In one of my radios, the trimmer capacitors (10 to 40 pF) are cylindrical in shape, 10mm in diameter and 8 mm high. Each has a circular cap on top with a central adjusting screw. Paint or wax (??) has been daubed on the screw and sometimes down the side of the cap.
When the adjusting screw is turned ....... is the top cap supposed to rotate also?? Or only the screw itself??

G'day Ian,

I've seen both happen myself. It may be best to send a picture and I will upload it to your post.

If it's the plasic encapsulated type (photo will confirm) with two screws at the base holding it down. Only the top adjusting screw moves & you should be able to see the top plate move.

The plates are very similar in design to the open type where the upper plates rotate about a central fixed screw.

Marc

Photo added.

What you should see on those is the brass bit you can see at the top (which are the plates of the condenser) spinning in sync with the screw. These were (and still are in modern electronics) sealed with a dab of coloured paint.

Thanks guys,

These trimmer caps are soldered in place. Due to their size 10 - 40pF, I had assumed some type of mica cap.

After I did an RF / Oscillator alignment on the 2 to 6.5MHz band, there is now a lot of noise between stations. The 6 to 15MHz band, which I haven't aligned, is silent between the stations. Whilst the 2 - 6.5MHz band is more sensitive to noise, it produces a higher volume compared with the 6 to 15MHz band ( constant signal generator input).

I was wondering whether I have simply improved the alignment, or improved it too much, or stuffed something in the process eg trimmer caps. I followed the Manufacturers' Alignment procedure, and it does not include any mention of de-tuning, stagger tuning etc..

Is this normal?? Any advice welcomed.

These trimmers were found in many kits published by the electronics magazines right up to the 1990's and are probably still sold today at Jaycar. They are pretty much built like a traditional tuning condenser but they have thermoplastic insulators in place of an air dialectric due to their small size.

Mica trimmers look more like a postage stamp with a screw going through it and come in several sizes.

After studying the photo a bit harder I notice a trimpot at bottom left and it suggests that this receiver isn't very old by comparison. In fact I must admit I've yet to see a radio fitted with that trimpot or the trimcaps you enquire about.

A rare set?

I would only use the sig gen as a reference for the frequency. And that is not always neccessary.

I was lucky today calibrating a bandmaster as WWVH was coming in well. That is a standard frequency and time signal transmitter. I often use it to check if a sig gen is on frequency.

It is better when adjusting the aerial trimmers to have just the aerial attached. Most are built for an antenna of around 25 feet.

That set looks like a TRF or amp preceeding the converter. (1st two gangs) then the osc.) Normally on BC band you set the ant. trimmers around 1500 kHz (2AY abt 1495kHz) and the SW in a similar gang position.

Peak tant ones there on a week station. Frequency may be able to be set from WWVH on 2.5; 5; 10 (30metres),12, 15; MHz depending on what the radio covers.

Those trimmers are still made. Metal screwdrivers are not reccommended for adjusting.

Marc

Thanks Brad and Marc,

The radio is one of my Quad AM 11 tuners, 1960's I think, superhet, no power supply or audio amp. The photo is off a web site, but both of my are in identical condition. Circuit diagram, if interested:-

http://www.saturn-sound.com/images%20-%20cct%20dia/quad%20am11%20-%20overseas%20am%20tuner%20-%20cct%20dia%27.jpg

The trim pot at bottom left is part of a switchable rejection Filter (9kHz for UK and 10kHz for Overseas models).

Re metal screwdrivers - the trimmer caps that I adjusted were immovable with my "filed down knitting needle". Had to use a metal screwdriver initially.

I will try re-tuning on a weak station to see how various settings affect the sensitivity.

I usually recommend silicone spray for easing moving parts as this product does not attract dust the way oil based sprays do however I am not sure about the dialectric qualities of any lubricants and their application in this case may throw out the value of these trimmers quite a bit.