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Old 12th Nov 2017, 3:13 pm   #61
kalee20
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Default Re: Oh No! Not another howler.

Quote:
Originally Posted by Boater Sam View Post
Please see post 47.
Thanks! Actually, this (and other posts) came up while I was preparing my own post 49. It's a lively thread!

Quote:
Originally Posted by Argus25 View Post
Probably most AC41's don't do it, but with a borderline condition such as a subtle amount of tube gas or heater cathode leakage, its enough to trip it off. I agree though, all the usual suspects should be checked first. It might be though that in this set, one of the IF coils was made with accidental reverse polarity, could be just enough to do it in conjunction with other factors.
Really good suggestion! In that case, the thing would always have been howl-prone.

Or could be TATG oscillator action, as Lawrence says.

Boater Sam has made it clear he's a chap with knowledge and experience, and even so, he's stumped so far! It almost sounds like there is borderline design, in which case we need another AC41, as-manufactured, just for comparison!
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Old 13th Nov 2017, 11:40 pm   #62
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Arrow Re: Oh No! Not another howler.

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Originally Posted by Boater Sam View Post
There may be all sorts of reasons for this radio to misbehave. I have satisfied myself that it is not a component that I can change that is at fault: they have all been checked one way or another.
As the 8k2 grid resistor has stopped the oscillation and the set is as sensitive and selective as the 2 others of the same model that I have, it has been moved to the "done" pile. I am not going back to spend more time on a 20 radio that is working fine.
Re: your last paragraph: IMHO, a very sensible conclusion. There are times when sheer pragmatism must dictate and call the shots.

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Originally Posted by Boater Sam View Post
This one is an AC41: possibly the simplest Bush chassis of the period.
With all due respects to Bush Radio (especially of this vintage), it has always seemed to me that their radios were designed and built with cost very much in mind. Which implies that the design and assembly of this set could have been highly marginal. With the passage of many years, many tiny and subtle changes will have occurred - typically of a mechanical nature - all of which could easily be adequate to push the functionality of such a design 'over the edge' at a much later date.

Finally, I admire your patience & perseverance in attempting to find the cause.

Al
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Old 14th Nov 2017, 9:48 am   #63
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Default Re: Oh No! Not another howler.

Thanks, Al.
There was maybe a suggestion in an earlier post from Lawrence that I should continue ad nauseum to find the root of the fault rather than find a solution.
However, I now don't have the time or persistence to do so, fixed is fixed.
In an ideal world I suppose there is always a better solution.
Is the addition of one resistor in any way worse than using a substitute valve of a different number for a hard to find replacement?
I used to be a pain in the butt perfectionist but advancing years and experience has taught me that nothing in this world is perfect, nobody gives a damn so why worry about the little things.
The radio would have been just so much recyclable waste anyway if I had not spent some time and cash on getting it working. It is all a fruitless effort anyway, the generations after me will not show any appreciation for my efforts.
When all the available programs have been withdrawn, these sets are going to be worthless unless we all use illegal pantry transmitters, or carve them up to fit USB ports or Bluetooth. No, they will be consigned to the bin.

The next AC41 is on the bench. Sadly it doesn't howl or motorboat. Its so insensitive that it is hard to hear any transmissions at all. It appears to have been well twiddled, something I avoid doing unless absolutely necessary.
The wiring is in much better condition however, even the bulbs work!
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Old 14th Nov 2017, 12:57 pm   #64
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Default Re: Oh No! Not another howler.

I suspect that you've introduced a design improvement that a Bush engineer would have devised had they known that some samples of the AC41 inclined towards IF instability with (very) long term component drift, perhaps a minor change in IF alignment.

I have a few sets of different makes where the IF amp is only marginally stable on weak signals when the AGC turns it to maximum gain: tuning becomes very
sharp and has that character of a TRF with its reaction set just below howl point. I assume that it's due to stray coupling in the wiring. I tend not to mess with them, but rather marvel at their unusual sensitivity and selectivity, but I can well imagine that a bit of a tweak in the IF alignment might well start up a reaction-type howl.

Typical examples in my collection are one or two 'All American Fives' and a couple of 1950s Smiths Radiomobile car radios. In both cases, high sensitivity for use with a short aerial is a requirement, and the compact layouts probably encouraged a bit of helpful positive
feedback. It would be interesting to know whether any had to be reworked in factory test due to instability.

Martin
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Old 14th Nov 2017, 1:04 pm   #65
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Default Re: Oh No! Not another howler.

