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Old 4th May 2019, 11:22 pm   #21
Synchrodyne
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Default Re: Bush VHF-41 Refurb.

Use of the ECH81 triode as an additional AF amplifier on FM (in FM-only and FM-AM receivers) and on the “gram” input on receivers generally was, as I understand it, included in the original “mission statement” for this valve. For example it was used this way (for FM and gram) in the Pye FenMan II. The same triode was also specified for use as a self-oscillating mixer on FM, in which case Philips envisaged that it would be preceded by an EF80 or EC92 RF amplifier. Whether any UK setmaker used it this way I don’t know. (The original Philips FM-AM valve set did not include the ECC85 - which came from Telefunken, I think – so a different approach was needed. However, it would appear that Philips did add the ECC85 to its range quite early on.)

In the case at interest, possibly an external RF preamplifier (or aerial booster amplifier) might have some chance of improving overall FM performance by getting a higher signal level into the receiver, enabling it to operate well along its quieting curve. Although the fully broadband front end is more likely to get into cross-modulation difficulties at lower signal strengths than typical sets, so one might need to be judicious in respect of the extra gain applied.


Cheers,
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Old 7th May 2019, 2:12 pm   #22
davidgem1406
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Default Re: Bush VHF-41 Refurb.

Thanks Synchrodyne,

Taken on board, an aerial amplifier may be another way to do this as you suggest.

Best wishes,
Dave
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Old 10th May 2019, 9:27 pm   #23
davidgem1406
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Default Re: Bush VHF-41 Refurb.

I have now completed this radio, including the cabinet.
It is now all built up again and working well, including the FM that seems to be much improved in the evening with a good volume on stronger stations.

It took me a while to sort out where to align the pointer drives on the dial cord. I eventually adjusted them so that the pointer aligned with the end marking at the right hand side of the scale with the tuning gang fully closed, that seems to be correct.

However there is a problem with FM in that each station has 2 peaks, it tunes in to the station then fades and tunes to a second peak. The 2 peaks are very close together.

Does anybody know what the cause of this might be, and the possible fix?

Regards,
Dave.
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Old 10th May 2019, 9:39 pm   #24
Nuvistor
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Default Re: Bush VHF-41 Refurb.

IF and Detector alignment, or are you picking up another transmitter on a slightly different frequency?
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Old 12th May 2019, 4:50 pm   #25
davidgem1406
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Default Re: Bush VHF-41 Refurb.

Hi Frank,

I did realign the FM tuner, once the set was built up, after it was striped down to replace all the caps. I also checked the IF's and peaked those. They were just a very small tweak though.

There are probably to many stations for it to be other transmitters in all cases.
I could check each peak with a signal generator and see if each pair have the same frequency or 2 values.

Dave,
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Old 18th May 2019, 11:47 am   #26
davidgem1406
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Default Re: Bush VHF-41 Refurb.

I have now done the full alignment of the FM, RF and IF.
Following the manufacturers procedure as given in the service information using an unmodulated signal generator and a VVM.

The results are an improvement in overall response on good signal. The problem of having 2 peaks for each station is better but still apparent.

I wonder if having an IF frequency of 19.5MHz may have something to do with this happening.

Thanks,
Dave.
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