Odd problem with Bush AC11, got me thinking.

[boater sam]

               

I have a few Bush AC11 sets lying around in fairly tidy order and thought I knew all there was to know about them. Wrong!
Decided to have a quick fix of one before I pack up for the summer, if we get one.
Nice clean chassis, original valves set, no signs of ever having been repaired.
I love the way these chassis come out, off with 4 push on knobs,2 plugs take care of speaker, dial lamps and frame aerial, 2 screws to release the cabinet and the fabulous wedge in dial string idea makes it a doddle. No way similar to a Philips!

Set works on all wavebands after a fashion, the usual UL41 gargling when asked to work a bit harder but there is a suddenness about the tuning, almost like an FM set with a bad discriminator capacitor. Very difficult to accurately tune in stations at the high end of the medium wave and they seem to drift away, needing tuning correction.

Quick voltage check reveals low HT with a burnt looking smoothing resistor between the electrolytic reservoir and smoother, and a slightly leaky waxy coupling cap on the UL41 grid as usual.
Change both parts and the HT is about right, but still odd on the tuning and then I noticed that as the tuning faded or drifted off the HT dropped a bit, only 4 to 8 volts or so.  I would of expected it to rise as the audio output fell, not the reverse.

While the soldering iron was hot I remover the connections to the UL41 pin 4 which Bush used as a tag for the signal grid stopper and coupling cap. Its a naughty connection and can lead to valve problems, Mullard designated this pin as an internal connection not as no connection and I believe some valves actually have an electrode connected to this pin.
Its the right hand of the 2 red wires on the valve base in the centre of the component picture, the 0.01uf coupling cap is above it on the tag board and the grid stopper resistor is between pin 4 and pin 6 on the chassis. The cooked 10k resistor is the large jasper far right.
   


So what causes fading audio output and difficulties on finding the best tuning point? What can alter the working conditions of the valves so much that the HT current rises and falls?

Ah. Automatic Volume Control? Maybe so, its a starting point anyway. Shorting down the AVC line to chassis stabilized  the HT.
On these sets there are effectively 2  AVC line decoupling caps, a 0.1uf at the frequency changer end and a 0.05uf at the IF valve. Both were a bit leaky. Changing them resolved the drift and fade and made tuning a lot easier.

Now this is a new fault experience to me, I would not of thought that the AVC would have such an effect on reception or on the DC working conditions of the valves.
We live and learn.

[ppppenguin]

The AVC alters the DC conditions of alll the IF/RF valves to which it's connected so there will be a change in HT current as the AVC voltage changes. I'm a little surprised it can swing the HT by 8V.

Unless you are sure they are in positions where a bit of leakage won't hurt it's easier just to change all the waxies.

[Radio Fixer]

Must be > than 10M leakage not to cause problems. I always change them anyway.

Gary

[boater sam]

I am of a mind to replace as few capacitors as possible these days. The exceptions are the dreaded crumbling Hunts and any wax ones that test as leaky.
Many caps are in places where mild leakage is not an issue and I prefer to maintain originality as far as possible, to the extent of re-stuffing.
In this set there are several wax caps that are still serviceable and in positions which are not critical, in the end its a personal choice but I cannot agree with a blanket change policy.

[Joe]

My heart agrees with you, Sam, but my head doesn't.

Are you testing the caps for leakage at the voltages and temperatures to which they will be subjected? I've come across quite a lot that seem to test OK yet changing them brings about a great improvement in performance.

As you say, it's a personal choice and good luck with it either way.

— Joe

[Old Sparky]

If I may add my two pennorth, I always regard waxed paper caps ( and those 'orrible 'unts!!)' as 'replace on sight' in any valve equipment. I metaphorically wear my service engineer's hat - i.e. when I was a service engineer, if a new or improved component became available to replace an item with known issues, we always replaced the old part with the later version. This I still do today. and have very seldom restuffed old capacitors, except on the very few occasions when I've repaired a particularly rare radio.

[pwdrive]

Nothing unusual with leaking wax capacitors upsetting the AGC rail, the leakages can be cumulative, most vari mu valves have a high slope, doesn't take too much bias voltage change to change the gain.

Lawrence.

[boater sam]

Everyone to their own ideas. I agree for long term use, bringing a set up to date with new improved parts is a grand idea. But it removes all originality and where do you draw the line? Its the road sweepers broom again. There is also a cost implication, I get through a ridiculous amount of parts in a year and even with circumspect buying on ebay from the Far East its expensive.
I like to look at a rare old set after repair and be unable to tell it has been touched. Or if its a common set, be able to see a neat and tidy repair.
I did say that this one was a fast fix exercise, it took less than an hour from start to finish. The  three replacement capacitors are obviously new parts, and the one 5W resistor was a reclaimed part, but all fitted as the originals were.

To get back on topic, the thread seems to have been hijacked into a discussion on preferences, has anyone had a similar experience where the HT voltage decreased with less output, or indeed any explanation of why this could occur?

[pwdrive]

I suppose it depends where the HT voltage is measured from eg: pre HT filter or post HT filter, the DC current draw by the output stage should be reasonably constant if it's operating in Class A, after the HT filter most of the valves will be under AGC control so the HT at that point could vary as the bias due to AGC action is varied on those valves.

A good description for AGC design/circuits can be found in RDH4 here: http://www.paleoelectronics.com/RDH4/CHAPTR27.PDF

Lawrence.

[pwdrive]

Just an addition to post#9:

Suppose the HT line feeding the mixer and IF valves is 110 volts and that voltage is derived from the main HT via a 10k filter resistor and that the combined current draw of those two valves is 11mA, an increase of 2mA current draw will cause the voltage to drop by 20 volts.

If the HT at the rectifiers cathode is poorly regulated for whatever reason then that voltage can change also.

Lawrence.