Hum Won't Go Away

12/14/2010 12:05:44 PMDale

Greetings - I have a "hum problem" with a Motorola model 5T22Y-1 that I have'nt been able to solve. The "hum" sounds just like the typical filter capacitor problem -- it is independent of volume control and independent of tuning frequency. The radio receives fine, but there is a an overriding "hum". I have replaced the filter capacitors (and verified that the new capacitors are good by haywiring additional new capacitors in parallel). I have verified that the cathode of all tubes is NOT shorted to the heaters or other components, and have replaced every tube with known good tubes. I have measured the resistance from the cathode pin on all tube sockets to all other pins -- no problems. Any suggestions? (I do not have a scope to look at specific signal integrity.) Any suggestion will be greatly appreciated. THANKS!

12/14/2010 12:33:00 PMNorm Leal

Hi Dale

Were original filter caps removed? Did you use exact same place for negative of new replacement caps? Even though B- connecting to a different point in a radio can allow hum.

http://techpreservation.dyndns.org/schematics/58/58_Motorola_5T22Y1_112.djvu

Norm

:Greetings - I have a "hum problem" with a Motorola model 5T22Y-1 that I have'nt been able to solve. The "hum" sounds just like the typical filter capacitor problem -- it is independent of volume control and independent of tuning frequency. The radio receives fine, but there is a an overriding "hum". I have replaced the filter capacitors (and verified that the new capacitors are good by haywiring additional new capacitors in parallel). I have verified that the cathode of all tubes is NOT shorted to the heaters or other components, and have replaced every tube with known good tubes. I have measured the resistance from the cathode pin on all tube sockets to all other pins -- no problems. Any suggestions? (I do not have a scope to look at specific signal integrity.) Any suggestion will be greatly appreciated. THANKS!
:

12/14/2010 12:36:38 PMLewis L

:Hi Dale
:
: Were original filter caps removed? Did you use exact same place for negative of new replacement caps? Even though B- connecting to a different point in a radio can allow hum.
:
:http://techpreservation.dyndns.org/schematics/58/58_Motorola_5T22Y1_112.djvu
:
:Norm
:
::Greetings - I have a "hum problem" with a Motorola model 5T22Y-1 that I have'nt been able to solve. The "hum" sounds just like the typical filter capacitor problem -- it is independent of volume control and independent of tuning frequency. The radio receives fine, but there is a an overriding "hum". I have replaced the filter capacitors (and verified that the new capacitors are good by haywiring additional new capacitors in parallel). I have verified that the cathode of all tubes is NOT shorted to the heaters or other components, and have replaced every tube with known good tubes. I have measured the resistance from the cathode pin on all tube sockets to all other pins -- no problems. Any suggestions? (I do not have a scope to look at specific signal integrity.) Any suggestion will be greatly appreciated. THANKS!

Short the grid of the output tube to ground (or cathode). What happens to the hum??
Lewis
::
:
:

12/14/2010 1:04:02 PMDale

Lewis - When I short the grid of the output tube (pin 2 of the 50C5) the hum goes away -- what does this imply?

::Hi Dale
::
:: Were original filter caps removed? Did you use exact same place for negative of new replacement caps? Even though B- connecting to a different point in a radio can allow hum.
::
::http://techpreservation.dyndns.org/schematics/58/58_Motorola_5T22Y1_112.djvu
::
::Norm
::
:::Greetings - I have a "hum problem" with a Motorola model 5T22Y-1 that I have'nt been able to solve. The "hum" sounds just like the typical filter capacitor problem -- it is independent of volume control and independent of tuning frequency. The radio receives fine, but there is a an overriding "hum". I have replaced the filter capacitors (and verified that the new capacitors are good by haywiring additional new capacitors in parallel). I have verified that the cathode of all tubes is NOT shorted to the heaters or other components, and have replaced every tube with known good tubes. I have measured the resistance from the cathode pin on all tube sockets to all other pins -- no problems. Any suggestions? (I do not have a scope to look at specific signal integrity.) Any suggestion will be greatly appreciated. THANKS!
:
:
:
:
:Short the grid of the output tube to ground (or cathode). What happens to the hum??
:Lewis
:::
::
::
:
:

12/14/2010 1:14:55 PMNorm Leal

Dale

When hum goes away shorting grid of an output tube it's generated before that point. Has 12AT6/V6 tube been replaced? Does it have a shield? Are wires around the volume control short or shielded?

