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Old 12th May 2019, 7:12 pm   #1
Electronpusher0
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Location: Bognor Regis, West Sussex, UK.
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Default Rent a set 551 (Strad) MW problems

I am restoring a Rent-a-set 551 which is basically a Strad 553 without SW in a 511 case.
I have posted about this under wanted when I was looking for identification and for a back.

The restoration is going well and LW is fully working (after changing 2 valves and making an adaptor for the double diode triode (which I will post about separately)

However MW, while working, can only be made to tune a single station.
The problem seems to be that L3 can be peaked to optimise any single station but if the set is then tuned to any other station it is deaf unless L3 is re-peaked for the new station.

I attach the relevant section of the schematic for the 553 and a modified schematic which reflects the actual circuit of the set I have.

Trimmer T2 is missing, as indeed is T1, T3 and T4, T1 and T4 are associated with SW so I would expect them to be missing. There is no evidence that T2 and T3 were ever fitted.

Any suggestions would be appreciated.

Peter
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Old 12th May 2019, 7:20 pm   #2
Boater Sam
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Default Re: Rent a set 551 (Strad) MW problems

Is the local oscillator tracking as you tune on medium wave?
Its an odd one, what happens if you put the aerial onto the mixer grid direct?
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Old 12th May 2019, 8:23 pm   #3
Electronpusher0
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Default Re: Rent a set 551 (Strad) MW problems

I checked the Local Oscillator with my 'scope. It does track although the wave shape is more triangular and lower voltage than on LW.
I will try putting the aerial onto the mixer grid direct tomorrow.
Thanks for the tip.

Peter
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Old 14th May 2019, 5:53 pm   #4
Electronpusher0
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Default Re: Rent a set 551 (Strad) MW problems

OK, it is now working as it should.
The problem was a combination of filthy, hence noisy and intermittent wave change switch and the phantom twiddler.
I stripped the wavechange switch and cleaned the contacts with flux cleaner which dissolves the black "crud", then reassemble and a light touch of servisol.
After a complete re-alignment of the local oscillator and front end all was well.
I did not have to touch the IF beyond injecting the IF freq and verifying it worked.
Interestingly the IF is set to 470kHz whereas the manual for the Strad 553 says it should be 465kHz.

Peter
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Old 14th May 2019, 6:00 pm   #5
G6Tanuki
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Default Re: Rent a set 551 (Strad) MW problems

Quote:
Originally Posted by Electronpusher0 View Post
Interestingly the IF is set to 470kHz whereas the manual for the Strad 553 says it should be 465kHz.

Peter
I wonder how accurate the typical radio-serviceman's signal-generator would have been back in the 40s and 50s?

5KHz error in a free-running sig-gen developing over the years could easily be imagined, since I doubt they bothered with annual calibration-traceable-to-an-accredited-standard back then, or even bothered to check them against a crystal reference.
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