Champion TRF - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 5th Dec 2019, 5:54 pm

My interpretation of the circuit diagram in Newnes, which is available from this site is that in the Champion 784B R5 and R6 are connected to the AC input. This was not what I found in my radio. I attributed this to the endeavours of The Phantom. I have made the wiring as per the 784B apart from the AC mains switch being on the Live conductor now. The new 1K resistor is not likely to short out against anything. One of the tags of the active section broke, which I have had happen before with old brass.

I now have to replace out of tolerance resistors. Is it more likely to be components that were of a high resistance value to start with?
So what values are going to be the worst for drifting out of tolerance?

Hartley118 · 5th Dec 2019, 6:16 pm

The general guidance for carbon composition resistors is that high values (say above 10K) tend to drift higher and low values (say below 1K) tend to drift lower.

I'd suggest though that it typically doesn't make a ha'porth of difference because the original designer didn't generally sweat over his slide rule calculating values - most would have been a 'wet finger in the air' choice. I used to take trouble correcting resistor values in vintage kit and was normally disappointed that it produced no significant improvement.

If you were dealing with a calibrated lab test instrument, it would be a different matter, but unless you detect an fault symptom attributable to a drifted resistor, I'd leave them alone. If you have a problem like a reluctant oscillator or unbalanced push-pull output valves, then it can be worth looking at the resistors. Otherwise, if it ain't broke, don't fix it.

A current example of resistor value tolerance that I have on the bench is a pair of Heathkit MA12 10-watt amplifiers. I was checking circuit voltages and, comparing the two, found some surprising discrepancies. I was confident I'd found the reason when I spotted that what should be a 22K resistor in the HT of the left-hand channel actually had a 220R fitted instead (an error made more than half a century ago by the original assembler). So I eagerly soldered in the right value. Did it cure the voltage discrepancies? No way!

Martin

Neil Purling · 6th Dec 2019, 1:24 am

The audio still sounds distorted & I have already done the caps. I was feeling suspicious about the high-value resistors. I have another Champion TRF, but that has other issues beyond audio fidelity.
When it was working it was not as insensitive as I expected (862 New Minuet)
I was expecting similar performance from this radio.

5th Dec 2019, 5:54 pm	#21
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,086	Re: Champion TRF My interpretation of the circuit diagram in Newnes, which is available from this site is that in the Champion 784B R5 and R6 are connected to the AC input. This was not what I found in my radio. I attributed this to the endeavours of The Phantom. I have made the wiring as per the 784B apart from the AC mains switch being on the Live conductor now. The new 1K resistor is not likely to short out against anything. One of the tags of the active section broke, which I have had happen before with old brass. I now have to replace out of tolerance resistors. Is it more likely to be components that were of a high resistance value to start with? So what values are going to be the worst for drifting out of tolerance?

5th Dec 2019, 6:16 pm	#22
Hartley118 Nonode Join Date: Aug 2015 Location: Cambridge, Cambs. UK. Posts: 2,198	Re: Champion TRF The general guidance for carbon composition resistors is that high values (say above 10K) tend to drift higher and low values (say below 1K) tend to drift lower. I'd suggest though that it typically doesn't make a ha'porth of difference because the original designer didn't generally sweat over his slide rule calculating values - most would have been a 'wet finger in the air' choice. I used to take trouble correcting resistor values in vintage kit and was normally disappointed that it produced no significant improvement. If you were dealing with a calibrated lab test instrument, it would be a different matter, but unless you detect an fault symptom attributable to a drifted resistor, I'd leave them alone. If you have a problem like a reluctant oscillator or unbalanced push-pull output valves, then it can be worth looking at the resistors. Otherwise, if it ain't broke, don't fix it. A current example of resistor value tolerance that I have on the bench is a pair of Heathkit MA12 10-watt amplifiers. I was checking circuit voltages and, comparing the two, found some surprising discrepancies. I was confident I'd found the reason when I spotted that what should be a 22K resistor in the HT of the left-hand channel actually had a 220R fitted instead (an error made more than half a century ago by the original assembler). So I eagerly soldered in the right value. Did it cure the voltage discrepancies? No way! Martin __________________ BVWS Member Last edited by Hartley118; 5th Dec 2019 at 6:18 pm. Reason: blunder

6th Dec 2019, 1:24 am	#23
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,086	Re: Champion TRF The audio still sounds distorted & I have already done the caps. I was feeling suspicious about the high-value resistors. I have another Champion TRF, but that has other issues beyond audio fidelity. When it was working it was not as insensitive as I expected (862 New Minuet) I was expecting similar performance from this radio.