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Old 15th Sep 2018, 8:16 am   #1
crestavega
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Default Eddystone statesman ... UC734B??

bit of a kink with my Stateman repair.
I replaced TR1 (originally a suspect 40841) with a BF982 with the result that the source, and both gate voltages are now hanging around 0.2V (rather than 6V)
this seems slightly more acceptable but it's still not working.
I investigated TR2
the schematic shows TR2 being a BF245B, in a TO92 package, but my radio PCB provides for TR2 being a TO72 package (albeit with one pin grounded)
indeed, the device I found occupying TR2's position is a UC734B. this shows signs of being a previously repaired part.
can anyone shed any light on UC734B? I cannot find a pinout.


realise this is more appropriate to the Eddystone usergrp forum so I have joined it. pending approval.
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Old 15th Sep 2018, 9:11 am   #2
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Default Re: Eddy TR2 ... UC734B??

How, and I really do mean; how the dickens, can the Gate of TR2 be standing at -2VDC wrt ground?
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Old 15th Sep 2018, 10:35 am   #3
Synchrodyne
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Default Re: Eddy TR2 ... UC734B??

Quote:
Originally Posted by crestavega View Post
can anyone shed any light on UC734B? I cannot find a pinout.
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Eddystone used the UC734B jfet in several models. Inter alia it was the lower unit (signal input) in a mixed cascode (3N128 upper, with AGC bias) used as RF amplifier in the EC958, 1830 and 1000-series models. Presumably it was adequately specified for critical applications.

Cheers,

Last edited by Synchrodyne; 15th Sep 2018 at 10:41 am.
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Old 16th Sep 2018, 12:31 am   #4
Synchrodyne
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Default Re: Eddystone statesman ... UC734B??

Some background on the Union Carbide UC734 jfet. The UC734B might have been a selected version.


Cheers,
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Old 17th Sep 2018, 12:07 pm   #5
crestavega
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Default Re: Eddystone statesman ... UC734B??

curioser and curioser.
thanks for the background, the FET indeed looks like a suitable part for everything.

I have now discovered that TR2 (LO) and TR3 (AGC amp) on my unit are also UC734B's, contrary to the published owner's technical manual which specifies a BJT for both these.
The FET's seem to my fairly ignorant brain appropriate parts for both, at least in bandwidth terms. at 9s each they must have been expensive parts back in the day.

I seem to have major problems though, the transistor voltages are way out of spec.

pls could someone put me right on an apparently insoluble problem:
How can the voltage on the Collector of Q3 be 3.5V when it designed to have a direct DC connection to the 9V supply line?
Upon looking at the diagram closely, I may have misidentified the collector, but in any case, as I read it, the only component standing between the "top left" terminal of TR3 and the +9VDC power input is a choke, which is a short circuit at DC.
I am indeed getting 8.8V at this point.
help very much appreciated
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Old 17th Sep 2018, 10:56 pm   #6
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Arrow Re: Eddystone statesman ... UC734B??

Yes, strange indeed. Three things spring to mind:
1. The choke will have a d.c. resistance: may be relevant.
2. Q3 - which is identified as TR3 - doesn't have a 'collector' since it is an F.E.T.: an F.E.T. has a drain in that 'position'.
3. Did an earlier model of the Statesman have a bipolar transistor in that position? If so, perhaps the author of the documentation got a bit confused when transcribing the write-up from the earlier model to this one.

Al.
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Old 18th Sep 2018, 8:48 am   #7
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Default Re: Eddystone statesman ... UC734B??

Thank you for this. I am almost tearing my hair!
1. indeed, the choke will have a dc resistance. The max drain current stated on the data sheet for TR3 (a BF245B per the diagram) is 100mA. if 100mA is flowing in the drain of TR3, the choke would have to have a resistance of 55 Ohms to drop 9V to 3.5V. and this is at max Id. the designers would never have approached max Id, (would they?) so let's assume drain current of 50mA, therefore choke should be around the 100 ohm mark, and capable of dissipating 0.25W. ..... this.... seems unlikely doesnt it?

2. Mea culpa. at first glance it looked like a BJT, but indeed by design this is a FET.

3. I wonder. I shall dig a bit try to find out. certainly, the part shown in the manual & diagram is different to what I have got: in both TR2 and TR3 my radio has a UC734 in the 4-pin tin-can package; and furhtermore my PCB is marked and drilled and printed for this package, whereas the manual specifies BF245 - a "modern" TO92 3-pin plastic package.
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