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Old 6th Jun 2017, 10:09 pm   #1
EdWilliams
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Default Bush VHF 62

I bought this set as a follow up restoration to the Philips 462A I completed earlier this year, because I thought I should have a go at a VHF set.

It clearly had come from a good home; the seller said it had originally been bought by his grandfather and he was able to give me the original guarantee, advertising leaflet, operating instructions and receipt for 33 Gns dated January 1958. The case was close to unmarked.

I know that you shouldn't apply power to an old set but the seller told me he had already tried this. So I did try it out as soon as I got it home, with the result that it received nothing on MW/LW on the internal ferrite rod, and only worked very quietly with 10 m of wire connected to the rear antenna socket. Deaf as a post basically. On VHF a background hiss could be heard between stations, but when tuned to a station the reception was very quiet and completely unintelligible. The volume and tone controls were horribly scratchy. All quite promising really.

About a month later I made a start on it by taking the chassis out of the case. All components looked original and in good condition, with an undisturbed layer of dust covering everything. I attacked the volume and tone pots with switch cleaner, and also sprayed the piano key switch mechanism with the same for good measure. Then I ordered replacement capacitors to replace the waxies and the Hunts. The grid coupling capacitor for the EL84 output valve was definitely leaky as I could measure +1.7 V on the grid side of this component.

When the parts arrived I tried the set again before replacing anything. It could now receive MW/LW weakly on the ferrite rod - perhaps squirting the waveband switches had helped. On the other hand it had started motorboating on VHF. The volume and tone controls now worked properly.

I then set about replacing the capacitors one by one, trying the set each time on MW to see if improvements were being made. I clearly know little about valve circuits as my predictions about which capacitors would help were completely wrong. In the end it was C45 and C37 (Trader sheet, both 0.04 uF) which seemed to help the most. After all the Hunts and waxies were gone it was working really well, with plenty of volume available.

I then connected up the internal VHF aerial and found that FM was now working properly. I was actually a bit disappointed by this; I was looking forward to being able to bore friends and colleagues with heroic tales of FM discriminator realignment and front end valve replacement, but it didn't need this and I really cannot fault it's FM performance.

I took the old Hunts capacitors to work and measured them on a Fluke 171 DVM. Most of them apparently measured several times higher than their nominal value but two of the 0.04 uF components appeared completely open circuit.

There is still some voltage on the grid of the EL84; I asked about this on the forum and was told it was common for these valves to get a bit leaky in old age. The cathode current is actually a bit below the Trader sheet value so nothing is in danger at the moment. I will keep an eye on it. The HT voltages are all close to spec so presumably the rectifier is still reasonably healthy.

There is no hum on the set and the electrolytic can (a triple) stays cool, so I did not change these parts.

The pointer alignment against MW/LW stations was wrong and on full capacitance it did not reach the calibration dots on the dial. So I moved the pointer on the string a little. This then made the pointer alignment on VHF go even more wrong in the opposite direction, but this was fixed by slackening the screw on the gang drum, adjusting until correct, and then retightening. There were witness marks on the drum so someone had been here before!


Other than this, all I have done is polish up the knobs and give it all a good clean. The dial light was a bit flickery occasionally; I found the centre contact on the bulb had gone flat over the years so I blobbed some new solder onto it to build it up again - fixed the problem.

In truth I am not that impressed with the MW/LW performance; it's OK but I think the Philips 462A is better. What's the point of a directional ferrite rod in a heavy tabletop radio? Even on a really strong MW local station the magic eye (quite dim unfortunately) only closes to about three quarters. On the other hand the VHF performance is really excellent; very clear and undistorted with good sensitivity. The magic eye closes completely on all stations. I don't care about the lack of coverage above 100 MHz as in my opinion there is nothing worth listening to up there anyway.

I'm quite pleased with the outcome; with VHF available it's actually useful. It's a good looking thing and gathers favourable comments from others. My kids though (8 to 12) think I'm barking mad for spending time and money on something so old - I will have to concentrate more on their education and cultural development.

Pictures below.

Ed
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Old 6th Jun 2017, 10:24 pm   #2
Andrewausfa
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Default Re: Bush VHF 62

Nice job Ed, that's come up looking very smart.

Regards - Andrew
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Old 7th Jun 2017, 9:56 am   #3
Leon Crampin
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Default Re: Bush VHF 62

Well done, your findings and fixes match closely the work I have done on various VHF61 receivers which use the same chassis.

