Bush VHF64 - FM re-alignment required? But can I do it myself?

[Nickthedentist]

Hello everyone,

I've just finished re-capping a lovely VHF64 which I bought from Steve (Music-Centre). All the Mouldseals and TCC tubular waxies replaced, as well as the three smaller electrolytics (and yes, I did get the discriminator cap the right way round). None of this should have altered the alignment AFAIK. Valves are the originals (well, the ones supplied with the set, at any rate).

I haven't tried it on the AM bands as I'll be using it exclusively on FM. Performance on FM is far, far better than it was, now with good sensitivity and loads of volume reserve. I was very relieved by this, as I have heard that some people have had problems with low gain on this model.

However, it's quite sibilant and distorted, and the strongest/cleanest signal is well to one side of what the eye indicates as optimal tuning. With a bit of wire connected to the aerial terminal, I get a peak of about -10V on the magic eye's control grid.

This is where my knowledge of vintage radio ends and I'm a bit stuck now

Should I try swapping valves? I have a VHF61 which I believe uses the same line-up and is working perfectly.

Or is it time to try re-aligning (part of) the set. I have a Fluke DMM, and Avo 8 (calibration unknown but seems to work), Hameg 20MHz scope (again, calibration uncertain) and an Advance SG62B signal generator which I'm in the process of checking-over, which apparently covers 150KHz to 220MHz with switchable 400Hz amplitude modulation, see here: http://www.thevalvepage.com/servdata/testeq/advance.htm

All ideas gratefully received.

N.

[G6Tanuki]

The sibilance and distortion you mention could be down to the aerial you're using: early VHF receivers often gave this sort of effect because of 'multipath' effects (the audio equivalent of ghosting on a TV).

Though you may have a strong signal that doesn't help if a significant amount of that strength is due to signals arriving after they have bounced off central-heating pipework, the neighbours' caravan, the metal-cladding of the supermarket at the end of the road....

Try it with a decent *directional* outdoor antenna aimed and suitably polarised for your local station.

Could also be worth changing the discriminator-diode valve as if one of the diodes has turned a bit less-diodey than the other it'll unbalance things.

[Nickthedentist]

Thanks for your input.

I have to say that I have a dozen other restored VHF sets from the same era at home and none of them have noticeable multipath distortion, even with the crudest of aerials so I suspect it's not that. I also feel that the fact that the reception is weak and distorted when tuned "correctly" using the tuning indicator suggests a problem with the set.

I will definitely try a new EABC80 though. Interestingly, this one has a different typeface (though the same Mullard emblem) on it to the others, so it may already be a replacement.

Nick.

[paulsherwin]

You should certainly try another EABC80.

If that doesn't help then the discriminator coil is probably misaligned. You can adjust this by ear but you really need to use a wobbulator and scope. You may be able to lash up a wobbulator using your Linex microtransmitter and a sine wave signal from a computer sound card.

Depending on your hearing you may be able to adjust for an undistorted sine wave without a scope, but the scope makes the job much easier and more precise.

As you will be aware, misalignment is unusual if the set hasn't been twiddled with.

[ronbryan]

Nick

The fact that the cleanest signal is not at the tuning point that yields the strongest signal (maximum -ve voltage at the eye grid), indicates that the discriminator is unbalanced. Unbalanced diodes in the EABC80 can be easily checked by swapping the valve with a known good one, but I expect that the FM IF circuits need re-aligning.

The procedure in the Bush service sheet recommends (for a given IF transformer) damping the IFT winding that is not being adjusted with a 1k resistor and I have found that this approach works well and prevents the IFs being set up with too narrow a peak.

You will need to borrow an AVO8 for setting up the discriminator balance, again as described in the Bush service sheet. You could try just setting up the discriminator balance, as you have little to lose by doing that first, but you may end up doing the full IF alignment

If the cores of the IFTs are a bit stiff, a squirt of Servisol on the core may make them move more freely. You can try to work it in by twiddling the core a little. You will also need a decent trimming tool that fits the slot.

Ron

[Nuvistor]

I don't know what the coil formers are made out of but if polystyrene be careful what you lubricate them with.
Frank

[Leon Crampin]

Nick, below is an old VHF61 post I made which covers some of the alignment points which I have found to be most critical in the restoration of these sets. I have found that RF/IF alignment with the notable exception of the ratio detector is seldom required.

Choose a good, well matched EABC80 and align the discriminator for balance and then don't change the valve. The VHF61 and 64 are virtually RF-identical. You can use an unmodulated 10.7 MHz signal from a generator to balance the discriminator. This will not give as symmetrical a curve as would the use of a wobbulator, but it will make the set sound fully acceptable with optimal results at the tuned maximum:

(Previous post).

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. 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.

[Nickthedentist]

Originally Posted by paulsherwin

You should certainly try another EABC80.

Thank you Paul. I have, albeit a used one, and although it was a little better, the results were still poor.

Originally Posted by paulsherwin

...using your Linex microtransmitter...

Good idea, I'd never have thought of that.

Originally Posted by paulsherwin

As you will be aware, misalignment is unusual if the set hasn't been twiddled with.

I agree, but I have a feeling someone's had a play with this one, albeit a good few years ago. The slots in the cores do NOT look "factory fresh"

[Nickthedentist]

Ron, your advice is a goldmine of useful fact, and I really appreciate your explanation and suggestions.