That is very plausible Martin, thank you.
This would tie in with the set's behaviour when tuned in to a signal, the AGC stabilised the valve and the howl stopped or reduced substantially.
There are so few components around this IF valve, not a lot to go wrong!
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Old 14th Nov 2017, 7:17 pm   #66
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Arrow Re: Oh No! Not another howler.

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. . . when tuned in to a signal, the AGC stabilized the valve and the howl stopped or reduced substantially.
Correct me if I'm wrong, but the valve that was stabilized by the AGC action is the I.F. amplifier, yes?

In which case that strongly suggests that the fixed -ve bias on that valve is insufficient. That bias is usually set by its cathode resistor, known as 'automatic bias'. (I assume you've checked its Ohmic value). If the decoupling capacitor across that R is leaky, the net resistance will be reduced . . . thus increasing the gain of that stage . . . possibly leading to its instability.

Another possibility is that there is a leak of HT+ from 'something' placing a small +ve bias on that valve's control grid - that 'something' could be due to leakage from the IFT's primary winding to its secondary, albeit an unlikely scenario.

Just a couple of thoughts.

Al.
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Old 14th Nov 2017, 7:26 pm   #67
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Default Re: Oh No! Not another howler.

Good guess Al, but cathode is grounded! Another cost reduction measure. If you have a look at the circuit you will see that there are very few components.
As an aside, the next AC41, lets call it set "B", has odd periods of instability when trying to align the IFs, so you may be correct. But the stopper resistor stays in set "A".
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Old 14th Nov 2017, 7:38 pm   #68
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Arrow Re: Oh No! Not another howler.

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Good guess Al, but cathode is grounded! Another cost reduction measure.
Bit more that a 'cost reduction measure': that's a design failure. The lack of a fixed bias means that when signals from the aerial reach a certain magnitude and the AGC applied to the mixer is overcome, the grid of that I.F. valve will be driven +ve. By adding an R in its control grid, in effect, that is same as introducing a small -ve bias on strong signals.

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Old 14th Nov 2017, 8:09 pm   #69
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Default Re: Oh No! Not another howler.

Some valves were designed to run like that, to be honest. It sounds crummy, and it is... But it keeps things simple.

As for the signal being strong enough to overcome AGC bias, well if it's that strong, it'll generate more AGC! There is a fixed ratio between IF signal voltage, and AGC voltage, determined by the gain in the stage, the IFT ratio, and the detector. Whether this ratio is enough to always guarantee the AGC bias being sufficient to stop the grid going positive on the signal peaks, is another matter.

Adding a bit of R will give a bit of negative bias, as Al says, although here the DC grid-circuit resistance is already high because of the AGC feed. So if grid current is drawn, this is going to increase the bias anyway, and charge up the AGC decoupling capacitors negatively.
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Old 14th Nov 2017, 8:15 pm   #70
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Default Re: Oh No! Not another howler.

Yes, no cathode resistor, the AGC delay operating voltage is taken from the HT via a 20Meg resistor to the anode of one of the diodes in the EBF80, the diode conducts due to that feed and reduces the AGC voltage to a minimum on weaker signals, stronger signals cause the diode to cut off, thus increasing the AGC voltage.

With the HT feed resistor to the AGC delay diode having such a high resistance it probably wouldn't need much leakage from any +ve voltage source to the diodes anode to send the AGC voltage to zero or +ve on weak signals, due to the circuit impedances that might be difficult to measure unless the pot. divider and battery trick is used.

Just dreamin'

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Old 15th Nov 2017, 12:04 am   #71
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Arrow Re: Oh No! Not another howler.

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Originally Posted by kalee20 View Post
As for the signal being strong enough to overcome AGC bias, well if it's that strong, it'll generate more AGC!
I follow your reasoning: it assumes a proportional relationship between signal strength and the magnitude of the AGC. Since in this set, AGC is applied to the mixer only, it makes we wonder if the amount of back-bias that the mixer gets from the AGC when a strong signal is received is adequate to reduce the gain of the mixer-I.F. amp. chain, resulting in the I.F. amp. being over-driven.

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Old 15th Nov 2017, 7:29 am   #72
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Default Re: Oh No! Not another howler.

Nope, it's applied to the IF valve too (although there as confusion earlier on this, but see post 47).

Which also means there's enough DC resistance in the grid circuit to generate extra bias if grid current flows.
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Old 15th Nov 2017, 9:24 am   #73
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Default Re: Oh No! Not another howler.

