invicta: Invicta29 and all sets with DAF96, DL96

I recently restored an Invicta 29 (A Pye Chassis same as Invicta 28, PAM 712, PAM716 and Pye Jewel Case with PCB version).

The question I asked people was where should the sceen can be?

I have seen both photographs

Screen on DAF96 (possibly wrong)
(RMorg Pye Jewel Case 1955)

and also

Screen on DL96 (probably right)
(Invicta29, also Pye)

Notice the 1955 and 1957 chassis are identical. Also I have seen in various photographs of the similar chassis the screen on DL96 or DAF96.

Well the DAF96 is sensitive and renowned for "Microphony", so I thought also since the RMorg photo showed it, that the DAF96 was correct location.

But I noticed that for a about 1/3rd of the mid range of the volume the Radio would often "squeal", always when "cold" and less so after an hour or two.

So I swapped screen shield (which has a soldered earth wire) to the DL96 output pentode and from cold the problem of "feedback squeal" at certain volume levels is cured.

 

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Hello Michael,

It isn't clear from the documentation for this and the similar PYE models which valve was originally fitted with a screen can.  All B7G miniature battery valves tend to be microphonic but in my experience with valve portables with this series of valves it is the DAF96 which is most likely to squeal and was fitted with a screen can, indeed I tried three of them in a PYE portable recently and they were all microphonic. However I suggest that you fit the screen can on whichever valve squeals first !

Regards ....... Howard

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Thanks for the comment. My money too would have been on the DAF96, I had moved it to the DAF96 when fixing the radio and moving it back cured the problem. Really it's capacitive pickup I think, not microphonic as at higher volume it was OK.

Perhaps all the valves should have had shields and as a cost reduction they only put it on DAF96 or DL96 depending which worked better!

On my model the c22 had been replaced by RS part maybe in 1970s but only a small part of band worked due to damage of the tuning capacitor. I used very broad tweezers and meter for resistance with the unit removed from chassis. I was surprised that I was able to fix it. I also loosened the tension grub at rear and used that to move the plates back and forth too and eventually locking it in best compromise of position and stiffness to turn.

All the remaining paper waxed capacitors were definately faulty. Gain was very erratic as one is on AGC line till I replaced them.

 

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One possible explanation for the squeal with the volume near a central position and for the useful shielding effect on the DL96, is that there is stray feedback from the DL96 plate to the volume control wiper and it's wiring, including the coupling capacitor C20, the grid bias resistor R8 and the control grid of the DAF96.

The inductive load of the DL96 and the two stages of amplification agravate the conditions for instability between the DL96 plate and the DAF96 grid.

At mid-volume, the wiper impedance is highest, thus it is the most sensitive position for stray feedback.

In a volume type of potentiometer, the resistance characteristic has a logarithmic taper. The highest impedance point should not correspond with a 50% knob position.

Dressing the volume control wiper wiring close to grounded metal parts may also cure the problem.

A common trick to shield the wiper wire is to wrap solid wire around it in spiral and ground one end of this wire as a shield.

-Joe

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Look at mine, probaly never tempered with, 

PYE P114BQ

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• shield not tempered with (257 KB)

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