The AM Forum

THE AM BULLETIN BOARD => QSO => Topic started by: kd1nw on December 08, 2017, 07:54:06 AM



Title: Santa came early?
Post by: kd1nw on December 08, 2017, 07:54:06 AM
I just discovered the SOB-301 I just finished working on has the AM filter installed instead of the CW filter. I guess when I have the SB400 working I'll have to get it on AM. I had no intention of doing that, I was actually going to use the pair as a vintage CW station. I think the 400 is working actually, it just isn't showing grid drive on the meter when it should show around 0.5 mA. There is grid signal though, I can hear it loudly using a receiver so its probably not an issue. My enthusiasm for this pair is growing more and more.


Title: Re: Santa came early?
Post by: WBear2GCR on December 08, 2017, 09:22:50 AM
Did not think the SB-401 or 301 for that matter had an AM position??


Title: Re: Santa came early?
Post by: kd1nw on December 08, 2017, 10:18:33 AM
yep, the 301 has an am position but the 400 transmitter does not. nice to catch you on 3885 this morning


Title: Re: Santa came early?
Post by: w8khk on December 08, 2017, 10:28:44 AM
yep, the 301 has an am position but the 400 transmitter does not. nice to catch you on 3885 this morning

The HW and SB 100 series of transceivers can make nice AM audio with the mod published on the AM window here:

http://www.amwindow.org/tech/htm/hw100plusmods.htm

You might want to try this mod on the SB-400.  Simple, easily reversible.  No need for a special filter on the transmitter side.


Title: Re: Santa came early?
Post by: kd1nw on December 08, 2017, 12:42:19 PM
cool, thanks for the link


Title: Re: Santa came early?
Post by: kd1nw on December 09, 2017, 08:27:16 AM
Also, thanks GFZ, I looked and can't find a series resistor feeding the screen. I only have the SB401 schematic which shows R17 680 ohm. I traced all the way back to the relays. I see the 10k 2W R37 which read 10K but no other resistor between screens and relays. I tested the resistance of the screens to ground and I'm getting 20K the schematic calls out at least 10K after 5 secs of reading so it seems okay. I'm reading about 215 volts on the screens pin 3 and -170 on Pin 5. I tried to reply via PM but its blocked
 
update: i adjusted the bias per the manual and everything is okay now. grid appox 0.5 and tunes up to 100W into a dummy load on 80 meters. Also, I was reading an old 73 magazine review / intro of the SB401 and they said the screen series resistor was introduced on the 401 to reduce distortion products by 4-6dB or more. Thanks!

https://www.nostalgickitscentral.com/heath/73-index/articles/SB-401-7-67.pdf

73
Kevin


Title: Re: Santa came early?
Post by: w3jn on December 10, 2017, 07:20:43 PM
Youu can easily convert the SOB-401 to pretty high quality AM by just bypassing the SSB filter, and unbalancing the balanced modulator for about 25 watts carrier.  I did this and fed line level audio right to the balanced modulator and re-wired the mode switch so as to route the signals appropriately when it was in the CW mode. 

Problem with this was the balance crept on the modulator as things heated up, and it really needed a balance control on the front panel.  Had to keep after it while in a QSO to keep the carrier level down.

It sounded excellent though.


Title: Re: Santa came early?
Post by: kd1nw on December 12, 2017, 06:31:57 PM
thanks for the tips and things to look out for. i'll have to at least try it and see how it goes. if i dont like it i can always put it back to original.  i have the feeling i would like the sound but not losing ssb or cw, plus i didnt even know about the balance issue.

73 kevin
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands