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Post by briano on Aug 15, 2023 16:19:43 GMT 12
ROLLS RADIO, TYPE XR
QUESTIONS1. What would cause the Local Oscillator to die after ten minutes and, what components should I suspect? 2. Any thoughts on where to obtain a replacement tuning friction wheel? EXPLANATION1. The radio is lively for the first few minutes, and then the radio station fades away. Anode voltage readings stay the same at +270 volts for each valve. Screen voltages are maintained on +100 v. However, the oscillator grid voltage on the 2A7 mixer stage (second photo) dies from negative voltage to zero volts. I placed a small transistor radio near the chassis and tuned in at the top of the AM band to receive the Rolls Radio's Local Oscillator. When the Rolls radio died, the Local Oscillator signal being picked up on the transistor radio, disappeared at exactly the same time. 2. The tuning friction wheel is perished and tuning slips. (Third photo) RADIO DETAILSThree-bands, MW and two SW Six-valves Rola 8" 2500 ohm electromagnetic loudspeaker VALVES58 RF 2A7 RF/Mixer 58 IF 2B7 Detector/1st AF Amp 2A5 Audio Power Amp 80 Twin Diode Rectifier WORK SO FARThis radio belongs to a friend of my wife. Work done so far: - Cleaned very dusty cabinet and chassis
- Remove small post-manufactured burn't out transformer
- Removed post-manufacture silicon diodes wired under rectifier valve
- Replaced out-of-tolerance resistors
- Replaced all paper/wax capacitors
- Fitted two new 10uF electrolytic capacitors
- Original mains transformer tested okay
- Fitted New Old Stock valves, except the 2B7 detector/AF pre-amp.
- Unwrapped crude joints going to speaker and to three dial-lamps and repaired.
- Radio turned on with only 80 rectifier valve only. Voltages checked out okay
- Remainder of valves plugged in, shield cans fitted.
- Manually tuned with hand on large mechanical cogged wheel - friction wheel is perished.
- Radio goes extremely well for about ten minutes and then station fades away
- Determined Local Oscillator is stopping
Your help on why the local Oscillator is dying and, what I can do about the perished tuning friction wheel, will be most appreciated. Thank you. Cheers, Brian Oliver
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Post by Peter Walsham on Aug 15, 2023 17:44:09 GMT 12
Hello Brian
I would start by checking (by substitution) the 25pF Osc. coupling capacitor, then check for leakage to earth of the Osc. trimmer, tuning cap. & padder capacitor - also checking for leakage of HT from primary to secondary of the osc. coil. The fact that you have tested the voltage on the grid of the oscillator is very much proof that the oscillator is stopping - and there's not a lot of components in that part of the circuit. Unfortunately, I can't offer a solution for the perished dial drive rubber.
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Post by briano on Aug 15, 2023 21:17:02 GMT 12
Hello Peter
Thank you for your response. Most helpful.
1. Any suggestions where I may be able to purchase the 25pF silver mica capacitor from?
2. If I couldn't get exactly 25pF, would a lower or higher value suit best?
3. What voltage rating would you advise?
Kind regards, Brian
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Post by Peter Walsham on Aug 15, 2023 21:49:39 GMT 12
A 27pF, 50 Volt ceramic capacitor from Jaycar (cat. no: RC-5317) will do the job very nicely. It isn't subject to any high voltage, so a 50 Volt capacitor can be used. I meant to add in my previous message that old silver mica capacitors are no longer to be trusted. They were generally regarded as being very reliable, but this is no longer so. I recently did a random 'study' of about 100 old mica capacitors I had in stock, and nearly all of them tested well below their stated values - some less than 50%. It wouldn't surprise me in the least if the mica capacitor in your set has gone low in value. You could possibly use a value quite a bit higher than 27pF (say up to 100pF or even higher, if you have one in stock). No harm will be caused if you go too high in value. The oscillator may 'squeg' (due to excessive feedback), but if the radios oscillator continues to oscillate without stopping, it may well prove the original capacitor is suspect.
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Post by Richard on Aug 16, 2023 8:11:51 GMT 12
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Post by briano on Aug 16, 2023 11:45:01 GMT 12
Hi Peter and Richard. Thank you for your replies. Peter - Thank you for your recommendation on the ceramic capacitor ex Jaycar. Awesome! Interesting on your study into the old mica capacitors. I have rummaged through my tray of about 70 ceramic capacitors, and I think I have found one that I can perform a test with. I think it is 56 pF? I upload a photo of it here. Not sure what the voltage rating is though? Richard - Thank you for your link to the thread on friction drives. Fantastic - looks like you have done some work in this area. Most helpful, thank you. Attachments:
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Post by Peter Walsham on Aug 16, 2023 12:34:46 GMT 12
I think that capacitor might be a 560pF. I'm not sure what the voltage rating of it is either, but it will do to test the faulty capacitor theory out anyway. The 'N750' written on the bottom line indicates its temperature stability factor. In your radio, this isn't going to be of much importance. Put the capacitor in & see what happens.
