Classic Hifi Care

You didn't say what was so onerous about the
Beitman's, so here's other docs that may give
some assistance.

Mr. Mogul, you posted exactly what I need, a resistance chart. The schematic you posted is the Rider's schematic that I already have. The problem I have with the Riders drawing is that there were no resistance measurements listed and there is a "60 volt" specification that is not specific as to just what element should have the 60 volts. I'll print and run that resistance chart and see what that tells me.

Looking further, the voltage readings given in the above drawing are totally different than what is on Rider's.

The schematic was from Sams, a bit different
than the Riders, it looks like.

I appreciate it in whatever form it is. Sams may have changed nomenclature and added their own comments. The resistance chart is the big deal for me. I can't tell you how many times my butt has been pulled from the fire by a resistance chart. Sams was good about that.

You and others have suggested all the possible sources of the problem. Let us know what you find.

Chris Campbell

I don't know what the trouble with this set is. I have no idea. The only thing I know for sure is that I sure feel stupid that I don't know what the trouble is with such a simple set. How hard can this be? I'm fixing to learn something when I DO figure out what is ailing this thing, I can tell you that.....

I reckon I'll make charts and go thru all the checks I can find like I would a TV or a big stereo.

I ran the resistance chart on this chassis. I found something out of spec, but I have no idea what it means. I am supposed to have 120K ohms resistance to common negative on pins 3 and 4 of the 12SA7. These two pins check infinite, open, many megaohms of resistance. Pin 6 also checks open to common negative. Pins 3 and 4 have the required 100 VDC B+ on them, and pin 6 still has its 70 volts. This makes no sense to me. How can a pin have B+ with no continuity to common negative? Pins 3 and 4 are the mixer. They get B+ from the primary of the IF transformer. What is going on with this radio? A miswire? This thing acts like a Zenith.......

Maybe time to take a break from it and do
something that will be an easy win ?

BTW, here's a cute ad I found of it.

https://www.rfcafe.com/references/radio ... g-post.jpg

As a side note, Noblitt-Sparks made car mufflers as well.

Greg, maybe some of that Manitou got into your 444?

DG

I have subbed the 12SA7 in the past, but not recently. Pentagrid converter tubes can be a finicky lot, one may not work in this radio, but might work perfectly well in another, and vise versa. I also have weak audio. Another volume control issue? In a typical AA5 or six with two IF transformers, the majority of the signal amplification is done in the IF section. In this radio, the largest gain is in the detector/first audio tube, the 12SQ7. Another idea might be to use the signal generator to see if I can get a signal thru this thing, section by section, starting at the speaker and moving back. I also should replace the speaker. I haven't decided yet whether a wholesale replacement of resistors will help. The drawing gives a 10% tolerance, while every resistor is out more than that. I don't think resistors are the problem. I am GUESSING that there is a miswire someplace, maybe before I got it. But I don't know that......

According to what I have read on the subject, these four-tube sets were made for local reception. DX-ing is not their strong suite, due to the fact that there is no IF amplifier. You need a strong signal to make them work. That said, will component values be more critical in these sets? Drifted resistors don't normally make a lot of difference in a radio, they usually have a wide tolerance range. This set wants 10%. I haven't found a text yet that deals with the specific issue I'm having.

We had a Christmas party to go to today, so my shop time was limited. I did find a couple of things wrong, but they didn't fix the set. There was a cold solder joint in the B+ line to pins 3 and 4 of the 12SA7. Fixing this gave me the required resistance to ground. Then, the terminal on pin 6 of the 12SA7 had gotten pushed over against a filament wire on the rectifier that had about 85 volts on it. I don't know yet whether it damaged the oscillator coil or not. My next step is to find out why I have such weak audio output. All I get is a very slight hum when I "tickle the grid" of the first audio tube. Very slight hum and incredibly low volume. I'll figure this out, then I'll fire up the signal generator to see if I can get a signal thru this thing anyplace. The output transformer is mounted to the speaker frame. I have another transformer on a part set ( I think) and also a speaker that should work. I'll start with this and see what I've got. The voltages I have checked all check good.

I've spent most of the afternoon trying to replace the speaker and output transformer in this dinky little set. Everything is crammed in so tight, you have to remove parts to replace other parts, then put those parts all back on to even test what you just did. I robbed the speaker and output transformer from a three-way RCA chassis I had here in my parts box. Got that all put in nice and neat, then found the transformer was open. I had it marked "good" but that was some time back. I have another transformer in a little Philco chassis that should work. It checks good. I may have a bad tube socket in the converter tube, which may be contributing to the 15 VDC I still have on pin 6. I can wiggle the pin 6 terminal and the voltage comes and goes. I am beginning to think the oscillator coil is bad, since I have no negative voltage on pin 5 of the converter tube. I don't see any other shorts. The volume control checks good. I replaced a few of the resistors, with no change at all in performance. There is a LOT wrong with this little radio. This is one of those cases where one fault makes two or three other faults almost impossible to find. I have another oscillator coil, out of a parts set. It checks good, but I'm not sure how to adapt it to this chassis. It is adjustable, the original is not. I'm not even sure it will work.