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Originally Posted by kalee20 View Post

Which also means there's enough DC resistance in the grid circuit to generate extra bias if grid current flows.
Ah, it is now because I have inserted resistance to the grid, before there was none at all.
So perhaps my innovative fix is not so heinous at all.

Set "B" is being difficult as well. As it had been twiddled I set about realigning it first.
The IF transformers will peak on one coil but not the other and there is a heterodyne whistle at one place on the tuning cap.
The MW oscillator coil will not tune to 600kc/s as it should either.
Seems the coils have suffered damp or there are mica caps that have drifted.
Got poor reception on MW and LW but it needs work, its also not loud.
Checked all the valves by substitution, HT is well up to the mark.
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Old 15th Nov 2017, 10:10 am   #74
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Default Re: Oh No! Not another howler.

Unless I'm wrong I think the 8.2k resistor you fitted in the grid circuit is just the top half of a potential divider, the bottom half being the valve Cg-k and any stray.

Lawrence.

Last edited by ms660; 15th Nov 2017 at 10:15 am. Reason: addition
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Old 15th Nov 2017, 12:36 pm   #75
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Arrow Re: Oh No! Not another howler.

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Nope, it's [the AGC] applied to the IF valve too (although there as confusion earlier on this, but see post 47).
Really? I see. In that case, most of my recent arguments and theorizing are erroneous.

Al.
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Old 16th Nov 2017, 1:05 pm   #76
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Default Re: Oh No! Not another howler.

The plot thickens.
Set "A" howler, now working fine with its g1 IF amp stopper resistor which has been suggested is a bodge fix.
By shuffling the manufacturers sheets today fixing the other (B) set, I mistakenly tried to follow the circuit for the DAC41, the similar AC/DC set.
And guess what? The V2, IF amp has a 2k2 grid stopper as made!
So I feel vindicated, if Bush found the need in the DAC41, the stability of the IF amp must of been marginal.

Now set "B" is working, not well but its progress.
The 1st IF transformer would not peak on the secondary, so I removed it and opened it up. This involves winding the adjuster fully in to get it out of the can, so it will need realigning fully.
The padder cap, a mica 100pf, had broken off at one end. This is possibly the original fault that the twiddler tried to fix, even a manufacturing fault.
Resoldered and reassembled it now peaks properly.
Unfortunately the twiddler hs wrecked several of the oscillator and aerial coil cores. I have managed to remove and reverse some of them but not all. I will try soaking them in acetone to loosen them.
But the point has been proven and I may just put it back on the "may do" pile. The set is not in good condition, some woodworm, no back, and scratches. It may well get sold on as is.
Besides I'm bored with Bush short superhets with less than good performance, I go away in 6 days and need to pack the workshop up for the winter. And its getting cold and damp here.
These 2 sets have taken an inordinate amount of time to get working and are not anything special, performance is not the best and they are obviously made very much to a low budget.
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Old 16th Nov 2017, 1:56 pm   #77
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Default Re: Oh No! Not another howler.

If you want a low-price short superhet may I suggest Ferguson 208U?

UCH41, UAF42, UL41, UY41. 3 wavebands. Very easy layout to work on.

Cut-price maybe - but the oscillator and RF tuning are almost no-expenses-spared, separate coils for each band; variable inductance AND trimmer capacitance for optimum tracking. Makes it an astonishingly good performer.
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Old 16th Nov 2017, 2:05 pm   #78
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Default Re: Oh No! Not another howler.

Thank you for the recommendation.
But, no, I have no desires for a short superhet and AC/DC sets I dislike, too much like TVs which is where I came in 55 years ago!
My current weaknesses are prewar or early VHF sets but then I have too many already.
The sets like these Bush will get culled next year, anyone fancy any?
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Old 16th Nov 2017, 7:04 pm   #79
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Default Re: Oh No! Not another howler.

Well, well, an official bodge. Just goes to show that practice doesn't always behave nicely and follow the theory.
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Old 16th Nov 2017, 7:38 pm   #80
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Default Re: Oh No! Not another howler.

Various Bush radio circuits have a grid stopper for the IF amp valve, but some like the DAC31 don't (according to the Trader sheet) even though it uses the UCH42 and the UBF80 like the DAC41, interestingly though it uses an anode tap on the 2nd IF transformer, the rest of the circuit and components are more or less the same as the DAC41.

The AC31 also has no grid stopper but has the anode tapped 2nd IF transformer.

Lawrence.
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