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Old 20th Aug 2009, 10:51 am   #1
'LIVEWIRE?'
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Default BUSH VHF90A has been dropped-now no reception!

Having been through this radio with the proverbial toothcomb, I am at a loss to know what has happened. There's nos sign of internal damage- i.e. no loose or missing tuning cores from I.F. and other cans, no cracked or dislodged components, the tuning cores can be moved through the RF & Osc. coils from LF to HF by rotating the control as per usual, all the valves are OK, and all DC voltages are normal, but no signals are being received, just an almost constant crackling along with the usual FM 'hiss', the cause of which I cannot pin down. The radio appears to be oscillating, since, with the cover off the FM Tuner box I can detect it on another FM Radio. Can anyone shed any light on this, please?
At the moment I am without a working FM sig. gen.(haven't had time to repair mine!) which compounds the problem.
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Old 20th Aug 2009, 11:15 am   #2
Tim
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Default Re: BUSH VHF90A has been dropped-now no reception!

You can still look for a null across the discriminator even with an AM signal at the FM IF frequency.
Er, apart from disconnected secondaries of transformers or loose,broken valve electrodes, I can't think of anything you haven't already tried. The alignment might have suffered, but of course you'll have to fix your FM genny first to check that.
Good luck anyway, this seems like a multiple cup of coffee and several hot bath problem!
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Old 20th Aug 2009, 12:08 pm   #3
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Default Re: BUSH VHF90A has been dropped-now no reception!

I'm not familiar with the VHF90A so this suggestion may be way off the mark, but have you checked that the vanes of the tuning capacitor aren't shorting? A strong jolt may have altered the adjustment enough for them to be shorting through their entire rotation thereby preventing any reception whilst the set appears otherwise fine.

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Last edited by Robert Darwent; 20th Aug 2009 at 12:17 pm.
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Old 20th Aug 2009, 2:26 pm   #4
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Default Re: BUSH VHF90A has been dropped-now no reception!

If it's been dropped, then something's come off somewhere. The VHF90A is a VHF only set and the service data is up top there. I will be using the manufacturer's data.

Resolder everything - even if it looks OK. Check valve sockets. The tuning is inductive and it goes through 2 coils, check here.

Also check the caps across the IFT's. One or more may be off.

The set may be oscillating, but is it on the right frequency. Check that and any trimmers that may have slipped during the fall. (How far did it fall?)

Cheers,

Steve P.
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Old 20th Aug 2009, 7:50 pm   #5
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Default Re: BUSH VHF90A has been dropped-now no reception!

the problem may me electrical leakage between the contacts on the wavechange switch. try connecting a digital meter between pin 2 (control grid) of the UCH81 and the chassis and switch to medium wave.if the reading is positive then the switch is your problem
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Old 20th Aug 2009, 8:28 pm   #6
Robert Darwent
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Default Re: BUSH VHF90A has been dropped-now no reception!

Quote:
Originally Posted by Laurdys View Post
...contacts on the wavechange switch. ...the UCH81 ...switch to medium wave.
Don't know what set's circuit diagram you've been looking at but it isn't the correct one? The VHF90A is a VHF band only set.

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Old 20th Aug 2009, 8:57 pm   #7
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Default Re: BUSH VHF90A has been dropped-now no reception!

I had one of these once that had me foxed for ages, just a faint hiss and the odd little crackle, turned out to be a shorted variable trimmer capacitor, one of the ceramic square ones with a screw in the middle, it had a hairline crack which caused insulation breakdown.
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Old 20th Aug 2009, 9:55 pm   #8
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Default Re: BUSH VHF90A has been dropped-now no reception!

Thanks for the various suggestions, guys. I don't know how far it fell, Steve, only that it was dropped during a house move, and must have fallen on it's front, since there is a small chip on the top of the cabinet above the tuning dial, but I will check, amongst other things, TC2(having already checked TC1 and the tuning inductors)and go over the solder joints again. I guess this one is a case of perseverance until I find the fault!!(I'll post results on here when I do)
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Old 22nd Aug 2009, 4:41 pm   #9
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Default Re: BUSH VHF90A has been dropped-now no reception!

Quote:
Originally Posted by Robert G0UHF View Post
Quote:
Originally Posted by Laurdys View Post
...contacts on the wavechange switch. ...the UCH81 ...switch to medium wave.
Don't know what set's circuit diagram you've been looking at but it isn't the correct one? The VHF90A is a VHF band only set.

Regards
oops i've been looking at the service data for the VHF90
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Old 23rd Aug 2009, 3:23 pm   #10
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Default Re: BUSH VHF90A has been dropped-now no reception!

