Bush TV62 Fine Tuning Control and Audio Alignment Questions

... One thing that is also noticeable is that whilst there is a good picture displayed throughout the full range of the fine tuning control the sound can only be tuned in right at one end of the control and it doesn't seem to peak ...

What do you mean by a 'good' picture? As you tune through the channel, the frequency response should alter considerably from good to really poor and smeary. The correct tuning point is that at which the full resolution is obtained just before the picture level drops dramatically and is replaced by bars across the screen if there is any sound content.

As for the sound itself, there are no sound specific contacts in the tuner unit and the sudden coming and going might be, as you say, be a poor contact somewhere - try placing a finger on the top of the pip of each valve in the sound chain in turn the, applying gentle sideways pressure, try to move the pip around in a circle. This should cause all the valve pins to move in their sockets and clearly show up any that need cleaning. Another possible cause could be the inter-chassis connector at the rear of the timebase chassis.

One possible cause is IF instability. While the fault is on, carefully tap both sound IF valves and see if there is a ringing sound from the speaker.

Don't rule out Mr Twiddler! Remove the IF chassis and study the dust iron cores in the IF cans and rejectors to see if there is any sign that they have been interfered with - the slots should be perfect with no trace of wear or damage. Under no circumstances whatsoever should you be tempted to adjust them yourself in ant way!

I will explain how the (fine) tuning works in another post but you might like to try moving your Aurora to Band III - Channel 9, say. There should be a slight overlap between Channels 8, 9 & 10 which might help you to explore the tuning characteristics in more detail and help to determine if mechanical adjustment is required.

The tuner used in this set features incremental tuning. Unlike a Turret tuner, which has separate coil sets for each individual channel, this tuner only has two sets of coils, one for Band I and the other for Band III. Within each coil set is a wand fitted with a series of brass and dust iron cores. Moving the wand in to or out of it's associated coil set allows it to tune from one end of the band to the other.

At the rear end of the coil sets, a strong spring inside the coil former attempts to push the tuning wand out of the of the other end. Rotary cams on the tuning mechanism regulate this in a controlled way to tune to individual channels. Apart from routine cleaning of the silver plated contacts on band switch, the mechanism used on these tuners is very reliable.

(Click on images to enlarge)

At the front of the tuner, a moulded block contains the cams which push against the spring loaded tuning wands by means of push rods running against a series of steps, one for each channel, interconnected by sloped sections. A continuous slope on the cam returns the push rod to the other end of its channel while the other band is in use. A third cam operates the sliding band switch.

Fine tuning is accomplished by a screw thread which moves the entire cam block backwards and forwards by a small amount, thus also moving the tuning point either side of its nominal position.

This close-up of the cam mechanism with the surface contours may help to see what is going on:

Terry,

Many thanks for the details of the mechanical permeability tuning arrangements used on these sets. The description and accompanying images are excellent!

I have only briefly had the cover off of the tuner section in order to replace the local/distant wire link, which was somewhat perished above the chassis, and didn't spend much time looking at the internals.

From what I know about the sets history I am fairly hopeful that Mr Twiddler hasn't been at work and I don't want to get into attempting any re-alignment unless it becomes unavoidable, having first eliminated any mechanical or component faults.

I hope to have some time this weekend to investigate further and I will let you know how I get on.

I will first pay some more attention to the effect of the fine tuning control on the picture. From my recollection however I didn't notice any significant change in the picture quality from good to really smeary or dramatic drops in picture level as you have described. Perhaps there is something wrong mechanically although there doesn't seem much that could go wrong with the fine tuning arrangement.

You mention routine cleaning of the silver plated contacts on the band switch. The band switch isn't at all obvious, where labelled, on the tuner image. Is it perhaps located above the chassis? I'm sure I'll find out when I next get to look at the set!

Thanks again for all the information and fault finding suggestions.

Best regards,

Richard

You mention routine cleaning of the silver plated contacts on the band switch. The band switch isn't at all obvious, where labelled, on the tuner image. Is it perhaps located above the chassis?

