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Sigtroniks SPA-400

Had some funny behaviour out of my SPA-400 intercom yesterday. Transmitted speech on both boxes suddenly unreadable. Flicking the switch to off cleared it instantly. (Apparently, switching it off causes a fall back to a direct connection between pilot’s mic and the radio).

Does anyone know what’s inside an SPA-400? I have not been able to find a schematic or maintenance manual despite considerable searching. I’d just like to know if there’s a relay or something else that could go non-linear and cause the unreadable distortion on the audio. Of course, on the return flight, it worked perfectly!

PS Yes I know I should throw it away and get a decent audio panel. I know!


EGBW / KPRC, United Kingdom

There are no relays and they are usually quite reliable. Switching it to off, also disables all the intercom complete. How was the intercom operation? Being unshielded, and commonly installed using wrong practices make this unit more suspectable to interference from other sources. Other then that, they are quite good and economical.


Hello Jesse and thanks for the reply. Intercom operation was normal throughout. The speech distortion was only evident on keying P1or P2 mic, when it could heard in the side tone and by the ground station. This lasted for the last 5 mins of flight and persisted after the engine had been stopped. Switching off allowed normal P1 operation. By the time of the subsequent flight the fault had cleared.

EGBW / KPRC, United Kingdom

SPA-400s are inexpensive so if you have a suspicion that it’s an internal fault, I think maybe the easiest way to diagnose the issue would be to stick a new one in and see if the problem disappears.

If you want a loaner SPA-400 I have one there you can try it, and send it back to me if you don’t need it.

Buying, Selling, Flying
EISG, Ireland

Three of these are going to be removed from my fleet in the new year……… any takers ?

William, thanks for that kind offer. Of course, it’s worked perfectly since!

EGBW / KPRC, United Kingdom

Returning to this thread of 8 years ago (!) does anyone have a schematic of this box? (Yes, it’s playing up again. Yes, I need a new audio panel. I know)
Thanks, David

EGBW / KPRC, United Kingdom

With most electronics, with no info, do this order:

  • repair mechanical damage (bad contacts etc)
  • fix PCB corrosion
  • replace all electrolytic capacitors
  • check supply voltages on known chips and fix the power supply (7805 is popular in GA, even though its max Vin spec is drastically exceeded during engine starter dis-engagement)
Shoreham EGKA, United Kingdom
9 Posts
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