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Old 19th Sep 2019, 10:48 pm   #13
space_charged
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Join Date: Dec 2014
Location: West Midlands, UK.
Posts: 286
Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

I have just acquired one of these sets. I was collecting a vintage telly (Philips 17TG106U) and the seller had some wirelesses he wanted rid of.

The A133 was in a very poor state but I felt sorry for it! Once on the bench it was found to be complete and all valves present and correct. The usual wax caps were replaced and the HT smoothing capacitor was removed. An attempt at slow, current limited, re-forming was made. This failed and I decided not to persist but replace it and put in modern caps. The original was put back, but out of circuit.

The Radio and Television Servicing Manuals circuit shows the HT capacitor to be 32+32uF and the one in my set is indeed 32uF. I'd be a bit worried for the rectifier with 50uF.

Actually, I didn't have 32uF so fitted a pair of 22uF 450V caps. There is no trace of hum from the set.

Despite the usual wax caps being replaced, the set didn't work first time. There was a nice crackle/his from the set when I attached an aerial, but no stations were found on any band.

That is of course "classical" for a functioning IF but a local oscillator that doesn't. With a sig-gen on my bench, I thought it worthwhile to switch it on and set it to 1KHz modulated 470KHz. Bringing the output probe near the aerial brought in a loud 1KHz tone.

As it was dead on all wavebands that lead me to suspect C9 (65pF) in the local oscillator circuit. Changing that produced a nicely working set. In the first pass of capacitor replacements, I don't usually replace low value caps.

Since I'd got the sig-gen on, I thought I might as well sweep the IF and peak it up. The sig-gen is connected to my 'scope for use as a wobbulator.

It was slightly off 470KHz and the peak was a bit asymetric, suggesting the two transformers were at slightly different frequencies. Each transformer has two cores to be adjusted. Its an iterative process and, if you are used to it, fairly easy. Its possible to peak each transformer separately, but with only two transformers, I just do it in one. Different story (elsewhere here) with my Eddystone 770, which had faulty IF transfomers. Each stage had to be fault found and peaked, then peaked end-to end.

Just finished stripping the varnish off the veneer. That will be varnished tomorrow...
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