A couple of observations: The usefulness of the eye on VHF can be enhanced by attenuating the DC signal to it from the FM detector with a couple of high value resistors. The set can then be tuned accurately on strong VHF signals. Watch the EL84 for grid current. Connect a DVM from g1 to chassis and monitor the DC potential with the volume control at minimum. Around 200mV is fairly normal for this valve, but if it increases steadily over a half-hour period, the set may suffer from excessive HT current draw. My two "fixes" for this are either to use a Toshiba 6BQ5 which is a far higher quality equivalent to the EL84, or substitute the more easily obtainable 6CH6. Using this valve means a minor re-wire to the base, and shunting the cathode bias resistor to give a total of 100 Ohms. The 6CH6 is reliable and gives equally good, if not better audio.

The question of AM performance is interesting. Why is it so lousy on these sets - with 2 IF stages? Obviously a fixed ferrite rod is stupid, as is the coupling of the external AM aerial with a shunt capacitor and resistor to attenuate the signal. The Germans knew far better and used a panel-rotatable rod and in some sets, a switchable set of aerial coils for external connection.

One day, I'll go through it all. I suspect proper AM aerial coils and some attention to the AM IF stages to reduce the damping on the tuned circuits may be the answer. Any suggestions or observations are welcome.

Leon.
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Old 7th Jun 2017, 10:34 am   #4
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Default Re: Bush VHF 62

Very very nice .... and well worth the effort - chloroform the kids !!!
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Old 7th Jun 2017, 8:43 pm   #5
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Default Re: Bush VHF 62

My kids (girls, 9 and 12) treat my VHF61 with some respect, as I still haven't really sorted it out and hence the back is not screwed on. So I told them 'stick your hand in there and you'll die'. They like to regale their friends with this sober instruction when they come round (perhaps I should add I don't rely on other people's kids ability to take instruction like I rely on my own ).
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Old 7th Jun 2017, 8:52 pm   #6
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Default Re: Bush VHF 62

"When they come round ...."? Exactly how long are they unconscious for Mark? !!!
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Old 10th Jun 2017, 9:35 pm   #7
Phil G4SPZ
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Default Re: Bush VHF 62

Well done, Ed. Nice useful set and a good write-up.
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Old 11th Jun 2017, 9:10 am   #8
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Default Re: Bush VHF 62

Very nice work.

These sets look lovely if their cabinet is as smart as yours. Sadly, most aren't.

As you've noted, VHF performance is excellent, but there are much better sets to listen to AM broadcasts on.
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Old 19th Jul 2017, 10:49 am   #9
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Default Re: Bush VHF 62

Lovely set, must track one down for myself. I find something very homely about 1950's VHF sets, probabily because my parents had a similar looking one that seemed to be on all the time when I was a kid.
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Old 19th Jul 2017, 1:18 pm   #10
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Default Re: Bush VHF 62

Quote:
Originally Posted by EdWilliams View Post
I took the old Hunts capacitors to work and measured them on a Fluke 171 DVM. Most of them apparently measured several times higher than their nominal value
This is common with most digital meters no matter how good they are. On a proper capacitance bridge, they would have measured very leaky and possibly it would not have been possible to obtain a good reading.

I believe most digital meters measure capacitance by applying a small voltage and measuring the charge/discharge time. If the capacitor is leaky, the charge/discharge time will be wrong so the meter will give a false reading.

A proper capacitance bridge works differently and compares the value of the test component against a known component in a bridge circuit.


A good repair! I must look out for one of these myself. I'm told the FM performance is very good.
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Old 19th Jul 2017, 6:02 pm   #11
Phil G4SPZ
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Default Re: Bush VHF 62

You really can't beat a Megger for testing capacitors at something approaching their working voltage. Do that first; any DC leakage means you simply replace the capacitor, without wasting time measuring its value!
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Old 19th Jul 2017, 6:26 pm   #12
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Default Re: Bush VHF 62

A really nice approach. None of this ripping it apart and then spending six months sorting out all the man made faults. I think you should have a go at a nice 405 line television receiver to go with it. Regards, John.
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Old 23rd Jul 2017, 8:35 pm   #13
EdWilliams
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Default Re: Bush VHF 62

I don't think I would be safe with a 405 line TV!

I'm currently plotting my next move. Having done a couple of post war sets I'm toying with the idea of something pre-war for the next project, perhaps accompanied by a low power am transmitter kit to make sure there would be something worth listening to.
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Old 23rd Jul 2017, 8:48 pm   #14
Phil G4SPZ
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Default Re: Bush VHF 62

Ed, look up the 'MiniMod' by Ian Listin-Smith in the Forum. Probably the cheapest and nicest-sounding low-power AM modulator you'll find. And you can build one yourself!
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