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Old 6th May 2009, 8:59 am   #1
wireful3
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Default VHF 70 Deaf on LW

Can anyone give me a hint on what to do with my VHF70?

MW performance is quite lively but LW disappointing. Radio4 only gives modest volume at full setting and spreads over quite a range of the tuning dial. I don't get any sign of the European stations that are usually even stronger than Radio4.

The Service sheet gives very sketchy alignment information and it appears on LW to be only adjustment of the oscillator to get alignment with the tuning dial. The good performance on MW that has also much better selectivity suggests there is nothing much astray.

Thanks in hope

John
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Old 6th May 2009, 10:29 am   #2
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Default Re: VHF 70 Deaf on LW

According to the circuit diagram there are certainly seperate RF stage trimmers for LW and MW on this set. It would be very unusual if there wasn't.

There are a bank of 4 trimmers under the chassis. TC3 (40pF) is nearest the tuning control spindle with TC4 (80pF) next to it. I'd guess from the greater value that TC4 is the one for LW but it won't hurt to try a small adjustment to peak both.

Dave
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Last edited by Boom; 6th May 2009 at 10:55 am.
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Old 6th May 2009, 11:25 am   #3
wireful3
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Default Re: VHF 70 Deaf on LW

Dave Many thanks for your advice, I found the aerial trimmers and adjusting TC4 made a big improvement. It is odd that the Bush service sheets only mention the oscillator adjustments. It now acts more sensibly, it stil needs full setting of the volume control but that may be the weak Radio4 signal.

It is still not as enthusiastic on the other stations as some of my sets but as I have it in bits on the bench it is not all that easy to rotate the aerial rod. It is certainly more acceptable now so perhaps I will leave it.

Regards
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Old 6th May 2009, 11:28 am   #4
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Default Re: VHF 70 Deaf on LW

Glad to help John. Did you know that sliding the coil along the ferrite rod can also make a big difference? There is probably a defined set up proceedure for this but I don't know it.

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Old 7th May 2009, 9:50 am   #5
wireful3
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Default Re: VHF 70 Deaf on LW

Yes Dave, I have used the method on Roberts and Hacker portables before. I think the adjustment sequence is similar to that used on a conventional set with iron dust cores. Trimmer to set the HF end and moving the coil to set the LF end. Though I can't remember the order. The coils on this one are very well secured with wax so as the performance is so much improved by finding that the LW aerial coil could be trimmed I think I won't risk disturbing them.

One thing I have learned is that even though the manufacturers service sheets look very comprehensive they may have omissions. Incidentally since starting this I have found a Trader service sheet and this describes the LW set up including the Aerial trimmer!

Mods , There may be other things that could improve performance even more but I think this thread can be closed as Dave has solved the main worry.
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