Graeme,

Another thought: by using the voltage info on the circuit diagram
(HT1 at 240v, 'cold' end of R8 at 165v) and the resistance values,
the current draw of V1 (total) should be around 9.5mA under the
conditions stipulated in the manual - this is very close to the 'no
signal, AGC off' value of 10mA measured in my set, so perhaps my RF
stage is working ok after all....! the current draw 'problem' may
just be an artifact of the ~120v mains being applied to the 110v
tapping after all: with 110v applied from my variac, I measure 108mA
total HT current for the set under no signal conditions and 99mA
with a strong signal (albeit this is with the 220ohm resistors still
fitted at R74 and R75, so the current would be a bit higher with the
standard 140ohm units re-fitted).

73's

Gerry

PS - if you speak to Bill Cooke again please pass on my thanks for
his input on this issue and, of course, for his legacy from all his
years at Eddystone!

--- In eddystone_radio_users@..., "Gerry O'Hara"
<gerryohara@...> wrote:
>
> Many thanks Graeme. Good advice from Bill regarding letting the
fault
> develop, though my main concern is damaging the mains transformer
(exact
> replacements are not too common this side of the pond) and the set
does run
> about 15% over the rating under low-signal conditions - this may
simply be a
> result of our ~120v mains being applied to the 110v tapping -
hence me
> trying to reduce the HT voltage by using higher-value resistors in
the
> rectifier anodes (220ohm in place of the 140ohm ones fitted as
standard),
> though I am intrigued as to why the cascode stage anode current
varies so
> much with varying the bias, either using the RF gain control or by
the AGC
> (<2mA under strong signal conditions to >10.5mA under no signal
conditions).
>
> 73's
>
> Gerry
>
> _____
>
> From: eddystone_radio_users@...
> [mailto:eddystone_radio_users@...] On Behalf Of
graeme wormald
> Sent: December 2, 2006 4:55 PM
> To: eddystone_radio_users@...
> Subject: Re: [eddystone_radio_users] S940 HT Current Draw
>
> Hello Gerry, I've spoken to Bill Cooke (who designed the 940) and
he says
> the mains tranny is good for at least a 15% overload, and that the
set
> proved very reliable from that point of view when in service. He
says if
> you are happy with the performance don't go looking for trouble!
If there
> is an incipient fault keep the set running until it shows
itself . . .
>
> 73, Graeme.
>
>
> ----- Original Message -----
> From: Gerry O'Hara <mailto:gerryohara@...>
> To: eddystone_radio_
<mailto:eddystone_radio_users@...>
> users@...
> Sent: Wednesday, November 29, 2006 2:29 AM
> Subject: RE: [eddystone_radio_users] S940 HT Current Draw
>
> Hello Graeme, many thanks for replying to my post.
> Yes, I have read the 'Cascode Catastrophe' article and I have
replaced all
> the V1 stage passive components except for R3 (already had been
replaced by
> somebody and tested ok), R4 (tested ok), R7 (tested ok), C11, C12
and C15
> (all tested ok). So the two 100kohm V1b grid bias resistors are
new, as are
> R8 and R9, C13 and C14: the static DC voltages as noted on the
circuit
> diagram are 'spot on' when measured on Band 5 with the AGC off,
aerial
> shorted to ground and RF gain fully on as per the manual. Also,
changing
> the valve makes no difference: the current draw of this stage
under no
> signal conditions seems excessive.
> I have been corresponding with Mike Cassidy who is kindly taking
some
> current and voltage measurements in his original-condition (working
> perfectly) S.940 under various signal conditions/control settings
for
> comparison - he notes that his set draws around 110mA HT current,
which is
> right 'on the wire' of the mains transformer spec. Do you know
what HT
> current the 3937P mains transformer is really capable of providing
(it is
> marked as 110mA on the connection plate but I am pretty sure it
will give
> more).
> 73's
> Gerry
>
> _____
>
> From: eddystone_radio_users@...
> [mailto:eddystone_radio_users@...] On Behalf Of
graeme wormald
> Sent: November 28, 2006 12:52 PM
> To: eddystone_radio_users@...
> Subject: Re: [eddystone_radio_users] S940 HT Current Draw
> Hello Gerry,
> I've had a glitch on my e-mail so I don't know if my message
reached you.
> So I'll send it again!
> Re the cascode on the 940, did you see and apply the article from
Lighthouse
> Issue 78, April 2003. page 42, "Cascode Catastrophe"??
> If not, take a look now; print it off the site.
> Graeme, G3GGL
> ----- Original Message -----
> From: g8guh <mailto:gerryohara@...>
> To: eddystone_radio_
<mailto:eddystone_radio_users@...>
> users@...
> Sent: Saturday, November 25, 2006 4:45 AM
> Subject: [eddystone_radio_users] S940 HT Current Draw
> I recently repaired an S940 that has an HT current draw that is on
> the high side: even increasing the two HT surge limiting resistors
> (between the transformer and the 5Z4G rectifier plates) to 220ohm
> (from 140ohm) - to try to mitigate for a 117v AC supply on the
110v
> primary of the mains transformer (there is no 120v tapping) -
> results in 110mA HT current draw when a strong station is tuned
in,
> with an HT2 of 240v (ie when the RF and IF valves are biased off
by
> the AGC circuit) and over 125mA when no signal is applied, when
the
> HT2 line drops to 225v. Most of the current draw variation is in
> the cascode RF stage at 12mA (range from 8mA to 20mA) and 3mA from
> the pentode RF stage (range from 7mA to 10mA). All the voltages in
> these and other stages of the set check out well within tolerance
> (most passive components in the first RF stage were replaced
during
> the repairs). I note that the 6ES8 max anode current spec is cited
> at only 15mA, so that tube is working really hard under no signal
> conditions (a bit worrying), but my other concern is that the
> transformer HT secondary is rated for only 110mA (it is type
3937P)
> and the 5Z4G rectifier is rated at only 125mA maximum plate
> current. The set appears to be working ok otherwise but I am
> concerned over stressing the 6ES8, the mains transformer and the
> rectifier.
>
> I cannot find reference to what the normal HT current draw for the
> S940 should be in the manual or other literature. Can anyone
> comment on this?
>
> Thanks, Gerry, G8GUH, Vancouver, Canada
>