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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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3rd Jun 2020, 5:48 pm | #1 |
Pentode
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Sonora SF5 - Low HT
Hi all
This is a follow up after a request for a circuit diagram for this French radio. I now have it thanks to help on this site OK, so its from 1936. Quick cold checks no obvious shorts on primary side and visuals looked ok though noted the original "42" model output valve had been replace by an EL84 some time back. The original socket is in place, looks like the base of the old valve retained and a B9A (?) socket with EL84 mounted.....no attempts seem to have been made re circuit component value changes. Anyway gave it a gentle start with a bulb limiter (100w bulb). Valves glowing and faint crackle from the speaker when volume control twiddled. Voltage checks on secondary of mains tranny giving just over 300v on each side of the winding ie 2 x 300v to the "80" full wave rectifier valve. However, the DC from the valve was a lowly 34 volts. I disconnected ALL the loading on the DC side of the valve (except the filter cap) and it bumped up to about 270v. As soon as I connect in the output valve or the earlier stages of the set the voltage plummets again. I've checked the other HT DC feeds to the remains of the set and no shorts apparent or anything getting obviously hot. Forgive my ignorance here but can rectifier valves give a good output but plummet when on load due to the valve being worn out?? Unfortunately I don't have a valve tester and was gonna stick a couple of 1N4xxx silicon rectifier diodes in place of the 80 valve but unsure what resistor I should use to drop the volts sufficiently for the set. Please see attached pics and screen snip of power supply. Any thoughts please?? Kind regards Rich |
3rd Jun 2020, 5:57 pm | #2 |
Dekatron
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Re: Sonora SF5 - Low HT
With NO load on the HT and 300Vac on the rectifier anodes, I'd expect 400V+ on the reservoir cap. Does sound like a very tired 80.
A couple of 1N4007s and say 220R 5W should be OK IF the reservoir and smoothing capacitors are rated at least 450V and in good health... use a lamp limiter too for initial investigations!
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3rd Jun 2020, 9:10 pm | #3 |
Pentode
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Re: Sonora SF5 - Low HT
Thanks Herald1360 - i've found a new old stock "80" at Langrex for a tenner so ordered it anyway as i think your suspicions are correct re the valve not being able to deliver some oomph. I'll try the diodes+resistor option anyway - the reservoir cap is 500V rated and i've broken it in gently via the limiter. I'll retain the limiter for testing.
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3rd Jun 2020, 9:34 pm | #4 |
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Re: Sonora SF5 - Low HT
A radio of this age is almost certainly going to need the audio coupling capacitor to the grid of the OP valve changed in order to stop DC leakage onto the grid of the OP valve.
The other problem is going to be leakage in the smoothing capacitors, this will have the effect of pulling too much current through the energising coil and causing the voltage to drop. I would advise rectifying those issues before you put another more capable rectifier into the radio. Or you may find the higher current capabilities of the new valve will be enough to damage the speaker energising coil or the output transformer, or the mains transformer secondary. What made you choose to identify your radio as an SF5, as that radio appears to have 3 bars across the speaker whereas yours has 2 bars. Mike |
3rd Jun 2020, 9:47 pm | #5 |
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Re: Sonora SF5 - Low HT
I think it would be a wise move also to replace the 0.02uF capacitor between the anode of the output valve and 0V- if this fails, the output transformer, field coil (exc dyn) speaker, rectifier and mains transformer are all in serious jeopardy. A 1kV rated capacitor should be used to give a healthy margin of DC HT + peak AC excursion. Sorry if the replacement list is starting to build up, but it's still cheap insurance.
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4th Jun 2020, 1:10 am | #6 |
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Re: Sonora SF5 - Low HT
You could test the rectifier with something like a 15W pigmy bulb. That would represent a load approximating to an average set.
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4th Jun 2020, 7:32 am | #7 | |
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Re: Sonora SF5 - Low HT
Quote:
Mike |
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4th Jun 2020, 8:42 am | #8 | |
Pentode
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Re: Sonora SF5 - Low HT
Quote:
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4th Jun 2020, 8:50 am | #9 | |
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Re: Sonora SF5 - Low HT
Quote:
Is it safe to leave it out of the circuit altogether, given the switch it's associated with is knackered? |
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4th Jun 2020, 9:08 am | #10 |
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Re: Sonora SF5 - Low HT
Yes leave it out for now , as I suggested replace all the paper capacitors ,and poss the smoothing , I have a full circuit and layout , of the two bar one vm1. it looks like the output could be an 80 or a 6f6 . .Mick.