Norm

:Lewis - When I short the grid of the output tube (pin 2 of the 50C5) the hum goes away -- what does this imply?
:
:::Hi Dale
:::
::: Were original filter caps removed? Did you use exact same place for negative of new replacement caps? Even though B- connecting to a different point in a radio can allow hum.
:::
:::http://techpreservation.dyndns.org/schematics/58/58_Motorola_5T22Y1_112.djvu
:::
:::Norm
:::
::::Greetings - I have a "hum problem" with a Motorola model 5T22Y-1 that I have'nt been able to solve. The "hum" sounds just like the typical filter capacitor problem -- it is independent of volume control and independent of tuning frequency. The radio receives fine, but there is a an overriding "hum". I have replaced the filter capacitors (and verified that the new capacitors are good by haywiring additional new capacitors in parallel). I have verified that the cathode of all tubes is NOT shorted to the heaters or other components, and have replaced every tube with known good tubes. I have measured the resistance from the cathode pin on all tube sockets to all other pins -- no problems. Any suggestions? (I do not have a scope to look at specific signal integrity.) Any suggestion will be greatly appreciated. THANKS!
::
::
::
::
::Short the grid of the output tube to ground (or cathode). What happens to the hum??
::Lewis
::::
:::
:::
::
::
:
:

12/14/2010 1:28:59 PMDale

Norm - The 12AV6 was replaced. It is shielded (with a spiral wire shield) and I've verified that there is a good connection of the shield to ground. The volume control is mounted directly on the printed circuit board, so it has short leads. Actually, except for the wires to the antenna, there are no wires on the chassis -- all printed circuit board connections.

:Dale
:
: When hum goes away shorting grid of an output tube it's generated before that point. Has 12AT6/V6 tube been replaced? Does it have a shield? Are wires around the volume control short or shielded?
:
:Norm
:
::Lewis - When I short the grid of the output tube (pin 2 of the 50C5) the hum goes away -- what does this imply?
::
::::Hi Dale
::::
:::: Were original filter caps removed? Did you use exact same place for negative of new replacement caps? Even though B- connecting to a different point in a radio can allow hum.
::::
::::http://techpreservation.dyndns.org/schematics/58/58_Motorola_5T22Y1_112.djvu
::::
::::Norm
::::
:::::Greetings - I have a "hum problem" with a Motorola model 5T22Y-1 that I have'nt been able to solve. The "hum" sounds just like the typical filter capacitor problem -- it is independent of volume control and independent of tuning frequency. The radio receives fine, but there is a an overriding "hum". I have replaced the filter capacitors (and verified that the new capacitors are good by haywiring additional new capacitors in parallel). I have verified that the cathode of all tubes is NOT shorted to the heaters or other components, and have replaced every tube with known good tubes. I have measured the resistance from the cathode pin on all tube sockets to all other pins -- no problems. Any suggestions? (I do not have a scope to look at specific signal integrity.) Any suggestion will be greatly appreciated. THANKS!
:::
:::
:::
:::
:::Short the grid of the output tube to ground (or cathode). What happens to the hum??
:::Lewis
:::::
::::
::::
:::
:::
::
::
:
:

12/14/2010 1:40:45 PMLewis L

:Lewis - When I short the grid of the output tube (pin 2 of the 50C5) the hum goes away -- what does this imply?

The first audio is now a "section of interest". Ground the grid, check the cathode, I shall peruse the schematic and think some more.
Lewis

:
:::Hi Dale
:::
::: Were original filter caps removed? Did you use exact same place for negative of new replacement caps? Even though B- connecting to a different point in a radio can allow hum.
:::
:::http://techpreservation.dyndns.org/schematics/58/58_Motorola_5T22Y1_112.djvu
:::
:::Norm
:::
::::Greetings - I have a "hum problem" with a Motorola model 5T22Y-1 that I have'nt been able to solve. The "hum" sounds just like the typical filter capacitor problem -- it is independent of volume control and independent of tuning frequency. The radio receives fine, but there is a an overriding "hum". I have replaced the filter capacitors (and verified that the new capacitors are good by haywiring additional new capacitors in parallel). I have verified that the cathode of all tubes is NOT shorted to the heaters or other components, and have replaced every tube with known good tubes. I have measured the resistance from the cathode pin on all tube sockets to all other pins -- no problems. Any suggestions? (I do not have a scope to look at specific signal integrity.) Any suggestion will be greatly appreciated. THANKS!
::
::
::
::
::Short the grid of the output tube to ground (or cathode). What happens to the hum??
::Lewis
::::
:::
:::
::
::
:
:

12/14/2010 1:00:59 PMDale

Norm - yes, the original filter capacitors were removed -- it was a metal can with three electrolytics. I used the same printed circuit board hole/connection for the negative I used three discrete electrolytics with the negative wires tied together and inserted in the original negative connection on the printed circuit board. The three positive capacitor connections are inserted in the three original holes.

