Steve
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Post by Steve on Mar 9, 2021 8:19:51 GMT 12
I'm repairing a 1938/39 Golden Knight model CKU which appears to have originally been a reasonably high end all-wave 6-volt vibrator powered set - with push pull 6G6G's in the output driving an early Rola PM speaker.
Things I've noted so far:
one of the 6G6's has been smashed the RF valve lead on to the cap has been cut completely off and is missing the tuning drive is slipping the volume control 'display' cord has snapped one of the power supply caps has blown completely apart the tone control has come loose in the chassis and has twisted itself into oblivion - The hot wire from the incoming power cord is just twisted around the on-off switch terminal and had started to come untwisted and was about ready to touch metal
1938ish radio with an EZ81 rectifier!
The second-to-last point is a timely reminder to never assume you can plug one of these sets in without checking underneath first... At first glance this radio looks fantastic and I almost fell into the trap of assuming the insides would match the outsides, and fitting a new 6G6G then powering it up - which could have been fun  - it seems like someone with cabinet skills may have restored it cosmetically and then just left the chassis. I'm hoping the owner might be able to elaborate on that.
And the last point about the EZ81 seems fine, and you might assume a repairman bunged that in some time in the late 50's once AC mains was more prevalent - until you look at how its fitted - the socket looks like it was riveted in at the factory - and the wiring is factory-neat and tidy as well... it's almost like it went back to Radio Ltd 20 years after it was made for a conversion to 230V mains... which is not something I've seen before. Did they do that?
All in all, it's an interesting set... I doubt I'll ever know its history, but its certainly one that has made me wonder...
Cheers, Steve
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Post by Richard on Mar 9, 2021 16:04:54 GMT 12
That tuning mechanism in those "Radio ltd" models was a very poor design. Strange about the EZ81, I would think a serviceman may have modified it?, back in the day. Good luck with the repair.
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Steve
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Post by Steve on Mar 10, 2021 15:52:12 GMT 12
Little bit of progress made, I've replaced the caps in the power supply section and checked the resistors - despite the heat damage the 2k power resistor is reading almost bang-on value - but the smoothing caps were VERY leaky so that would likely explain the heat it was experiencing.
And the audio output stage is done - there was a giant TEERAD 0.5uF cap on the HT line that had exploded here so its nice to get this all tidied up
I haven't powered it back up yet - I'll sort the missing grid cap wire on the RF section first because I expect it should fire up and sing with that connected... but expectations and reality are not often on the same page - so we'll see  But I did put my finger on the cap of the RF valve when I briefly powered it up through a dim bulb the other night and get a good strong buzz through the speaker so it seems like all stages are operational to some degree.
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Steve
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Post by Steve on Mar 21, 2021 13:31:07 GMT 12
So I'm having a wee struggle with the volume indicator on this set. Its a spring-loaded indicator and I've adjusted it as close to the edge of the springs capabilities as I can - but even then the spring is pulled tight enough at zero volume that the friction of the pot can't hold it at fully off - the spring pulls it back up again. Its a replacement pot and its reasonably stiff, but just not enough to sit and stay (at zero... over the space of about 5 seconds it pulls up a bit as you can see in the two photos below.
The disc with the volume level indication printed on it was quite stiff with old grease so it lagged behind the control so much the cord would fall off the drum before the spring pressure allowed it to catch up - so I freed that up a bit which helped that problem but introduced the new problem... and I'm no really sure how to fix it other than to disconnect it entirely. I suspect an old pot with a heavier duty construction would probably not have this problem.
before, and a few seconds after Has anyone had, and solved, this problem? - I assume this mechanism is the same across all these 1938/39 higher-end dial mechanisms? I'll keep tinkering with it but I really want it off the bench so I can get on to the next set so the indicator might just end up in the too-hard basket given the radio is here as a repair for someone (not a restoration) and I don't want to sink more hours into it for no real gain.
Cheers, Steve
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Steve
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Post by Steve on Mar 21, 2021 20:02:12 GMT 12
Just when you think you know what you're doing... I replaced the cathode bias resistor because it was out of spec, along with waaay too many other changes all at once without powering up to check progress. 220 ohms, which is a bit low for a 6D8 - the datasheet suggests 300 ohm... so I replaced it with a 270 ohm resistor and new decoupling cap.
5 points if you can spot my error...
Powered it up, totally dead. Audio section working fine, IF working sort of, RF dead... voltage checks were all ok or near enough... until I checked the cathode voltage on the 6B8 - over 100V!
Sigh...
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Post by dada on Mar 21, 2021 22:28:29 GMT 12
colour decoding specs needed? Well done! Chrs, DC
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Post by Richard on Mar 22, 2021 19:15:29 GMT 12
270K instead of 270 Ohm , need to go to spec savers As for the new pot , not being as stiff as the old one, can you reduce the tension on the spring? or a new spring? or use a felt washer between the cabinet and the knob to introduce some friction? |
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Steve
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Post by Steve on Mar 25, 2021 15:07:08 GMT 12
yep, I don't know what I was thinking (or wasn't thinking as the case may be) when I fitted that resistor... Once sorted it all burst into life quite happily.
Re the volume indicator: I couldn't use less tension on the spring, or the dial cord goes slack at the other end of the travel... and falls off the drum! I ended up stuffing an old felt into the shaft hole around the shaft to create some more friction in the end, and that worked perfectly (I couldn't jam the knob against the case and use a felt washer that way because the shafts all sit slightly proud of the case and I couldn't push them in far enough to do that.
I've got to say the performance of this set was not quite as good as I expected, and the tuning cap has a huge dead zone at one end of the travel - the plates separate completely for about 10 degrees of travel meaning I had to mess around a lot to get the full range from around 540 up to 1600, and the tracking was pretty bad.
Its working pretty well now and is ready to go back to the owner - its a great looking old set. The tone is pretty nice too.
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Post by radioholic on Mar 31, 2021 22:49:54 GMT 12
I've had that same problem with spring-loaded tone / volume indicators. Its a very poorly designed system.
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- it seems like someone with cabinet skills may have restored it cosmetically and then just left the chassis. I'm hoping the owner might be able to elaborate on that.
But I did put my finger on the cap of the RF valve when I briefly powered it up through a dim bulb the other night and get a good strong buzz through the speaker so it seems like all stages are operational to some degree.
