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Old 15th Dec 2011, 11:59 am   #1
'LIVEWIRE?'
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Question Crackling Bush VHF90A

For quite a while now I have been trying to find the cause of a constant crackling on this VHF-only radio and, in fact, have posted threads before relating to it.

Most recently I replaced the skirted valve holder, along with the components attached to it, to no avail, after having replaced C37 and C40 (reservoir/smoothing caps) a number of other capacitors and out of tolerance resistors, the UCC85, and the o/p transistor (the latter with a known good one from a different make of scrapped radio, the original having an o/c primary).

Amongst the caps. replaced are C1, C2 (aerial isolating) C3, C4, C5, C6 and C10 in the VHF tuner, all of these being those red tubular (hi-K?) ceramic types, with modern ceramics.

The only parts in the VHF tuner I haven't replaced are C7, 8 and 9 (2 x 22pF and 1 x 5.6pF) across the Osc. Coil, R1, R2, and R4.

The crackle doesn't occur if I temporarily disconnect the HT supply to the VHF Tuner, which convinced me that the fault lies therein, but I could be wrong. Has anyone any suggestions as to what else I can do? The radio is otherwise working with reasonable sensitivity, and tunes ca. 88 to 101MHz, but it's frustrating not being able to cure this fault.
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Old 15th Dec 2011, 3:10 pm   #2
Freya
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Default Re: Crackling Bush VHF90A

I would be looking at the switch S1A and S1B maybe next ?

stephen
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Old 15th Dec 2011, 3:25 pm   #3
geofy
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Default Re: Crackling Bush VHF90A

This presumably is MW as well as VHF. As well as the MW/VHF above switch, check that the VHF body is earthed, check the coils for poor soldering.
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Old 15th Dec 2011, 5:05 pm   #4
Steve_P
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Default Re: Crackling Bush VHF90A

This is the VHF/FM only version isn't it. R2 and R4 are distinct possibilities here. Try swapping R1 and R3 as well. I take it you've cleaned and resoldered the valve base?

These sets have a tuned coil system. Give it a clean...!

Cheers,

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Old 15th Dec 2011, 8:29 pm   #5
'LIVEWIRE?'
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Default Re: Crackling Bush VHF90A

R2 & R4 are next on my list to change, Steve, and if you mean V1 base, as stated, I have fitted a brand new one, since I thought the cause of the fault might be there(i.e. loose or dirty socket pins, and, since this holder is screwed, rather than riveted, to the chassis, it
wasn't too difficult to change it. To gain access to some of the components in the tuner, I have removed the tuner drive cord & spring, and cleaned the coil pack whilst so doing. As some members will know, the VHF tuner box is secured to the main chassis with nuts & bolts, and these are all clean & tight, though it'll do no harm to check them again. Oddly I never had any of these problems with a VHF62, which uses the same FM Tuner, the only difference being the use of a valve(ECC85)with a 6.3v heater in the latter.
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Old 18th Dec 2011, 4:58 pm   #6
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Default Re: Crackling Bush VHF90A

Does the fault occur when the set is off tune? I'm wondering about a problem in the IF stages which can only be heard when there is a signal available to add the crackle to.

Disconnect the tuner and inject 10.7MHz from a signal generator and see if the fault is still there or, if you don't have access to a generator, feed it with the IF from another receiver, taking obvious safety precautions with the live chassis, of course.

If you still have the VHF62, perhaps you might consider a tuner swap to see which set ends up with the fault?
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Old 18th Dec 2011, 10:32 pm   #7
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Default Re: Crackling Bush VHF90A

Unfortunately I no longer have the VHF62. It was one I repaired for someone else maybe 2
years ago. I must admit I hadn't thought of the possibility of an I.F. problem, but will check that. My sig. gen. is out of order, but I can rig up a coupling from another FM radio
to see what happens. The beauty of this forum is that we hopefully help each other with our suggestions and tips. Two (or more) brains are often better than one!
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Old 19th Dec 2011, 1:17 am   #8
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Default Re: Crackling Bush VHF90A

I agree with terrykc, disconnnecting the tuner will also remove any signal to add a crackle to.

You have done all the usual DC voltage checks on all valves?
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