Classic Hifi Care

This is my next project. I bought this little 4-tube set at an auction in the rain. It needs paint and TLC. It is also kicking my butt. After you all stop laughing, here is the situation. This thing will not say a word. No reception at all. Everything I have checked, checks good. It acts to me like the oscillator is not running. It's been a couple years since I messed with it, but memory recalls that the oscillator showed to be working with negative voltage on the grid of the oscillator. There is an approximately 20 foot hank antenna on it. I can get a signal at IF from the antenna coil to the speaker. All element voltages were good. I need to start over and make every measurement again. I was in a weak signal area in Missouri, so that may have had an effect on it. Out here with a good signal, who knows? I am going to go thru everything again and will take pictures and share results, so stand by.

Arvin made a lot of tiny radios. I've got a two-tube metal-cased version with no remaining model ID. It's presumably a TRF device. Your 4-tuber is likely a superhet with (1) a selenium rectifier, or (2) no IF amp/2nd IFT.

Chris Campbell

Mine is a four-tuber with a tube rectifier. 35Z5GT, 12SA7GT, 12SQ7GT and 50L6GT is the tube complement. No IF amplifier, so it takes a good antenna to get enough gain. Everything is jammed pretty tight. Not much there, but I couldn't get it to work......

electra225 wrote: ↑Wed Jun 08, 2022 2:26 am Not much there, but I couldn't get it to work......

Most of us need lessons in humility from time to time.

Chris Campbell

I found the print for this radio. I have the print for every radio I owned at one point. I have my VTVM warming up. I want to find what I have been overlooking. This is a four-tube radio, for the love of Mike. It has maybe a half dozen parts. I took a Stereo Theater apart to the last board and put it back together so it works. Come on, Greg......

Why do the simplest ones have to be the hardest to fix?

The primary of the oscillator coil appears to be intermittently open. There was a mica cap connected between the coil and pin 5 the oscillator grid of the 12SA7 tube. I changed that mica caps just to be sure it was not the trouble. I am suspecting green crud in the oscillator coil, since this set came from the midwest and I bought it in the rain. That coil may have been wet at some point. Sometimes I get negative voltage on the oscillator grid, sometimes not. There is another trouble in addition to that, however. If the oscillator running intermittently was the only trouble, I should have an intermittent radio reception. R10, a 2200 ohm 1-watt resistor must have drifted high, since I only have 70 volts on the screen of the 50L6 instead of the specified 100 volts. This is not the reason for the inoperative set, however, since I have noise in the speaker. I have good B+ otherwise and all the other element voltages are really close. I was powering the chassis on my Variac at 117 volts. I need to do some digging. I believe I have an oscillator coil on a junker chassis around here somewhere.

Nope, no joy on finding a good oscillator coil on a junker chassis. Why do I save this stuff?....

Have any of you used one of these oscillator coils from AES? This is about my only option for a replacement coil, but they are spendy. What say ye?

https://www.tubesandmore.com/products/c ... oscillator

Looks like it's worth trying. Does your schematic give enough values for the original coil to know if this one is close?

Chris Campbell

No. NO specs on the original that I can see. The replacement that is not open surely has to be better than the one I have now. From what I read, one adjusts the slug in the replacement oscillator coil to set the 600 kc setting, and the trimmer on the antenna side of the tuner to set the 1400 kc setting. Do all oscillator coils have a red dot on them to help with identifying the terminals? I don't see a red dot on the one in the radio.

Their diagram shows which terminal goes to what. Just trace out the Arvin circuit to identify "top' and "bottom" if each original winding, and match the new one up. The red dot is just so you can ID the new coil's terminals. The old coil's terminals are defined by where they are connected in the set.

