I have recapped and replaced out of tolerance resistors and so on. Radio plays nicely on fairly strong stations. The problem is weak stations produce squealing when tuned in. I have touched and moved many wires which make the squeal go away but just moves up or down the scale a tiny bit while still on the same weak station. I have swapped out the oscillator, IF and detector tubes with no change. Grid wire to the detector is inside the tube shield. Like I say the stronger stations don't have this problem. An alignment has been done but noting the antenna trimmer does nothing. Signal generator set very low. Any idea what is causing the squealing?
All correct shields must be in place, all tubes correct no subs of any kind.
Check any soldered, riveted ground connections, DC measures of ground cannot detect a ground that has hidden corrosion, solder those riveted grounds to the chassis.
An open coil or a coil with resistance higher than the published specification.
A band switch that has been treated with a lubricant may be leaking. Sam lube on tube sockets can cause this. Clean all excess lube off with 91% alcohol, gently blow dry do not power until all alcohol is gone.
Check the AVC circuit for stray leakage and possible incorrect parts in the circuit.
Do, look for stray wire snips and larger solder blobs causing shorts. A misplaced bypass cap can be problematic too...
Look for calibration errors, a mica may be failing or failed. Calibration is often a clue other than mis-alignment.
GL
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
I have taken out the 2nd IF and found a problem or not. I believe the coils are litz wire. There is a very small strand tied to the wire leading out of the coil. That strand it a lot smaller than the coil wire. Problem is the coil wire is less than an inch long and is the inside of the coil. How am I ever going to solder that? I'm thinking that single strand might be my squealing. I have never tried to solder litz wire. I am concerned I may burn up the wire and then the coil is done for. Any suggestions?
I'm pretty sure I now have the litz wire soldered. This did not make any difference. Back in April I rewound the secondary of the oscillator and because the radio did not receive stations. After the rewind it did but I didn't pay attention to what the reception was like. I then prepared the radio for cleaning and restoration over the summer. It has been sitting since then until now. As I have said the radio plays good on strong stations but squeals on weak stations. I have gone over it with a fine tooth comb using the advice from Chas. Radio still squeals so I am starting to suspect the oscillator secondary rewind. Wondering if I did it backwards. Can anyone verify how it is supposed to be wound? I'm pretty sure I have the number of turns correct.
If a coil was wound backwards, the oscillator would not work at all.
Old school litz has plain enamel insulation it must be either mechanically or chemically removed, the modern litz has fusible insulation a touch of the iron burns it away like rosin.
Any coil with litz the has a published resistance will assume all strands are connected. If a strand opens, the resistance will rise and the coil will tune more broadly.
Glad you did the usual routine, sorry it didn't work.
Download this Riders publication, The first chapters are teaching, the last chapter is application to superhetrodyne radios. It is written for technicians.
I got my signal tracer out to see if I could tell at what stage the squeal was coming from. It is present on the plate of the oscillator (6A7) tube. What I thought was strange is I had to have the volume turned up on the radio before I would hear anything on the tracer. Why is that?
Your IF transformers are tuned to the resonant frequency they were designed for. When you observe the bell curve they display, the transformers pass signals at maximum voltage at this frequency. Sometimes it's too high and creates an unwanted oscillation. The fact that it occurs with distant stations seems to be the clue because the AVC has biased the rf gain of the first three tubes very high. Detuning the IF slightly will flatten the bell curve, reducing the gain slightly at resonance. When you do this it may be covering the real reason for the oscillation, such as a bad ground connection or mistaken capacitor value in the AVC (bakelite block part 7).
Rod, I had checked out your suggestions but did not help. I did solder the ground rivets to the chassis as Chas suggested.
Good news! The squealing is gone, now that I have corrected my wiring mistake. I had the oscillator grid wire connected to the top connection of the the antenna coil and the pin 5 of the antenna coil connected to the top connection. So basically backwards I guess. I didn't think I was going to solve this one.
My radio has what is called a wave trap but the alignment instructions don't say what to do with it so I just peaked it for the best signal at 600. Hopefully that is correct.
Quote:I just peaked it for the best signal at 600.
NULL it for IF frequency, originally it would have been null for 500khz., to reduce signals from the distress frequency. There should be a defined dip for 500, may also dip at IF. Any "stray" IF would be blocked by the trap.
Glad to hear the problem is now gone!
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
Usually a wave trap is for keeping IF signals from entering the antenna and from leaving the antenna. You feed an IF signal into the antenna and adjust the trap for minimum. But I don't see a trap in the schematic.
Well, a wave trap is a trimmer in series with the coil so at the resonant frequency it provides a short to ground. This looks like something else, more like a broad boost, depending on which band it's resonant at. Perhaps for SW. In this case I would tune it to maximum signal like you did.
![[-]](https://philcoradio.com/phorum/images/bootbb/collapse.png "[-]")
