AA5 hum - Printable Version
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AA5 hum - fenbach - 10-18-2018
i recapped a crosley 10-135 AA5 dashboard radio [120 volt] and aligned it but can't seem to completely eliminate the hum. between stations, anyway. reception isn't all that great either, altho turning the radio helps. anyone know of a way to reduce the hum? and maybe improve the reception??
RE: AA5 hum - Radioroslyn - 10-18-2018
Sounds like it's rf interference. Unplug any switching power supplies and other sources of interference.
RE: AA5 hum - Jayce - 10-19-2018
Like said above, check for RF interference first, also try flipping the plug in the outlet. AA5 radios always have a bit of hum, no matter what you do to them. Usually it's not super noticeable, but it's there. It seems to depend on the circuit. Also, internal antennas tend to be directional. There is a reason why most of consoles with internal antennas are made so the antenna can be turned back and forth. Of course, that seemed to be forgotten after the war!
RE: AA5 hum - Chas - 10-19-2018
This one?
http://www.nostalgiaair.org/pagesbymodel/170/M0003170.pdf
Check the 50C5 for heater/cathode leakage and the 12BE6 too.
If the pressed copper loop got wet (cleaned) or the radio was once stored in a damp location. Place the loop in an oven 150F or so for a couple of hours. Check all the loop winding's for continuity. If the set got a polarized cord, change back to a parallel, that will allow the cord to be reversed to explore the AC line as an RF return.
If the chassis shows even the slightest corrosion and has riveted grounding lugs or lugs on tube socket rings, solder those to the chassis with a high wattage iron to get good wetting of solder.
Do look for devices on the AC power line as a source of hum as Terry suggested. The consider an alignment to bring up the sensitivity. Local noise can cause the AVC to cut back the IF gain...
Most is moot if the home is covered in stucco, brick, stone, aluminum siding or foil vapor barrier, radio in the cellar or surrounded by high rise buildings...
It appears this radio has the troublesome IF cans, well, if the set is working without crashing noises the IF cans will be O.K. for a while longer.
Oh, C16's, the filters, are connected to B- not the chassis...
YMMV
Chas
RE: AA5 hum - fenbach - 10-19-2018
thanks folks,
chas mentioning the IF cans reminds me that i may be lucky to still be picking up signals. so to speak. me, not the radio, that is. I fried the low voltage side of the first IF transformer by attaching a scope probe to contact 3 of the first IF coil = grid of V2 [as i recall] and connecting the ground clip to the ground side of the external antennal loop. baffling to me. of course, i'd be glad if someone could explain why the low voltage side of the IF transformer blew. i heard a pop and saw a puff of smoke. ebay to the rescue. i am using my isolating transformer now!
for what it's worth, i put on a polarized plug with the hot wire connected to the switch but did try reversing the plug. still hummed.
RE: AA5 hum - fenbach - 10-19-2018
PS. yes, chas, your link to nostalgia air is the radio i have.
RE: AA5 hum - Chas - 10-20-2018
Ouch on the IF, funny, I'm also a guy who has an isolation transformer, but seldom uses it.
Your accident is a reminder that isolating the hot chassis set not only saves a life but parts as well.
Sit down with the schematic and consider what happened when the AC (line) found its way to take out the IF, I think that more damage was done, possibly a resistor dramatically changed in value or an antenna coil has gone open, even though signals are being received but not well. Is the AVC circuit working?
I just hate it when you try to fix something and only make it worse.
I could tell you a story, just finished this week, how I "fixed" a gas stove. I wanted to improve the ignition and stop the pilot flame from blowing out during ignition. Some 110$ later, new thermocouple and new Junkers Zündelektrode, the spark still won't light the gas, so I'm still torching it. The blowing pilot stopped on it own, despite spending hours fabricating a draft baffle. Oh, well at least it works and I didn't damage the porcelain... I had to get the Zündelektrode from ebay Germany...
Other radio related shorty, nice AC/DC table radio just dirty worked great, decided to wash the filty back cover/loop. Yep, one of those copper foil embossed types. All done looks super, turned it on dead as a door nail. What, so I injected a signal, as the radio warmed, it improved slowly, noticed when I came in contact with the antenna circuit signals improved. I got a test loop and tack soldered. Wow Normal! Looked at the foil loop, yep still damp, into the oven cooked until well done, Reinstalled perfect!
