Bush VHF 61/62 FM RF Alignment

[DoubleWound]

I have a Bush VHF 62 that is way out of alignment on the FM tuning scale, eg Classic Radio appearing at the Radio 2 area. Obviously this is too far out to mechanically adjust on the drum mechanism.

Suspect components (on the Trader Sheet) may include C5, C6, C7 and C8.

However, the purpose of my post is to ask if anyone has the knowledge to describe the process of adjusting C7 and C9 which is not covered in the Bush or Trader Sheets but merely described as a factory operation.

Hoping someone can help.

[Mr 1936]

Hi

To be that far out, the LO would have to have gone up in frequency by about 12 MHz or 12%, which implies the total tuning capacitance has dropped by about 20% e.g a capacitor has gone to a low value or open circuit.

There is also another (less likely) possibility, that you are receiving on the "wrong" side of the LO, if for some reason the LO is about 9 MHz lower than it should be.

Example: LO is 90.2 MHz. Low side mix will receive on 79.5 MHz, High side mix will receive on 100.9 MHz (The "Image")

However, I think it's rather unlikely that the tuning capacitance or inductance has gone up unless the phantom bodger has been at it !

[Leon Crampin]

Beware when aligning the FM IF stages of these receivers. Some service data (Trader, I think) has the locations of the primary and secondary slugs of the final FM IF transformer transposed. The data published by Bush is OK. Obviously your present problems relate to the FM front end.

Leon.

[DoubleWound]

Thanks for those clues Mr1936.

The FM box was unmolested at the time of my initial post but a couple of hours ago I "adjusted" C8 and C9. Now Radio 2 is closer to 100MHz and it seems that Classic Radio is now at the Radio 2 end.

Poking C8 gives intermittent fault and the identical C9 is looks as though it possesses some green corrosion.

The actual FM reception is excellent, just in the wrong place on the dial.

I will see if I can determine the LO frequency although my scope is limited to 100MHz.

If the 3-15pF trimmers are the culprits I may not find alternatives that easily.

The factory process for adjusting these trimmers remains a mystery.

[mark2collection]

Our VHF61 was crackling and had FM stations in the wrong place, Classic FM at 96 Megs, for example.

The set had been re-capped & was ok, but became worse over time (years). One of the fibre washers on one of the ceramic trimmers had become conductive (the one carrying HT inside the enclosed FM module), the other trimmer was also covered in some form of oil. One of the carbon comp's had gone up in value too, which was soon replaced.

Both ceramic trimmers had to be removed, cleaned & nylon washer used instead of the original fibre washers.

Once reassembled, I tuned a good radio to a known station/frequency (in the middle of the FM band), at one end of the workshop (so I knew what to listen out for), & then set the dial pointer on the Bush to match the correct frequency. Then adjusted the once covered oily ceramic trimmer until the correct station appeared, then trimmed the other until the magic eye had reached its 'peak'. Tuning the Bush up & down the dial showed all receivable stations were where they should be, and I was using the in-built aerial at the time, too.

I didn't have to move the ceramics far, just a mild nudge. That was 3 years ago now, set is still bang on.

If you have a frequency genny or pantry transmitter, so much the better. Not a particularly fancy way of doing it, just making do & mending ...

Mark

[Mr 1936]

Hi again Doublewound.

I have now had a look at the service data and had some more thoughts.

The quoted tuning range is 87.5 to 100 MHz, and the IF is 10.7 MHz. If the LO is supposed to be on the high side (most were), that gives an LO range of 98.2 to 110.7 MHz.

The tuning is achieved by a fixed capacitor and variable inductor L4. The total capacitance is around 25 pF and the inductance will be something like 105 nH to 83 nH. These are rough figures, to give you an idea of what you are dealing with.

Unlike a variable capacitor arrangement with end stops, the "slug on a string" setup would show a maximum inductance with the slug in the middle of the winding, and diminishing inductance if it is pulled out in EITHER direction. So it's possible if the drive cord is mechanically out of kilter for the tuning to go backwards, or even down then back up again.

The FM tuning range is set by adjusting the slug/string mechanically at the bottom of the band. At the top of the band it will be affected by both C7 and C9. The idea is that a bit of LO is fed back from the anode circuit of the mixer in antiphase, such as to neutralise the LO signal being fed back to the RF amplifier and thence radiated from the antenna.

No doubt Bush had their own test jig and procedure to minimise LO radiation whilst getting the tuning range right, your only choice is to do it by trial and error.

[DoubleWound]

Thanks so much for the feedback Mr 1936.

I've had problems in the past with Cossor sets utilizing two slugs on a single string and your explanation of inductance variation has made things clearer to me.

I believe I found the oscillator trace at the junction of trimmers C7/C9 and the variation of frequency across the dial made no sense to me.

For the hell of experimentation, using FFT there was a nice 10.7MHz peak feeding into V2.

