Panasonic 730 AM/FM Six tube radio -Loud Hum even after Cap replacement

[malomarski]

Hello folks,

I spent the day replacing nearly all of the capacitors on an old (1962)
Panasonic AM/FM radio hoping the humming would go away...

Unfortunately it didn't.

I have scoured all of Google trying to find additional advice, but did
not find anything beyond replacing "filter capacitors" (already done)

Please see my impromptu webpage which has a pic of the radio and the
schematic:

http://www.geocities.com/omar5193/panasonic.html

If you have any ideas (whether good or bad) please post as reply to
group here.

Thanks for your help and knowledge!

Malomarski

[Bill M]

Post by malomarski
Hello folks,
I spent the day replacing nearly all of the capacitors on an old (1962)
Panasonic AM/FM radio hoping the humming would go away...
Unfortunately it didn't.

Heater to Cathode leakage in the 50C5 perchance? Not uncommon, even in NOS.
Is that a selenium rectifier that I see? (X1) You might want to check
the voltage on its output where it says 130v. When they go bad they
drop voltage excessively and this can throw off everything resulting in
hum...sometimes.

-Bill

[Luke]

I have to agree on the 50C5. Had that happen to one of my radios. After
changing the caps I was surprosed at how easy a fix it turned out to be.
Luke

Post by Bill M

Post by malomarski
Hello folks,
I spent the day replacing nearly all of the capacitors on an old (1962)
Panasonic AM/FM radio hoping the humming would go away...
Unfortunately it didn't.

Heater to Cathode leakage in the 50C5 perchance? Not uncommon, even in NOS.
Is that a selenium rectifier that I see? (X1) You might want to check the
voltage on its output where it says 130v. When they go bad they drop
voltage excessively and this can throw off everything resulting in
hum...sometimes.
-Bill

[Jim Strickland]

Post by Luke
I have to agree on the 50C5. Had that happen to one of my radios. After
changing the caps I was surprosed at how easy a fix it turned out to be.
Luke

Post by Bill M

Post by malomarski
Hello folks,
I spent the day replacing nearly all of the capacitors on an old (1962)
Panasonic AM/FM radio hoping the humming would go away...
Unfortunately it didn't.

Heater to Cathode leakage in the 50C5 perchance? Not uncommon, even in NOS.
Is that a selenium rectifier that I see? (X1) You might want to check
the voltage on its output where it says 130v. When they go bad they
drop voltage excessively and this can throw off everything resulting in
hum...sometimes.
-Bill

If replacing the 50C5 as previously suggested doesn't get it, and you
check that your rectifier is both producing the correct voltages *and*
rectifying (a scope helps here, otherwise you might try putting a VOM
set on AC from B+ to ground. I don't think you should get anything -
someone yell if I'm wrong, okay?) - if all that checks out, you might
look at the tone control/switch. I'd try unsoldering the wires from
the switch and connecting them together, then plug the radio in. If
your humming is gone, you've got some kind of coupling going on from
your AC (the switch) to your tone control (audio).

[d***@juno.com]

Post by malomarski
Hello folks,
I spent the day replacing nearly all of the capacitors on an old (1962)
Panasonic AM/FM radio hoping the humming would go away...
Unfortunately it didn't.
I have scoured all of Google trying to find additional advice, but did
not find anything beyond replacing "filter capacitors" (already done)
Please see my impromptu webpage which has a pic of the radio and the
http://www.geocities.com/omar5193/panasonic.html
If you have any ideas (whether good or bad) please post as reply to
group here.
Thanks for your help and knowledge!
Malomarski

Just curious, does the volume control have any effect on the prob? Just
finished a very similar Panasonic set, a little different in the audio
section.
I noticed that the vol pot was not grounded, you would get a hum
touching the vol control shaft..

[malomarski]

Post by malomarski

Post by malomarski
Hello folks,
I spent the day replacing nearly all of the capacitors on an old

(1962)

Post by malomarski
Panasonic AM/FM radio hoping the humming would go away...
Unfortunately it didn't.
Malomarski

Just curious, does the volume control have any effect on the prob? Just
finished a very similar Panasonic set, a little different in the audio
section.
I noticed that the vol pot was not grounded, you would get a hum
touching the vol control shaft..

Thanks for the input. No, I don't think that the volume control has any
major effect on the humming, but I'll have to take a look at it this
weekend and that'll be the first thing I check...

I hope it is because that would be fairly easy to correct...

Thanks.
Malomarski

[robert casey]

Post by malomarski
If you have any ideas (whether good or bad) please post as reply to
group here.

Shorted rectifier diode? Bad ground?

[malomarski]

Post by robert casey

Post by malomarski
If you have any ideas (whether good or bad) please post as reply to
group here.

Shorted rectifier diode? Bad ground?

I haven't checked the rectifier diode yet, and not really sure how I
would go about checking one per se.
I did replace one on a Zenith TransOceanic about a year ago though...

