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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
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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 ?

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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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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Old 19th Jan 2019, 1:04 am   #11
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Default Re: Stella ST113U- distorted sound causes ?

My friend was able to test the heaters of all four valves and they were fine although he advised me that a complete test would be definitive but also take longer, for which we did not have enough time.

Coming back to the 'big fat' 220 ohm resistor, it is measuring 286 ohms after I desoldered it. I think I ought to replace it, but is it a 2 watt resistor ? ( I don't seem to have this in my parts box).

Also I will replace both .01 uF capacitors as advised and I seem to have the right ones. But just to be on the safe side, I am posting photos of them before I take out the old ones. Please advise if they are suitable replacements. They are marked 103J. Thanks.
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Old 19th Jan 2019, 3:12 am   #12
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Default Re: Stella ST113U- distorted sound causes ?

You already knew the valve heaters were OK...
The 220R measuring high is out of tolerance, and probably 1 or 2W, but not by enough to worry about at this stage.
The green capacitor in your picture may only be rated at 50V (too low), though some new ones of 400V (OK) also look like that. Was it sold as 400V, or salvaged from transistor equipment?
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Old 19th Jan 2019, 8:20 am   #13
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Default Re: Stella ST113U- distorted sound causes ?

I wouldn't use that green capacitor...nothing wrong with them but, as stated above, only 50V which is nowhere near high enough. You need 400V rating or above. There are some very good polyester types available on eBay at 400V or 630V which are ideal and not expensive. The 220 ohm looks like a 2 watt type....don't be tempted to use a lower wattage in this position.
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Old 19th Jan 2019, 8:38 am   #14
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Default Re: Stella ST113U- distorted sound causes ?

As MildMartin says the green cap is going to be a low voltage one.
You have to use a 400 volt or 630 volt cap for C24. Also an axial type will fit better.
Look for 0.01uF 400v (or 630v) film cap. (0.01uF, 10nF and 10,000pF are all the same value...)
This is an example you can get them from lots of sources https://uk.farnell.com/vishay/mkt181...pet/dp/1166851

Is that the resistor from the set?
Alan
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Old 19th Jan 2019, 8:54 am   #15
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Default Re: Stella ST113U- distorted sound causes ?

Do the capacitors actually need changing ?

I thought this set was from the 60's and by that time I thought Philip's had moved to polyester and ceramic capacitors.

Worth checking and or a picture because in general these later types dont need replacement like paper capacitors.

Cheers

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Old 19th Jan 2019, 10:08 am   #16
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Default Re: Stella ST113U- distorted sound causes ?

Quote:
Originally Posted by snowman_al View Post
As MildMartin says the green cap is going to be a low voltage one.
You have to use a 400 volt or 630 volt cap for C24. Also an axial type will fit better.
Look for 0.01uF 400v (or 630v) film cap. (0.01uF, 10nF and 10,000pF are all the same value...)
This is an example you can get them from lots of sources https://uk.farnell.com/vishay/mkt181...pet/dp/1166851

Is that the resistor from the set?
Alan
Thanks. The resistor is the one I desoldered from the set.
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Old 19th Jan 2019, 10:13 am   #17
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Default Re: Stella ST113U- distorted sound causes ?

Quote:
Originally Posted by G8UWM-MildMartin View Post
You already knew the valve heaters were OK...
The 220R measuring high is out of tolerance, and probably 1 or 2W, but not by enough to worry about at this stage.
The green capacitor in your picture may only be rated at 50V (too low), though some new ones of 400V (OK) also look like that. Was it sold as 400V, or salvaged from transistor equipment?
The green cap is from a lot of assorted caps I bought from eBay. I used it once in another project and I should have at least one more hiding somewhere. There is no indication of the voltage ratings of any of the caps, although if you have ever looked inside an old CFL bulb base, you will probably see similar sized ones.
Having said that, I will buy some 400 V rated ones.

Last edited by Jolly 7; 19th Jan 2019 at 10:23 am.
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Old 19th Jan 2019, 12:43 pm   #18
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Default Re: Stella ST113U- distorted sound causes ?

Quote:
Originally Posted by Jolly 7 View Post
The resistor is the one I desoldered from the set.
I see, a 2 watt metal film would be fine, but it is easier to find 3 watt examples and they will match the original size better too. As before this is an example, plenty of choice from other sources. https://uk.farnell.com/multicomp/mor...ide/dp/1357907 (Refit the insulation from the original, they act to keep the resistor off the board and aid air flow.)

Re C24, cobaltblue makes a valid point, if the board was from Philips it might well have a ceramic or poly cap fitted. A picture of the board would prove it.

And I have learned something too.
There is a 'code' for marking caps called the ''EIA System''.
There is a table here http://www.iequalscdvdt.com/Markings_and_Codes.html half way down the page, next to the yellow caps.
Have a look at the top line of your green cap, perhaps 2E is 250 volts?

For this job stick with the 400 volt or better yellow ploy types though. Alan
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Old 19th Jan 2019, 1:52 pm   #19
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Default Re: Stella ST113U- distorted sound causes ?

Thanks, 2E does refer to 250V as per the EIA codes. The top line of the cap reads CY2E.
And the J in103J stands for 5% tolerance.

Here are some pictures from inside the radio. If the orange ?polyester caps are modern, I guess I don't need to replace any of them then ?

I am not sure what the parts are from which some red substance has dripped out. Is this a leak ?
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Old 19th Jan 2019, 2:05 pm   #20
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Default Re: Stella ST113U- distorted sound causes ?

Those caps are ceramic and very likely OK. It may even use one of those for AF coupling in which case it's probably OK. Check to see where C24 is from the layout shown in the circuit information.
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