Bill's Quad woes cont.

[Bufo Bill]

I see, what I meant was that whatever is happening seems to destroy the valve, with sound slowly disappearing until the valve no longer works at all (the effect is cumulative). I can try the old one again to check if my description sounds unlikely?
Cheers from Bill.

[Bufo Bill]

I will try for some better pictures tonight.
Bill.

[Bufo Bill]

You can hopefully distinguish the subject of these pictures from my bare backside, a characteristic somewhat lacking in my previous efforts.
Bill.

[PJL]

It looks untouched with Hunts capacitors that leak badly and carbon resistors whose value drifts. It really needs a full restoration to get it to reliable working order, replacing capacitors and checking resistor values.

Putting the old valve back and checking the voltages on the valves when the fault is present might allow us to identify the current culprit but it is not going to be working properly like this.

[vidjoman]

Looks like a Rifa smoke bomb has replaced the original Dubillier suppressor on the mains switch. Remove it or replace with a new one before it goes off.

[Bufo Bill]

Thanks guys. Do you think I can manage this myself or should I get someone more experienced in? As an example of my lack of knowledge, I am not sure what type of capacitors I should replace these with, I know to get a similar value, but ceramic, tantalum, electrolytic etc bewilders me a bit. I would love to have a go, just not sure if I'm up to it.
Cheers from Bill.

[Techman]

I agree that unit needs a complete overhaul.

There's lots of information on this forum on types of capacitors and how to do various repairs if you're prepared to take the time out to do a bit of reading and study.

Everything you ever needed to know is right here at your fingertips - it's just down to you to do the 'leg work', if not, then send it to someone else to fix is my advice.

[Bufo Bill]

Cheers Techman, will get some reading in!
Bill.

[Techman]

Just to get you started, these are the type of capacitors ideal for general replacement, but others are available from other places. Capacitors used as suppressors across the mains are 'special' types and are listed elsewhere.
https://www.cricklewoodelectronics.c...-to-1000V.html

[Radio Wrangler]

Capacitors across the mains and from mains to earth are indeed special. Mostly they are metallised paper.

There are transients every now and then on the mains. Small ones quite frequently, bery large ones are rarer. Essentially, no capacitor has enough voltage rating to stand everything which could hit it, and still have enough capacitance to be useful. The advantage of paper is that it burns back from where high voltage has punctured it, and the metal coating melts back from the hole. So the capacitor takes a hit it can't handle, but survives without going short-circuit. With time its capacitance drops as successive transients each zap away a bit more of its area.

The golden Rifa capacitors are actually very good at this. Their problem is the outer case crazes over time and allows moisture in, the resi is history... and smelly.

Aluminium electrolytic capacitors use aluminium oxide as the dielectric. By the cunning technique of using an aluminium foil as one electrode, and matting its surface to a huge increase in surface area considering the foil's outer dimensions, aluminium oxide is formed on its surface. The oxide follows the microscopically craggy surface intimately the oxise is also rather thin. The problem is how to make another electrode that is equally intimate to the top side of the oxide. The answer was to use a conductive liquid. And to connect to the liquid, a foil of naked aluminium goes over the top. To stop the foil getting too close to the other, a sheet of insulating separator goes in the liquid. A second, wet, separator goes on top of the whole thing and it's rolled up. This way both sides of both foils are useful.

They are assembles with etched (textured) foil in both places, with natural oxide from air contact on both.

Once wet, rolled and canned, they are 'formed' by electrolysis, THe oxide goes from one foil, leaving it naked, while oxide builds on the other.

Problems: The conductive liquid is water-based and water escapes as vapour, leaking past seals. They dry out and eventually fail. The conductive liquid isn't good enough, it contributes losses as series resistance. They are polarised. They need DC bias on the the right way round to stop electrolysis reversing. If that isn't enough they can't be made to very accurate values.

Advantages: They don't half pack in a lot of microfarads for a given voltage and volume.