No - it is, in the main, obscured by the component references that have been overlaid on the upper part of the switch, compounded by part of the lower half being hidden under one of the coil assemblies! Here is a part of the Bush photograph with one of the upper and lower sections circled in yellow:

The switches are in two rows with the sliding contacts on either side of a paxolin strip, each section being a one pole - two way switch (although the lower section in the picture exceptionally only has two fixed contacts!

I've attempted to recreate one switch section from the photograph - see here:

When you look inside the tuner, the switch should be quite obvious!

As for 'routine cleaning of the silver plated contacts', this might have been every couple of years if the set was in an industrial area like the one I grew up and worked in in South Essex when the siver plating would turn jet black, whereas I've seen tuners recovered from ex-rental sets only a few miles north with absolutely pristine plating! So, by 'routine', I really mean 'as required'!

Thanks for the clarification Terry,

Now it is clear to me exactly what I should be looking for in terms of the band switch contacts.
As you say the sliding contacts should be obvious enough when I next take a look inside the tuner and operate the channel selector switch between the two bands.

Regards, Richard

... The switches are in two rows with the sliding contacts on either side of a paxolin strip ...

Its over 45 years since I last saw one of these sets and it has occurred to me that I might have made a slight error. The mechanical arrangement of the tuner went through several iterations - including later conversion to push-button tuning, for which it was eminently suitable! However, I was puzzled by the wide separation of the two sets of contacts in that tuner until it dawned on me that the two rows in the photograph operated in opposite directions!

I think you will find a rocker mechanism between what is two physically separate switch sections that didn't exist in later models!

... operate the channel selector switch between the two bands ...

Something worth noting, Richard, that is easily overlooked by even seasoned engineers who are more familiar with 13 channel turrets is that, if cleaning the switch, it is only necessary to repeatedly move the selector back and forth between either channels 1 & 13 or 5 & 6 to operate the switch!

On the later 'Bush' Button models it is even easier - simply release all the buttons and move the switch rocker bar backwards and forwards!

Terry,

Again some more very useful information. I was wondering why the two contact sets apparent in the photo seemed to be spaced rather too far apart!

Terry,

I didn't have much free time to work on the TV62 at the weekend, but I did make some progress.

The band selector switch contacts weren't particularly dirty, but I've cleaned them anyway.
I also replaced another Hunts capacitor that I found hiding under a tag strip on the tuner and IF chassis.

I then confused myself for a while, thinking that the channel selector control and its toothed wheel was out of step with the channels until I realised that I was looking at it with the chassis upside down. Doh!

Having figured that out, I then proceeded to try the set with the Aurora output on each of the channels in turn. As before, on the Band I channels the sound tunes in right at one end of the fine tuning control. On Band 3 however the tuning behaves much more as you have described it with a clearly defined correct tuning point for the vision and sound signal. So perhaps I don't really have a tuning problem after all.

Having briefly tested all of the channels I then became distracted by the fact that the volume control pot was quite noisy in operation. As such I removed the time-base and power chassis again in order to extract the combined brightness / volume control / on-off switch. I've cleaned the pots and the switch, but have yet to re-fit it.

Regards,

Richard

I've never experienced a problem with the hundreds of Bush sets with these tuners that have passed through my hands but, in your first post you implied that there might be an adjustment for the cam assembly:

... The alignment instructions for sound talk for the Bush TV53 series talks about setting the fine tuning control at mid-position on Channel 6 ...

However, this is purely the position to have the tuner in while alignment is carried out - there is no actual adjustment described there.

An interesting experiment would be to set up the Aurora and TV for Channel 2 then, with the tuning as accurate as possible, try to turn the Channel Selector very slightly - you might need to remove the locating spring on the toothed selector wheel to be able to do this easily - so that the push rod leaves the flat section of the cam and moves onto the sloped section. I think it will need to be moving towards Channel 1 for this but what you need to do is to try to quantify how much additional movement - if any - is required to reach the correct tuning point. If a measurable improvement is noted, the problem is how to rectify the situation.

