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G8MNY > TECH 08.11.26 18:23l 62 Lines 2884 Bytes #5 (0) @ WW
BID : 10550_GB7CIP
Subj: HT diodes in Drake L-4/4B PA
Path: JH4XSY<N3HYM<IW2OHX<PI8LAP<VE2PKT<VE3KPG<GB7CIP
Sent: 261108/0914Z @:GB7CIP.#32.GBR.EURO #:10550 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New Nov 26)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿﾚﾄﾄﾄﾄﾄﾄﾄｿ
ｳﾚﾄﾄﾄｿ _ _ ｳｳ ｳ
ｳﾀﾄﾄﾄﾙ (_) (_) ｳｳ ｳ
ｳﾚﾄﾄﾄｿ _ ｳｳ ｳ
ｳﾀﾄﾄﾄﾙ (_) � ｳ ｳ
ｳ o [|]ｳｳ ｳ
ﾀﾄﾂﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾂﾄﾙﾀﾂﾂﾄﾄﾄﾂﾂﾙ
This USA made Drake PA as a pair of 3-500z valves at 2000 & 3000 Volts in the
earier (1960) L-4, and 1800 & 2800 Volts in the later (1965) L-4B version.
ﾚﾄﾄｴ>ﾃｴ>ﾃｴ>ﾃｴ>ﾃｴ>ﾃｴ>ﾃｴ>ﾃﾄﾄﾂﾄﾄﾄﾄﾂﾄRfuse>+HT
CW Tune ｳ 270K ===120u
Lﾄo-oﾄﾄo/oﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ\ oﾄﾄﾄｿ_ ﾃﾄﾄﾄﾄﾄｿ ﾃﾄﾄﾄﾄｴ
Fuse 1 on ｳ oSSB _)||(850V ｳ 270K ===120u
ｳ ｳTUNEｳ_ ||(on ｳ ﾃﾄﾄﾄﾄｴ
ｳ_ _ ｳ______)||(TUNE ｳ 270K ===120u
115V)||(_5.5V ｳ_ ||(mode ｳ ﾃﾄﾄﾄﾄｴ
_)||(_30A )||(or ｳ 270K ===120u
ｳ_ || _ 115V)||(1.2kVｳ ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾅﾄﾄﾄﾄｴ
115V)||( 18V _____)||(on ｳ ｳ 270K ===120u
_)||(_0.5A [o-o__ ||(SSB ｳ ｳ ﾃﾄﾄﾄﾄｴ
ｳ Fuse 2 )||(Mode ｳ ｳ 270K ===120u
Transformerｳ 115V)||(@ 2A ｳ ｳ ﾃﾄﾄﾄﾄｴ
in PAｳ _)||( ｳ ｳ 270K ===120u
Nﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ HTﾀﾄﾄﾄﾄﾄ)ﾄﾄﾙ ﾃﾄﾄﾄﾄｴ
Transformer ﾚﾄﾄﾄﾙ 270K ===120u
ﾀｴ<ﾃｴ<ﾃｴ<ﾃｴ<ﾃｴ<ﾃｴ<ﾃｴ<ﾃﾄﾄﾁﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄ>-HT
The diodes are 1N5407 on the L-4 and smaller 1 amp 1N4007 with 1kV 1nF across
each on the L-4 (not shown). The R fuse is large a distroyable carbon 8R!
FAULTS
I have had 2 diodes fail in the heavily used L-4B PSU, but so far none in the
earier version with the bigger diodes & no added suppression caps. There are
different diodes used both in long strings of 7 on tags strips next to the 4
caps + & - capacitor banks.
Symptons were a bruning smell thought originaly to be from the PA, but when I
saw only half HT, I investigated the PSU and saw a broken 1N4007 diode and a
1nF cap unsoldered. I tested the cap and replace the diode all was well. But
later after some mains arcing, I retested the PSU and found another faulty
diode, this time a shot circuit that would not have show up other than with a
meter!
The mains acring was found to be the Hi/Lo mains power switch that was dirty,
and needed some spray contact cleaner to sort out.
See my Tech bul on "ALC line for Drake L-4B PA", "PTT Line for Drake L-4B PA",
"Power Factor of Drake L-4/4B PA", "Drake PA 3-500 valve repair" & "Using 2 HF
PAs".
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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