Tantalum electrolytics are blocks of sintered tantalum dust as one electrode. They are processed to grow Tantalum Pentoxide on the huge 3-dimensional surface of the particles. Again a liquid electrode is needse to get into the interstices and form the other electrode. Once again high capacitance density, lower ESR than comparable aluminium types. Polarised. Prone to dramatic failure. Easily triggered by large pulse currents. They have low stray inductance which helps at high frequencies. Tantalun dust prices are scary. Avoid like the plague!

Paper and metallised paper are fine if moisture is kept out. Wound construction gives inductance, though. Degrade in storage just as fast as in use.

Plastic film many different plastics, superb insulation can suffer if water gets in.
Polystyrene does an irreversible value shift at about 70C
Polyester ones exhibit "Dielectric Absorption" sometimes called "soakage" Charge one up. Discharge it with a short circuit, remove the short after all current has stopped and the damned thing charges itself back part way up! Not good for timing circuits and ADCs.

Ceramic Many different materials.

NP0 and C0G are low tempco parts for RF use. Good accuracy, stable, values up to about 1000pF

X7R are rougher parts, more drifty, more capacitance per unit volume. significant temperature drift but good decouplers.

There are more extreme ceramics, fitting more uF into the space but beware of terrible temperature drift, their values even change with voltage, and to crown it all, they are as microphonic as hell. Decouplers for the desperate.

Oh

Oscon These are aluminium electrolytics with conductive polymers instead of the water based liquid.

So electrolytics in PSUs, cathode decouplers in output stages.

Plastic film for signal paths, tone controls etc

NP0 for any smaller values as an alternative to the older mica RF types

That should set you going!

David

[Bufo Bill]

Wow, thanks guys! David, thank you so much for writing all that out for me, that will be invaluable, thanks so much for your patience.
Cheers from Bill.

[Bufo Bill]

Techman that's a great resource, many thanks.
Cheers from Bill.

[samjmann]

Any chance of putting David's post in the sticky section? I know I'll refer to it again!

Thanks, SJM.

[Ted Kendall]

Good idea.

[Bufo Bill]

Hi all, changed most badly drifting resistors today, but came up against one inside the canister containing the two linked coils marked on the schematic. I measured the resistance between the two points marked on the instruction manual without opening the can itself and it registers as drifting high. My questions are as follows: do I get an accurate reading on this resistor from the contacts on the exterior of the can; also, I am a bit worried about what lives inside the can, delicate easily damaged parts or is it easy to get to the resistor inside?
Will post my readings and other useful info shortly.
Cheers from Bill.

[Bufo Bill]

Hi, the coils are marked L1 and L2 on the schematic, the resistor in question is R37, specified as 15k but registers as 18k on my meter from the exterior connections. If further clarification is required please let me know.
Bill.

[Radio Wrangler]

well, you've got connections going right to the resistor, so you haven't got any coil resistance in that 18k. It's not terribly far out, and it will only cause a small shift in the curve of the filter. It won't have any effect with the filter out, and with the filter in you adjust the slope pot by ear until you like the result, so maybe the knob winds up in a slightly different position. The perfectionist in me says why not get it right. The pragmatist asks who will notice. Can you live with knowing?

Can you open the can without damaging what's inside AND can you put it back together unblemished?

OK, the perfectionist in me would machine a new can out of solid or go looking for one of those old aluminium screw-top pill canisters.

David

[Bufo Bill]

Thanks David, the voice of reason as always. There is a screw holding the can in place, but I have visions of coils of hair fine wire and breaking there of. I think I'll leave it be.
Cheers from Bill.

[Bufo Bill]

Hi all, looking at my notes again and think I may have found part of the valve problem. Back when I was taking readings off the valve pins, I had trouble getting a reading off pin nine which is grid 1. When I measured the resistors, R22 should be 1.5 MOhm, but measures over 2MOhm.
To illustrate, I have included a snippet of the schematic, I hope this is okay, feel free to delete if you need to mods.
I still plan on replacing the caps, do you think this is a sound idea?
Cheers from Bill.

[Herald1360]

Might help if R22 featured in the snippet!