Radio question

[dhuebert]

I have an old Nortern Electric radio that I have been working on for the past couple of days. It is the 12BE6/50C5 tubeset. I recapped it and replaced the 35W4 socket. When the tuner is off station it squeals with a dropping pitch as you tune in the station. Once it is tuned the station comes in clear but the slightest drift causes the squeal or oscillation to come back with increasing frequency as you move off station. I haven't aligned the radio yet and I'm thinking this is a local oscillator problem.
Any hints?

http://www.northernelectric.ca/radios/ne5708/ne5708.htm

Don

[patrickn]

This is exactly how a communications receiver behaves with the BFO (beat frequency oscillator) turned on. You hear a "beat frequency" as you tune in a station that gets lower and lower in pitch until the station is perfectly tuned in. At this point the beat frequency is 0Hz in theory, so you can't hear it. Usually you get some distortion with audio signals though if the BFO is on.

So, since I don't know anything about this radio or the schematic, I'd see if you have a spurious oscillation very close to the IF (usually 455kc) at the detector or some stage before it. This would act very much like a BFO.

Hopefully this helps to some extent
-Patrick

[erichayes]

Hi All,

Don, what Patrick said.

If, however, there is no guilty BFO present, check the 12BE6 for heater to cathode leakage (or simply replace it temporarily): H-K leakage can raise all sorts of Hell with the LO.

If the tube passes muster, check the value of the resistor between the cathode and first grid on the BE6--it should be between around 18 and 33K. Also check the value of (or replace) the cap between the first grid and the stator of the osc. section tuning cap--it should be around 100 pF or less.

In the IF stage, check the value of the cathode resistor on the amp tube (usually a 12BA6 or 12AU6). It should be no greater than around 150Ω.

Good luck.

Just spotted your link. Unless there has been modification (why?) internally, this radio doesn't have or need a BFO.

[EWBrown]

The other possibility is that there is some unwanted feedback in the 12BA6 IF stage, this can be caused by aging components, or improper alignment of the two IF cans - if they are all "spot on" at 455 KC, then sometomes underired and uncontrolled feedback can occur (emulating a tubed-grid, tuned-plate oscillator). The best approach, is to "stagger-tune" the IF trannies slightly, which may slightly reduce the sensitivity, but results in a wider IF bandwidth and better audio recovery.

The three basic reauirements for a simple oscillator are feedback, tuned circuit and amplification. Which leads to:

In ny early radio-hacking days, we used to have a saying that Amplifiers Oscillate, and Oscillators Don't. Basically a corrolary (sp?) of Murphy's Law.

The other "usual suspect" is the AVC circuit, the 12AV6 / 12AT6 has two diodes, (as well as one triode), one diode serves as an AM detector, the other one generates a small negative control voltage which is proportional to the received signal strength, and it "plays" with the biasing of the 12BA6 IF stage (and perhaps the 12BE6 pentagrid converter) in order to make stronger or weaker signals have approximately the same volume levels. This AVC voltage is lightly filtered, just enough to keep the audio component from "pumping the IF stage, and the RC constant is usually around 1/4 to 1/2 second. If the filtering cap is open, then there is no RC constant, and this can result another unwanted feedback path.

The components in these old 5-tube radio designs were the cheapest available, and the ceramic disc caps, paper and wax caps, and of course, electrolytics are all susceptible to aging problems. Back when these radios were menufactured, no one expected them to still be working 50 years later...

There is a "funny" thing with some of the older AM radios... In the 1930s the broadcast band was 550-1500 KC, and police radios operated on 1500 - 1700 KC. After WWII, the police moved up to the 30-50 MC band, and AM expanded to 1600 KC (and there was some lingering county and state police activity on 1610 - 1800 KC) . Later on, in the 1970s, the first cordless phones used frequencies between 1620 and 1780 KC for the base, and around 49.83 MHz for the handset. Needless to say, the lower frequencies could travel for quite a distance,...

In the last decade, the AM band was expanded up to 1700 KC (and down to 530 KC). So, as a totally unplanned and unexpectred "bonus" some of these vintage AM table radios, which included the old police band, are now fully "exdpanded AM band" capable. My "loctal" tube based AC/DC Philco AM / Police table radio has the "full" band coverage, and works quite well.

HTH

/ed NB in NH

[dhuebert]

I spent a good couple of hours replacing caps and a tube socket. Some DPT (dreaded previous tech) had wired a new muti-stage cap incorrectly so the radio didn't work at all when I got it. I sent for the schematic and ripped it apart and put it back together right. The beat frequency was disappointing but thought I would ask around before proceeding. I have lots of tubes to try. The worst part of the job was finding out the speaker was no good. Fortunately it is not the kind that has the transformer welded to it, I still have to make up a new mounting bracket tho.

Just to babble on a bit further... I got into the whole tube amp hobby because I had so much fun fixing these old radios. I bought a Westinghouse 5T108 for my garage about 20 years ago

http://www.radiotiques.com/collection.a ... be&Page=14

Which stopped working in ~2000. Well, I thought, I make my living fixing electronics, this shouldn't be too hard. And it wasn't! I couldn't have done it without the internet, tho. Somehow I stumbled on Shannon's site and the rest is tube history.

I would like to extend a personal thanks to Gar Gillies for all the help he has given me over the last few years. It is amazing to have a man of his stature to talk to. It has been like having Leo Fender or Jim Marshall next door to consult with. Gar started a radio repair business here in Winnipeg in, get this, 1939! Now that's experience!

http://www.garnetamps.com/garnet2.htm

Don

[DerekVa]

I have an Zenith 8S154 that I need to rebuild. It looks like this one, but not so nice. It has some water damage around the bottom and the unit is currently non-funcitioning (although I've verified that at least the heaters work). It probably needs a full rebuild but I don't have the time and I don't know enough about the field coil speaker to tell whether or not that's what's broken. It was a gift from my wife's late grandmother who had it under a pile of junk in her trailer. Right now it is sitting in the living room since it at least *looks* nice.

There's a guy (or at least there used to be) who was about 30 minutes from my house who pre-dates the invention of fire and rebuilt old radios and amps. I should look him up and see if he's still alive and if he could rebuild this beast (the circuit at least). Perhaps I could strip and repaint the carcass and refinish it...

Dammit, now you've got me thinking about another project! Yellow_Light_Colorz_PDT_21

Oh well, I guess it'll just drop into the queue behind the chassifying of my K12-M amp and the ST-35 mono amplifiers.

-Derek

p.s. Angela instruments has a good deal on Elna Cerefines (the big black cans) - I just picked up two 220/220mf @ 350V cans for $60.00/pair and am thinking about grabbing a couple of 47mf @ 500V for the parts bin