RF switching PIN diode experts despartely sought!

[Chris Wilson]

I have a used Alpha 86 HF amateur band amplifier on the bench. It appeared to be working fine when I received it, but subsequently I have found an issue that may be related to the PIN diode RF switching that is used to enable fast CW with break in.

It has two operating conditions. STANDBY where RF goes in to the antenna socket and out to the transceiver via the relay K3. So when this relay is closed RF comes in from the antenna, via the closed relay contacts, to the transceiver. in this state RX is fine. In its OPERATE mode K3 opens and RF is sent via, as far as I can read the schematic, the RX PIN diodes D3, D4, and D5. In this state the RX signal is attenuated some 3dB. More so on the higher bands.

But I have just found another anomaly, in the OPERATE status if I move the TUNE and LOAD capacitor positions from where the amp is happy to load up and TX into a dummy load, but not transmitting, just receiving via the transceiver, the attenuation reduces or virtually goes altogether, depending on band. As such I am wondering if the big, rare and expensive UR4001 stud mount PIN diodes are leaking the RX signal back in to the tank circuit? I could REALLY do with someone that understands the schematic and PIN diodes far better than I to advise. If you didn't hear the drop in RX receive sensitivity, or look at the S meter, you might think, as I initially did, there was nowt wrong with it.

I find it hard to believe the PIN diode set up normally attenuates 3dB!

I was tempted to convert it to two vacuum relays and bypass the fast PIN diode switching, a fairly common mod, but I would rather leave it in the more valuable original state if possible. I attach two schematics, if anyone would be able to find the time and inclination to study them I would be extremely grateful as I don't want to spend a lot of dosh on fitting new stud mount PIN diodes if it might be just D3, D4 and D5. These are also near unobtanium, but Alpha now say it's OK to replace them with "low capacitance" 1N4007's. I can find nothing lower than 8pF ones, so i guess they mean those. Alpha have shut for the virus with no signs of opening again any time soon.



Thanks a lot!

[cdev]

?   ?

http://www.sm5bsz.com/pindiode.htm


http://www.sm5bsz.com/linuxdsp/tx/safe/tx_control.htm

Don't know if this is exactly what you need, but Leif the author of Linrad has a lot on his site about using PIN diodes for QSK.
Recently its emerged that an 1n4007 could substitute for a PIN diode in many applications.

Just poke around on his site, maybe you'll find something useful.

http://www.sm5bsz.com/linuxdsp/rxhfa/rfamp_hfa.htm