hum.

I switched out the tubes, but the L channel hum
is still there.

What about spraying out the Left Vol pot again with
electronics cleaner ?

Seems worth a go.

[Ken Doyle]

Might be worth a try to compare voltage readings between the two channels.

[Hi-Fi-Mogul]

Might be worth a try to compare voltage readings between the two channels.

I have not done that. Will do today. Thanks for the reminder.

[electra225]

You also might check the tube on the channel with the most hum for grid emissions. Measure the grid voltage as the tube gets warmer to see if the grid voltage gets more positive. Tube testers won't find grid emissions.