Roberts RT1 which has been dropped

[Old Sparky]

I have now, thanks to the suggestions of the helpful people on this forum, sorted the problems with the tuning gang. IPA and the hairdryer treatment, plus a very small adjustment to the moving vanes did the trick.

[Murphyv310]

Good result.
Well done and many thanks for the compliment.

[Old Sparky]

Yes, despite the crackd/broken PCB, which I may reinforce by glueing a piece of plain veroboard over the affected area, the RT1 works weel, though i shall have to tweak the oscillator circuits to bring the tuning back into line. At the moment stations on MW appear somewhat LF of their correct positions, e.g. R. Gloucester appears at ca. 220M, when it's frequency (1413kHz) = 212M. Before anyone comments, yes I've set the pointer for correct travel, so I suspect that gently bending one of the T/cs Vanes has altered the minimum capacitance.

[Refugee]

Well done for getting it fixed tuning caps are not easy things to fix.
I would expect maximum capacitance to be affected by adjusting the vanes. The vanes are far apart at minimum capacitance.
For frequency it would be the other way round.
It is easy to get confused though.

[Old Sparky]

You're right, Refugee, but nonethe less, as noted, the stations at the mid to HF end of the dial are about 8-10Metres LF of their correct positon, and, as I haven't touched anything which affects the alignment other than the tuning gang, I can but carefully check the alignment. I have the Trader service sheet and a good quality RF Sig. Gen. so it's easily done.

[Refugee]

Have you checked that the aerial coils have not shifted when the set was dropped?

[Old Sparky]

The aerial coils are secure in their original positions, and, by, judicious afdjustment, I've minimised the tracking errors, although not eliminate them entirely. If I adjust the oscillator coil to bring Radio 5 to the correct position, then Radio 4 (LW) is about 20kHZ LF of where it should be. Another factor is that there is oscillation around 1413kHZ, which, by careful adjustment I've minimised.

[Old Sparky]

A quick calculation shows that 1413kHz is 3kHz Higher than 3 x the I.F., which is 470kHz, so it would appear that this radio is picking up the 3rd harmonic of the I.F and this is causing a 3kHz beat frequency. This doesn't happen with any other radios of my acquaintance. Perhaps judicious adjustment of the I.F. to 471kHz will cure this problem!

[Murphyv310]

Could be faulty decoupling capacitors, just could be the cause of both issues.

[Old Sparky]

Good Point, Trevor. The radio is almost 60 years old, so I'll replace the decouplers first. (Why didn't I think of that??)