Identifican request - 3 Valve mains superhet

[pwdrive]

I haven't looked at it for two days on account of going out, and my 6 Volt LT batery needing recharging.       I know that the cans would come open easily enough if removed from the chassis, but they can only be removed by brute force, and having been removed, I doubt if I could get them back!     When it gets light, I will take a photograph to show you why!    I could check them by disconnecting them underneath, and a connecting each half in turn to a home-made transitron oscillator to see exactly what frequency each  LC circuit oscillates at!      Plenty other things to look at before I have to do that though - the dreadful sate of the wiring for a start.      
Bob

Doing that oscillator test might not give you the frequency that they actually resonate at when they are connected in the receiver.

Lawrence.

[Sparks]

Possibly not, but tests like this have often been of the greatest use to me. For instance, if one LC circuit was considerably different to the others, that would indicate a fault. In any case, I do not believe there is anything wrong with the IF cans - there are plenty other dodgy areas to investigate before them.
Bob

[pwdrive]

Possibly not, but tests like this have often been of the greatest use to me.   For instance, if one LC circuit was considerably different to the others, that would indicate a fault.    In any case, I do not believe there is anything wrong with the IF cans - there are plenty other dodgy areas to investigate before them.
Bob

Yes, I can understand that but IF transformers are relatively high Q, if one tuned circuit is a few kHz out with respect to the other then performance will suffer, best non intrusive way to check is to sweep the sig gen through and note the frequency it peaks at, if the cores haven't been tweeked then the chances are that all's well but that's not to say that the tuning capacitors or inductors don't go faulty, they some times can but not very often.

Lawrence.

[Sparks]

The solution came quickly, and unexpectedly (as it has often done with me). On trying it this morning, I found it 100% stable. I turned the HT up to 200 Volts, and the volume increased accordingly, and the stability was maitained. It is apparently funtioning perfectly! Reason - the low battery LT supply. I tried a reduced LT voltage and the instability came back. The instability was due to the LT battery running down. I had checked the battery before I started the repair, and it registered at 6.3 Volts, but two days ago, when I checked it on load to the set, it was 4 Volts, although it went back to 6 Volts off load! This would not have hapenned if I had used 6.3 Volts ac from a transformer, but I like to have an independent battery supply on projects like this. If the repair had gone quickly and smoothly, I would have gained no pleasure from it. But as it has given me a lot of brain exercise, I am now feeling pretty good about it, espcially as I have cured a large number of genuine faults along the way. Just a matter or re-wiring, and tidying it up now!
Bob

[Sparks]

Here are the fixing methods of the IF cans, as you will see, they are clamped under lugs that are part of the main chassis.   They could only be removed by brute force, and then would probably be impossible to get back in again.      The tray shows what I have taken out or replaced so far.   It really was in an awful mess!
Bob
 

[Sparks]

I have now cleared the final fault! With the full LT supply restored, as I said, it became stable again, but that was with the AGC capacitor shorted out. When I took the short off, it became partly unstable again if the HT was too high. Some final checking with the meter located a dry joint on one end of the V2 control grid decoupling capacitor (C7 0.1uF). As soon as this was made good, the set was fully stable on 220 Volts HT and the AGC operational (temporary short removed).
All that remains now is to rewire, and build a new PSU.
Bob

[Crackle]

Excellent work

Mike

[Refugee]

Well done.
I did say that the AGC capacitor might be duff back in post #41.

[Sparks]

The AGC capacitor was not duff - it was a brand new 0.1uF capacitor that was replaced early on in the repair! In fact the only capacitor I did not replace in the whole set was the 100pF aerial capacitor, and that will be replaced shortly because although it is still on spec, it looks awful, being one of those misshapen waxy types! The problem, as I said, was a dry joint at one end of it. I can only think that the 70-year-old solder had "gone off" somewhat, and when I soldered the new capacitor in by just melting the original solder, it produced a dry joint. I removed all the ancient solder, and re-made the joint.
Bob

[Refugee]

Well the "duff" was close enough to the electrical part of the capacitor. Modern rework flux works very well on old solder where the flux has evaporated long ago. It only takes a few seconds longer to clean the old solder off and start again though.
It is fixed and that is the important bit.
Well done.