Help needed spark problem

[iw5ci]

Hi, it's my first post in this forum , i am IW5CI from Florence.
Here in italy we have a nice AM net on 40 meters on saturday and sunday morning at 7190-7195.
I am setting up a "real" AM transmitter a GELOSO 222 that uses 2 807' as modulators and a 6146 as RF final : this is the scheme:

http://www.webalice.it/iw1pue/geloso/schema_g-22-tr.htm

I have a problem, every time i put the radio in tx or i return in RX (by a rotary switch) i see sparks in the rotary switch and so i think i am slowly burning the contacts.

I am shure there is some component fault that causes this problem, but i don't' know where to start troubleshooting. Can someone give me an hand?

73!

[w3jn]

It looks like as the switch is rotated from receive to transmit, a wiper grounds the screen supply of the 807 modulators thru a 33 ohm 1 watt resistor.  That *cannot* be right.

The switch also switches the primary of the 807 plate supply transformer.  That's probably where the sparks are coming from.  You can reduce this by putting a .1 uf/1KV capacitor across those contacts.

[WA1GFZ]

Put a 100 ohm resistor in series with the cap so it acts like a snubber. The best fix so the switch lasts for ever is to add a relay with larger contacts to handle the switch functions. A 10 amp multiple pole relay would operate with the antenna transfer relay.

[flintstone mop]

Welcome To AMFONE........IW5CI...........Nickname Andy?

Glad that you joined us.
I'm waiting for responses from Amateurs using A.M. in the Philippines.

Welcome aboard

Fred