Good morning ALL, at present I am restoring an astor radio model NJ. It has a tapped potentiometer as the volume control, if I wire the pot in as shown in the circuit diagram I am rewarded with a high pitched squeal irrespective of the position of the vol control knob. I have reverted to earthing one end of the control and bypassing the section of the circuit using the tap. This works ok. Can someone explain how a tapped pot works so I can rectify the problem. The circuit is available on Kevin Chants site. Regards bowler

Hello Bowler
The basic idea of a tapped volume control is to increase the bass response as the volume is turned down. This makes it sound less tinny as the volume is turned down. Looking at your circuit it seems that the volume control is incorporated into feedback from the output transformer secondary. This feedback is used to increase sound quality. However if the feedback goes wrong you can get oscillation of the output valve (6V6) resulting in a squeal. So not having direct experience of this model all I can suggest is to carefully investigate all of the components in this area and look for wiring errors. Also, sometimes the layout of the wiring can cause feedback so try and keep the route of the wiring and location of components as original as possible.
Good luck

Thanks STC830. Bowler

Yes the negative feedback goes via the pot tap.
Did you replace or re-wire the output transformer? If so, reverse either the secondary or the primary connections.

Thanks for your help Ian, I had changed the speaker transformer. Taking your advice I reversed the the leads and now have a fully functioning radio. bowler

That issue is very common, it does not matter, if you reverse the primary, or secondary. Both, especially on Philips sets, will normally end badly, as it goes into oscillation.

Sorry for not remembering that solution - have heard of it enough here but haven't come across it myself - just got lucky replacing the output transformer in such sets I guess.

Will have to look out for any such problem changing the tone control which is in the positive feedback circuit of my STC830. The voltage for this feedback is derived from a voltage divider across primary of the output transformer. Too many other issues to attend to on this radio to get to this.

Your not the only one to fall foul of that. I salvaged a Philips 132L from a recycle depot; Some goose had changed the speaker transformer and in the process drilled a hole through the cone, doing enough damage to cause it to pole. The two wires to the secondary were not connected, probably due to no knowledge of what had been done nor, any on how to fix it. So it got flipped, as it is a common method of saving face.

Being a six inch speaker which is not that common, I manged to score a NOS one that fitted a Mitsubishi Magna, and it nicely, despite a higher wattage rating, these are a technically, much more advanced speaker. That only helped the sound.

On a HMV set here I have actually made a "stepper" for a missing tapped volume pot 2M using a multi position switch.

On a HMV set here I have actually made a "stepper" for a missing tapped volume pot 2M using a multi position switch

What a brilliant idea! I guess there is some maths involved to achieve a logarithmic taper, but it wouldn't have to be perfect.

Harold

Some have made a tapped pot from a normal pot by using conductive paint to create the tap
https://www.vintage-radio.net/forum/showthread.php?t=162751
https://antiqueradios.com/forums/viewtopic.php?t=100223

I suppose this would depend on whether the log profile was the same before and after the tap on the original tapped pots.

Actually no maths, the biggest PIB was that like many non-German circuits the info on the circuit was scant; albeit at a cost I could get the paperwork from England.

After scratching through several hundred salvaged pots I ended up with of course none of them. But I did note a multi position programable switch.

So, as I had just made a pot shaft coupler for another job, it dawned on me to get a 2M log pot I spotted and direct couple it to the switch with the new coupler. Then with a digital meter, I recorded the resistance at each step of the switch. Then it was just a case of daisy chaining resistors. I have no idea what the tapping was, however, that may only need the substitution of one resistor for two, to adjust; that's why I went that way.

Marc

Had what I presume to be a feedback problem when my STC830 gave a couple of blats recently: one ended by turn off, the other ended by itself.

I guess this is because my most recent job on the radio has been renovating the cases of the volume and tone control which were very rusty externally. This involved removing them from the chassis, disassembling (difficult, the interference fit with tabs type), kill the rust on the metal caps and spray with silver paint. Also clean the internals with electronic cleaner, relubricate the tracks with electronic spray lubricant and the pick-up tracks for the slider feeds with silicon grease. Then reassemble and reinstall, remaking the shielded cotton covered wiring to the volume control and unshielded wiring to the tone control. These controls done separately and the radio tried successfully after each.

Then renovated the metal shielding cap for the the 6J7 (metal type valve audio preamp for a 6F6 output) to which the shielded wire goes from the volume control. The shield of this wire is connected to the shield cap and the wire to the usual valve cap connector in this case held captive in the shield cap with a piece of bakelized sheet. The cap was cleaned and polished and the shielded wire fitted new black fabric spaghetti as the old was tattered and in pieces. The shielded wire was then resoldered and the radio tried out OK.

The next day while running it for a while when looking at the effect of line voltage on filament voltage, another issue, it got the blats. Tapped around for a while trying to reproduce it without success, but found that the 6J7 was microphonic and replaced it with a good spare 6J7. No blats since, but another possibility is that the shielded wire is old, and battered where it goes through the chassis to the 6J7 cap and gets flexed when off the valve . Have considered replacing this but it runs right across the chassis by an indirect route behind other components and soldered to the chassis every 50mm or so - a good 400mm long. Would be a right coot getting a high wattage soldering iron in for the job, or to patch in a piece.

Have run the radio for many years with various 6J7Gs in the preamplifier spot unshielded apart from the cap shield and it came to me with one in that location. So hardly seems susceptible to oscillation. 6J7 in specified on the valve layout label, and no other shield than the top cap shield is provided.

So theories are in order of decreasing favouritism:

- microphonic 6J7 activated by the speaker which is sitting on the bench top without any enclosure

- some problem with the shielded feed to the 6J7

- some problem with renovated volume control, or renovated tone control which is in the positive feedback loop for the 6F6.

Will just have to wait and see what happens.

A rattle like Blat! can be the speaker poling. The fun thing is that it is often a specific note at a specific amplitude and that makes it hard to ping. This can also be a motorboat that pulls the cone in.

That Motorboat can be slow and the cone can be seen slowly moving in and out. Where there is negative feedback from the output transformer secondary. Oscillation can occur if the phasing is wrong, normally caused by the reversal of the primary wires, or those of the secondary.

My method for proving poling speakers is to use the function generator to supply & sweep audio frequency at specific single frequencies at differing amplitude. The rattle will occur at lower frequencies.

When I hear microphonic's; 6AV6 being a horror often its not the tube but a fault

Marc

Thanks for those thoughts Marcc.
When I say blat I mean continuous loud unmusical noise.. Certainly not slow cone movement - did not notice if the cone was pulled in. It does not feel as if it is poling when the cone is pushed in by hand.

Never made a noise like this before so that is why I think it might be due to the last things that I did if not the microphonic 6J5.

Do 6J5G and GT valves need a shield? Have not been able to find a direct statement of that need. If I want to continue using them (my good steel 6J5 is a little weak) I might need to devise a whole valve shield where none was provided.

Oh, when you say it was a "blat", do you mean a fart?

I had an AWA portable that would do this, after an extensive overhaul. Turned out to be RF feedback from the audio to the antenna. Added 47pf ceramic from the 1S5 anode to ground - that fixed it. Probably due to the new caps being MUCH smaller than the originals, less capacitance to ground.

Does it need a shield? Does it work without it? With GT shaped bottles you can cut one from a tin can and run a short piece of braid to the chassis.