Chris Campbell

I found a good oscillator coil in a junker chassis and put it in this chassis. That made no difference. I have three volts negative on the grid of the 12SA7, a bit low, but better than zero. I'd expect to see five volts negative or more negative on the grid. It does not get more negative as I move the dial to higher frequencies. I have 80 volts on the cathode of the 12SA7. That doesn't sound right to me. It is possible the original oscillator coil was bad, but whatever is putting 80 volts on the cathode took the oscillator coil out. It appears the oscillator is somewhat running, although I can't hear it in another radio. How hard can a four-tube radio be?

I have used a generic oscillator coil from AES they work , but you basically have to re-align the trimmers and peak the IF's on the set ... if you have no IF coils , then I have no idea if it would work ... I have used their generic ant coils as well those worked great , this was years ago working a philco that had green crusties in the coils ... a common problem if they are not waxed from the factory

This little set is in my list of radio cabinets to repaint. I dug the chassis out and made another stab at trying to get it to work. I can redo a Stereo Theater, a Concert Grand, TV sets, but a little, simple 4-tube radio is kicking my butt, and not just recently, but for YEARS, it has kicked my butt......

I struggle mightily when trying to troubleshoot oscillator coils and band switches. Following a schematic drawing of these items is my biggest problem. I would appreciate some hands-on technical advice when it comes to these items. The main problem I see with this chassis is that I have 60 volts on the cathode, pin 6, of the 12SA7 oscillator tube. At this point, I have determined that the oscillator coil itself may not be the issue. Something is killing the oscillator. The speaker on this set is in horrible condition, so audio is weak, but that is not my concern at the moment. I can get noise thru the set from pin 6 of the oscillator on to the speaker. This leads me to believe the oscillator coil itself if good and I have subbed it (I THINK I WIRED IT RIGHT ) with no improvement. How and why can I get 60 volts on pin 6? How is that even possible? R8 and C7 are the only paths I see for B+ to get to pin 6 of the oscillator tube. I'm overlooking something. Maybe a component by component value check is in order? Other than pin 6, the element voltages I have checked are okay. I need to fire up the signal generator then see where I can get a signal thru to the speaker. I really need to figure out what ails this simple little radio....

Schematic from Nostalgia Air (also added to downloads): http://www.nostalgiaair.org/Resources/d ... 012518.pdf

Can you post some pictures of this little cutie?

You are saying you get 60V on pin 6 (Cathode) of 12SA7, which connects to "floating ground" through the primary of T2? Something is wonky.
Are you using the floating ground as reference? Have you swapped out the 12SA7 (maybe it's shorted?) I assume primary of T2 has continuity?

I am saying exactly that. I have 60 volts DC on the cathode measured from common negative, floating ground, from one terminal of the power switch, where the filter caps negatives are connected, whatever they want to call it. Measuring continuity on T2 is where I am having an issue. I can't find continuity FROM anywhere TO anywhere on T2. Hence my frustration.....

I got the same readings when I subbed in another oscillator coil....

I have the drawing you posted. Not only was this a cheap radio when it was built, but the drawing is equally cheap. Not much help, when you get to looking at it.

This thing is really rough when you see it in pictures.

It needs a lot of help, but it's a midget radio and I don't have one. Every resistor in this set except one is more than 20% drifted out of value. The AVC resistor is out more than 20%, but will that kill the oscillator? Best I can tell, the oscillator coil is not open. The voltages on the 12SA7 are not right, but I haven't found a cause yet. Jeez, there isn't much that can go wrong. More than one thing, maybe? I found a speaker I can make work. Something is killing the oscillator.....

On the bright side, there is a man on ARF wanting knobs and a back for one of these. This POS has all its original ones, probably worth more than the whole radio is....

It is a cute little compact radio with a speaker that has seen better days. I have no clue on what is wrong with it, but I wish you luck figuring it out and I am sure there are members here that can help. Have you decided on a color yet?

Bill

If I can get it working well, I might take it in to Metro and have them chrome the cabinet. I have also thought of painting it a gawdy, bright color, like hot pink or something like that. I really haven't decided on a color yet.

Chrome would be cool.

Bill