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Old 08-10-2017, 10:23 PM
tubetwister tubetwister is offline
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Join Date: Oct 2010
Location: west blue coast conservative
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Quote:
Originally Posted by Electronic M View Post
Heck of a lot to respond to there....

Quick hum diagnosis tip. If you can disconnect the B+ with a lytic still in circuit giving a couple secs B+, but keep the heaters going on AC if the hum persists when the B+ is disconnected from AC that indicates heater/line cord hum leakage (or perhaps field coil issues if it does not use a PM speaker).

If hum seems to be in the heater circuit try unplugging the set and see if it hums for the second or two it runs with the plug pulled.

Standby usually cuts off B+ while leaving heaters on. If audio/hum persists in standby then there is something wrong with the standby mode.
Quote:
.Heck of a lot to respond to there...
Thanks for the plausible reply here

Aside from my old man rambling and way to wordy OP ..Nothing complex at all here but no answers or opinions to my specific questions but in all fairness maybe better answers now but it's only an AA4 aka AA5 radio without a diode tube > 1961 to 1965 Hallicrafters S120 and any USA mid century vintage radio enthusiast or old dog enthusiast like me should know what it is and the preceding S38 full AA5 radio and the late 1930's Zeniths and some others and the boat anchors ofc OTOH millennials maybe and probably not just because of timing


We should all know those and the rest were PM speakers for ~ decades prior to 1965 like maybe a Catalin madness or Plastik AA5 radio or '40's Zenith T.O. anyway apart from a 1952 Seeburg 100 B happy days juke box I owned , salvaged and repaired in H.S with good tunes and usually up to date selection labels.



Beyond multiband tuning coils , a BFO and such ,a phono plug or a treble cut a 1935 AC/DC - AA5 AA5 is an 1965 AC/DC - AA5 /AA4 but with a PM speaker and mid century 7 pin small tubes on the latter.


Quote:

Quick hum diagnosis tip. If you can disconnect the B+ with a lytic still in circuit giving a couple secs B+, but keep the heaters going on AC if the hum persists when the B+ is disconnected from AC that indicates heater/line cord hum leakage (or perhaps field coil issues if it does not use a PM speaker).

If hum seems to be in the heater circuit try unplugging the set and see if it hums for the second or two it runs with the plug pulled.

Standby usually cuts off B+ while leaving heaters on. If audio/hum persists in standby then there is something wrong with the standby mode.
If you unplug the radio playing normally the hum vanishes instantly while it still plays .thus the heater/line cord hum leakage you brought sounds plausible but I'm thinking more heater leakage but both may be a good place to look ,thank you again



Apart from the standby noise at any volume, above zero ( fault there too or unlikely related fault ) ................................,

I'm thinking common mode AC hum may be DC ripple or heater/cathode ,hum is present at both REC and STBY maybe a faulty no name new lytic of three or all of them the Pay Bay seller put in and or AC potential leak in the B+ or any DC or audio grid or something in there


OTOH NOW that I remembered the hum clears at unplug [ON],maybe a cathode or (heater leak like you brought) in a tube or HK short in a tube diesel jeep mentioned here .

Even with the unplug [on] result may be pointing away from the Lytics if I am thinking well I will put my DVM on ACV at B+ and DC and see what I can but I don't know the AA5 tolerance for that like 1.0- 1.5 volts on an automotive alternator .



Variable amplitude and cadence with volume Standby noise is in addition to common mode AC hum at STBY and REC. but not an issue if REC is unaffected here ,its never going to be valuable radio on the 21st century AFAIK anyway and a lot of pay bay sellers are asking high on these. IMO,they may think it's really a decent SWL radio

STBY noise is like an uncontrolled variable @ loudness control. oscillation (s) and maybe harmonics @ not a stable one as you vary the loudness pot and at the schematic STBY switch doesn't seem to open the Plate B+ @ any tubes but more something at AVC 1st audio tube (or to it ) to the volume pot or .001 film cap there but maybe from the AVC /first audio tube or IF but not a control grid or plate

I need to revisit the official schematic .pdf here about that. but AFAIK looking at the dia. STBY SW . is definitely NOT tube plate voltage interrupt in the schematic keeping in mind this goes back 4+ decades for me and I was never into AA5 beyond a typical and nearly deaf Hallicrafters . S38 AA5 and more deaf National radio S54 and just Lytic caps ,dial lamps and tubes and on those & new strings on the S38's . the S120 will be getting a bandspread tuning string also .

The common mode hum is maybe leaking in after the standby SW or just included somehow ,

Beyond that I read the .001 film cap at the Vol. pot can make these deaf or hum and commonly does .


Maybe importantly the 50C5 beam power control grid cap is supposed to be .001 uf Film cap in the S120 schematic and .001 uf -.002 uf at AA5 reference diagrams ,it is a .0047 Mica cap and definitely OE so whatever that cap is doing or not has been since day one and no opinion on that or NOS tubes just because of the age of the tubes and no emission tester here maybe I should wait or just put may caps on order in it and see if I want to change out the Lytics and (now) moreover the tubes before lytics depending on the results or my ouija board :-\


Again I am getting 6 Volts at the 50C5 beam power AF control grid instead of zero or (neg volts) specified Imagine all that and maybe a hum since new and an uniformed buyer perhaps or is that a stretch electronically ?


Another question I posted is the zero to negative volts 50C5 beam power control grid voltage spec. valid at normal radio play volume power on or @volume [OFF] power [ON] or no? IOW do I need to close the volume pot for a valid result

Last edited by tubetwister; 08-15-2017 at 09:07 PM.
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