Cossor melody maker 524

[ultrascan]

I have a cossor 524 melody maker, mw sw and longwave are fine but vhf is low output, I have replaced all those hunts caps in the bottom chassis the if appears to work ok even realighned the strip, I have fed an external tuner to the input but made little improvement, any ideas welcome.

[Crackle]

Where did you connect the external tuner, to the gram input? Or do you mean you connected an external dipole aerial to the VHF areal sockets?
Have you checked the 5uf electrolytic in the discriminator circuit? On the Trader service sheet 1221, it is C46 and described as "FM Limiting".
Failing that I have had problems with the triple diode triode type valves V4, changing it on some of my sets has made a big improvement. An equivalent valve is the EABC80.
I normally shy away from fiddling with the alignment of FM IF transformers, you can really mess things up if you dont do it properly.

Mike

[boater sam]

I have one of these on the bench as we speak, not the best of sets. The vhf valve, ECC88, is often low emission due to cathode poisoning, the HT is off when in AM mode.
The valve sockets are the nasty paxolin ones, they need careful cleaning with a pipecleaner and the valve pins polishing too.
Beware the plastic gear on the tuning, it strips very easily.

[Terry]

 The vhf valve, ECC88, is often low emission due to cathode poisoning, the HT is off when in AM mode.

That's poor design!

To prevent a recurrence, connect a high value resistor - 1MΩ or so - across the switch contacts so that a trickle of anode current continues to flow in the valve(s) in the FM tuner head when operating on AM.

[Mike Watterson]

Does heater on and HT off really give cathode poisoning?
I've heard the reverse claimed too, but most Transievers with Valve PA leave HT on and turn off the heater for RX only.

I have the RCA "Book" and others and find little on this subject.

[Terry]

Does heater on and HT off really give cathode poisoning?

Yes or, at least, that is what I was taught.

Theory is that the space charge forms above the cathode in the normal way but it has nowhere to go. So, because of the shortage of electrons in the cathode, which would normally be satisfied by the current flowing through the valve, the cathode becomes increasingly positively charged, attracting the electrons back at high speed. 

The constant - and continual - bombardment by the electrons eventually damages the emissive layer of the cathode, 'poisoning' it.

The term is incorrect in these circumstances as cathode poisoning is normally caused by impurities in the cathode material but the outcome is the same: the cathode 'dies'.

[boater sam]

ECC85 of course, typo.