Quote:
Originally Posted by Laurdys View Post
oops i've been looking at the service data for the VHF90
No problem, what's a dropped 'A' between friends. I'm guilty of the same thing on more than one occasion too!

Regards
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Old 23rd Aug 2009, 5:03 pm   #11
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Default Re: BUSH VHF90A has been dropped-now no reception!

If you can hear the "normal" VHF hiss/noise (this usually originates from the mixer stage), so the fault may well be prior to this stage.
I would think.
Your description suggests that most of the set is working, and I would be thinking of the very early stages, the VHF front end.
If the fault is due to dropping and not just coincidental, have you looked at the connections to the aerial socket, and if you carefully touch, via a small screw driver with an insulated handle, around the front end tuning coils area, do you get any signs of life.
Dont know the set, presume it uses something like a Double Triode at the front end, is this warm to touch or stone cold.
Have you measured any voltages around the VHF amp/oscillator ?
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Old 23rd Aug 2009, 9:35 pm   #12
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Default Re: BUSH VHF90A has been dropped-now no reception!

This Radio, in common with many others of that era, uses a UCC85, which I have substituted to no avail. I have also checked the aerial tuning coils, isolating capacitors, etc., and measured the voltages around the front end, which are much as stated in the service data. My feeling is that the fault is indeed somewhere in the front end, but, despite checking most components in the Tuner Box, I cannot pin down the cause, which, as you can all imagine is very frustrating! That having been said I'm grateful for any suggestions received

Last edited by 'LIVEWIRE?'; 23rd Aug 2009 at 9:38 pm. Reason: Forgot to mention that I'd checked voltages!!
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Old 23rd Aug 2009, 10:24 pm   #13
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Default Re: BUSH VHF90A has been dropped-now no reception!

It could be the IF's I suppose. Have you got another set you can borrow the VHF signal on.Wire the VHF box on the working set to the faulty set in place of the 'faulty' VHF box. Switch both sets on and see if the VHF comes on.

If it does, the VHF box is faulty. If not, the IF is faulty.

Cheers,

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Old 24th Aug 2009, 10:00 am   #14
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Default Re: BUSH VHF90A has been dropped-now no reception!

Steve, I wish I'd thought of that whilst I had a Bush VHF61 in for repair. Other than an ECC85 instead of a UCC85 the two Front ends are identical, so it would have been quite easy electrically to swap tuners. Oh, well, never mind. I'm virtually certain the fault is in the tuner box anyhow, since there is plenty of gain through the IF & AF Stages.
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Old 24th Aug 2009, 11:27 am   #15
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Default Re: BUSH VHF90A has been dropped-now no reception!

Hi !
Coul you "hear" the 10.7 MHz IF ?
Get a traffic receiver and tune it to 10.7 and put it near the Bush. If your front end is working you can get the audio signal in the traffic receiver. Distorded because it is not FM demodulated but AM but it will show !
This way you can tell for sure that the VHF head gives a decent job or not.
If your valves were old you could have broken an internal connection in one of them. I've battery valves with broken mica and wire loose inside due to transportation and rough handling.
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Old 24th Aug 2009, 12:49 pm   #16
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Default Re: BUSH VHF90A has been dropped-now no reception!

This model has permeability tuning I think (ferrites moving inside coils) so no tuning cap to damage.
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Old 24th Aug 2009, 1:18 pm   #17
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Default Re: BUSH VHF90A has been dropped-now no reception!

Quite right Paul. But is the tuning OK and are the coils OK.

You think that the fault is in the RF box, but we really need proof of this.

Cheers,

Steve P.
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Old 24th Aug 2009, 7:56 pm   #18
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Default Re: BUSH VHF90A has been dropped-now no reception!

The fault WAS in the Tuner Box, but it was partly MY fault. I had removed the pulleys and tension spring associated with the tuning drive in order to check components obstructed by them, and, when reassembling the drive had set it incorrectly. Having reset the cores earlier today, the radio now receives all usual stations between 88 & 103 + Mhz at good strength on it's internal aerial(The manual says 87.5 to 100), so I've gained about 3Mhz{should say Mc/s with these old sets!} somehow, but will leave that as it is. All is now fine, the one remaining puzzle being the almost constant crackling until the set has thoughroughly warmed up(abt. 10 minutes) This is barely audible on station, though.
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Old 24th Aug 2009, 8:19 pm   #19
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Default Re: BUSH VHF90A has been dropped-now no reception!

EXACTLY the same fault as my VHF80c! It's in the workshop shed at the moment having a rest as it very nearly met it's maker aftre me spending hours and hours on it!
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