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G8MNY > TECH 24.03.24 10:31l 74 Lines 3299 Bytes #999 (0) @ WW
BID : 3989_GB7CIP
Read: GUEST
Subj: Drake PA 3-500 valve repair
Path: IW8PGT<IZ3LSV<I0OJJ<GB7CIP
Sent: 240324/0907Z @:GB7CIP.#32.GBR.EURO #:3989 [Caterham Surrey GBR] $:3989_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New Mar 24)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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THE FAULT
I came across a common fault with a many PAs including the Drake L4 Series, an
expensive valve that goes intermittent "heater to grid short". I read somewhere
this is a common failure of directly heated cathode PA valves. Symptoms are a
very low gain & standing current on Rx, as the +120V cathode bias is shorted
keeping the valve on, making electron shot RF noise on Rx.
On testing the separate HT PSU with a 2A mains variac wired straight to
transformer, I found several issues including on the 120V resistor attenuated
rail dropped from 2.7kV, missing. But that was not it!
Still using the variac this time wired direct to the PA (no 2.7kV HT), I found
the 120V rail line on the PTT Relay was shorted to ground when the PA was on
its side! Putting PA upside down the short cleared, but only until the heaters
were powered up. With the variac I was able to find the voltage were short
occurred.
Now to do internal surgery inside the valve & not damage it!
Äo-oÄÄÄÄo/oÄÄÂ RF drive >ÄÄ´ÃÄ¿ _³_ 2x 3-500 _³_
Fuse 1 on ³ ³ /ÄÁÄ\ /ÄÁÄ\
Trip ³ ÚÄÄÄÄÄÄÄÄÄÂÄÄÄÄ¿ ÚÄÄÂÄÄ´ |- - -|ÄÂÄÄ¿ |- - -|ÄÂÄÄ¿
Ú Ä Ä >ÀÄÄÄÄ¿ ³5.3V === ())))) ³ ³ | /\ |=== ) | /\ |=== )
| 115V)||(______ ÃÄÄ¿ :::::: === ³ \³__³/ ³ ) \³__³/ ³ )
| )||(30A ³ === Á ())))) ³ ³ ³ ³ Á Á ³ ³ Á Á
Variac ÚÄÄÄÄÙ||ÀÄÄÄÄÄÄ)ÄÄÁÄÄÄÄÙRFC ³ ³ ÀÄÄÄÁÄÄ)ÄÄÄÄÄÄÄÄÄÙ ³
Mains ÃÄÄÄÄ¿|| __ ³ ÀÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÙ
| ³115V)||( 18V ³ 5V
| FAN )||(_0.5A ³ RxÚÄÄÄÄÄÄÄ<+120V
À Ä Ä >ÃÄÄÄÄÙ ³___/ Relay
³ 250VA Heater Tx³
³ Transformer ÃÄÄÄÄÄÄÄ>Anode
³ in PA 1R Meter I
ÄÄÄÄÄÄÄÄÄÄÄÄÄÁ Á
THE FIX
I tried several amps at a few volts between the negative grid to cathode, to no
effect.
What did work was to use a 68,000uF charged up to 20V between the heater
transformer centre tap and a hard earthed grid (not to burn out the grid choke
of few ohms). I then turned up the mains variac until the short reoccurred, I
had to do this a few times to make sure the intermittent short had been blown
off!
The pulse current had to be high enough to blast away the touching a grid wire,
but not damage the 15A thick heater wire, & also not crack the glass!
CONCLUSION
Saved a few hundred dollars of PA valve, so worth a try? I see about 1kW now.
Never run or test directly heated power valves of their side!
Never over run the heater voltage (measured on valve base).
Never over temperature the Anode as that can cook the heaters too!
See my Tech bul on "ALC line for Drake L-4B PA", "PTT Line for Drake L-4B PA"
"Power Factor of Drake L-4B PA" & "Valve Heater delay Drake L-4B PA".
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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