Even if it doesn't seem to recreate the original friction function, that saves the dial string if you
tune up to the end of the dial at full speed, it's still a nice alternative. 👍

Martin

Interesting hack!

Krolroger wrote:

You will need two 0,5 mod gears, one of 20 teeth, and the other of 45 teeth.  RS Components (in the UK) do them and the part numbers are 5217023 and 5217124.  They are made of Delrin.

The bore of the larger gear will have to be drilled out to 6mm to accommodate the axle.  For accuracy, I would recommend that this is done on a lathe, not a drill press. The smaller gear is a press fit on the shaft.

🙂 A lathe? Who's got a mini-lathe standing around as a hobbyist?

A good drill press should be fine as long as the workpiece remains movable so that the drill can center itself, don't you think?
Is the big gear a press fit too? If so, I'd enlarge the hole with a 5.8mm drill and see how that works. Heating the gear to some 80 or 100C might help with mounting, since Delrin has a thermal expansion coefficient good ten times bigger than that of steel or brass.

Let us know your actual process, incl. successes and fails. Thanks!

I tried this on a drill press with not such a great result.  Too much run out on my Chinese drill spindle, I guess.  It might be fine if you have a Meddings or Fobco or similar quality kit.

I don't have a lathe either so I put a call out on a model engineering forum and was swamped with offers of help from good people who wouldn't even accept a bottle of wine for their trouble.

You need to remove the flywheel and assemble from the back.  Both gears press on.

Getting the dial cord on was a bit of a faff.  The manual calls for a string length of 160cm to be wound round the capstan and the various pulleys and magically knotted by fingers more nimble than mine with the knot in precisely the right place.  Or you could pre-tie the knot but then it feels too tight to slip over the rim of the capstan.  The vintage radio forums are full of arcane information on dial cord knots, if that is your thing.

The solution I adopted was to use a spring to tension the cord.  It's not banjo-tight as I suspect it was intended to be, but it shows no signs of slippage.  The knots are superglued.

I fitted a new mains flex.  The tuner front end can be unscrewed and moved out of the way which makes just enough room to get a soldering iron onto the switch.  There is no shrouding on the switch (and indeed elsewhere) to prevent inadvertent contact with line voltage, but I guess that was normal for 1969.

Krolroger wrote:

I tried this on a drill press with not such a great result.  Too much run out on my Chinese drill spindle, I guess.  It might be fine if you have a Meddings or Fobco or similar quality kit.

Granted, a damaged drill press may cause some problems, but any standard product - even a no-name made-in-China low-cost type - should be fine. The average accuracy you get on a lathe and on a drill press should be around +/-0.05mm. Add another 0.05mm for the drill and you'll see that you need to go for a 5.9 or 5.8mm drill to get a 6mm hole. For a real engineering fit you normally need a reamer anyways, i.e. you drill a 5.8mm hole and then you use the appropriate reamer to get a transition fit or press fit as needed.

Moving on to the pre-amp, I am getting a lot of noise on the right channel.  Noise on the tape input is not a problem so I am assuming a fault somewhere on the pre-amp.  The noise is a crackle and whine and hiss, evident at high volume levels.  The left channel has some slight hiss only.

I replaced the both BC114 transistors on the right channel which I believe can be noisy and that has lessened the hiss component.  On the circuit diagram, four of the resistors on each channel are marked Beyschlag, which I take to be a specification for low noise resistors.  Though they measured fine, I have replaced them with 0,6W metal film just in case they were contributing to the problem.

I haven't touched the caps yet.  Could the tants or styroflex caps be contributing to the whine/crackle?  

Hoping to solve this without digging out the oscilloscope.

Simon

Yes, tantals can become noisy, but it could also be something else, - a contact failure perhaps.
I wouldn't suspect the styroflex caps. They are usually incredibly reliable.

Well, I have gone through the phono pre-amp board and I still can't find the source of the whine on the right channel, a bit like a boiling kettle whistle, though other white noise has been lessened with the replacement of the low noise resistors with similar.  I've changed the two right channel BC114 and the 2,2uF tant.

I'm assuming this is some sort of oscillation.  When I press the Mono switch it lowers the pitch.  When I release the Mono switch, sometimes the oscillation has disappeared - until I depress Mono again.  The whistle is more apparent at volume levels below maximum.  It is attenuated/increased by the treble control.

The fault is not apparent on tape input which means it must be prior to the first amplification stage.

Taking Martin's advice of possible contact failure, I have reflowed any solder points I can get to round the Phono and Mono switches and the preamp board contacts, and cleaned (as far as possible) both switches with Kontakt 60 - without improvement.

Next step advice gratefully received.

Simon

Are the power supply filter cap(s) good?
And you didn't you fit low-ESR caps anywhere in the output stage or power supply, right?

Martin