Identifican request - 3 Valve mains superhet

[pwdrive]

Make sure the IF transformers are tuned to the same frequency, but get the HT up to normal first.

Lawrence.

[Sparks]

As long as the wire is connected to the anode of V2 (IF amp), I can turn the HT up to the full 200 V and it works perfectly. Take the wire off, and it is back to shrieking and whistling. The slug holes of both IF transformers are visible in the sides and are sealed with wax that has obviously never been disturbed, so at the moment, I am reluctant to mess about with them. Neither can I open the IF cans to check the capacitors because they are clamped firmly in place with a lip of the chassis steel pressing on the lower rims. If I forced them out, I would not be able to replace them. I put the sig gen on the grid of the EF39 and it seems to indicate everything is OK with the IF and output stage. Just can't figure out how the instabilty is hapenning at the moment. With the wire connected to the anode, I am amazed at the stability, volume, clarity and performance of the set!
Bob

[pwdrive]

As has been mentioned, make sure the metalized coating and its connection on the valve is good.

Lawrence.

[Sparks]

As I said, the metalising was complete, and connected.    However, I have sorted it out now, although what I have done will probably not be accepted as a solution!      On studying the circuit, it appears that the supressor grid of the EF39 (IF amp) is used as a diode anode to produce the AGC.      I decided (probably incorrectly), that a malfunction of the AGC was increasing the gain to the state of instability.      So - I shorted the AGC capacitor out, thereby disabling the AGC.    The shrieking and howling stopped immediately.     On 200 Volts HT, the tone was a bit rough, but on turning it down gradually, things began to improve.   Finally, the optimum performace was obtained with an HT voltage of only 50 Volts.   This gave more than adequate volume, excellent tone and selectivity.     This suits me fine, because I can build an isolated PSU providing the required 50 Volts, that I consider a lot safer than the specified 220 Volts HT on a 73-year-old receiver!
I can now concentrate on replacing the wiring.   I have already restored the screen dropper circuit on V2 to the original configuration of sharing the screen dropper and decoupler of V1 with V2.        
Maybe the condition was caused by the fact that the Westector was faulty, and I replaced it with a gremanium didoe that was probably superior to the Westector when it was working!
Bob    

[Sparks]

With 50 Volts HT, the HT current total is 8mA. When set at 80 Volts, I get extra volume, still with excellent performance for 13mA total HT current!
Bob

[pwdrive]

I would do 220 volts HT, much better performance, better conversion gain, more power output, but...'tis up to you.

Lawrence.

[Crackle]

My WTCR works very well and sounds great on the standard 240v mains, it was a very clever yet simple design.
You must still have a fault somewhere. Have you changed all the wax capacitors?
I have some westector diodes somewhere if you want one.
Mike

[Sparks]

Crackle - Thanks for offer, but I have another Westector, but didn't realise it was one Your third image along shows one exactly the same as I have in my diodes box!

There may well still be a fault on it, but I have changed ALL the capacitors except the aerial series capacitor (and that would not effect anything as it checks OK and is only 100pF).

PW Drive - As it stands at the moment, increasing the HT to 220V reduces the volume and makes it unstablel. I am amazed at the performance with 50 Volts HT, and it is safer as well. On a MW set, the lack of AGC will not affect anything very much, but I will investigate the phenomena when I get further along with the repair. There may be some resistors gone off spec, but they are extremely hard to identify as their colours are all faded, and difficult to disconnect from the double tag strip!

Bob

[pwdrive]

A lack of AGC voltage when receiving strong signals can cause the mixer/IF/Det to be overloaded, if there's a problem when running with normal HT then that suggests that there's fault with the receiver.

Lawrence.

[Terry]

  There may be some resistors gone off spec, but they are extremely hard to identify as their colours are all faded,

Bob, this might help:

http://www.radio-workshop.co.uk/service/...ver-ac.pdf