Bush VHF 61 Alignment

[PaulR]

I am trying to align the VHF IF on my Bush set. It all goes OK until I get the the final secondary. I believe that the Trader sheet has this the wrong way round and that the secondary is under the chassis. However, when I connect everything as per the trader sheet I get no milliamps at all reading in the ammeter. Adjusting the core has no effect although if I set up the s/g and voltmeter as for the other transformers I can get the secondary to peak. It now works better but I think it is still not completely right.

Is there another mistake on the sheet or should I have another go?

On a more general point, I find it really difficult and fiddly to connect the s/g to the relevant valve pins from underneath and have to resort to temporary soldering. Is there a better way to do this?

Thanks

Paul

Paul, The VHF61 is a really nice set to align, as it can be done with the chassis in place (although the early ones do not have access holes drilled in the bottom for the AM oscillator adjustments).

The Trader sheet is incorrect. The secondary of the ratio detector transformer (largest can) is at the bottom. All other secondaries are at the top. I would suggest the following:

Carefully check, or preferably replace the ratio detector stabilising capacitor C56. De-Hunt the entire set. Check that the EABC80 has two good, matched FM diodes. Use acetone to soften the white locking paint on the cores, and use the correct tool to move them. A DVM is very much better than a milliammeter due to its higher input impedance. I use a matched pair (to 1%) of 100k resistors across the stabilising capacitor, setting the secondary for a null between their junction and the free end of the tertiary winding. If the set is a long way out, you will have to iterate with the primary adjustment, but always finish with a null at the secondary, making sure it’s the real one (definite swing positive and negative on adjustment). If your set is correctly aligned, apart from sounding good, the eye will overlap on a reasonable FM signal. I fit a 4:1 attenuator in the FM feed to the eye to make it more useful.

To connect to the valve pins, I use flexible Hirschmann probes with the crossover wire ends. These are brilliant, and worth their cost (available RS/Farnell). If you haven’t got these, remove the valve and put a few strands of cut down connecting wire into the grid pin, and carefully replace the valve.

Finally, check the value of the EL84 bias resistor, these go high due to overcurrent caused by leaky coupling capacitors. I shunt these resistors, as they are usually wrong, but stable. Keep an eye on the EL84 for grid current.

Regards, Leon.

[ronbryan]

Leon

How closely do you recommend that the FM detector diodes in an EABC80 be matched, for good results when aligning the ratio detector?

A quick check on my Philips B4X23A/15 yielded a difference of about 2V DC in forward drop between the diodes, when receiving a signal strong enough to give 52V across the fm stabilising capacitor.

Is this about average matching, or way out?

Ron

[telstar]

Hi. I have the actual Bush manual for this radio. If I can be of help to you, please let me know via p. message, and can sort something out from there.
Best regards
Mike

[PaulR]

Thanks all,

This set had certainly seen much better days when I bought it for £5 frrom a local sale room. It looked as if it had finished up at a motor repair garage or similar judging from the filth all over it. I originally intended just to replace the caps necessary to make it work in order to keep it as original as possible, but eventually decided to de-Hunt it as it was totally unreliable until the last Hunts had gone!

It worked fairly well with its new caps, but not as well as I have read it should do, so I eventually decided to re-align it. I didnt realise this could be done with it still in its case and so spent ages going from under to over the chassis whilst trying to keep the speaker wires and the other connections attached.

I used "point X" and earth for the resistors, which seems to be the same as connecting across the stabilising capacitor. I connected a microammeter as per the Trader sheet. Would a volt meter be connected similarly, and is there an obvious place to connect it whilst leaving the set in its case?

Finally, is it necessary to short the other half of each transformer whilst adjusting it as instructed by the Trader sheet? I gave up trying to do this as it was too fiddly and was still able to get them to peak.

The eye does not overlap even when on a roof aerial so there is still clealy some way to go before it is properly set up.

It has a brand new EABC80

I want to get this set working as perfectly as possible as it is in the sitting room and I show it off to visitors to demonstrate how well this vintage technology can work.

