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Old 27th Jul 2009, 10:01 am   #1
'LIVEWIRE?'
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Question Bush VHF61 Quiet on VHF

Some time ago I overhauled a Bush VHF61, replacing the main Reservoir/Smoothing Capacitor, other electrolytics and 'waxies', plus some out of tolerance resistors. The Radio now works fine, except that the volume control on FM needs to be set between half & 3/4 max. for reasonable listening level in a quiet room, whereas on AM a setting about 1/4 above minimum is plenty. The internal FM aerial is being used(I don't have an outside VHF Aerial). The EABC80, EF89 & ECC85 have all been replaced, but I have NOT openend to VHF Tuner Box. DC Voltages are what I would expect them to be. Any suggestions as to the cause of this problem?
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Old 27th Jul 2009, 12:20 pm   #2
Leon Crampin
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Default Re: Bush VHF61 Quiet on VHF

My first point of investigation would be both EF89s. These valves have a shorter life than the others (even the ECC85) and fail with very low emission. Then check the ECC85 by substitution and the EABC80 for ratio detector diode balance. A poor diode is surprisingly common and makes the set impossible to align correctly.

When working correctly, on a reasonable signal (these sets have lots of gain on FM) the audio level should be at least as high as that on AM. The eye will also overlap on a strong signal - I fit a 3:1 attenuator in the FM DC feed to the eye to make it more useful as a tuning aid.

The AM performance of these sets is very poor indeed, even with a good set of valves and correct alignment. A fixed rod aerial is a joke on a big mains set and the external AM aerial provision (bottom coupling to a single tuned circuit) is hopeless.

The Bush sets of this era (last of the "horizontal chassis" AC only FM sets) are my favourites - the extra EF89 really transforms the FM performance. A correctly set up VHF64 with a working tweeter and a couple of anti-hum mods is the best of them all.

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Old 27th Jul 2009, 9:08 pm   #3
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Default Re: Bush VHF61 Quiet on VHF

The signals from several stations, including R2, 3, 4, & Classic FM do not close the 'magic eye' display, so I guess you could be right, Leon, although I have already replaced the ECC85, EF89, & EABC80, so, in a way, I'm now more puzzled than I was before I posted my query!
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Old 27th Jul 2009, 9:57 pm   #4
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Default Re: Bush VHF61 Quiet on VHF

Has the set any evidence of damp storage ? I have had a number of VHF64/94s which exhibited low gain + extended bandwith at IF
Having ruled out any obvious faults, I came to the conclusion that damp in the IFTs had possibly reduced their Q. I could find no other sane explanation.
As Leon says, you need BOTH EF89s to be 100% Changing both of these for new valves can yield another 12 db of gain compared with aged ones.
You could also disable S24 (on the trader sheet) as this appears to decrease the gain when switched to FM by slightly increasing the negative feedback.

Where I am in Surrey, the average levels of R2/3/4 seem much lower than the IBA stations. I find I need an external aerial to really make these close the magic eye.

Andy

PS is it just me or is the sound quality of the '61 pretty dire anyway ( as it has no 'tweeter' ) - despite the applied feedback
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Old 27th Jul 2009, 10:12 pm   #5
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Default Re: Bush VHF61 Quiet on VHF

Hadn't thought about the IFTs, Andy. The set was in the cellar where I have my workshop for several weeks, but has been in a warm & dry environment at my home for a couple of months at least since then, and there's no evidence of dampness present. I'll think about disabling S24(or whatever it's number is on the BUSH diagram) As to the sound quality I would describe it as Ordinary, I guess. A Tweeter would improve matters here, esp. on FM
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Old 27th Jul 2009, 10:17 pm   #6
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Default Re: Bush VHF61 Quiet on VHF

Like yours mine is also quieter on vhf compared with the am bands. I have not done much with my set though and is still full of the crumbling hunts caps. Even so I think the sound quality is pretty good especially for a little set like this.
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Old 28th Jul 2009, 9:22 am   #7
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Default Re: Bush VHF61 Quiet on VHF

If changing tubes doesn't help, you might want to check C4. I have recently completed the restoration of a Murphy A242 and A252. The A242 suffered from exactly the same problem as yours, where the A252's FM gain was hot -- too hot. After about a week of listening to the A252 in my office, it stopped working with nothing but howls on the FM band. The solution for both the A242 (poor FM gain) and the A252 (to much gain and eventually oscillation) turned out to be the same component (even though their designs are different, they are similar). I just had a look at the schematic of the Bush VHF61 and it's similar to the Murphy's. I would have a look at C4 (540 pF?, I can't quite read the value on the schematic I'm looking). In the Murphy's, this capacitor is critical to the gain of VHF; too low and it breaks into oscillation; too high and gain is killed. In the Murphy's this capacitor was a Hunts. After I pulled out the capacitors, I measured them and sure enough in the A252 the capacitance had dropped by about 150 pF where in the A242 it had gone up.
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Old 28th Jul 2009, 9:29 am   #8
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Default Re: Bush VHF61 Quiet on VHF

Hi,

One common fault on FM is the deterioration of the electrolytic capacitor (positive to ground) across the ratio detector transformer. This usually gives distorted audio. The IFT's don't need to have a high Q for FM - in fact a common mod is to damp them with resistors to cater for the wider deviation on current FM transmissions. When sorted the VHF61/62 can sound pretty good on FM - I use mine daily.