12/14/2010 1:45:15 PMW8TBK John

:Norm - yes, the original filter capacitors were removed -- it was a metal can with three electrolytics. I used the same printed circuit board hole/connection for the negative I used three discrete electrolytics with the negative wires tied together and inserted in the original negative connection on the printed circuit board. The three positive capacitor connections are inserted in the three original holes.
:
::Hi Dale
::
:: Were original filter caps removed? Did you use exact same place for negative of new replacement caps? Even though B- connecting to a different point in a radio can allow hum.
::
::http://techpreservation.dyndns.org/schematics/58/58_Motorola_5T22Y1_112.djvu
::
::Norm
::
:::Greetings - I have a "hum problem" with a Motorola model 5T22Y-1 that I have'nt been able to solve. The "hum" sounds just like the typical filter capacitor problem -- it is independent of volume control and independent of tuning frequency. The radio receives fine, but there is a an overriding "hum". I have replaced the filter capacitors (and verified that the new capacitors are good by haywiring additional new capacitors in parallel). I have verified that the cathode of all tubes is NOT shorted to the heaters or other components, and have replaced every tube with known good tubes. I have measured the resistance from the cathode pin on all tube sockets to all other pins -- no problems. Any suggestions? (I do not have a scope to look at specific signal integrity.) Any suggestion will be greatly appreciated. THANKS!
:::
::
::
:
:Check for a crack in the copper tracks on the circuit board. Sometimes it takes a resistance check to find a crack.
John

12/14/2010 1:50:57 PMWarren

This is a double sided PC board. Has feed through points top and bottom. Have had problems before with that. Use a pencil iron with small tip. Re-solder volume control at the top side to PC board, and grounding points from negative end of filter capacitors.

12/14/2010 2:31:15 PMEdd

Sir Dale. . . . .

I initially dumped post info into laptop and was disrupted from coming back to it . . Sir Norm's was being the onliest reply at that earlier time.

I'm putting my formerly prepared reply on, only now . . . I do see that others have since jumped onto it . . so there might be some overlays of consensus.

(I also concur on vias and "ribbets"
from one side of the Pee Cee Board to the other . . . particularly if the set should have been a Gentle Eclectic !)

Divide et impera . . . . .(Divide and conquer) . . . . .

Looking that the sets 12AV6, notice that a good ground is as nearby as its pin 2 so use the end of a 1/8 inch bladed pocket sized standard screwdriver to temporarily test short between that grounded connection and that tubes 1st
grid . . . pin # 1.

That would then be shutting down all input from the front of the set . . . so NOW . . .is the hum still present ?

If so, then the problem is on further towards the end of the sets circuitry OR being power supply derived OR possibly being * ground loop associated.

(* But not so likely, since we would now be getting on down into the LOW gain section of the set.)

Now we move on down to the next stage and that will be the 50C5, and the closest bona- fide-certified ground that I am seeing near to there is the pin #12 of that interstage coupling mono-block circuit.

So this might now require a short jumper wire to reach, but connect it between that ground AND either pin 2 or 5 of the 50C5 and listen up for the hummmmmmmm again . . . . . ???

If still present, that is signifying power supply derivation as the next suspicion, so, as to completely eliminate your 50C5, take one of those loose electrolytics which you were so freely shunting the others with and charge one up to probably whats gonna' be 'bouts one hunna lebenteen volts from the B+ supply.

And then you have the set operational, making its hummmmm and then have the test capacitors neg lead to ground and bring its free + lead up to the plate of the 50C5 (#7) . . . and touch to it.

Just after the "pop" of the voltages equalizing out . . . .I now ask you . . . is the hummmm now present ?

If so, we have earlier eliminated the front end, then we eliminated the AF output stage and just now, we have eliminated the final aspect of any problem being with the 50C5.

Soooooooo we are delving right into the consideration of the power supply as being the culprit.

You need to take the old three section electrolytic out of circuit, and individually sub in three individiual electrolytics in order to be able to now fully evaluate the power supply again.

HOWEVER . . .it would be initially permissible to lift only the FIRST section of the electrolytic by solder sucking its terminal free from still electrically connecting to the PCB and putting in your substitute 40 or so ufd capacitor to the foil land just opened up from the old capacitors sections presence.

Now in the re-evaluation of the set . . . any better on hummmm abatement ?. . . . if so . . . . expect a like proportional improvement with the replacement of the original electrolytic caps other sections.

Standing by . . . . while you proceed to find out . . . . WHASSSSSSUUUPPPP ?

73's de Edd