Anyone Know How to Fix Radios?
 

Finished a Stromberg 400H. Did little more than recapping, and it works well. Then I got another 400H and did the same--recapping, replacing bad tubes & resistors. This is a link to an album showing both sets:

https://picasaweb.google.com/coldrb/...eat=directlink

But the second one doesn't work well. For example, each station is received at equal strength at two frequencies 60 kilocycles apart. All I can do is multimeter testing. So I put the chassis side-by-side and checked voltages and continuity. Now I have data with anomalities which I don't understand (The only thing I've figured out is that the second set may have a short in the capacitor can I rebuilt [C26]).

Hopefully, with a simple 5-tube radio, someone can get me pointed in the right direction. I've posted schematic, wiring diagram, voltage data, continuity data. These are in the album, but these single-pic links give better resolution.

Schematic:
https://lh5.googleusercontent.com/-E...%252520100.jpg

Wiring Diagram:
https://lh3.googleusercontent.com/-h...%252520126.jpg

Voltage Data:
https://lh5.googleusercontent.com/-z...%252520120.jpg

Continuity Data:
https://lh5.googleusercontent.com/-c...%252520122.jpg

Other Tests:
https://lh3.googleusercontent.com/--...%252520124.jpg


If anyone has time to look at this stuff, I'd really appreciate advice.

- Winky

Without looking at any of your data I'm thinking that somebody monkeyed with the IF transformers before you got the radio and it's not on the prescribed IF frequency. First you have to know what that frequency is, should be in the repair data somewhere. It "might" be 455 kc but early sets varied.

It would be easy enough to figure out with a signal generator set to the IF frequency but if you don't have that, I'm thinking you could take the IF signal from the output of the good radio and feed it to the input of the IF of the problem set. This might be done with minimal coupling, like wrapping a wire around the IF tube of the good set and running over and putting the wire near the input IF transformer wiring of the problem set. Yank the oscillator tube from the problem set. If the problem set is right on IF, the signal should feed through. If not, adjusting the IF transformers on the problem set should pick it up. Mark IF position screws and write down how many turns so you can put it back if no dice.

If the coupling on an IF or RF transformer is too tight, you can get double humps, similar to 60KHz apart, like what you described. Looks like the IFs are tuned by variable trimmer caps, but if they were adjustable slugs I'd say that they were set too close together. I see a :gimmick" on the RF coil in the schematic diagram, if that is too much capacitence (too tightly wound) it might produce the double humps.

Double humps have their uses, one can produce say a pair of humps with a separation of say 5KHz, which can make for higher fidelity (higher audio frequencies) from radio stations. But be careful that the carrier isn't attenuated too much, or distortion will result,

Thanks, guys. Of course, I don't have a signal generator, but someday it may come to that. With good set/bad set both on the bench right now, I'll work on the work-arounds and let you know happens. At the very least I'll probably learn something.
-Winky.

This is a long shot, but I'll toss it out anyway inasmuch as you have a sample set for comparison. If it's possible to take the cans off the IF xfmrs for inspection, look and see (in the problem set) if the primary and secondary windings ("tires") have migrated closer to each other than in the sample set.

I had this happen once, not in IF transformers, but in the plate choke of a transmitter. It was a 2.5 mhy RF choke that carried the plate current of the final stage. It consisted of four 'tires' in series, spaced apart on the form. The wax had softened and allowed the 'tires' to gradually slide together, drawn by their mutual magnetic field. Conceivably the upper 'tire' in an IF could slide down under gravity if the wax were softened sufficiently. Like I say, just a long shot. oc

Looked into the suggestions by WA2ise and old_coot88, and all that looks OK. I was about to follow up on Reece's recommendation of getting the signal with a loose link from the good set, but I got carried away with trying to learn what I was doing. Then the Frequency Fairy appeared and promised to put a signal generator under my pillow next week. Before I can use that, however, I need to know how to assess or measure signal or output strength. Is this something I can hear, or am I measuring audio output from the voice coil, or what? In case it might be useful, I have a 1960s analog Micronta multimeter with an output jack.

I've posted specs, alignment instructions, etc. in the last 5 or 10 images in this album:

https://picasaweb.google.com/coldrb/...eat=directlink

Viewing Picasa Web albums: (1) Click an image from the thumbnails. (2) Click "full screen" at top left. (3) Use navigation bar at bottom of image or use back button to return to album thumbnails. (4) If you're not in full-screen mode, a blue navigation arrow will appear if you move the cursor to the right or left of the image.

- Winky

You can do this aurally. Pull the radio's oscillator tube so it won't interfere with the signal generator Set the SG to the proper IF frequency of the radio and just lay the cable from the SG near the IF tube. The SG should be set to produce a modulated signal, with a tone. You want a weak signal so AVC doesn't kick in. Weak signals allow sharpest tuning peaks. Adjust the last IF transformer for strongest signal. Then move the cable near to the bottom of the first IF transformer and repeat. If the signal is too strong turn down the SG output and/or move the cable a little farther away.

After you've peaked both IF transformers, weaken the signal and peak again. Then shut off the SG and plug the oscillator tube back in and see if the dial lines up and tunes properly. You can tweak the oscillator and antenna trimmers on the tuning cap as per any alignment.

If you haven't played with a SG before you might want to experiment with it "broadcasting" at different spots on the AM band and see how the controls work to vary the signal. The SG is just a mini transmitter. I often find I don't need a direct connection to a radio as just having the SG cable in the vicinity is enough.

