Last update
04 April 2002.

I don't know how many, if at all, you have done any sound modifications for overseas televisions to convert them from 6.0MHz to 5.5MHz sound. But the first one I did, I noticed that as I screwed the slug into the sound detector coil the sound level increased until it got to the point were there was a bit more physical resistance in turning the slug, and the sound level was still not near a acceptable level, so naturally I applied a little more energy, only to hear the munching sound of the screw driver chewing up the slug into bits, and smaller bits. Then I remembered what somebody had told me a few years earlier and thought, there may be a tiny bit of truth when they said I was a "silly @*$%^#". So then I decided to use a bit of theory to make out that I was a bit more intelligent than you average "silly @*$%^#" and work out how to do the job properly the first time.
If we take a 6.0MHz tuned circuit and increased the value of the capacitor we will get that tuned circuit at a lower frequency, if we increased the value by the right amount, hopefully we might just get it tuned at 5.5MHz, then the slug would not have to be adjusted and that means no more broken slug. But what is the correct amount? That is were you have to apply a bit of mathematics to the frequency formula, but! since I am writing this article I will do it for you, and save you the time, "Don't you think I'm good to you?"

Radio-Electronics people should know the next formula.

What we have to do is to put "F" & "C" on one side of the formula and every thing else on the other side, so we can get rid of it.
(since we would like "2_*pi"and "L" to remain a constant after the conversion).

Now we get to the good part by using the F²*C and disregarding the rest of the formula, we can say that;

What we have just worked out, is that we have to increase the original capacitor value by 19% to change a tuned circuit from 6.0MHz to 5.5MHz. or to save time go to the next highest preferred value in the E12 range

Since we could easy substitute "L" for "C". this would suggest that 19% is past the breaking point of the slug. I have seen the maximum variation figures for adjusting a slug in a coil but, I just can not recall it for this article at the moment, I think it is between 5% and 10%
Getting back to the practical side,

One would of thought that after they invented the ceramic filter, that the television designers would of used them for sound traps, sound take off and sound detector. And your right, they have used them there, but when I was putting this article together all I could find looking in modern circuits is inductor-capacitor network for the sound detector while using a ceramic filters for sound traps and sound take off. I am not too sure why it has worked out that way, or if I am going blind, but I would be interested to learn. (about the detector coils and also if I am going blind)
If you pull out your old PYE CT104 service manual and look in the parts list you see that the tolerance for the sound take off is ±40KHz while the sound detector filter has a smaller tolerance of ±6KHz. Is it that it is cheaper to use coils than a filter with ±6KHz tolerance.

Marking & color found on some 5.5MHz sound filters,

Note: If you find a filter with 6.0MB on it, you replace it with 5.5MB or, If you find a filter with 6.0MC on it, you replace it with 5.5MC

One of the sets I modified was a UK Sharp C-1410 which looked very much like NZ C-3700,C-3705,C-4200 series, this is one of those sets that uses two filters and a sound detector coil. If you look at the C-3700 circuit you will see (assume) that there is a capacitor inside the can of the coil Experience has told me not to take coils apart to see what's inside, not even to read the capacitor value. take my word of it, it is better to guess what value might be inside, and then put a capacitor across the coil and see how the sound level improves. I guessed the value might be about 47pF The nearest capacitor I could find to a 9pF (47pF*1.19=55.93pF -47pF) in stock was a 12pF, this was placed across the coil which almost worked well, A small tweak with the slug worked excellent.

Another set was a South Africa Panasonic TC361. This set looked very much like a 14" version of New Zealand 26" M4 with a few minor changes for example the I.F. board was wired in and mounted on it's side, instead of been plugged in. The customer report said "Detector coils broken" after pulling out the M4 circuit it was revealed that the detector coil slug was one of the few slug that had not being broken, the broken slugs were in the I.F. coils. A quick check revealed that who ever tweaked and broke the slugs had not changed the quality of the picture to any noticeable effect. So guess what your strange author did next, the logical, pulled out a old M4 I.F. board and swapped the detector capacitor over, a change from 39pF to 47pF, and after doing the other changes and the set was turned on. The set was found to have normal sound level. No tweaking was needed for that slug.

Now having said all that the easiest way if all else fails is to put a trimmer capacitor on the track side of the printed circuit board, since writing the article, I was asked to modify a TRIAD TV after looking around their workshop for a few minutes I could not find a suitable capacitor, but I did find a trimmer capacitor from a National/Panasonic M7/M8 Chassis IF board. A 3-30pf trimmer capacitor seems to work out good, and that gets adjusted instead of the slug for the highest sound level.

Problems,
We all know to make a sound conversion look professional you have to replace the Sound Traps & Sound Take off then modify the Sound Detector coils, yet for some reason you can only buy a sound take off filter from our main TV & VCR parts supplier in New Zealand.

The only problems I have run into in the past is (1) Converting the modulator in the VCR, (while I have succeeded with lots of others) (2) and once had a two legged filter when most are three legged one (but solved that). I have also modified a Hong Kong 6.0 MHz NICAM to NZ 5.5MHz. This Sound trap issue in NZ has left a horrible taste in my mouth that would be unlikely to undo, and as normal it is all my fault.

The parts problem,
Now here is a very strange story, In New Zealand the TV and VCR parts supply companies only sell "sound take" off, to get the "sound traps" you have to either get them from TV and VCR parts suppliers in Australia or from the agents of one brand or another. This seems to tell me, either the TV and VCR parts supply companies in New Zealand do not know their theory or they are tring to make us less efficiently at doing our work.