Jon
New Member
Posts: 35 
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Post by Jon on Sept 3, 2021 12:27:06 GMT 12
Hi all, I would really appreciate some help with my Cash Radio that I am restoring. Having re caped the set and replaced all out of tolerance resistors (which was all bar 1!) the radio seems to play quite well and tunes across the dial, but I have two issues. Firstly, the radio would appear to have an unamplified RF stage if this is the correct term? that would appear to have been disconnected form the circuit yet the coils both seem to be good. So, with my limited knowledge I don’t see why this has been done or how to re implement it into the circuit. Secondly, I am unable to turn the volume right down, at its lowest setting it is still quite loud and shunting the wiper of the volume pot to ground has no effect. I have also substituted valves from my spares box, but this has made no difference as well. Also, I am unsure of the IF frequency, as I can find no information on the set at all so have set them two 460 kc as this seemed to be where they had been tuned originally. I don’t think that I have made any error when working on the radio, but it is always possible! I have drawn the circuit diagram (attached) to try to help me understand what is happening but I am now lost and hoping that someone can cast an expert eye and point me in the right direction or have a circuit diagram for this radio that they are willing to share. Thanks in advance Jon |
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Post by Philconut on Sept 3, 2021 19:27:29 GMT 12
Hi Jon, Firstly, you have marked the volume control value as 8k. This loads the IF transformer heavily. I would have expected the value to be 500k or even 1 meg. Also the circuitry around the 75 seems a bit unusual with the 500k resistor to the cathode.
I have attached the circuit of a Pacemaker I am working on at the moment that might help. The IF frequency is not all that critical because you can align the rest of the set to track properly. We have a very strong BC station here in Christchurch on 1593kC that creates a heterodyne on 675kC when the IF is set at 455kC, so I align to 465kC & all is well.
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Jon
New Member
Posts: 35
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Post by Jon on Sept 3, 2021 20:32:03 GMT 12
Hi Philconut, thank you for your reply.
I have just double checked the volume pot and indeed it is 8k so I will get back on to the set over the weekend an change for .5 or 1 meg. I have looked at many circit diagrams to compare what I have with this set and thank you for the diagram you sent and will see if I can work things out.
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Post by Philconut on Sept 3, 2021 22:31:40 GMT 12
I'm not at all sure that the 500k resistor should go to the cathode of the 75. It could well be that someone before you has "got at" the set. It often happens. - can't find a circuit to be sure. I would be inclined to make the volume control 500k and connect the 500pf cap across it as an RF bypass but you will need a blocking condenser and grid leak resistor exactly as per the circuit I posted. Also, because of the very high slope of the 75, there is little need for the biasing network in the cathode circuit. It will cause a small decrease in amplification of the 75 but nothing to be concerned about. It's all challenging and interesting stuff!
Cheers, John
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Post by Richard on Sept 4, 2021 10:17:58 GMT 12
The 500k from the IF to the cathode is the "Diode load resistor", so that is correct, however there should be a .01uF cap inbetween the bottom end of that resistor and the top end of the volume pot which should be 1 Meg in value. This make the Vol pot look high impedance relative to the diode load resistor, which allows the .01uF cap to discharge DC wise in-between peak modulation, so that it doesn't become saturated and cause audio distortion.
You can also reduce the diode load resistor to say 330k, as this wont cause much of a audio level reduction.
also missing on your schematic ,is the AGC filter cap, from the AGC bus to ground, it should be around .05uF , the filter resistor could be increased from 500k to 1M, to also make it high impedance relative to the diode load.
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Post by Richard on Sept 4, 2021 10:40:31 GMT 12
 ve modified your drawing to show what i was meaning. I haven't checked the rest of your drawing. I would also question the 75 tube as it could be causing audio leakage from the diodes internally to the plate?? |
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Post by Richard on Sept 4, 2021 11:01:59 GMT 12
Hi Jon, Firstly, you have marked the volume control value as 8k. This loads the IF transformer heavily. I would have expected the value to be 500k or even 1 meg. Also the circuitry around the 75 seems a bit unusual with the 500k resistor to the cathode.
I have attached the circuit of a Pacemaker I am working on at the moment that might help. The IF frequency is not all that critical because you can align the rest of the set to track properly. We have a very strong BC station here in Christchurch on 1593kC that creates a heterodyne on 675kC when the IF is set at 455kC, so I align to 465kC & all is well.
The pacemaker schematic shows the 500k Vol pot is also the diode load resistor. |
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Post by Peter Walsham on Sept 4, 2021 14:27:10 GMT 12
Having the diode load resistor returning to the cathode of the detector, instead of it returning to earth, provides an AGC delay voltage of about (guessed) 0.45 of a volt. With a replacement 500k volume control connected directly in the place of the 8k pot will seriously load the AGC voltage. I would suggest leaving the 500k diode load resistor connected as it is shown, but replace the 8k Ohm pot. with a 500k (or even a 1m) Ohm pot., but fit a coupling capacitor (say 0.02uF) between the AGC line & the 'top' end of the volume control. Having the diode load resistor (as the volume control pot) connected to earth makes for simple AGC control. Leaving the circuit as it is (having replaced the 8k Ohm pot. with a 500k Ohm one), means that when the volume control is turned up (with the radio tuned to a strong signal) the negative AGC voltage will likely bias the 75 valve close to cut-off. Currently, with the 8k Ohm volume control, there is no chance of any AGC voltage being developed.
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Post by Philconut on Sept 4, 2021 14:42:44 GMT 12
With the 500k resistor connected to the cathode instead of ground, there will also be a small audio voltage on the cathode which will explain the signal being present in the output even though the volume control is at zero.
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Post by Peter Walsham on Sept 4, 2021 15:19:33 GMT 12
Most radios I have seen that have the diode load resistor returned to the cathode of the detector valve have had an electrolytic bypass capacitor (a 25uF) connected across the cathode resistor as well as a 'paper' capacitor. The wonder of it is that the radio produces any audio at all.
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Jon
New Member
Posts: 35
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Post by Jon on Sept 4, 2021 20:25:15 GMT 12
Hi all and thank you for your help.
I am happy to report that I now have a working radio, and a working volume control.
I fitted a 1 meg volume pot and replaced the 500k filter resistor with 1 meg as James had suggested. I have to admit to missing the .05 AGC filter cap and the .01 cap to the volume pot from my diagram. Stupidity because, I had initially drawn the circuit into sections with these included, and I stitched it all together for the full version, but still managed to miss them!
This made a considerable difference, but not silence with the volume pot at minimum. I still had audio but, at a much lower volume than previously but it sounded muffled until you increased the volume slightly which then restored good tone.
The eureka moment came when I fitted a 25uF cap across the cathode bypass as Peter mentioned and John had remaked upon. Now there is silence at min vol, and no muffled sound, and the radio plays very well.
So, a good learning experience for me, thanks for your help.
Jon
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Post by Peter Walsham on Sept 4, 2021 20:33:34 GMT 12
Great to hear that it's now going well.
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Post by Philconut on Sept 4, 2021 22:13:44 GMT 12
An excellent result.
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