Not that I have taken a lesson from a local ebay seller, he never touched the sets, wiped them down with, are you ready? Mayonnaise! Took the ebay pictures and always got $50 or more for his everyday table radios...
Oh, general replacement metalized caps have a very poor dv/dt rating, in fact they are not rated. A pulse, even one less than the voltage rating can rupture the metalizing and the cap can loose capacity. In an AC/DC set this can be the grid cap. on the output tube, a tone cap on the plate of the output or a noise bypass on the same plate. Consider the IF going open, severe noise pulse...
GL
Chas
RE: AA5 hum - gregb - 10-21-2018
I just repaired an AA5 set for a customer yesterday and it had rather weak reception and I have seen this problem before. There is a low value capacitor, approx. 100pf, in the grid circuit of the convertor tube and it had gone bad. I have had three sets now that had poor reception after a recap and changing that cap brings it back to life. They are usually a silver mica or a ceramic cap so are not normally changed in a rebuild. Might be worth a try in your case.
Gregb
RE: AA5 hum - David - 11-04-2018
Living in Florida with our nice HUMID air have seen my fair share of failed silver mica and ceramic caps, have started replacing them when doing a recap.
RE: AA5 hum - Jayce - 11-04-2018
Actually, if the climate keeps up what it is doing, we are all going to have the nice, HUMID weather every summer. Ohio was miserable this summer and I am wondering what that is going to do to about everything. We replaced on of those caps in a Zenith AM-FM radio not long ago and the guy was definitely shocked it was bad.
RE: AA5 hum - 35Z5 - 11-05-2018
Philco had best circuit to eliminate filter cap hum(Look up 46-250). A 30 mfd on cathode, followed by a 220 ohm and 25mfd(this is B+ point for output xfomer). Finally connects to a 1K ohm & 20 mfd that feeds audio output screen and other circuits in radio. I generally use three 33 mfd caps.
RE: AA5 hum - morzh - 11-05-2018
Also I noticed in this sch they did not place an X-cap across the AC plug. If all else fails and it seems to be an RF interference, try 0.01uF - 0.04uF X (or XY)-type cap across the AC input.
RE: AA5 hum - Squire - 11-08-2018
I did one of these that was an ungodly corroded mess and was very pleased with the way it came out. Ditto on putting an X-Cap across the line. Also make sure the two tube shields are well grounded to the chassis. Resolder both ends of the wire connections that run from the shields to the chassis.
RE: AA5 hum - fenbach - 03-16-2019
as to the IF coil I fried, if all else fails, read the instructions! seems the Crosley service manual spells out, in bold print no less, UNDER NO CIRCUMSTACES SHOULD A GROUND BE CONNECTED TO THIS RECEIVER. in other words, USE AN ISOLATION TRANSFORMER before connecting a [grounded] scope to the chassis. even so, even after using an isolation transformer, I connected the scope ground to the neutral wire. maybe someone can tell me if I was well-advised to do that. or not. I say neutral wire because the isolation transformer has a polarized outlet.
as to the hum, I discovered [with a scope] that the signal from the audio transformer [with the antenna disconnected] looked like a sine wave with the peaks flattened a bit. unlike the signal from a radio that didn't hum. those peaks were narrow and pointy. I switched transformers and NO HUM! while I am pretty much a rookie, at fixing 120V radios anyway, I've never had a transformer cause a hum. nor read any mention of it. i'm curious to know what might be "wrong" with the transformer. the resistances are good. and it did work but introduced the hum even with the volume turned down.
thanks as always.
PS I put an X-Y cap across the AC line. it had no effect on the hum.
PPS the reception was poor because the solder joint on the antenna lead to the stator side of the tuning cap was loose. something this rookie discovered entirely by accident.
RE: AA5 hum - Arran - 03-16-2019
Bad connection to the loop antenna can cause hum, as can a heater to cathode short on the 50C5, but the H to K short would not go away without changing the tube.
Regards
Arran