Hello all,

I've recently restored a Zenith 5-S-127 that receives beautifully, except that there is some sort of parasitic oscillation going on. It whistles, squeals and motorboats LOUDLY on the broadcastand band, though it is present to some extent on the shortwave bands as well. The problem mostly goes away when I touch the antenna lead. Adjusting the wave trap cap makes matters worse. Any suggestions?

Thanks in advance,
Larry

Hi, Larry. Here's the schematic: http://www.nostalgiaair.org/PagesByModel/888/M0024888.pdf

I guess you recapped the set, right? And tube shields are in place?

The wavetrap was for blocking radio signals that were close to the IF. It's no longer required, and you should be able to short across it.

You could try pulling tubes, one at a time, starting at the front end, to see if you can localize where the oscillation is originating. Don't pull the rectifier.

I'm suspecting the RF section ahead of the first detector. Go over that with a fine-toothed comb.

:Hello all,
:
:I've recently restored a Zenith 5-S-127 that receives beautifully, except that there is some sort of parasitic oscillation going on. It whistles, squeals and motorboats LOUDLY on the broadcastand band, though it is present to some extent on the shortwave bands as well. The problem mostly goes away when I touch the antenna lead. Adjusting the wave trap cap makes matters worse. Any suggestions?
:
:Thanks in advance,
:Larry

Hey Doug,

Yes, I've totally recapped and aligned the set. All tubes in the set are shielded. I'lhappens. Thanks!

-Larry

:Hi, Larry. Here's the schematic: http://www.nostalgiaair.org/PagesByModel/888/M0024888.pdf
:
:I guess you recapped the set, right? And tube shields are in place?
:
:The wavetrap was for blocking radio signals that were close to the IF. It's no longer required, and you should be able to short across it.
:
:You could try pulling tubes, one at a time, starting at the front end, to see if you can localize where the oscillation is originating. Don't pull the rectifier.
:
:I'm suspecting the RF section ahea

d of the first detector. Go over that with a fine-toothed comb.
:
::Hello all,
::
::I've recently restored a Zenith 5-S-127 that receives beautifully, except that there is some sort of parasitic oscillation going on. It whistles, squeals and motorboats LOUDLY on the broadcastand band, though it is present to some extent on the shortwave bands as well. The problem mostly goes away when I touch the antenna lead. Adjusting the wave trap cap makes matters worse. Any suggestions?
::
::Thanks in advance,
::Larry

:Hello all,
:
:I've recently restored a Zenith 5-S-127 that receives beautifully, except that there is some sort of parasitic oscillation going on. It whistles, squeals and motorboats LOUDLY on the broadcastand band, though it is present to some extent on the shortwave bands as well. The problem mostly goes away when I touch the antenna lead. Adjusting the wave trap cap makes matters worse. Any suggestions?
:
:Thanks in advance,
:Larry

Hi Larry,
I have done two 5S127's and run into similar problems, but not the motoboating.
Doug's suggestion about tube shields is dead on. On one of the 5S127's I had to connect a copper solder braid from chassis to the tube shield to keep the radio from howeling. Oddly it wasn't an issue on the other. The shield held more tightly on that one.
Poor conneciton in form socket to tube on the 6K7G can cause this, too.
One thing I found on both 5S127's was that the IF tuning instructions are in error. If you follow them you get a squeal if you are tuned slightly off a radio station's signal, because the IF is peaked too sharply.
I found the fix was to follow the tuning proceedure and then on each IF transformer, detune one screw about 1/8 to 1/4 turn clockwise, and the other 1/8-1/4 turn counterclockwise. On both sensitivity, selectivity, and tone quality are great! I had one playing two nights ago and my monter-in-law asked if that fancy radio was a satellite service!
When you get it working string your outside antenna in the direction of Toronto. They have a great radio station (740 KHz) there that plays the big bands in the evenings.

Let me know how you do!

Best Regards,

Bill G.

Can't say that satellite radio can even begin to match the fidelity of AM radio, much more FM. Everything sounds like its coming over a cell phone. Seems to me that modern people, in an effort to keep up with trends, find this terribly distorted medium better sounding than real audio. Regarding oscillation, though, be sure that C3 is new and leak free. Shield the plate leads of the 6A8 and 6A7 well with a long close wound coil spring. Keep the antenna lead away from all other leads within the set. A shield around the antenn wiring may help. If you are getting howling when you properly align the IF transformers, then you are getting feedback, and need to attend to proper wire dress and shielding. Audio fidelity will be slightly better when the IF transformers are detuned, but selectivity will suffer.

Thomas

:Can't say that satellite radio can even begin to match the fidelity of AM radio, much more FM. Everything sounds like its coming over a cell phone. Seems to me that modern people, in an effort to keep up with trends, find this terribly distorted medium better sounding than real audio. Regarding oscillation, though, be sure that C3 is new and leak free. Shield the plate leads of the 6A8 and 6A7 well with a long close wound coil spring. Keep the antenna lead away from all other leads within the set. A shield around the antenn wiring may help. If you are getting howling when you properly align the IF transformers, then you are getting feedback, and need to attend to proper wire dress and shielding. Audio fidelity will be slightly better when the IF transformers are detuned, but selectivity will suffer.
:
:Thomas
I think neither my mother-in-law nor I have really heard satellite radio.
I will look into C3, I am sure I changed it. I assume you mean grid leads on the 6K7G and 6A8G. These didn't come with spring shields. What would make them necessary now?

All the Best,

Bill G.

Well, it doesn't sound all that great, though I've heard people claim that it's better than FM radio (it isn't...it's terrible and fuzzy sounding). I did intend to say "shield the plate leads." However, if you wish, you may shield the grid leads as well. This certainly can help. Be sure to shield all RF plate and grid leads (control grid). There isn't really a need to shield the oscillator grid's lead. It's already oscillating. Shielding may help, but probably won't.

Radios howl especially when the antenna lead is brought near any of the other circuitry. Shield the wiring inside of the set, especially if it comes near plate leads.

All spring shields should be connected to the chassis.

T.

:Well, it doesn't sound all that great, though I've heard people claim that it's better than FM radio (it isn't...it's terrible and fuzzy sounding). I did intend to say "shield the plate leads." However, if you wish, you may shield the grid leads as well. This certainly can help. Be sure to shield all RF plate and grid leads (control grid). There isn't really a need to shield the oscillator grid's lead. It's already oscillating. Shielding may help, but probably won't.
:
:Radios howl especially when the antenna lead is brought near any of the other circuitry. Shield the wiring inside of the set, especially if it comes near plate leads.
:
:All spring shields should be connected to the chassis.
:
:T.
Many thanks Tom,
This sounds quite reasonable, shield the plate leads. Possible the reson it is needed on some radios and not others is the placement of wires in the chassis.

All the Best,

Bill

Plates typically radiate the most energy, which is why they should be shielded. Shielding the grids is an excellent idea, too, but in some sensitive cases, this can severely change the capacitance of the grid circuit, which will throw off tuning. I've seen grid leads in audio circuits with shields, but I don't recall often seeing them with shields in an RF circuit. I'm sure it can be done...like in a television...where coaxial cable is used with a lot of space between the center conductor and the shield.