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Old 19th Apr 2019, 8:32 pm   #1
davidgem1406
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Default Bush VHF-41 Refurb.

I have recently completed refurbishing of a Bush VHF-41 chassis.
All the Hunts moulded capacitors have been replaced (The moulding was all cracked up or missing), including the FM tuner.

All electrolytic's replaced except the main smoother, that's fine and happy.
A number of out of speck resistors also replaced.
The EABC80 valve base was replaced.

The chassis is now completed and generally working. However I have 3 problems to solve.

First the drive cord has been replaced but things are not very clear as to attaching the 2 pointer carriers.
It just says "with the tuning gang at fully closed attach a temporary pointer" to align with the datum mark.

What I actually did was to fit the 2 pointer carriers to the drive cord.
First the AM carrier with the gang fully closed set to the datum line.
Then the FM carrier set to the datum line with the gang fully open.
That seems the only way it can be done.

So, to do the alignment for the AM we use the AM carrier set against the relevant frequencies as shown on the front of the chassis.
For the FM we also use the AM carrier but set it against the FM frequencies.

My question here being is this the correct way to do this. It seems to be the same result as using a temporary pointer.

Next the AM is working well and with plenty of volume but it seems to be rather poor in noise rejection as we have a sound like sizzling bacon in a pan over the whole band that is quite prominent even with the stronger stations.

Finally we have a problem with the FM, although this is working to be able to listen to, even the stronger signals, we have to have the volume at maximum. So the FM audio is well below any sensibly usable level.
I have read that these sets are known to have a low FM volume as opposed to the AM volume. However it surely cannot be as low as mine is.

I have checked all the valve voltages and they are what I would reasonably expect them to be against that given in the service sheet.

Some valves were low, they have been replaced. EABC80, ECH81, 2x EF80 All others tested good.

Any help and suggestions would be appreciated.

Best regards,
Dave
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Old 19th Apr 2019, 10:23 pm   #2
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Default Re: Bush VHF-41 Refurb.

For the AM noise problem, confirm it’s not something external to the set, I.e. computers, switch mode PSU, lights etc.
For the FM low volume is it better with a proper aerial?
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Old 20th Apr 2019, 8:20 am   #3
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Default Re: Bush VHF-41 Refurb.

This was Bush Radio's first VHF/FM receiver. The VHF41 was a poor performer on FM especially in the London area back in the days when the transmissions were originally horizontally polarized.
A really good aerial signal is required to obtain reasonable listening volume and tuning drift over a period was also a problem. None of my customers used the FM facility simply tuning to MW/LW.
A private wire to Wrotham may cure the problem!
Most front ends with twin EF80 valves were poor except the Philips who managed to do the job correctly. Good luck with it, nice looking radio. John.
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Old 20th Apr 2019, 4:29 pm   #4
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Default Re: Bush VHF-41 Refurb.

Hi Frank,
I guess you mean a 75 ohm aerial on the roof for a proper aerial?
If so I don't have such an aerial. I use a long wire that goes round the room I use.
Usually that will supply a quite adequate FM signal for older valve sets.

Without an aerial there is zero to be had.


Hi John,
Thanks for that info, somehow I knew somebody would say that.
It seems as though the FM is junk and not really worth having.
Any idea as to the Philips sets that used 2 x EF80?

Regards,
Dave.
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Old 20th Apr 2019, 5:40 pm   #5
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Default Re: Bush VHF-41 Refurb.

Worth trying a dipole, just use a spare length of coax and strip the outer pvc and braid back to give the correct length using the centre and braid as the elements. Pin the wire to the wall in the best reception position.
It’s never going to be a record breaker on VHF but it may help.
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Old 21st Apr 2019, 2:56 pm   #6
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Default Re: Bush VHF-41 Refurb.

I will see what I may have, doubt it will be 75 ohm coax though, but I may have some 50 ohm for the feeder and use insulated wires as the dipole elements.

Dave
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Old 21st Apr 2019, 10:07 pm   #7
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Default Re: Bush VHF-41 Refurb.

Philips did seem to make a much better job of FM with 2x EF80 - the B3G63A working well but they used them in quite a few.
Steve.

https://www.radiomuseum.org/r/philips_b3g63a.html
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Old 22nd Apr 2019, 10:24 pm   #8
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Default Re: Bush VHF-41 Refurb.

