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Old 7th Nov 2017, 10:35 pm   #21
Boater Sam
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Default Re: Oh No! Not another howler.

There is no approach to howling really, it seems takes off as soon as there is enough signal into the EL41 to amplify and hear, I suspect that it is oscillating all the time really.
The various extra decouplers were grounded on an assortment of chassis grounds, there are around 7 or 8 around the valve bases. One is on the electrolytic can clamp mounting, and it is the neg for the caps too, the can is not isolated, not a good idea, but the bolts are tight and clean
I will try monitoring the AGC with the EL41 out, that is a good idea I had not thought of.
The reason I think its the IF amp regenerating is that when the wavechange switch is in gram, the RF valves are positively disconnected because the oscillator becomes 1st AF stage, and then there is no trace of oscillation anywhere yet the HT is still being applied the the frequency changer and IF amp.
There is so little in this chassis that Its improbable that its the wiring or layout, and as I said its all virgin.
With it in oscillation I could still receive the stronger stations but the audio was heavily modulated, but the motorboating slight reduced until it went off tune.
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Old 8th Nov 2017, 12:35 am   #22
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Default Re: Oh No! Not another howler.

Have you checked the valve socket pins. If the screen one is cracked and the others are all right that would cause a lot of unwanted noise on an otherwise working set.
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Old 8th Nov 2017, 10:20 am   #23
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Default Re: Oh No! Not another howler.

Now, that is a good point, the B9A valve holder is one of those nasty paxolin ones and I haven't checked. That could be why the screen voltages are a bit high too. Thank you.
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Old 8th Nov 2017, 2:22 pm   #24
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Arrow Re: Oh No! Not another howler.

Is that B9A valve socket one of those that uses split pins? If so, change it! I've had no end of trouble with that type and always replace them on sight with the 'wrap around the valve pin' type.

Al.
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Old 8th Nov 2017, 2:30 pm   #25
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Default Re: Oh No! Not another howler.

Hi Al, I think it is worse than that, its the flat paxolin one made in 2 layers with the flat brass tags in between. Seems firm enough on the valve but I will investigate.
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Old 8th Nov 2017, 4:33 pm   #26
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Default Re: Oh No! Not another howler.

Hi Sam - ah yes! I know the ones you mean. I haven't come across those for ages & ages - but they do have the appearance of "How cheaply can we make a B9A valve socket?"
So yes - I'd change it. Not a lot of effort so to do - and besides, it eliminates another variable.

Quote:
Originally Posted by Boater Sam View Post
Seems firm enough on the valve but I will investigate.
Nine pins: if only one valve pin is loose in its socket, physically, you wouldn't notice the difference.

Al.
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Last edited by Skywave; 8th Nov 2017 at 4:36 pm. Reason: Add quote.
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Old 8th Nov 2017, 7:28 pm   #27
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Default Re: Oh No! Not another howler.

Taking all the previous comments on board, had a thorough dig into this set this afternoon.
Removed and tested every valve, cleaned all the pins, cleaned and inspected the sockets and checked that the IF amp valve socket had a grip on every pin.
Checked all valve voltages, close enough to spec on the Avo 8.
Released and re-tightened all ground tag bolts and nuts, went over soldered joints with a magnifying glass.
Shorted out the extra grid stopper on the IF valve, the howling immediately started as before. Dabbed decoupling 0.1uf caps all over, even cutting out the signal, the crazy thing still howls.
Set it up with a tuner running into the gram sockets, clean audio at good volume.
Scoped the anodes to find that the EBF80 is oscillating whenever it had HT no matter what I did with the volume control or the wavechange switch or decouplers.
Rechecked all components around the first 2 valves, checked the IF transformers had not been twiddled. I can find nothing faulty in this set.

So, like it or not, this set is finished and the 8k2 grid stopper is staying in as it now works just fine, even short wave is working really well.

But the story goes on. I have just dug out another AC41 from the shrubbery to compare the chassis, and this one is oscillating too! The symptoms are very similar though not so severe.

We shall see how this one responds, next week. Watch this space please.
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Old 8th Nov 2017, 7:32 pm   #28
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Default Re: Oh No! Not another howler.

There are times like this when you can sometimes feel you need to employ the services of an Exorcist rather than a radio-technician.

Yes it can be very frustrating but the fact that you've found a second radio of the same type with the same problem can perhaps give you some sense of satisfaction that it's not something your restoration/refurb work has caused!
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Old 8th Nov 2017, 7:50 pm   #29
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Default Re: Oh No! Not another howler.

Just clear a good point made by Lawrence earlier, with my poor aerial and tuned to Radio 4 which is good here, the AGC is over 4v, fluctuating with program content, which would seem to be adequate.
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Old 8th Nov 2017, 11:08 pm   #30
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Default Re: Oh No! Not another howler.

Could it possibly be the paxolin the valve holder is made of has become conductive? - I have just started looking at a GEC BC4750 that uses these types and with the X61M valve out you can see a small area of the valve holder glowing/sparking!
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Old 9th Nov 2017, 12:53 am   #31
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Question Re: Oh No! Not another howler.

Quote:
Originally Posted by Boater Sam View Post
I have just dug out another AC41 from the shrubbery to compare the [two] chassis and this one is oscillating too! The symptoms are very similar though not so severe.
Quote:
Originally Posted by Boater Sam View Post
Just [to] clear a good point made by Lawrence earlier, with my poor aerial . . . (etc).
The fact that you have two sets that are unstable has caused me to ponder: is there a common link there? (Apart from you, that is! ). You say "my poor aerial". So presumably you are using what is colloquially termed a 'long wire'. Could it be that the lead-in from that aerial is draped across the bench and possibly the set, thus acting as a coupling medium for the various stages in the set? I do realise that that is a really long shot, but up to now you seemed to have explored the underneath of every stone . . . . .

