RE: [AMRadio] another SX-28 question
Many RF gain controls work by a voltage divider setup, a pot with the wiper going to the cathode of a stage, one terminal going to ground, the other to a positive voltage source. This allows the cathode voltage to go to ground (rf gain max), to a small positive voltage (rf gain minimum). Other circuits use the AGC setup on multiple stages. Don't assume the pot is good, or the correct value. The pinched wire could have ruined the pot, some PO could have changed the pot because of the pinched wire, etc. You need to reference the diagram and check values and connections, many older receivers were modified for better SSB reception (yuk!). If a pot went open, or was too high a value, the rf gain will be normal when the wiper is grounded, but as soon as its off the end point, its open... Many older receivers (not the SX28 I think) did different things with the gain control in SSB/CW mode, on my G76, the audio gain control turns into an RF gain control... On my SX17, the bfo is separate, and does not interact with the RF gain circuit. I think the RF gain control is the voltage divider type. I think there is also an AGC on off switch. Great sounding receivers, all those that Hallicrafters made with the nice push pull audio output, puts receivers that use a 6aq5 or some such to shame, and people don't know what they are missing, listening to good AM on the typical receiver, even high end modern stuff. It sounds better than side band, but it can be as much of a difference between a good and bad audio amp as between ssb and AM. The Scott SLRM receiver I have seems to have a real hi fi audio amp, with no driver transformer in the audio stages, and even a position on the selectivity control marked hi fi. Broad as a barn door! Push pull 25L6 tubes! Brett N2DTS -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of w5sum Sent: Tuesday, December 30, 2003 5:47 PM To: amradio@mailman.qth.net Subject: [AMRadio] another SX-28 question well 24 hours of work and she is coming along nicely. The audio section is fine now. Sound graaat.. She is not working on the top two bands but we'll cross that bridge later. the RF gain control was not working at all. I found a pinched wire that was grounded. This was the wire going to the cathode resistor of V-1. I fixed this, and even put a new pot in there for the RF Gain. It works BUT.. all you have to do is just barely turn it and it blanks out the front end. Could one of the cathode resistors have changed value, canceling out the work of the rf gain control? R-3 to V-1 or R8 to V-2? Or, possibly, C14 on V-1 or C18 on V-2? looks like they will be tough to get to if that is the case. your ideas gentleman? your The AM voice of Shreveport, Louisiana USA formerly WN5AIA and WB5AIA I got my Extra Class the old fashioned way... I earned it! ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio
Re: [AMRadio] another SX-28 question
the RF gain control was not working at all. I found a pinched wire that was grounded. This was the wire going to the cathode resistor of V-1. I fixed this, and even put a new pot in there for the RF Gain. It works BUT.. all you have to do is just barely turn it and it blanks out the front end. Could one of the cathode resistors have changed value, canceling out the work of the rf gain control? R-3 to V-1 or R8 to V-2? Or, possibly, C14 on V-1 or C18 on V-2? Just a silly-assed question, Ronnie - mainly because I don't remember... but is there an AGC control switch on the front of the SX-28? It should be ON. Without the AGC circuit being active, you could have the same effect - any signal would send the front-end to saturation. Happy New Year! 73 = Best Regards, -Geoff/W5OMR -- 7:56am up 2 days 14:23, 4 users, load average: 0.17, 0.65, 0.84
Re: [AMRadio] another SX-28 question
I spewed my drink all over the monitor when on Wednesday 31 December 2003 08:48, w5sum said (and I am -not- making this up!):: good morning Jeff yes there is an AVC switch and it is on... I think the problem is, that this is a non linear pot I dug up... measuring the resistance, it moves slowly for just a (little bit), then it JUMPS quickly... I'm going to change this out first Sounds like the culprit. While you're checking on things, make sure you know you're not addressing the whole list ;-) -- 8:52am up 2 days 15:20, 5 users, load average: 0.21, 0.16, 0.19
Re: [AMRadio] another SX-28 question
its an old age thing!! -- Original Message --- From: Geoff Edmonson [EMAIL PROTECTED] To: amradio@mailman.qth.net Sent: Wed, 31 Dec 2003 08:54:14 -0600 Subject: Re: [AMRadio] another SX-28 question I spewed my drink all over the monitor when on Wednesday 31 December 2003 08:48, w5sum said (and I am -not- making this up!):: good morning Jeff yes there is an AVC switch and it is on... I think the problem is, that this is a non linear pot I dug up... measuring the resistance, it moves slowly for just a (little bit), then it JUMPS quickly... I'm going to change this out first Sounds like the culprit. While you're checking on things, make sure you know you're not addressing the whole list ;-) -- 8:52am up 2 days 15:20, 5 users, load average: 0.21, 0.16, 0.19 ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio --- End of Original Message ---
[AMRadio] another SX-28 question
well 24 hours of work and she is coming along nicely. The audio section is fine now. Sound graaat.. She is not working on the top two bands but we'll cross that bridge later. the RF gain control was not working at all. I found a pinched wire that was grounded. This was the wire going to the cathode resistor of V-1. I fixed this, and even put a new pot in there for the RF Gain. It works BUT.. all you have to do is just barely turn it and it blanks out the front end. Could one of the cathode resistors have changed value, canceling out the work of the rf gain control? R-3 to V-1 or R8 to V-2? Or, possibly, C14 on V-1 or C18 on V-2? looks like they will be tough to get to if that is the case. your ideas gentleman? your The AM voice of Shreveport, Louisiana USA formerly WN5AIA and WB5AIA I got my Extra Class the old fashioned way... I earned it!