In the early days of FM, a lot of manufacturers were forced to use what valves were available. The ECC85 was developed for FM but I don't think it appeared in quantity (and the right price) until 1955. So early sets used the best valve available at the time, the EF80 which existed in their zillions as TV IF amplifiers etc. The EF80 is good up to about 115Mhz but its still pushing to it's limits in FM sets (since at 100Mhz, the FM oscillator is running at 110.7Mhz) which is another reason why most sets were only good up to 100Mhz. They have to be in first class condition as well. Even after the ECC85 was starting to be used, some manufacturers were 'locked in' to using certain valves...GEC for instance were using Marconi valves and Marconi didn't have an equivalent to the ECC85 so they used an EF80 equivalent.
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Old 23rd Apr 2019, 4:45 pm   #9
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Default Re: Bush VHF-41 Refurb.

Hello all,

Thanks for the info.
The 2 EF80 I have are NOS, when they arrived I tested them and they were right on with what the test data quoted.

Steve, thanks for the link to the Philips, I have downloaded the circuit to have a look and compare it to the Bush VHF-41 circuit, maybe the bush can be modded in some way.

Best regards,
Dave.
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Old 23rd Apr 2019, 5:12 pm   #10
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Default Re: Bush VHF-41 Refurb.

If the ECH81 is not good, FM sufferers as its the 1st FM IF amp on FM.
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Old 23rd Apr 2019, 5:25 pm   #11
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Default Re: Bush VHF-41 Refurb.

I doubt if it can be modified to improve performance. The VHF41 was a very poor performer on VHF/FM but I have no doubt there may be an example somewhere that is driving the output valve to overload with a bit of wet string, in a remote basement. There are always a couple of good ones.
There are plenty of makes of reasonable VHF performers around at low prices so worth making the Bush the first of many..Generally but not all UK VHF receivers were rotten performers even with the E/UCC85. More I.F. gain was required but price competition prevented it. Philips usually out performed them all. John.
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Old 23rd Apr 2019, 6:20 pm   #12
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Default Re: Bush VHF-41 Refurb.

I had a VHF41 in my youth and it struggled on FM. You could try using the EF184 in place of the EF80s for more gain, and possibly fit an EF183 in place of the EF85. IF stability was always a bit marginal on these sets though, so you might need to adjust the bias - which is changed for FM operation.

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Old 24th Apr 2019, 3:33 am   #13
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Default Re: Bush VHF-41 Refurb.

The Bush VHF41 was unusual in that it had what was effectively a single-gang front end, only the oscillator being tuned. The aerial input was aperiodic, which was quite common practice, but the interstage, normally tuned (as part of a two-gang circuit) was also aperiodic. This may have limited the RF gain as compared with the tuned interstage arrangement. The fully aperiodic front end in turn required an IF that was enough higher than the tuning span (12.5 MHz) to at least provide a hint of image rejection. That outruled use of the customary 10.7 MHz, and pointed to a somewhat higher number. On the basis of avoiding in-band IF harmonics, the ranges 16.67 to 17.5 and 20.0 to 21.875 MHz were available. The former was probably too low for adequate image rejection. Once the higher range was seen as preferable, Bush may have elected to compromise a little and go with the established 19.5 MHz TV SIF number, for which circuitry was already available. That put the IF 5th harmonic at 97.5 MHz, probably considered not too much of a problem given that the view from 1954-55 would have been that there would be no UK FM transmitters above around 96 MHz for quite a few years to come. But the 19.5 MHz IF meant that the available gain from the two-stage IF strip was somewhat reduced as compared with what could be achieved at 10.7 MHz. And at 10.7 MHz, such an IF strip was barely enough. Murphy was on record as saying that the required gain (to drive a ratio detector) could just about be achieved by a two-valve intermediate frequency amplifier, consisting of the heptode portion of the triode-heptode used for mixing in the A.M. portion of the set, and the variable slope intermediate frequency amplifier, the latter usually being one of the more recent types having a maximum slope of about 4 to 6 mA/volt. Thus the VHF41 was probably non-negligibly short of IF gain as well as of RF gain.

More generally, I doubt that the availability of suitable valves was an issue for the UK setmakers when they needed to develop FM receivers. By then, FM was a reasonably mature technology. A high performance FM front end could be obtained by using a cascode RF amplifier (e.g. ECC84) and a triode pentode (e.g. ECF80/ECF82) or double-triode (e.g. ECC81) frequency changer, preferably in a three-gang circuit. Cascode double triodes and triode pentodes capable of working at over 200 MHz had been available since 1951 from US sources and since 1953 from European sources. The ECC81 (as the 12AT7) had been introduced by GE in 1947 as an FM and TV frequency changer, and was available from European sources from c.1950. So, no excuses on the valve front. Respectable results could be also obtained with pentode RF amplifiers, and such were used during the 1950s by some of the major American setmakers , including GE, Zenith and RCA.