I think it very unlikely that the 20ρF Silver Mica cap across the Band I oscillator coil is causing the problem so I wonder if someone has disturbed the coil itself?

Take a look at L13, it is the coil nearest the cam assembly on the right hand side of the tuner (viewed from the front with the receiver unit inverted). Do any of the turns look as if they are loose on the former? They may look a bit irregular but this is normal (the coil pack is actually aligned in manufacture by distorting the turns to increase or decrease the spacing as required).

Tune in the Band I channel as well as is possible, then move a finger close to the coil, which should detune it slightly - take obvious safety precautions when doing this - there is no need to actually touch anything! Does this improve the tuning? Or make it worse?

Let us know what you discover and we'll take it from there ...

Terry,

Thanks again for your helpful suggestions.

Once again it won't be until the weekend before I get the opportunity to investigate the Band I tuning any further.

In the meantime I have come across the following info in the Bush Service Instructions for the TV62 describing some modifications that were made to the receiver unit of later TV62 models. I will check some of the components mentioned to see if my set is an early or a later build one.

It seems to me as though some of these design modifications might perhaps be of relevance? For some reason Bush felt the need to make these modifications to the Band I circuitry on the TV62.

Regards,

Richard

... I have come across the following info in the Bush Service Instructions for the TV62 describing some modifications that were made to the receiver unit of later TV62 models ... It seems to me as though some of these design modifications might perhaps be of relevance?

You can safely ignore these changes, Richard!

The TV62 is a bit of an odd ball and I wonder if it was released before the rest of the TV6x series (there is no corresponding TV52, whereas the rest of the TV5x and TV6x series have very marked similarities. The modifications you describe also correspond with RU 98, used in the rest of the TV6x series - they are not unique to the TV62!

I'm fascinated by the reference to 'the Turret Tuner' because this not a Turret Tuner - in fact, Bush never made a single example of such a device!

There's not much progress to report towards resolving the tuning issue.

With the Aurora and the TV set to Channel 5, as per my initial post the sound peaks right at the clockwise end of the fine tuning control.
If I move the channel selector switch part way between Channel 5 and Channel 4 I can find a point where the sound can be peaked at the centre of the fine tuning control. The Band I push rod has moved out slightly from its position with the channel selector on Ch.5.

On Band 3 the tuning error is even more pronounced. If the Aurora output is set to Channel 7 I have to select Channel 6 on the TV62 before I can tune in both the audio and video near to the centre of the fine tuning control.

So the question arises as to what might be causing these observed tuning issues.

If the problem seen between Channels 7 and 6 on Band 3 was happening on all channels then I would most likely conclude that the toothed channel selector was somehow mechanically out of step by one channel, but the Band I channels are not out by a complete channel.

My thinking at the moment is that the local oscillator is running at a higher frequency than it should as the fault is at least somewhat common to both Band I and Band III. I have been checking some components around V2, but haven't found any that are definitely faulty. [There are a couple of resistors supplying the anode of V2b that measure on the high side. R13 should be 3K3 and measures around 4K ohms; R14 should be 3K9 and measures around 4K5 ohms.]

Any further ideas as to where I should be focussing my attention would be welcomed.

Regards,

Richard

Interesting ...

You've now established that the problem is on both bands. Band III will appear to be worse than Band I because the tuner has to span 8 channels in Band III in roughly the same physical distance as the 5 channels of Band I.

You've established that the tuning is high on both bands which doesn't leave a lot of options!

There aren't many frequency determining components common to both bands - in fact, there are only two - C18 & C70. I assume that these will both be Silvered Mica types and unlikely to cause problems. The resistors you mention are both a bit on the high side, both being on the upper boundary of 20% tolerance components. I doubt they are the cause although you might want to replace them, anyway, as 10% components are specified in both positions.

I don't see this being valve trouble but you could try swapping the PCF80 in the tuner with the vision output valve.

I wonder if the problem might be mechanical? I've never known any problems with any of these tuners but is it possible that this one has been tampered with? The offset from the correct tuning position probably corresponds to the same amount of physical movement on both bands.