Good luck

Keith
http://tibblestone.users.btopenworld.com/Bush_VHF62.htm
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Old 28th Jul 2009, 9:47 am   #9
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Default Re: Bush VHF61 Quiet on VHF

Keith. Amongst the capacitors I changed was the electrolytic accross the ratio det. tx. I usually change these on sight, anyway. I have also replaced BOTH EF89s(but not the ECH81). C4 i will check, though, Jim. Hadn't thought about that one.
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Old 28th Jul 2009, 11:24 am   #10
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Default Re: Bush VHF61 Quiet on VHF

A question for Andy(setsappeal) ref post #4. I assume the switch labelled S24 in the Trader Sheet (which I don't have) is the one labelled S3C in the Bush Manual-i.e. it shorts out R25(Bush item No.) on AM. If I remember my theory correctly, reducing the resistance to ground at this point will increase the gain of the audio stages-right?

Last edited by 'LIVEWIRE?'; 28th Jul 2009 at 11:29 am. Reason: Correcting a Typographical error!
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Old 28th Jul 2009, 11:52 am   #11
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Default Re: Bush VHF61 Quiet on VHF

yes, you are correct on both counts. Being as the loop is frequency dependant, the effect may be subtle as it appears from the capacitor values that the feedback is for LF correction, no doubt to improve the rather cheap and nasty speaker fitted !


I'd be interested to hear how it sounds without the feedback ?

Andy
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Old 29th Jul 2009, 11:39 am   #12
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Default Re: Bush VHF61 Quiet on VHF

Andy, without modifying anything, except putting in a better ECH81 than that fitted, I am sitting here listening to Classic FM on the '61 as I type, and the sound, whilst not HiFi, is quite good, with no(audible)distortion. Changing the ECH81 improved the gain to the point where the 'eye' display is almost closing, whilst tuned to 100.1Mhz, using the set's internal aerial(and I'm in a ground floor flat over 40 miles from the Sutton Coldfield transmitter!)
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Old 29th Jul 2009, 1:51 pm   #13
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Default Re: Bush VHF61 Quiet on VHF

Hello Livewire !
Try tweaking a little with the slug of the discriminator coil. It could well improve the sound.
Of course a proper alignment is way better but if you changed the discriminator cap you may get good results by tweaking.
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Old 29th Jul 2009, 2:15 pm   #14
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Default Re: Bush VHF61 Quiet on VHF

I presume you've changed all the dodgy wax/paper caps? There are about 20 in total, some of which are slightly awkward to get at, but my sets were much louder and better performing when they were replaced.

Nick.
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Old 29th Jul 2009, 3:19 pm   #15
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Default Re: Bush VHF61 Quiet on VHF

Georges-at the moment the '61 has been running for over 4 hours tuned to Classic FM, with no sign of any distortion or drift, etc. As for the capacitors, Nick, I don't think I've missed any. There are no waxies in the FM Tuner, which I haven't even opened. As the radio is working well after being run continuously I'm inclined to leave well alone anything I haven't already changed or adjusted, though point taken about the radio being much louder when all the caps. were changed
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Old 29th Jul 2009, 4:40 pm   #16
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Default Re: Bush VHF61 Quiet on VHF

A couple of things to watch out for on the Bush VHF161/162 and its relations.

1. Decoupling capacitors. Being a well-designed set (apart from the AM aerial tuned circuit coupling, as noted earlier) nearly all of the stage HT feeds are RC decoupled. Open-circuit decouplers are a common problem and can cause loss of stage gain and instability. The worst ones are in the feed to the VHF tuner and the second IF/ratio detector stage. The latter can really damp the detector primary and make its alignment impossible, with attendant distortion.

2. EF89s do lose emission, and they can also develop leakage and grid current. The latter really damps the preceding IFT and can give you similar symptoms to shorted turns in the transformer.

However, get these little sets sorted out and they're fine performers. I use one as our 'house radio' and is used every day.

Cheers,
Neil
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Old 29th Jul 2009, 5:10 pm   #17
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Default Re: Bush VHF61 Quiet on VHF

Quote:
Originally Posted by 'LIVEWIRE?' View Post
the sound, whilst not HiFi, is quite good, with no(audible)distortion.
Fair comment

I accept the sound quality is reasonable, but a tweeter and a better main speaker could have made a big difference. I thought my '61 was quite good, until I did an A/B comparison with a Roberts. The BUSH really is rather dull!

Andy
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Old 29th Jul 2009, 10:54 pm   #18
Leon Crampin
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Default Re: Bush VHF61 Quiet on VHF

The VHF64 is, very loosely a '61/2 with better 'speakers and a tweeter. Of course, it all comes in a bigger box. Bush also did the honourable and correct thing and equipped the set with a decent power supply (bigger transformer and an EZ81 rectifier) enabling the EL84 to be run at a higher voltage.

RF-wise, the sets are just about identical (the '64 has SW - hopeless, of course) but if you can be bothered to get the hum out of the '64 it's a great sounding machine. Mine needed 3 rather fundamental mods to achieve a reasonably low hum level. Later sets might need 2 of them.

Hum on the '61 is much less of a problem because of the small 'speaker's limited bass response and the lower total HT current, which gives rise to less ripple.

Leon.

Last edited by Leon Crampin; 29th Jul 2009 at 10:58 pm. Reason: Poor grammar
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