I used the speaker from the RCA radio along with the original audio output transformer from the little Arvin radio. You have to mount the transformer kinda sideways so it will clear the 12SQ7 tube. I now have great audio strength, but I also have no signal getting thru and I have a sound like an open mic in the speaker. The 12SQ7 that is there is a glass tube. If I put my hand on that tube, the sound changes. I need to try a metal tube and see what happens. I tried getting a signal at 455khz thru the radio. I can get that tone, along with a tone at almost any other frequency from the generator. Something is wrong, either the generator isn't working, or there is a short someplace. The IF trimmers make no difference at all. If I can get any signal at all thru the set, does that clear the oscillator coil? This thing is quite the project.....

The metal 12SQ7 made no improvement.

I have disconnected pin 6 of the 12SA7. It still has 90 VCD on it! DISCONNECTED!!

I have come to the conclusion that the high voltage on the cathode, pin 6 of the 12SA7 is what took the oscillator coil out, and I have confirmed with three tests that the oscillator coil is for sure bad. Pin 7 of the 12SA7 is a heater terminal, spec'd to have 82 VAC on it. This is nearly identical to the DC I have on pin 6. I suspect there is a bridge between pins 6 and 7. Why, then, would the voltage on pin 6 read as DC if it was being fed by the AC on pin 7? I don't want to change the oscillator coil until I get this pin 6 situation sorted. I don't want to go to the trouble of changing the tube socket if I am on the wrong track. One of the rivets for the wafer tube socket is under the tuner....

All the voltages and resistance readings in this entire, massive, four-tube midget set are within reason except pins 5 and 6 on the 12SA7. There has been a lot wrong with this set and I think I'm getting closer. For pity's sake, I have done TV sets, a Stereo Theater, a CG other stuff, all more complicated that this little set. This thing is being a pain for some reason. I believe the tube sockets, oscillator coil, the antenna coil and the tuner are the only parts on this set I have not replaced. If anyone has any input on whether any of this makes sense and would care to share that input, I would be most grateful.

One more observation. The buzz/hum/noise I am getting in the speaker. I suspect that may be AC backfeeding into something. Again, if the tube socket was bad, we have a cathode terminal right next to a heater terminal with AC on it. This is what got me to thinking that something was going on in that tube socket. BTW, I don't have any continuity between pins 6 and 7 on an ohmmeter.

I'd use a megger on that socket.

A megger is a small crank-driven high voltage DC generator, and a calibrated meter reading in Ohms/Megohms in a single box. It has two alligator clamp leads to test whatever.

They are great for seeing if insulation in something electrical is breaking down under higher voltage than an ordinary Ohmeter using a few dry cells as a voltage source can.

Or just blow 120VAC across it with a test cord and see if it lets out any magic smoke...

Dennis

There is no other way pin 6 can get almost the same voltage as the AC on pin 7 other than due to leakage between the two tube socket pins. I should have come to this conclusion sooner. I let this little radio kick my butt. I worked on it so long without success that I just got stupid. I have four new octal tube sockets that are going in this radio chassis, one at a time, then test. All being well with this project, I'll work on the oscillator coil next. I took one out of a junker chassis, but it is lots smaller in size and will have to dangle by wires. I'd like to find something closer to what is there now. I'm not saying that changing tube sockets will fix it, but it sure won't hurt it.....

I've made a little progress.....

I have changed the 12SA7 socket and the rectifier socket. This little set is such a mess and I'm going to have a fortune in time in it. The wafer tube sockets are so brittle, they fall apart when you wiggle them even the slightest amount when you are soldering and unsoldering components. I decided the brown power cord wasn't going to look very good, so I got the bright idea of replacing it with a white power cord. Everything is so crammed together and my fingers aren't as young as they used to be. I hope I haven't killed the IF transformer by yanking around on the wires. The filament wires all have rubber wiring that the insulation falls off . So now I have to replace all the filament wires. I have changed all the resistors, all the capacitors except micas, most of the wiring, two (probably all four by the time I'm done) tube sockets, the speaker (that I've been trying all day to poke my finger thru) the oscillator coil. All this because I wanted a working four tube radio......

If this radio ever works again, it will be by the grace of God.....

It will work, I have confidence and it is not a Zenith so you have that going for you too.

Bill

I've got the Zenith done.....