Tuning condensor mountings.

[radiograham]

Does anybody know if you can get the type of rubber washers that were used for the mountings on tuning capacitors,In the past I have used ordinary grommets but these arent really suitable.does anyone know of an alternative?.

[G6Tanuki]

Must admit, I've never seen the need for or logic behind the flexible-tuning-capacitor-mounting thing: even the slightest degree of 'squidge' means there's additional backlash in the drive-path between knob and electronics that does nothing for tuning-accuracy.

You don't find these rubbery-things in 'proper' communications-receivers. When I come across a set of them I generally replace each failed grommet with a couple of hard. flat rubber tap-washers: a couple of packets of these:

https://www.screwfix.com/p/arctic-pr...s-5-pack/2070J

are cheap and effective. No more wobbly tuning-drive!

[Boater Sam]

Would Sugru sort it out? Sugru.com.

[Argus25]

Most of the time standard rubber grommets were what the manufacturer used in the first place. Though some used softer rubber. Except in cases where it was a unique shape moulding , Philco did this a lot. Normally the rubber would be hard enough that it didn't affect the tuning or add any backlash effect, but soft enough that it helped to prevent some acoustic feedback. So it was a kind of soft semitransparent brown rubber that turns to hard particles with time.
One thing that makes this worse is if "that capacitor" has been inadvertently made too high in value and the LF response of the radio's amplifier has been improved.
Another thing that can appear to make the V/c microphonic, is in short wave radios. At the high end of the band say around 17 to 19MHz, the converter valve, especially the 6A8, can become unstable and oscillate, though a 47R g1 resistor can help. Mechanical vibration then amplitude modulates the oscillation (carrier) and it makes the V/C super microphonic and stroking the vanes makes it sound like a Harp.

I would just use standard rubber grommets as replacements. The white ones are a tad softer than black.

Another trick to achieve the same effect is to put the speaker on grommets instead.

[ms660]

Oscillator microphony/rubber grommets, an article here well worth a read, W.W Dec. 1952, mag page 504 (part 1):

http://www.americanradiohistory.com/...ld-1952-12.pdf

Part 2 in W.W. Jan. 1953, page 35 but there are two of them, it's the one about 3/4 of the way down when scrolling):

http://www.americanradiohistory.com/...ld-1953-01.pdf

Lawrence.

[jonnybear]

I have used these in transistor radios to mount tuning capacitors :- they are tattoo grommets very soft and pliable, hole size is 3mm thickness is 4mm diameter is 8mm, I think you can get larger ones.
John

[radiograham]

Originally Posted by Argus25

Most of the time standard rubber grommets were what the manufacturer used in the first place. Though some used softer rubber. Except in cases where it was a unique shape moulding , Philco did this a lot. Normally the rubber would be hard enough that it didn't affect the tuning or add any backlash effect, but soft enough that it helped to prevent some acoustic feedback. So it was a kind of soft semitransparent brown rubber that turns to hard particles with time.
One thing that makes this worse is if "that capacitor" has been inadvertently made too high in value and the LF response of the radio's amplifier has been improved.
Another thing that can appear to make the V/c microphonic, is in short wave radios. At the high end of the band say around 17 to 19MHz, the converter valve, especially the 6A8, can become unstable and oscillate, though a 47R g1 resistor can help. Mechanical vibration then amplitude modulates the oscillation (carrier) and it makes the V/C super microphonic and stroking the vanes makes it sound like a Harp.

I would just use standard rubber grommets as replacements. The white ones are a tad softer than black.

Another trick to achieve the same effect is to put the speaker on grommets instead.

I Have found the ones that I have replaced in the past have had a metal bush in the centre through which the mounting bolt passes,the hole in the grommet is same diameter as said bush which isnt the case with an ordinary grommet of the same diameter,It is always bigger diameter than the bush which goes in centre, which is why I think they were made for this purpose alone. I Have used tap washers myself but have found these to be much harder than the originals.

[radiograham]

Originally Posted by jonnybear

I have used these in transistor radios to mount tuning capacitors :- they are tattoo grommets very soft and pliable, hole size is 3mm thickness is 4mm diameter is 8mm, I think you can get larger ones.

These are much closer to the original types that I have encountered in the past.They have the smaller inside diameter hole into which a metal bush fits and through which the mounting bolt goes,In a standard grommet this inside diameter is always too big even though the outside diameter is same as the original.

[kalee20]

I tend to use silicone rubber grommets. They're more expensive, but they are soft, and moreover will be the same in 50 years time as they are today.

[rontech]

I have made grommets from thick wall silicone rubber tubing. od about 12mm id about 8 mm.
Just cut to required thickness with Stanley knife.

Not sure where to obtain said tubing, mine came from a chemistry lab' years ago.

[Synchrodyne]

Originally Posted by ms660

Oscillator microphony/rubber grommets, an article here well worth a read, W.W Dec. 1952, mag page 504 (part 1):

http://www.americanradiohistory.com/...ld-1952-12.pdf

Part 2 in W.W. Jan. 1953, page 35 but there are two of them, it's the one about 3/4 of the way down when scrolling):

http://www.americanradiohistory.com/...ld-1953-01.pdf

Thanks – that is a most interesting article. It gives one an appreciation of the difficulties encountered in designing an integrated domestic receiver, where the speaker is in the same cabinet as the electronics. One could deduce that in some ways it was easier for the designers of communications receivers, which very often had the speaker in a separate cabinet, and which usually had a higher budget for more sophisticated oscillator circuitry, etc. The designer had to be something of a mechanical engineer as well as an electronic/electrical engineer, effectively concerned with component suspension characteristics and also what might be classed as NVH issues.

One point made in the article was that wood cabinets were better than moulded cabinets in respect of minimizing microphony – not really so surprising.

And the author, H. Stibbé, then a Bush staffer, had previously worked for Philips, where he had been involved in the development of a very low microphony double conversion bandspread domestic receiver. (My guess is that this was the Philips 681A/Mullard MAS231.)


Cheers,

[jonnybear]

Originally Posted by radiograham

These are much closer to the original types that I have encountered in the past.They have the smaller inside diameter hole into which a metal bush fits and through which the mounting bolt goes,In a standard grommet this inside diameter is always too big even though the outside diameter is same as the original.

If you require this size PM me I can send you some.
John