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Old 3rd Jan 2020, 7:13 pm   #1
Ken Taylor
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Default Sonorton repair

Hello all and Happy New Year.
I have just been handed a lovely old French radio, a Sonortron, for repair. Not sure of the exact model but it is similar to the 22525 (described here: http://www.radioman33.com/pages/post...e-1953-54.html ) but the knobs and lamps are swapped, and it has an IF transformer under the chassis, in spite of a there being a perfectly good cut out for it. I am not the first to visit - it has had a few soldering irons poked in it. I cannot however find a circuit diagram and I wonder if anyone can help.

Many thanks
Ken
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Old 4th Jan 2020, 6:07 pm   #2
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Default Re: Sonortron repair

The only one I can find is https://www.doctsf.com/document-mode...6#lg=1&slide=0

Mike
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Old 5th Jan 2020, 7:28 pm   #3
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Default Re: Sonortron repair

Thanks for that Mike - it's close. The output valve on this set is a 6BM5 rather than EL84 but the rest of the lineup matches. I have already drawn out a diagram and it does not entirely match. There is an open circuit wire wound resistor in the power supply and although the marking have gone, by measurement it was 1.5k. The EZ80 valve base has been replaced previously and the chassis is blackened around it. The transformer tapping was set to 220v so it was probably running with too high HT. I shall order capacitors and continue.

Just noticed the g2 resistor on the EBF80 discoloured and only reads 30k. I should have thought it would have been much higher. On this set it goes straight to the HT line.

Ken
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Old 5th Jan 2020, 8:09 pm   #4
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Default Re: Sonortron repair

Replace or do a high voltage leakage test on the G2 capacitor on the EBF80 it may be leaking.
Mike
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Old 6th Jan 2020, 12:43 pm   #5
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Default Re: Sonortron repair

Just realised I have misspelt the name of this radio - it is Sonorton! Should I start a new thread or can it be edited?

Thanks Mike - I am ordering new capacitors, I don't have a high voltage test source.
Ken
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Old 6th Jan 2020, 1:33 pm   #6
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Default Re: Sonorton repair

Thread title updated
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Old 7th Jan 2020, 5:19 pm   #7
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Default Re: Sonorton repair

Cheers Bill.
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Old 10th Jan 2020, 12:48 pm   #8
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Default Re: Sonorton repair

The EZ80 cathode and heater have been connected together in this set, I cannot find another circuit where this has been done so can anyone suggest a reason why I should not separate them ?

Ken
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Old 11th Jan 2020, 7:09 pm   #9
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Default Re: Sonorton repair

While waiting for the final few components to arrive, I thought I would test the valves (ECH81, EBF80, EF80, 6BM5, EZ80 and EM80). Now I have an AVO CT160 which has been sitting in a neighbour's garage since it was pulled out of the lab we both worked at. I have never switched it on so did a search on this wonderful forum which came up with a most comprehensive thread on what is the recommended procedure https://www.vintage-radio.net/forum/...ad.php?t=69885. The tester is serial number 1707 and looks untouched inside. The manual is the 16th edition, 1964.

Switches have been sprayed with switch cleaner, resistors and capacitors given a quick in-circuit check, and it is suggested I disconnect the meter to protect it. I'll put it on the Variac in the morning.

Ken
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Old 13th Jan 2020, 12:07 pm   #10
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Default Re: Sonorton repair

Quote:
Originally Posted by Ken Taylor View Post
The EZ80 cathode and heater have been connected together in this set, I cannot find another circuit where this has been done so can anyone suggest a reason why I should not separate them ?

Ken
The cathode and heater of indirectly heated rectifiers were connected this way when the h-k insulation wasn't anything special and a separate rectifier heater winding was needed anyway. The unusual aspect of this set is the presence of a separate winding for an EZ80 which was designed to be safe on the same heater feed as the rest of the set. Separating the heater and cathode would be pointless but wouldn't cause any problems.
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Old 14th Jan 2020, 6:28 pm   #11
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Smile Re: Sonorton repair

Thanks for the reply, that is what I expected. I have now completely rewired the power supply section, using the separate transformer winding for the EZ80 heaters and doing away with the bare wires used for the previous repair. The o/c 1.5k resistor between the electrolytics has been replaced and the caps too - easier than reformatting. The set works and is quite sensitive.
However, there is a fair old hum which is not affected by the volume control and hanging the 'scope on the HT rail shows about 50v p-p ripple. I had replaced the caps with their original values - 8 and 16uF, should I have used bigger ones?