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Post by briano on Aug 16, 2023 16:34:54 GMT 12
Hi Peter
Thank you for your suggestion to use the ceramic capacitor on hand to test.
I fitted and the radio ran fine for 25 minutes before the Local Oscillator stopped and radio faded out. I turned off for one minute and switched back on. The radio then went for 6 minutes, faded briefly and then came back. Then the radio died after 2 minutes.
I wonder if the fault is temperature related? Although the radio did last longer from cold than before.
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Post by Peter Walsham on Aug 16, 2023 17:54:00 GMT 12
MMMmmm, well that appears to rule out the oscillator feedback capacitor as being the prime suspect. Did you check for any resistance (high Ohms range on an analogue multimeter) from the oscillator grid coil to earth? Have you checked the resistances of the coil primary & secondary windings (what are the resistances?)? Have you refitted the old 2A7 valve & 're-tried' that? I agree that the fault does sound like it's a heat related one, however the valve is the only thing that would heat up. Does the oscillator drop out on the short wave bands as well?
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Post by stanley on Aug 16, 2023 18:55:46 GMT 12
Some thoughts of what I would test:
Do you have access to an insulation resistance tester (megger)? They are very useful for finding things that have been tracking or leaking. (like the mica capacitor breaking down)
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Post by davidballantynezl1tfg on Aug 17, 2023 8:04:27 GMT 12
You say the station fades away. Do you mean it gradually gets weaker then gone completely? If so the filament may be going out. Dry joint at socket, pin not tight enough, dry joint in valve pin ,sometimes file the end of the pin & resolder fixes that, dud valve. I note on your circuit the high osc. plate voltage. David
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Post by Peter Walsham on Aug 17, 2023 9:42:43 GMT 12
David makes a very good point.
Another thing you might try is to measure the 2A7's heater voltage - but at the valve pins themselves where they protrude from its socket, not just the socket terminals. The 2A7 'only' has 2.5 Volts available to heat it and a very small amount of valve socket contact resistance might not let enough current to flow to heat the valve properly. Have a look at how bright the 2A7's heater is when the radio is running & compare it (as best as you can) to when the radio stops.
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Post by briano on Aug 17, 2023 16:34:20 GMT 12
Hi Peter, Stanley, and David
Thank you for your further suggestions. Most helpful. I think we are onto the cause - an intermittent filament termination on the base.
But first, I checked for any resistance from the oscillator grid coil to earth and was completely open circuit on my multimeter's highest range. The original 2A7 was dead and has a loud rattle when shaken, so I had no options to swap. The oscillator dropped out on short-wave as well.
I tapped all joints as per Stanley's suggestion, checked the new resistors. Unfortunately I don't have access to a megger.
Your suggestion David about, "dry joint in valve pin", and your suggestions Peter about, "how bright the 2A7's heater is" led me to investigate. In fact, when I turned the set on today, the oscillator was dead. And, I could not see a glow in the 2A7, and was the only valve that felt cold. Pulling the valve out the filament was open circuit. This despite a check I did on it before I used where I had a DC resistance of 0.5 ohms.
So, I filed the heater pins and then re-soldered being careful not to apply too much heat. Resistance became 6 ohms. I put back into the set - and was still dead. Checking the heater resistance I found it had become open circuit again. So, this time I applied lots of heat and fresh solder. Resistance now was 1 ohm, and the radio ran for 10 minutes before it died.
In conclusion, I believe the NOS 2A7 mixer valve, is faulty. I purchased this last December from "Antique Electronic Supply" in Phoenix, when visiting our son in the same city. I will have another go at re-attacking the base and try and use a solder-sucker to expose the wires better. However, I'm tending to think I should procure another 2A7. I can import from the U.S. - unless anyone knows of a local source?
Once again, thank you for all your help. It feels good to know what is going on with that radio. It was getting a bit frustrating, ha ha-!!
Cheers, Brian
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Steve
Society Members
vintageradio.co.nz
Posts: 734
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Post by Steve on Aug 18, 2023 11:19:12 GMT 12
Hi Brian, I believe I probably have a few 2A7's kicking about - I'm particularly fond of that lineup, and the 6.3V version that followed it, and have a few sets with them in it... I'll have a look over the weekend.
Cheers, Steve
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Post by briano on Aug 18, 2023 12:03:50 GMT 12
Hi Steve Thank you for your kind offer to check. Yes, after doing work on the heater pins this morning - solder sucking the hollow pins to expose the wires, and re-soldering - the radio turns on but, after a minute or so fades out. A few minutes later it fades in briefly before fading out again. Using a surface thermometer, the temperature, within an inch of the top of the valve, goes up when working, and down with the fault. I think the filaments are partially working when the local oscillator stops, but not enough to keep it going. I upload a photo showing work on the heater pins in progress. Cheers, Brian Attachments:
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