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Old 6th Sep 2018, 10:29 pm   #1
crestavega
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Default 40841 equiv?

repairing an eddystone statesman. think tr1 has failed (believe it is rf preamp.. preselector? on the AM wavebands) .
it is a dual gate FET 40841. are there any equivs?
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Old 7th Sep 2018, 12:29 pm   #2
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Default Re: 40841 equiv?

BF982 appears to be closest electrically that I can find.

40841's are listed for sale here:

https://www.box73.de/product_info.php?products_id=2412
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Old 7th Sep 2018, 12:33 pm   #3
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Default Re: 40841 equiv?

BF982 @ £2.48 inc on Ebay.
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Old 11th Sep 2018, 12:47 am   #4
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Default Re: 40841 equiv?

please see attached schematic and my measured voltages.
both gates and the Source of TR1 are all high, around 6V.
I removed TR1 to test it out of circuit and I was expecting it was going to be all shorty but it seemed to check out as a FET out of circuit with the basic voltmeter gate-opening test.

still, when I removed it, and powered the radio back on, both gates and source all around 0V, so that 6V must be due to a malfunctioning TR1 I guess.. Unless R1,R2,R3 and R4 all check out ok (off-load at least)
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Old 11th Sep 2018, 4:25 am   #5
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Default Re: 40841 equiv?

As I understand it, the RCA 40841 was the “economy” model in the 3N187 family of nominally 12 000 micromho slope dual-protected gate mosfets. The series appears to have been graded as follows:

40673 Full specification
40820 TV VHF RF
40821 TV VHF mixer
40822 FM RF
40823 FM mixer
40841 Economy, general purpose

The 40841 was also slated for use, with the two gates tied together, where single-protected gate devices were required. When RCA introduced the protected gate feature – which opened the door to widespread use - it elected to include it only for the dual-gate devices, keeping the unprotected single-gate devices for applications requiring super-high input impedances.

I suspect that the different versions were also used to allow different pricing levels. For example, I think that the Quad FM3 started out with the 40673 in both the RF and mixer positions, but soon switched to the 40822 and 40823 combination, which suggests that their unit prices were low enough as compared with that of the 40673 to offset the extra inventory cost of using two rather than one device type.

Eddystone I think used the 40673 in its higher-performance HF receiver front ends, so its use of the 40841 in the EB35 Mk III Statesman, probably done for cost reasons, also suggests that it viewed the latter as a less critical application.

Thus I think it reasonable to assume that any of the above listed variants, or any near-equivalent dual-gate mosfet would be an adequate substitute for the 40841.

It’s role in the Statesman was as AM mixer. That model did not have a tuned RF amplifier (for which I think we’d award Eddystone a “fail”). The 40841 had signal on gate 1 and oscillator on gate 2, as was usual for dual-gate mosfets in AM applications, but it also had agc on gate 1. Agc on mosfet mixers was unusual – and anyway agc on HF receiver mixers, at least at higher frequencies, was somewhat frowned upon. On RF and IF mosfet amplifiers, agc was normally applied on gate 2. Mitigating somewhat was that the mixer agc was wideband, with loop input obtained from the IF signal immediately after the mixer and before IF bandpass filter. The agc circuit required its own jfet amplifier. It looks as if Eddystone was taking the long route to make some amends for not including an RF amplifier….


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Old 11th Sep 2018, 9:00 am   #6
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Default Re: 40841 equiv?

interesting, very comprehensive reply and thanks.
showing my ignorance somewaht there.
I cannot seem to untangle where G2 and G1 are on the diagram, they seem somewhat at odds with the pinout.
I guess that following your explanation, the "upper" Gate must be G1, with incoming signal via C13... and the AGC voltage via R1....?

therefore the lower gate in the schematic is the Local Oscillator coming up via C34...?

therefore TR2 is the Local Oscillator, TR3 is the AGC amplifier

I confess that I had not actually looked farther than TR1 in the Eddystone schematic... on studying the whole schematic it is clear that the only thing between TR1 and the speaker is.... IC1... which is one of the "just add water" radio chips. so I guess it should have been obvious that there is no preselector amplifier

grateful for guidance & commentary on my leaps of faith here.
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Old 11th Sep 2018, 9:18 am   #7
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Default Re: 40841 equiv?

Yes, That's how I read the circuit.

Oscillator is on the lower gate of Tr1; RF and AGC are on the upper gate, and Tr3 is the AGC amplifier.

It might be that the lower gate is g1 and the upper gate is g2, the reverse of my earlier assumption. But that would seem to be a bit perverse in that the gain available with signal applied to g2 would - I think - be lower than that of g1. Given the lack of RF amplifier, I'd expect that Eddystone was looking for quite a bit of conversion gain.

But then there were valve mixers with osicllator on grid 1 and signal (and agc) on grid 3. (Eddystone did that on the 870, which used a 12BE6.) So it's an open question.

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Old 11th Sep 2018, 9:45 am   #8
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Default Re: 40841 equiv?

I looked at one or two other Eddystone circuits. It does seem to have drawn its dual-gate mosfets bass-ackwards, with g2 at the lower (source) end and g1 at the (upper) drain end. On that basis Tr1 in the Statesman would have g1 (signal and AGC) at the top and g2 (oscillator) at the bottom.

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Old 11th Sep 2018, 10:13 pm   #9
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Default Re: 40841 equiv?

I should have added that in those other Eddystone examples with apparently "upside down" gates on dual-gate mosfet drawings, the accompanying descriptions had signal going to gate 1 (upper on the diagram) and oscillator to gate 2 (lower on the diagram).

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Old 11th Sep 2018, 10:33 pm   #10
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Default Re: 40841 equiv?

Quote:
Originally Posted by crestavega View Post
... on studying the whole schematic it is clear that the only thing between TR1 and the speaker is.... IC1... which is one of the "just add water" radio chips.
That [“just add water”] is a nice way of describing it. The TBA570 was a fairly early example of its kind:

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I haven’t checked, but I imagine that Eddystone used the AM mixer section as an extra AM IF amplifier, given that the Stateman had an external AM frequency changer. That alternative may well have been covered in the application notes. (It was for the later and somewhat elusive TDA1071.)


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