Originally Posted by ronbryan

You will need to borrow an AVO8 for setting up the discriminator balance, again as described in the Bush service sheet. You could try just setting up the discriminator balance, as you have little to lose by doing that first, but you may end up doing the full IF alignment

Yes, the service sheet makes it seem quite simple. I have an AVO8, but not sure how well it works.

Originally Posted by ronbryan

If the cores of the IFTs are a bit stiff, a squirt of Servisol on the core may make them move more freely. You can try to work it in by twiddling the core a little. You will also need a decent trimming tool that fits the slot.

They're stuck fast. I will try some Servisol as you suggest. My collection of trimming tools is not very extensive but there's one that seems to fit.

[Nickthedentist]

Originally Posted by Nuvistor

I don't know what the coil formers are made out of but if polystyrene be careful what you lubricate them with.
Frank

I will, Frank. I think modern Servisol Super10 is much kinder to plastics than older formulations though.

[Nickthedentist]

Leon,

Thanks for posting. I consider you a real expert on these sets so I do appreciate your input.

Originally Posted by Leon Crampin

I have found that RF/IF alignment with the notable exception of the ratio detector is seldom required.

That's consoling.

Originally Posted by Leon Crampin

Choose a good, well matched EABC80...

Is there any way of doing that without a valve tester, other than finding one that works well in another set?

Originally Posted by Leon Crampin

You can use an unmodulated 10.7 MHz signal from a generator to balance the discriminator. This will not give as symmetrical a curve as would the use of a wobbulator, but it will make the set sound fully acceptable with optimal results at the tuned maximum

That sounds good. I have an old RF generator which I will have to check-over before I start on this.

I will be back soon...

[paulsherwin]

You can temporarily substitute a pair of solid state diodes in the ratio detector. Just hand select them for identical voltage drop. If the distortion goes away then you've found the problem.

From your description it does sound as if somebody's been twiddling though.

[AlanC]

I've just done a VHF62 using the 2x 100K resistor technique mentioned by Leon; I just used my DMM as the indicator. This particular VHF62 had the problem that if you tuned for maximum magic eye deflection (maximum volts on the test point) there was a sharp, distorted null in the audio at that point. I set up the alignment jig, let the set warm up for 15 minutes, then tuned in radio 3 (on a quiet passage, the next best thing to an unmodulated signal! ).

The voltage on the test point was about -12V; that between the discriminator output and the junction of the resistors was about -3.6v, I had no problem zeroing it- it took about half a turn of the bottom core in the discriminator transformer, if that. As the zero point was approached, the audio level peaked up and the distortion fell to a very low level.

As others have said, the EABC80 needs to be reasonably matched, or what you are doing is 'aligning out' the mismatch, which will result in a distorted 'S' curve.

[Nickthedentist]

Thanks again Paul, and Alan.

Originally Posted by AlanC

I've just done a VHF62 using the 2x 100K resistor technique mentioned by Leon; I just used my DMM as the indicator. This particular VHF62 had the problem that if you tuned for maximum magic eye deflection (maximum volts on the test point) there was a sharp, distorted null in the audio at that point. I set up the alignment jig, let the set warm up for 15 minutes, then tuned in radio 3 (on a quiet passage, the next best thing to an unmodulated signal!

The VHF62 is the same chassis as the VHF61 as mentioned by Leon earlier, of course. My set is exactly as you describe, i.e. a distorted null at maximum eye deflection.

Originally Posted by AlanC

As others have said, the EABC80 needs to be reasonably matched, or what you are doing is 'aligning out' the mismatch, which will result in a distorted 'S' curve.

Once again, is there any easy way to tell whether the diodes in the EABC80s I have to hand are reasonably matched?

[paulsherwin]

Compare performance with solid state diodes. It only takes a couple of minutes to hook them up.

[Nickthedentist]

I like your idea Paul, but how would I do it in practice?

I suppose the valve heaters are in parallel so I could pull the valve and poke the diodes' legs into the valveholder's contacts. What diodes would I need? 1N4148 not suitable, I imagine.

[paulsherwin]

Er, use a soldering iron?

Just select the diodes, disconnect the discriminator wires from the EABC80, connect the diodes, and see what happens.

[Nickthedentist]

Sorry to be a dimwit Paul.

I was keen to mess with the wiring as little as possible now it's all "done", but I suppose fitting couple of diodes would be easy enough to do without leaving a trail of melted PVC insulation.

What diodes should I use?

N.

[Leon Crampin]

To check an EABC80 without a valve tester, obtain a B9A valveholder. Wire the heater pins to the set's heater line. Connect both cathodes to earth, and connect each of the FM diode anodes (pins 1&2) to the set's HT line via two matched 47k Ohm resistors.

Power up and measure each diode anode voltage with respect to earth. I would reject a valve with a voltage mismatch of more than 15% - but the Philips tolerance is much wider, approaching 50%. This test runs the diodes at about half their rated current.

My valve tester shows these diodes to be generally matched to +/- 15% (usually better) unless the valve has been to the moon and back. Bad ones are really bad...

Leon.

[Nickthedentist]

Thanks Leon.

Lots to get my teeth into now.

Nick.