Vintage Japanese HiFi amp restoration issue.

[ms660]

I like the hum antenna capacitors that have been fitted....

Lawrence.

[TimMills]

Originally Posted by GrimJosef

Things are likely to be a bit more complex than that I'm afraid. First, the 'lower' section of the pot will be in parallel with whatever signal source is driving it, so the net impedance will be less (very probably a lot less) than the impedance of that part of the pot. Second, since it's a volume control pot it's likely to have a 'logarithmic' resistance profile. This means that if you rotate it through 75% of its range of travel you won't have 75% of the resistance in the bottom section. The log profile is often approximated by two linear sections. The first of these, up to 50% of the rotation, covers 10% of the resistance and the second, over the remaining 50% of the rotation, covers 90% of the resistance. So at 75% rotation you have the whole of the first section (=100kohm) plus half the upper section (=450kohm) which adds up to 550kohm.

Many thanks for the quick tutorial GJ.
The maths side of electronics does befuddle me somewhat, but what you've outlines makes good sense.

I am possibly going to replace the pot too at some stage, so a better understanding of them is definitely going to help me down the line.

Cheers

Tim

[Studio263]

Originally Posted by TimMills

I am possibly going to replace the pot too at some stage...

Trigger's broom? Soon it will be neither vintage nor Japanese...

[TimMills]

HaHa! A timely warning....
It came with Chinese and British valves and British resistors. It's previous owner had had a right old go at it. electronically it was a mess and completely non functional.
It now has a full compliment of 9 vintage Matsushita and Toshiba valves and is sounding superb.
Blore Edwards supplied me with a 1meg pot recently because I'd had no luck getting cleaner into the existing one and it was very rough, but I'm reluctant to swap it out now because, the new one has no loudness tap and I'm not sure what disconnecting that circuit would do, and also because I've just had another go at cleaning the existing one and it seems much better. Fingers crossed it lasts!

[G8HQP Dave]

Which side of the capacitor has DC is completely irrelevant to this discussion. If you see any website which says otherwise then you can completely ignore any advice from that site.

In almost all cases it doesn't matter which way you connect the capacitor. It only matters when the capacitor value is smallish, stray capacitance is highish, and one side of the circuit has significantly lower impedance than the other. None of these things is true for your circuit, so it doesn't matter. When it matters the capacitor should have its outer foil connected to the lower impedance side, or the input side (which will almost always be the same anyway). In your case the volume pot is the lower impedance side, but the capacitor value is high enough that orientation doesn't matter.

Marking the outer foil is largely used for marketing purposes when selling 'audio capacitors'. People selling components into other less fussy markets (such as medical, military, telecomms, instrumentation) only mark the outer foil when it is relevant.

This reminds me of the battery in an opthamascope, an ingenious design that had positive on both ends and a negative spring on the side. This was done to have charge on the bottom and the lamp on the top without complicated internal wiring. No need for orientation, just slip it in and all is well, customers where asking which way round it went so an (totally unnecessary) arrow was printed on the outside. This stopped enquires.

[Chris55000]

Hi!

Could you tell me the make and model so I can get the full circuit to refer to please?

The practice of marking non–polarised paper, etc., capacitors at one end to show the outer foil is a very, very old one going back to the 1940s, but the idea in those days was to 'earth' the outer–foil connection on all such capacitors was originally because it helped to cut down the risk of r.f. or i.f. instability in early t.r.f. (tuned radio frequency) or superhet receivers.

As other members have said, nowadays it is simply an "audiophool" fad to buy unnecessarily expensive "marked" capacitors in a unit like you have there, paying attention to the exsisting h.t. decoupling and filtering ("smoothing" in British radio terminology) bypassing anode and cathode circuits to earth is far more important!

For your unit, the Vishay Rodestein MKP 1813 series sold by RS, Farnell or Mouser make excellent replacements in nearly all valved amplification equipment!

Note–if you ever come across the beige/mustard–coloured cylindrical type of Mullard/Philips capacitor with black lettering around the body, these are very reliable and very rarely need replacement!

Chris Williams

PS!

The tap on the volume–control is for "loudness" compensation, a feature which helps to correct the bass response at lower volume settings. If you've got a really steady hand, a very fine drill bit and some 1 or 1.5mm screws you might be able to fashion a tapping on the replacement control but I have to confess I've never tried it!