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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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29th Sep 2009, 1:52 pm | #1 |
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Ekco PB189 Motor Tuned Radio Audio Distortion.
I've recently ben working on an Ekco PB189 Motor Tunes radio, using the Trader Service Sheet 765.
The set was initially tried with the original condensers in situ, it worked but there was some audio distortion. All of the condensers were then replaced but the distortion is still exactly the same. It increases as the volume increases and also the more "treble" the tone is set the more it is noticable. If the volume is increased too far and the tone too far up then it is very distorted, sounding a bit like a Dalek! When quieter with a more Bass seting it is listenable, but there is still a distortion noticable, especially when music is playing. The Trader sheet gives a valve line up of Ekco valves but it also mention in brackets that other valves may be fitted instead, and this set has the alternatives, for example, V6 can be either OP42 or else PENA4, and mind has PENA4. Since all of the condensers have been replaced I'm now rather puzzled as to what else it could be. C40 (image suppressor) had broken down, this is the adjustable screw type, connect the aerial one side resulted in a good signal, connecting the other side resulted in a very weak signal on MW and LW, but it didn;t effect SW. I bridged it with a 0.001uf condenser, still leaving it connected up in site, and the signal was then fine on SW, MW & LW. Can anyone make any suggestion please? Is it likely to be the audio output valve or transformer? The speaker is a permenant magnet type, which is dated 1937, so presumably original? The problem isn't with the speaker as that's been tried. Is it likely to be the alternative non-Ekco valves, although I'm told theyhave ben in the set since at least the early 70's, and probably much earlier, so that seems unlikely. The ones fitted are the alternatives stated in the service data. Aside from this the set works extremely well, the motor tuning is spot on, it's a really great set. I'd just be grateful of any pointers of what the problem may be and how to track it down. Thank you. Peter. |
29th Sep 2009, 1:58 pm | #2 |
Dekatron
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
Measure the HT and LT on all the valves. Check the circuit voltages. Post them here.
Cheers, Steve P.
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29th Sep 2009, 2:15 pm | #3 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
Thanks Steve, wll look in to that and post results here. Any ideas what the problem may be?
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29th Sep 2009, 2:22 pm | #4 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
Looking at the trader sheet, I'd look at R34, R35, R36, R37.
Do you get the same distortion when you feed a signal in at the Pick Up sockets? Cheers, Steve P.
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29th Sep 2009, 2:49 pm | #5 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
Thanks Steve, that's well worth investigating whilst I have the set apart.
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29th Sep 2009, 2:50 pm | #6 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
You can eliminate the -ve feedback by checking if the distortion is still there on SW. A dalek sound could be modulation distortion? Have you tried another TDD4?
Love the AFC on the motor tune...sounds like it's working too. |
29th Sep 2009, 2:59 pm | #7 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
Oh yes, as far as I can tell it's all working fine, an incredible set, bit like an early "electronic brain"!
I don't have a spare TDD4 I'm afraid. The sound on SW is pretty good actually, very minimal distortion, mind you, not a lot of music there, aminly people talking or otherwise such weird music from so far away that it's hard to tell if it's distorted or not! Modulation distortion sounds quite possible, would it be best to try and track down another TDD4 to eliminate that? |
29th Sep 2009, 3:02 pm | #8 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
These radios have those awfull 'Trebor mint' ceramic capacitors across the IFT's that always seem to be open circuit (you can normally see the break). It's well worth taking off the cans and checking them, if you haven't already
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29th Sep 2009, 3:13 pm | #9 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
Thanks for the tip Dave, no I hadn't done that, I wasn't aware of that problem. Are these housed in the coil cans on top of the chassis, or elsewhere?
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29th Sep 2009, 3:29 pm | #10 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
Best to follow Steve's suggestion next and try some audio in the pick-up input. If it's still distorted post the voltages on V5 and V6.
Did you replace the electrolytics? |
29th Sep 2009, 3:43 pm | #11 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
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29th Sep 2009, 5:02 pm | #12 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
Thanks for the tip about the Gram input, I'll try that. Thanks also for indicating the appropriate condensers, I'll see if I can track them down.
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4th Oct 2009, 9:45 am | #13 |
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Re: Ekco PB189 Motor Tuned Radio Audio Distortion.
I've now checked the above resistors R34, 35, 36 & 37 and they are all fine. I visually checked C4, 5, 17, 18 & 22, none had any visible damage, but, to be on the safe side, I replaced them anyway. There's no change at all with the distortion, it's still exactly the same. The HT is fine.
Any other ideas would be appreciated. Is this likely to be the audio output valve, transformer or something else? The audio output transformer appears to have an additional winding on it, not sure what that is for? Any suggestions welcomed. Many thanks, Peter. |