Although the 240v mains transformer setting is now selected the HT is nearly 300v. (measured with a DVM) This sounds too high to me and perhaps I should have used a bigger resistor too. Any advice welcome.

Interestingly, this French set has long, medium and shortwave bands, plus a separate position for just the 49m band.

73
Ken
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Old 14th Jan 2020, 7:54 pm   #12
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Default Re: Sonorton repair

The EZ80 can cope with up to 32uF reservoir so you could try an increase in value to see if it helps.

However, if you've rebuilt the PSU it's possible you may have introduced unwanted common mode paths in the HT filtering. It's something of a passion for me having been involved with some very low ripple magnet PSU designs and low level remote sensor monitoring circuits.

Check that the TX HT centre tap goes directly and only to the common of the filter caps. Take any chassis connection separately from that point. Similarly the cathode of the EZ80 should go directly and only to the reservoir capacitor tag (the heater connection excepted provided its supply winding is floating). The filter resistor should be connected directly between the tags and the HT feed should come separately from the smoothing cap tag. I know a lot of old radios play ducks and drakes here and get away with it, but good practice is better.

Essentially you don't want any ripple current induced voltages in the input wiring to the filter to get out of the rectifier-capacitor loop and the point where the ripple voltage is lowest is directly at the capacitor terminals.
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Old 15th Jan 2020, 1:29 pm   #13
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Default Re: Sonorton repair

Thanks Chris that is most interesting. It will take a bit of re working but it is possible and I'll try it.

Ken
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Old 16th Jan 2020, 11:54 am   #14
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Default Re: Sonorton repair

The heaters are all on one rail with the other side earthed so I imagine this would cause ac current to circulate in the chassis. When I have built amplifiers in the past I always used a twisted pair for the heaters with a centre tapped winding transformer. No centre tap on this set and I don't know if I can be bothered rewiring. New larger caps ordered and I'll see if they help first.
Ken
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Old 16th Jan 2020, 9:00 pm   #15
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Default Re: Sonorton repair

The hum could be caused by excessive anode current in the 6BM5, which could be due to any of several causes, the most likely being a leaky coupling cap. from the first audio amplifier to the grid of the output valve. You may have replaced this capacitor, but the cathode voltage, compard to the maker's service sheet will tell if there is excessive current being drawn. The valve may be "soft" i.e. gassy, or have heater/cathode leakage. Tony.
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Old 17th Jan 2020, 6:44 pm   #16
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Default Re: Sonorton repair

Ah! If only I had a maker's service sheet. I can measure the cathode voltage and determine the Ia and compare with data for the valve. Thanks for the suggestion, I'll do it this evening.

I still think the HT looks too high at 300v and I'm considering a higher value resistor in the smoothing circuit. It's 1.5k at present.
Ken
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Old 18th Jan 2020, 7:21 pm   #17
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Default Re: Sonorton repair

The cathode resistor measures 170 ohm and the voltage across it is 5v so the anode current looks about right. However, a slight rewiring as per Herald1360's posting and changing the reservoir capacitor to 33uF has reduced the hum dramatically. The anode voltage of the 6BM5 is 265v however, so I am going to up the filter resistor. I don't want this set coming back.
Ken
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Old 24th Jan 2020, 1:13 pm   #18
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Default Re: Sonorton repair

Well I put in a 2.2k 5w ceramic and the set still works. Those resistors get pretty hot don't they - should they be clamped to the chassis? I have just left it in mid air away from other components and wiring.
Ken
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Old 27th Jan 2020, 5:41 pm   #19
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Default Re: Sonorton repair

Fini. The owner is happy and has taken it away. He remembers it as a child. He did not know that that the French mains voltage is lower than in the UK.

Next - think I might have a look at that DAC90A that I found in my mother's garage...

Thanks to all who offered help and suggestions with the Sonorton.
Ken
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Old 28th Feb 2020, 3:12 pm   #20
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Default Re: Sonorton repair

Postscript.

The owner of this set had hoped to listen to French radio stations on his repaired set. Unfortunately all the French MW/LW stations have gone QRT, but we did hear Monte Carlo and Luxembourg through the noise on LW. I quickly wrapped about 25 turns of wire around a cardboard supermarket veg tray and connected it to a 500pF variable. It brought the LW stations in very well. So the owner built a wooden frame for the set to sit in and we wound the wire around that. It turned out even better and he is well pleased.

Ken
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