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Old 11th Jan 2019, 11:31 pm   #1
Jolly 7
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Default Stella ST113U- distorted sound causes ?

Currently I am trying to fix a Stella valve radio. I found out that the speaker is open. The radio works on both medium and long waves but produces distorted sound through a pair of high impedance headphones. Can anyone please suggest the most likely cause of the distorted sound. It uses four valves: UCH81, UBF89, UY85 and UCL82.
I am aware the speaker has an impedance of 300 ohms. Are there modern substitutes available ?
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Old 11th Jan 2019, 11:54 pm   #2
Tim
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Default Re: Stella ST113U- distorted sound causes ?

I would be very cautious about using headphones on a set like this, as it has no mains transformer, and the chassis is (hopefully) directly connected to mains neutral. Of course if you are unlucky then the chassis will be connected directly to mains live!
Of course there is no reason why you can't put a standard output transformer in, and run a loudspeaker.
You are also trying to shove 40-50 milliamps DC through your headphones, which might contribute to the distortion.
Can't remember off the top of my head what type the grid coupling capacitor is between the anode of the triode section of the UCL82 and the Pentode grid. Might be worth checking for +DC here( there shouldn't be any at the pentode grid, should be slightly negative). Positive voltage here could over drive the output section and cause bad distortion.
Also from memory, working on a few of these sets, the cathode bias resistor for the UCL82 Pentode section is a fat carbon resistor(180 ohms?) that tends to go LOW in value, changing the bias point of the valve and causing too much anode current to flow. Unfortunately this often damages the UCL82, which will often continue to pass a high current even when the circuit is corrected, and replacement is the only option.Possibly why your speaker has gone open circuit.
Also check out the cathode bypass capacitor if there is one.

AM radio isn't hi-fi, but it should be reasonable to listen to.
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Old 12th Jan 2019, 12:41 am   #3
Jolly 7
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Default Re: Stella ST113U- distorted sound causes ?

Thanks Tim
Fortunately the chassis is connected to neutral. This is my very first attempt at repairing a valve radio and this is the only set I have.
The UCL82 looks a bit black and was flickering when the radio was receiving stations. There is a big resistor ? 2 Watts correctly reading 330 ohms connected to the UCL82 base...which pin exactly I will need to find out.
I will investigate further and report back
All the best.
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Old 12th Jan 2019, 12:53 am   #4
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Default Re: Stella ST113U- distorted sound causes ?

Actually I think the speaker is 700 ohms. Personally I would forget trying to find one and fit an output transformer instead with a standard 3 or 4 ohm speaker. Virtually any output transformer will do to test. For a correct match you need a 43:1 transformer. Nearest is 45:1 for a 3 ohm speaker or 40:1 for a 4 ohm speaker. It isn't that critical and either will do for either speaker. An RS universal is probably the best way to go.
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Old 12th Jan 2019, 1:12 am   #5
Jolly 7
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Default Re: Stella ST113U- distorted sound causes ?

Quote:
Originally Posted by Sideband View Post
Actually I think the speaker is 700 ohms. Personally I would forget trying to find one and fit an output transformer instead with a standard 3 or 4 ohm speaker. Virtually any output transformer will do to test. For a correct match you need a 43:1 transformer. Nearest is 45:1 for a 3 ohm speaker or 40:1 for a 4 ohm speaker. It isn't that critical and either will do for either speaker. An RS universal is probably the best way to go.
Thanks. Yes I read up a bit about this set and the unusual 700 ohm speaker. Possibly a cost-saving measure ?
Please ignore the typo in post 1 saying it is 300 ohms.

Last edited by Jolly 7; 12th Jan 2019 at 1:39 am.
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Old 12th Jan 2019, 8:20 am   #6
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Default Re: Stella ST113U- distorted sound causes ?

Hi Jolly 7,
Seriously Be Very Careful here. Do Not connect head phones across the speaker connections.

One end of the connection will be from the HT+ supply.
That is connected directly to the mains via the rectifier (UY85). You effectively have 240 volts DC 'mains' there... Really not a good combination with head phones.

I do not have any data, but there is enough from the internet to see the cathode resistor is 220 ohms (R20) and is on pin 2 of the UCL82.
The coupling cap (C24) is 0.01uF 400 volts (or better).

Plan to replace them both.
Flashing inside the UCL82, if that is the case, will need a replacement too.
And as sidband says, get an output transformer and replacement speaker. Do not stress over matching too much.

Again take care here. Alan
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Old 12th Jan 2019, 8:24 am   #7
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Default Re: Stella ST113U- distorted sound causes ?

(Posts crossed-Snowman-al got in first while I was writing mine)

Sounds as though the UCL82 may be dodgy. When you say flickering, is that a bright light, like a spark?
The big resistor probably is the cathode resistor for the Pentode section. I would monitor the voltage across it with a 700 ohm anode load(2x1.5 k in parallel will be near enough) which will give you an indication of current flowing through the output valve. A quick ohms law calculation will give you the current which should be about 40mA.
You wouldn't want to burn out an output transformer too!

I would expect to see about 11 or 12 volts across it. The voltage should be fairly stable once the set warms up. It may well fluctuate when your flickering happens, indicating a bad UCL82


Info on UCL82( and the other valves in your set) at this excellent site.

http://r-type.org/exhib/aaa0477.htm
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Last edited by Tim; 12th Jan 2019 at 8:35 am.
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Old 12th Jan 2019, 8:31 am   #8
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Default Re: Stella ST113U- distorted sound causes ?

Are you sure it's a 330 ohm? If it is it may have been replaced in the past, or upped in an attempt to tame an already faulty UCL82

Check out the two capacitors in the mini diagram above too. C24 is the grid capacitor I mentioned, and if it's passing DC then the UCL82 will be rather uncomfortable.. If C25 was short or very low resistance that may also cause the speaker grief.
I would replace them both.
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Last edited by Tim; 12th Jan 2019 at 8:38 am.
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Old 12th Jan 2019, 8:57 am   #9
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Default Re: Stella ST113U- distorted sound causes ?

Quote:
Originally Posted by Tim View Post
Sounds as though the UCL82 may be dodgy. When you say flickering, is that a bright light, like a spark?
The big resistor probably is the cathode resistor for the Pentode section. I would monitor the voltage across it with a 700 ohm anode load(2x1.5 k in parallel will be near enough) which will give you an indication of current flowing through the output valve. A quick ohms law calculation will give you the current which should be about 40mA.
You wouldn't want to burn out an output transformer too!

I would expect to see about 11 or 12 volts across it. The voltage should be fairly stable once the set warms up. It may well fluctuate when your flickering happens, indicating a bad UCL82


Info on UCL82( and the other valves in your set) at this excellent site.

http://r-type.org/exhib/aaa0477.htm

Good info given there. If you are not sure about calculating the current, all you do is measure the voltage across the 220 ohm and then divide the voltage by the resistance and the result will be in amps...so for instance if the 220 ohm has 10 volts across it the result will be 0.045 amps which is 45 milliamps. This is the TOTAL current through the valve and also includes the screen grid current. Alternatively, measure the anode current directly by connecting your meter in series with the 700 ohm dummy load.
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Old 12th Jan 2019, 9:10 am   #10
Jolly 7
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Default Re: Stella ST113U- distorted sound causes ?

Thanks everyone for the concern and very informative replies. Thankfully I'll be seeing a retired electronics engineer friend this weekend in London and he may be able to test all the valves for me. He might even have an output transformer.
Of course I will be testing everything else as advised above before sticking any new valves in.
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