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:
Sent: December 2, 2006 4:55 PM
To:
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
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the ‘Cascode Catastrophe’ article and I have replaced all the V1 stage passive components except for R3 (already had been replaced by somebody and tested ok), R4 (tested ok), R7 (tested ok), C11, C12 and C15 (all tested ok).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:
[mailto: eddystone_radio_ users@yahoogroup s.co.uk ] On Behalf Of graeme wormald eddystone_radio_ users@yahoogroup s.co.uk
Sent: November 28, 2006 12:52 PM
To:eddystone_radio_ users@yahoogroup s.co.uk
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
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
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