I detect that you and I am similar in one respect: a passionate hatred of bizarre, unsolved mysteries in mis-behaving R.F. circuits.

Al.
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Old 9th Nov 2017, 1:04 am   #32
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Arrow Re: Oh No! Not another howler.

Quote:
Originally Posted by Boater Sam View Post
Scoped the anodes to find that the EBF80 is oscillating whenever it had HT, no matter what I did with the volume control nor the wave-change switch, nor the decouplers.
Not wishing to insult your intelligence nor your diligence, but you have tried a replacement EBF80, yes? Moreover, recently I had a fault with a Bush radio that turned out to be a defective ECH81. O.K., I was using second-hand 'pulls' from my valve store, but it wasn't until I fitted the third replacement that the fault was eventually cleared!

Al.
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Old 9th Nov 2017, 10:03 am   #33
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Default Re: Oh No! Not another howler.

Hi Al.
The valve bases are good, the paxolin one is spotless, the others are ceramic and clean. I had considered anode to grid conductance but can find no tracking and the pins are not close. I had tried more than one replacement 'pull' EBF80, the howl was exactly the same. There was no switch cleaner used on the bases, I have found that this can cause tracking particularly if it has not fully evaporated before applying power.
The fact that I could not cause any alteration in the howl no matter what I did lead me to consider that the feedback had to be across one stage, dropping the mains voltage with the lamp limiter was the only thing that changed the pitch.

The "aerial" leaves a lot to be desired, originally it was a bit of wire up from over the bench to the floor joists above, draped across the room at right angles to the bench, about 4m of horizontal. The workshop is the old corn mill room with the loft above, very old.
The ceiling got underdrawn with OSB, hiding the wire. It has had another down tail twisted on to the workshop set, far from ideal, but it has never caused any pickup problem before. The racket on the mains from the fluorescent light, the lights in adjacent units and all the milking parlour computers and pumps is bad but to run a wire outside the other way would be impossible, the silage trailers that use the track are over 14ft tall, more when they drive out with the rams up.

I am looking forward to the next AC41, if its the same problem I would be amazed, Bush must of sold thousands of these.

Thanks everyone for the input, much appreciated.
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Old 9th Nov 2017, 10:06 pm   #34
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Default Re: Oh No! Not another howler.

I understand that with diode-pentode and double diode-pentode valves like the EBF80, when used as an IF amplifier and demodulator, neutralization was desirable because of the feedback pathway due to capacitance between the demodulator diode anode and the pentode control grid. There was a discussion of this topic, with circuit details, in AWV Radionics #118 of 1946 March-April, page 33ff, available here: https://frank.pocnet.net/other/AWV_Radiotronics/.

If the receiver at interest has such a neutralization circuit, then perhaps something is astray there. If it does not (e.g. omitted for cost reasons), then one suspects that oscillation-free operation might be somewhat “statistical”, dependent upon component tolerances in summation not leaning too far away from “normal”. Were this feedback path the cause of oscillation, then perhaps it could be checked by temporarily shorting the diode anode to earth, which I think would stop the oscillation (although then there would be no signal getting to the AF section).


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Old 9th Nov 2017, 10:39 pm   #35
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Default Re: Oh No! Not another howler.

But if you short the diode anode to earth, you're also putting a pretty good IF short across the final IFT secondary, which somehow I think would stop any oscillation!
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Old 9th Nov 2017, 10:56 pm   #36
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Default Re: Oh No! Not another howler.

Ah yes - a senior moment there!. So oscillation caused by the diode anode-to-pentode grid feedback pathway might be hard to prove.


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Old 10th Nov 2017, 12:52 am   #37
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Default Re: Oh No! Not another howler.

Diode to pentode grid is probably the pathway that this set suffered from, it oscillated in isolation whenever there was HT.
There is no neutralisation that I can recognise, my grid stopper works perfectly.
How would you neutralise this stage in the conventional sense?
I had capacitors all round it, the only time the howl stopped is when I killed the signal on the detector diode or the pentode anode as there was nothing going to the audio amp. So that was not the right way!
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Old 10th Nov 2017, 1:31 am   #38
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Arrow Re: Oh No! Not another howler.

Neutralization: this might help as a 'starter' . . . .
http://www.vias.org/basicradio/basic_radio_28_04.html

Although that reference primarily refers to R.F., the essential principles apply at A.F., too.

Al.
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Old 10th Nov 2017, 1:38 am   #39
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Default Re: Oh No! Not another howler.

As a test you could disconnect the diode and fit a silicon one in its place if you want to see if there is audio during the test.
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Old 10th Nov 2017, 1:45 am   #40
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Arrow Re: Oh No! Not another howler.

Quote:
Originally Posted by Boater Sam View Post
Diode to pentode grid is probably the pathway that this set suffered from: it oscillated in isolation whenever there was HT.
Could the HT+ rail be a little too high?

I've also had I.F. amplifiers go unstable if their tuning was incorrect, especially if that tuning is substantially 'off-tune'. The way to tackle that is to damp the primaries and secondaries of all the transformers with something like 47 kΩ resistors across every winding: then tune them for correct freq., then remove those Rs and then make very slight adjustments to their tuning, as required.

The secondary winding of the last I.F.T sometimes has one wire to the detector diode, the other to the AGC diode. If those wires are transposed, instability can result.

A defective AGC line can cause similar trouble, since it is usual for an AGC voltage to be applied to the control grids of the I.F. valves.

Al.
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