Unless anybody else has any bright ideas, it has to be a toss up between this and C18/70 ...

Component positioning can be critical in VHF tuners, so I urge you to investigate how easily any component can be replaced before attempting the exercise ...

Terry,

At last I think that with your help the solution to this tuning problem is getting closer.

A new PCF80 valve didn't make any difference to the tuning.

I replaced the two resistors that I mentioned as reading high in my last post. Not unexpectedly I suppose that had no effect on the tuning issue. Whilst R14 was out of the circuit however I measured C18 as 58pF. My multi-meter doesn't read low values of capacitance very accurately, but this was clearly high for what should be a 47pF capacitor.

I didn't have a 47pF mica capacitor so I temporarily fitted a 45pF mica capacitor instead. With this capacitor fitted the tuning on Band I was much improved, with the signal peaking at around the centre of the fine tuning control. On Band 3 however either the oscillator had stopped working completely or the tuning was way out.

I should say at this point that the 45pF capacitor was not fitted properly. I had left it with full length leads and hadn't tried to fit it correctly in the place of the original. I reconnected the original capacitor and both bands were working once again albeit with the tuning offset. I've ordered a new 47pF silver mica capacitor and am hoping with this component fitted in the right position that the tuning will be OK on both bands.

Regards,

Richard

As a matter if interest, Richard, what value does your meter thing the 45pF cap is?

I'm a bit suspicious as it seems impossible for the capacity to increase - but there's always the first time, I suppose ...

45pF is within ~4% of 47pF which shouldn't make much difference but you cant take liberties with long leads like that in VHF tuners!

It's always a bit hit and miss trying to read low value caps on a multimeter. The length of the leads, hand capacitance and the input capacitance of the meter all play a big part in the final reading. Make up some very short leads (as short as practical to allow connection to the component and with good quality clips), attach clips firmly to the component leads, keep hands well away while taking the reading and you may find the readings more reliable.

I use a Marconi RCL bridge and that is fitted with leads just 2 inches long. Even on very low value capacitors (below 100pF), hand capacitance can affect the 'null' on the meter.

Thanks for the comments about measuring low value capacitors.

I don't remember the exact model of multi-meter that I use. It was one of a series of White Gold Precision multi-meters with inductance and capacitance ranges as well as transistor HFE measurement functions that I bought from Maplins quite a few years ago.

I made up some short-ish leads with flat blades that plug into the meter slots for capacitance / inductance measurement and small croc' clips on the other end.

The 45pF capacitor measured within a couple of pF. From recollection slightly low at 43 or 44 pF.

The original capacitor was measured in-situ, with one end disconnected.

Well it has taken a long time and much effort, but I have at last resolved this tuning problem.

Firstly I replaced the two capacitors in the local oscillator (C18 & C70) that were mentioned in an earlier post with new silver mica ones, but that made no noticeable difference.

I then resorted to following the full vision and sound alignment procedure. Some of the trimmers for the sound IF in particular needed some minor tweaking. The result was that the sound and vision then peaked together rather better, but still at the clockwise end of the fine tuning control.

Now to the fix. I removed a sprung metal plate from the inner end of the fine tuning control. This allowed me to remove a couple of end stops that fit onto the squared end of the fine tuning control. I was then able to freely turn the fine tuning control. My first attempt at mechanically repositioning the push rods was a disaster; I moved them in too much and lost both vision and sound! Fortunately I was able to tell where they were originally located by the clean areas on them and this allowed me to re-adjust the push rods to be just a little closer to the chassis than they had previously been. This restored both sound and vision and most pleasing of all the fine tuning control now peaks at about mid travel on all channels.

Although it shouldn't have affected the IF alignment I went through the alignment process once again in case any of the adjustments around the local oscillator had been affected by my mechanical adjustments.

I've no idea why this mechanical adjustment should have become necessary, but it has fixed the tuning problem for which I couldn't identify any other cause.

Regards,

Richard