Tomorrow I'll look at the set again and my inclination at this stage is to suspect failing trimmers or perhaps capacitors.

[DoubleWound]

In the absence of any information concerning the adjustment of C7 and C8 I did it my way by trial and error, simultaneously achieving an objective of getting reception up to 106 MHz. The set is very sensitive with the built in dipole and amazing with an external circular dipole.

Sweep tests show the IF response and S curve to be a little off but thanks to Bush the cores are sealed solid. The set sounds so good it's not worth the risk of trying to unseal the cores.
What a stupid idea to seal cores in this way - my recent alignment of a Fenman plus many other radios was so easy due to soft wax sealing of the cores. (Rant over and this thread may be closed).

[Leon Crampin]

Nearly all FM radios other than the earliest were designed with the oscillator running below signal frequency. This was because when Band III broadcasting began, the third harmonic of an FM set's oscillator otherwise fell within the band.

Leon.

[ronbryan]

DoubleWound

Acetone is the compound to unlock the cores in Bush FM radios if the sealant is that white, paint-like substance, should you decide to have a go at improving the alignment.

Ron

[DoubleWound]

Originally Posted by ronbryan

DoubleWound

Acetone is the compound to unlock the cores in Bush FM radios if the sealant is that white, paint-like substance, should you decide to have a go at improving the alignment.

Ron

Thanks Ron.

It is the white stuff and I do have acetone.

Let's see what I decide to do tomorrow - leave the set alone in good working order (after many hours of other work) or improve the alignment with the help of acetone, or melt the formers, or break the cores.

I do have some Bluetooth modules if I lose the IF cans.

[DoubleWound]

Thanks folks for the acetone tip. It worked well and the AM has now been peaked and the FM swept.

Reception and audio quality is superb.

[Boulevardier]

Originally Posted by DoubleWound

Thanks folks for the acetone tip. It worked well and the AM has now been peaked and the FM swept.

Reception and audio quality is superb.

Out of interest, what is the best way of applying the acetone to IF cores?

Mike

[DoubleWound]

I dabbed the internal threaded coil formers and top of cores with an acetone dipped artist brush, cleaning the said brush frequently.

After two or three applications the cores turned without any drama and remained firm enough to not require re-sealing after alignment.

There was no need to soak the area.

[Boulevardier]

Originally Posted by DoubleWound

I dabbed the internal threaded coil formers and top of cores with an acetone dipped artist brush, cleaning the said brush frequently.

After two or three applications the cores turned without any drama and remained firm enough to not require re-sealing after alignment.

There was no need to soak the area.

Thanks for that useful tip!

Mike

[Mr 1936]

Hi

This thread inspired me to dig out my own Bush VHF61 (S/N 209/36691), which hasn't been powered up for at least 10 years. A quick dust off, then gradual increase of mains voltage over 5 minutes using a Variac.

Rewarded with good reception on FM and AM using the built in aerials. This set has all its original components except for a "new" magic eye. Nevertheless I will probably partly re-cap it for reliability's sake. A realignment is also proposed, so thanks for the acetone tip.

A check of the LO frequency (looking at leakage from the aerial socket with a spectrum analyser) confirms that the LO is in fact on the LOW side (76.8 to 89.3 MHz). Thanks to Leon for pointing out that UK (and possibly European ?) sets of that era used the low side convention to reduce the chance of LO second harmonic upsetting TV on Band III.

I suspect that practice in the USA may have been different, as they were more worried about the LO fundamental landing in their Band I channels 5 and 6. Certainly, the US/Japanese equipment I have encountered from the transistor and then IC eras have all used a high side LO.

There are good reasons for and against both conventions, and I don't suppose it matters much now but it's sometimes quite useful to know which one you are dealing with!

[DoubleWound]

Originally Posted by Mr 1936

Hi

This thread inspired me to dig out my own Bush VHF61 (S/N 209/36691), which hasn't been powered up for at least 10 years. A quick dust off, then gradual increase of mains voltage over 5 minutes using a Variac.

Rewarded with good reception on FM and AM using the built in aerials. This set has all its original components except for a "new" magic eye. Nevertheless I will probably partly re-cap it for reliability's sake. A realignment is also proposed, so thanks for the acetone tip.

A check of the LO frequency (looking at leakage from the aerial socket with a spectrum analyser) confirms that the LO is in fact on the LOW side (76.8 to 89.3 MHz). Thanks to Leon for pointing out that UK (and possibly European ?) sets of that era used the low side convention to reduce the chance of LO second harmonic upsetting TV on Band III.

I suspect that practice in the USA may have been different, as they were more worried about the LO fundamental landing in their Band I channels 5 and 6. Certainly, the US/Japanese equipment I have encountered from the transistor and then IC eras have all used a high side LO.

There are good reasons for and against both conventions, and I don't suppose it matters much now but it's sometimes quite useful to know which one you are dealing with!

This is all very interesting to me and can't wait to hear of further developments.