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Old 26th May 2018, 10:17 am   #1
Join Date: May 2016
Location: Bath, Somerset, UK.
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Question Radio Rentals Model 58

I am working on an early Radio Rentals set which I believe has a model 58 chassis. After recapping the RF and IF stages the set is working on MW but will only tune to BBC Radio 5 Live (909 kHz). With an aerial this is very strong but no other stations are available and LW is completely dead. I suspect the local oscillator of IF stage but my experience in trouble shooting these is limited. Has anyone experienced this problem or able to offer some advice? any help will be greatly appreciated.

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Old 26th May 2018, 10:46 am   #2
Boater Sam
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Default Re: Radio Rentals Model 58

Why did you recap the RF and IF stages? Usually these have mica caps which are very reliable. You have probably upset the alignment.
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Old 26th May 2018, 11:25 am   #3
Station X
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Default Re: Radio Rentals Model 58

Does BBC R5 appear at the correct dial position?

Can other radios in your collection receive BBC R5 on both 606 and 909 kHz (assuming you're in a suitable location to receive both transmissions)?
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Old 27th May 2018, 9:10 am   #4
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Default Re: Radio Rentals Model 58

Did you get signals before you recapped it ?
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Old 27th May 2018, 5:14 pm   #5
mister valve
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Default Re: Radio Rentals Model 58

I assume that 'Radio 5' has a 909kHz 'Tx' near to you ? So is a strong signal, that is probably why you can tune it in.

This sounds like a classic case of the tuned circuits being badly out of alignment (as 'Boater Sam mentioned). Changing the 'Mica' caps in the tuned Circuits (R.F & I.F) will almost certainly detune them.

As mentioned above, Mica Capacitors (Condensers) are very reliable and normally retain their correct capacitive values and internal insulation so rarely need changing. But do change all the Paper Capacitors "waxies" regardless of condition, they will by now be in varying states of electrical leakage (particularly 'THAT' Capacitor which signal couples the Anode of the 'EBC33' to the Control Grid of the 'EL33'. The 'EL33' Valve is a high slope Pentode, so even the tiniest leakage of 'THAT' Capacitor can cause the EL33 Valve to run to saturation point - thus passing excessive current. This can cause the output Transformer and/or the Cathode bias resistor to burn out. Even the Mains Transformer and 'AZ31' rectifier could become casualties.
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Old 27th May 2018, 6:51 pm   #6
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Smile Re: Radio Rentals Model 58

Thank you all for your observations, I did not replace any mica caps, only the paper ones (0.1mf ones). I think I have located the problem. The Screen grid voltage on the EF39 (pin 4) was only 5V and not the 60v it should be according to the circuit data sheet. On measuring the 68K resistor I had a very high reading. Have not replaced this yet as I have had to order a suitable wattage resistor. I have been careful not to disturb the RF alignment so all should be well when the resistor is replaced. Once again thank you for your help, I will keep you posted.
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