The ECC85 appears to have been less an effort to obtain better FM front end performance than an exercise in parsimony, in that it facilitated the single-valve FM front end. Whilst the ECC81 would have done a good job in this role in core performance terms, much better screening between the triode sections was needed to minimize oscillator radiation, and the ECC85 provided this, although I think not without some performance penalty.

Perhaps the ECC85 role and relativity is well illustrated by the Pye FenMan models. For the higher-priced FenMan II, Pye chose a 3-gang FM front end using an EF80 and an ECF80, followed by a 3-stage IF strip and a Foster-Seeley discriminator. For the lower-priced FenMan I, it used a “standard” circuit with a 2-gang, ECC85 front end, a 2-stage IF strip and a ratio detector. Logically, one would have expected the FenMan II to have outperformed the FenMan I along most vectors. Just perhaps the latter had a slightly better initial part of the quieting curve because of its triode RF amplifier.


Cheers,
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Old 25th Apr 2019, 2:20 pm   #14
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Default Re: Bush VHF-41 Refurb.

Thanks all for the input.

Boater Sam, the ECH81 tests good to the test data.

John, After looking at comparing the circuits I agree, modding it is not an option.
It wont be the first of many as I don't collect them as My income would not support that and I have no place to keep the. I buy them with the intention of refurbishing and to sell on. I like doing it and it keeps me occupied. Maybe it also saves a few sets from the scrap yard.

Leon, I may get a couple of EF184's to try in place of the EF80's. Other than that I think that will be it.

Synchrodyne, Thanks for all that information, all very interesting and quite complex as well.

I now have to consider if I continue with it and finish the cabinet, so far it is upt to the stage of re-varnishing, so I suppose the answer must be yes.

Best regards,
Dave.
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Old 25th Apr 2019, 2:32 pm   #15
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Default Re: Bush VHF-41 Refurb.

I have lots of EF183 and EF184 if you need any.
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Old 2nd May 2019, 9:33 pm   #16
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Default Re: Bush VHF-41 Refurb.

Thank you Boater Sam, I will keep that in mind.

Today I took the bull by the horns and realigned the FM, both IF and RF.
I was not surprised that the RF was out as I had striped the tuner of to replace the hunts moulded capacitors.
However I was surprised to find the IF was off and did need to peaked up again.

The result was worth the effort as FM is now much better, particularly on the stronger stations.

What I was considering was how to put an extra stage of audio amplification, for FM only, in front of V5.
However I do not understand how the audio is derived as I expected it to feed to the top of the volume control, but when switched to FM by S1k it switches to an HT line so no audio there. I also do not understand how the VC VR1 still controls the volume as it is DC coupled to the triode grid.

Can somebody explain how the audio is obtained from the discriminator, and how the VC is able to turn the triode of V5 up or down.
And would it be possible to put an extra stage of amplification in front of V5 for the FM only.

I am using the manufacturers service information for references.

Many thanks,
Regards,
Dave.
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Old 2nd May 2019, 11:10 pm   #17
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Default Re: Bush VHF-41 Refurb.

The triode of the ECH81 is used as the first audio amplifier on FM while on AM it’s the LO. Follow the screen lead points B and C to the grid of the ECH81 triode. The anode of that valve feed the volume control with audio.
There is no DC on either the volume control or the grid of the EABC80, C44 and C45 stop that.
So there are already two stages of audio amplification on the FM audio before the output valve.
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Old 2nd May 2019, 11:16 pm   #18
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Default Re: Bush VHF-41 Refurb.

The triode in the ECH81 is used as an additional audio amplifier on FM in this set. The circuit diagram as drawn by Bush is not easy to follow.

Additional AF amplification on FM is unlikely to improve performance and will likely cause stability and hum problems.

Optimise the RF gain and accept that this model is a poor FM performer. All of its FM design defects were corrected on the VHF61/2, although these sets are in fact less good than the '41 on AM.

Leon.
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Old 2nd May 2019, 11:17 pm   #19
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Default Re: Bush VHF-41 Refurb.

Post crossed with Frank.
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Old 4th May 2019, 5:09 pm   #20
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Default Re: Bush VHF-41 Refurb.

Thanks guys, for the explanation, I have learnt something here that I had not come across before.
It looked a good idea at the time as a way of obtaining more output from the FM.

I must admit I am still thinking of giving it a try though.

Best wishes,
Dave.
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