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4th Jun 2020, 9:26 am | #11 |
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Re: Sonora SF5 - Low HT
Mick, the output valve is a 42 but was replaced at some point in the past with an EL84...though the original valveholder is intact. It seems from a quick look on interweb that 42 valves are quite hard to find (and probably expensive...). Is a 6f6 a direct pin for pin substitute/equivalent
The repairer doesn't seem to have changed any of the other components in the output circuit so i'm not sure the EL84 is 'right' for the job, but we'll see... |
4th Jun 2020, 10:03 am | #12 |
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Re: Sonora SF5 - Low HT
Sorry I don't know what happened there I know 80 is the rectifier ,It would be nice to restore it back to factory, I may have a 42 will let you know ,Mick.
Good news I have a few 42's I can let you have one tested 100% £7 inc p/p PM me with your posting details , if you want it , I noticed the 6f6 is octal ie not the same base as the 42 , Mick. Last edited by vinrads; 4th Jun 2020 at 10:22 am. |
4th Jun 2020, 10:27 am | #13 |
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Re: Sonora SF5 - Low HT
The capacitor from the anode of the OP valve to ground (the one that has blown up in your case) is I believe referred to as the tone correction capacitor. It has two functions, to reduce treble in the audio output, you sometimes see it connected via a variable resistor to give full tone control.
Another function is I believe to help stop any unwanted oscillation that might occur in the OP stage. In this case a safer way to connect the capacitor is across the primary of the OP transformer, rather than from the anode to ground. Mike |
4th Jun 2020, 11:29 am | #14 | |
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Re: Sonora SF5 - Low HT
Quote:
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4th Jun 2020, 11:46 am | #15 |
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Re: Sonora SF5 - Low HT
here is one, https://uk.farnell.com/vishay/s223m7...z5u/dp/2860155
But I would try a .01uf as I always find the tonal quality is better with a little more treble. Mike |
17th Jun 2020, 10:13 am | #16 |
Pentode
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Re: Sonora SF5 - Low HT
Hi everyone
OK, update on the Sonora radio. Thanks to Langrex and a kindly chap on here i've got replacement full wave rectifier and audio output valves back into the chassis. I now have a set that's working 'sort of'...
So, HT. Both my DMM and analogue meters are giving 345v DC on the cathode of the CV1708 f/w rectifier valve - this drops to around 294v on the anode of the output valve. The CV1708 replaced the original "80" valve - I was advised this was a suitable substitute (not on this website)... This voltage seems high. Any thoughts and/or suggestions on how to tame it? which 'may be answered by the next issue. The speaker cone is dragging - it's an energised coil speaker (not permanent magnet) and tbh I don't currently have time or patience to try and sort it out. I do have a permanent magnet speaker that would fit. If I ditched the original speaker what sort of resistor ohm/rating might be good to try? I'm also linking this with the high HT above. Medium wave is quiet. The set uses a strange set of 12 rotating cams across 12 contacts to change waveband so i'm trying to see if some aren't making contact. Wafer switches are so much easier! Long wave pulls in RTE and R4 but both are buried under a lot of static. I'm using around 7ft of wire for the aerial. I guess it needs more?? All paper caps have been changed for new ones and resistors checked though most were within 20%. Cheers Rich Last edited by richieyork; 17th Jun 2020 at 10:22 am. |
17th Jun 2020, 2:16 pm | #17 |
Dekatron
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Re: Sonora SF5 - Low HT
It will be far easier to fix the speaker than change to a permanent magnet one. Did you replace the suppression capacitors across the mains input with class Y types? Is the speaker in the set original?
Is it set for 250V? The high HT volts may in part be due to a later generation 80 valve but high HT normally means the set is not taking enough HT current. Check the value of the output valve cathode resistor then measure the cathode voltage so you can work out the current. |
17th Jun 2020, 2:33 pm | #18 |
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Re: Sonora SF5 - Low HT
Is the voltage selector correctly set?
What's the AC voltage on the HT secondary of the mains transformer? Resistance of HT Choke/speaker energising coil?
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17th Jun 2020, 2:58 pm | #19 | ||
Pentode
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Re: Sonora SF5 - Low HT
Quote:
The speaker looks to be the original. I'm put off trying to repair it as i'm unsure how to with this type of cone assembly - i've done a few hifi speaker re-centerings but this one has a centre piece that looks like a 45rpm record centre adaptor that's held down by a screw. How can I safely remove that from the cone? Quote:
The value of the output valve resistor measures correctly at around 400 Ohm. |
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17th Jun 2020, 3:06 pm | #20 | |
Pentode
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Re: Sonora SF5 - Low HT
Quote:
The voltage selector is set at 250v (originally was 220v as it originated from France). AC voltage on each of the anodes on the CV1708 is 336v Resistance of the speaker energising coil is 1.3k ohm I've just noticed an earlier repair (not mine!) has used 16 + 16uf smoothing/filter cap can - the circuit shows 8 + 8 uf - would this account for the higher than expected voltage?? Cheers Last edited by richieyork; 17th Jun 2020 at 3:16 pm. Reason: Noticed a wrong value component on the power supply? |
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