Many thanks. Nothing I read mentioned that obviously you're creating an audible tone. That will save me a great deal of time.
By the way--I mentioned I had a short in the capacitor can that I rebuilt. I'd actually switched the terminals of a 50V and a 250V cap, and the short circuit was the fried capacitor.

I've made some progress in aligning the Stromberg-Carlson 400H after a few days of trial and error with a signal generator. The "double hump" was gone after my first adjustment of the IF transformers. I followed the sequence in the alignment instructions, adjusting to get the maximum noise with the signal generator at 455 KHz. But when I turned the signal generator off, it didn't seem to affect the output at all. So, I don't know if I'm using the instrument properly or even if the instrument is working at all. I also did the wave trap adjustment.

There's a great deal of crackling noise when I turn the tuner knob, and when I have a station tuned in, the sound may go totally off (or come back on again) when I touch to tuner knob or the tuner itself. BUT...this only happens below 900 KHz.

I started looking for a bad connection, and I noticed that some of the tuner plates were actually touching when they were engaged, and I verified that with a meter.

https://lh4.googleusercontent.com/-r...%252520144.jpg

So the first thing I want to do is get the plates properly aligned. Does anyone have suggestions on doing this?

I may not need to do anything else after I straighten the plates, but I need to understand more about this signal generator. The instructions are a scant 7 lines, and they're written in Sino-English.

https://lh6.googleusercontent.com/-Z...%252520140.jpg

I understand how to set the frequency for RF output. There are three screwdriver adjustments--modulation level, RF level, and mod. frequency--which are preset at or near their max. Number 9 on the generator is actually labeled mod. in/audio out. I have no idea what the "mod. mode [internal/external]" button is for.

Alignment procedures:
https://lh5.googleusercontent.com/-q...%252520136.jpg
Alignment chart & specs:
https://lh3.googleusercontent.com/-n...%252520130.jpg
Schematic:
https://lh5.googleusercontent.com/-E...%252520100.jpg

Your advice is greatly appreciated.
-Winky

Can't tell from your picture but some tuning capacitors have a screw and lock nut on the rear of the frame behind the tuning shaft. Adjusting that will move the shaft and thus the rotor plates of the cap in and out to center them with respect to the stator plates.

If your cap doesn't have this adjustment feature, the rotor plates will have to be carefully bent so they don't touch the others. Blast clean the plates first with some electronic cleaner, like CRC electronic cleaner available cheap at auto parts stores. It has no oil in it to gum up things. Your ohmmeter is your friend in this instance as you check plate clearance.

Also, there will usually be a brass spring often on the center "wall" of the tuning cap between the two sets of stator plates. This spring is an electrical contact between the cap frame and the rotor shaft. This spring and the part of the rotor shaft that it bears against needs to be clean and shiny or you'll get crackling even if the cap plates don't touch.

Signal generators all work about the same. Just remember to keep the lowest signal that you can still hear with the volume on the radio under test turned up and you'll get the sharpest tuning. Internal modulation means that an audio oscillator inside the S/G provides a tone, otherwise you'd have an unmodulated carrier (like a station on the air with no music or speech going.) External modulation means you could feed an audio signal into the S/G and transmit that via the carrier that the S/G produces, like a mini-transmitter.

One trick to remember if you don't have a frequency counter: 455 x 2 = 910Khz. If you have a radio station locally on 910, you can tune the S/G around 455 and beat the first harmonic of the signal against the 910 station tuned in accurately on an operating radio. Tune the S/G for zero beat against the radio station. At zero beat when the squeal goes down to zero, you'll be tuned exactly to 455Khz.

Thanks, Reece. That trove of information will keep me busy for a while. Even though I knew enough to use the ohmmeter in the first place, I was envisioning checking for plate contact repeatedly with a thin paper shim. Um-m-m...I think your way is better.
- Winky

Since the tuning cap has coils connected across the rotor and stators effectively making almost a short, you may find it easier to unsolder the leads to the stators while checking for shorts between the plates.

Success!...and much easier than I anticipated. Got the CRC Electronic Cleaner at Autozone. Gave it a thorough cleaning and adjusted four plates visually. Tuning is smooth and quiet, and I'm done with this chassis. Except I'll probably play with the signal generator while I'm finishing the cabinet. Thanks for all the help.

Good job, Wink.:cool:

Good for you. The ones after this will be easy. We'll be waiting for the "after" pictures.

The "After" Pics
 

Here's my second Stromberg-Carlson 400H. This is my best work so far, so I packed it up and mailed it to my brother, K3KU, because I don't know anyone who can better appreciate this type of endeavor.

https://lh6.googleusercontent.com/-u...%252520150.jpg

I kept the original finish, only touched up the scratches and repainted the dark trim, then added two coats of lacquer, preserving the original decal. Even the feet are original. The only parts replaced were capacitors, some resistors, one tube, a few frayed wires, and the grill cloth.

https://lh6.googleusercontent.com/-T...C%25252072.jpg

https://lh4.googleusercontent.com/-y...%252520158.jpg

Although it was a pretty simple project, it stretched the limits of my skill and knowledge. It arrived safely (bubble-wrapped inside a box, then bubble-wrapped the first box inside a bigger box) and was, to the say the least, well received.

Thanks again to all who helped me with this project.

- Winky

That's nice! I know your brother will like it, shucks, anybody would!

Like the man with the deep voice would say on their ads, "There's nothing finer than a Stromberg Carlson".