Thanks.
Malomarski

[John Byrns]

Post by malomarski
Hello folks,
I spent the day replacing nearly all of the capacitors on an old (1962)
Panasonic AM/FM radio hoping the humming would go away...
Unfortunately it didn't.
I have scoured all of Google trying to find additional advice, but did
not find anything beyond replacing "filter capacitors" (already done)
Please see my impromptu webpage which has a pic of the radio and the
http://www.geocities.com/omar5193/panasonic.html
If you have any ideas (whether good or bad) please post as reply to
group here.

I don't have any suggestions to help you cure the hum problem in your
Panasonic 730 AM/FM Six tube radio, but I am curious if anyone can explain
the operation of the portion AM/FM switching circuit shown in the center
of the schematic around the switch labeled "M2 FRONT"? The circuit
doesn't make any sense to me as drawn, in the AM mode there is no DC
reference provided to the RF input grid of the 12AJ7 converter tube, the
usual filter capacitor is missing from the AGC line, and the AGC line is
inexplicably connected to the AM oscillator grid and the injection grid of
the converter tube. Does anyone know what is going on here?

This radio also seems to use all the tricks that are suggested from time
to time in this group for improving the sound of AC/DC radios. The heater
of the 12AL5 in the Ratio Detector is run with reduced heater voltage to
reduce the contact potential, although the trick is not used with the AM
detector, possibly because it would reduce the contact potential bias of
the 1st AF amplifier too much, which is in the same tube. Or perhaps it
is because the contact potential actually works to counteract the effect
of the AC loading that is greater than the DC loading on the detector,
thereby improving the modulation acceptance.

Negative feedback is also used, taken from the speaker terminals of the
output transformer back to the cathode of the AM detector/1st AF amplifier
tube. The feedback loop has an extra series capacitor that seems intended
to provide a bass boost at frequencies below about 160 Hz. It would have
been nice if they had provided a switch to short out this capacitor when
its effect is not desired.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

[robert casey]

Post by John Byrns
I don't have any suggestions to help you cure the hum problem in your
Panasonic 730 AM/FM Six tube radio, but I am curious if anyone can explain
the operation of the portion AM/FM switching circuit shown in the center
of the schematic around the switch labeled "M2 FRONT"? The circuit
doesn't make any sense to me as drawn, in the AM mode there is no DC
reference provided to the RF input grid of the 12AJ7 converter tube, the
usual filter capacitor is missing from the AGC line, and the AGC line is
inexplicably connected to the AM oscillator grid and the injection grid of
the converter tube. Does anyone know what is going on here?

Looks like a drafting error to me. C15 0.05uf (drawn near the AM
antenna) looks to be the AVC filter when the radio is in AM mode.
However that would kill the oscillator tank circuit. So I suspect
that it's an error. As for the connections to the 12AJ7, it
looks the same as the suggested circuit in the PDF data sheet
I downloaded from Frank's tube data web site. G1 gets the RF,
G3 the LO from the triode portion.

Post by John Byrns
This radio also seems to use all the tricks that are suggested from time
to time in this group for improving the sound of AC/DC radios. The heater
of the 12AL5 in the Ratio Detector is run with reduced heater voltage to
reduce the contact potential, although the trick is not used with the AM
detector, possibly because it would reduce the contact potential bias of
the 1st AF amplifier too much, which is in the same tube.

I spotted that too. Though the voltages noted on the heater string
don't really make sense with a 500 ohm resistor across the 12AL5
heater. Also I've been able to improve the diode detector in the
12AV6 without adversely affecting the triode by reducing its heater
voltage to 10V.

[Mark Robinson]

Hi,

To see if its a bad ground (as was suggested), try tacking a jumper from the
volume control side of C31 to ground. If the hum goes away then I'd look
the grounding and shielding. Does this radio use a printed circuit board?
If so, perhaps a cracked trace is the problem.

Mark

Post by malomarski
Hello folks,
I spent the day replacing nearly all of the capacitors on an old (1962)
Panasonic AM/FM radio hoping the humming would go away...
Unfortunately it didn't.
I have scoured all of Google trying to find additional advice, but did
not find anything beyond replacing "filter capacitors" (already done)
Please see my impromptu webpage which has a pic of the radio and the
http://www.geocities.com/omar5193/panasonic.html
If you have any ideas (whether good or bad) please post as reply to
group here.
Thanks for your help and knowledge!
Malomarski

[malomarski]

Post by Mark Robinson
Hi,
To see if its a bad ground (as was suggested), try tacking a jumper from the
volume control side of C31 to ground. If the hum goes away then I'd look
the grounding and shielding. Does this radio use a printed circuit board?
If so, perhaps a cracked trace is the problem.

I'll try that this weekend. No printed circuit board on this one, just
a nest of components that are hard to reach..
Thanks.
Malomarski