Thanks again

Paul

Paul, The Philips spec for the “FM” diode matching in the EABC80 is ra(1)/ra(2) = 0.65 to 1.5. This is rather wide, and I aim for a 10% match at about 5mA forward current. I do this on a valve tester, but if you have a spare B9A holder, wire the heaters, earth pins 3 & 7, and wire pins 1 & 2 separately to the 250V HT line of a receiver via two 47k resistors. Power up, and measure the anode voltage of each diode, relative to chassis using a DVM. The two voltages should match to 10%. Unless the diodes are really dreadful, these valves generally work, but a poor diode match makes it difficult to align the ratio detector, leading to an asymmetric “S” curve, bad limiting and poor audio.

Ron, It’s difficult to comment on the measurement you made with your Philips set as presumably the diodes would be rectifying the carrier during measurement. A static test is always more accurate, but the figures you give would suggest that the detector and its diodes are well balanced.

The VHF61 has a convenient test point above the chassis between the EABC80 and the final FM IF can, slightly towards the front of the set. This is connected to the negative end of the stabilising capacitor, and is used in alignment, both for peaking the IFs and balancing the ratio detector, where the two matched resistors are connected in series between the test point and earth. Thinking about your first post, a milliammeter would be hopelessly insensitive – you would need a 50 – 0 – 50 microammeter. Use a DVM in place of the microammeter; a DVM is very much better and more convenient. Finding a null with a DVM is easy.

Follow the Bush procedure and use the damping resistor if you want to get the FM IF bandwidth correct. (You might just have to buy some of those nice probes). I have dealt with a number of these sets, and in every case, if the white locking paint is undisturbed, the alignment will be generally OK. Usually, all that is required is a discriminator re-balance to take out any unequal drift in the EABC80 diode characteristics.

Let us know how you get on; the first one always takes the longest…

Regards, Leon.

[PaulR]

Thanks Leon,

I will get a DVM as you suggest - much easier than looking at a quivering needle- and have another go. The alignment was actually OK with only tiny adjustments, if any, needed. I will get some of those probes as well, they sound really useful.

Is there any way of connecting the other meter lead whilst the set is in its case for the discriminator adjustment?

Thanks

Paul

Paul, The other point you need to get at for setting the discriminator balance is readily accessible beneath the chassis when the set is in its cabinet. There is a 100 Ohm resistor connected to the tertiary winding which enables you to identify the correct transformer pin.

These were well designed sets.

Leon.

[PaulR]

Thanks, Leon.

What can I say - no wonder these sets are so popular. The only criticism I have is that the sound quality is not fantastic, I think the speaker is on the small side. My other VHF set, a Mcmichael FM55, has better quality sound. It looks nowhere near as good, though.

It may be a while before I can get the bits and pieces, but I will post the results.

Thanks again

Paul

Paul, I agree that the rather small ‘speaker is the weakest link in the VHF61 audio chain, but remember that this was the cheapest version using this chassis. The ’62 and the radiogram sound better.

If your set has ever been stored in damp conditions, check that the loudspeaker centering device has not come unstuck from the frame, near to the moving coil at the apex of the cone. A really good EL84 (hard to find) correctly biased is also important.

Finally, remember that this set has extension ‘speaker sockets. The results from a good external ‘speaker are excellent.

Leon.

[Paul Stenning]

The '62 has the same speaker though...

In a much better box.

[PaulR]

I assume I would need a 4ohm speaker to try the extension socket. A modern Hi Fi 8ohm speaker would not do?

Paul

Paul, A 3 or 4 Ohm extension 'speaker will give maximum power transfer when the internal 'speaker is switched off. An 8 Ohm 'speaker would be a poor match, and coupled with the fact that most 8 Ohm hi-fi 'speakers are very inefficient, you might have trouble driving it fully.

I have an old 3 Ohm Celestion Z10 10" unit in a wooden cabinet which I use as a test 'speaker - it makes the VHF61 sound really good.

Leon.

[PaulR]

I have at last bought a digital meter!

Before setting about the Bush, I thought I would try it out on my other VHF set, a McMichael FM55 which had been thoroughly mis-aligned by a previous owner.

I had previously aligned the main IF so I just tried the discriminator. It was very difficult to get a zero on the meter as even pressing on the core slightly with the alignment tool sent it off. The tiniest turn on the core made a huge difference. It did go from negative to positive but was not at all stable. Is this usual or am I doing something wrong? The set is not very happy. I can pick up the BBC national stations but with quite a lot of hiss and distortion when I would have thought I was right on the station. The distortion decreases a bit when tuned slightly off.

Hmm...

Paul