"Bob Henderson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Geoff and all
I have the faulty R-4C and a Ten Tec Orion connected to the same antenna. The unwanted bc interference is present on the R-4C but not on the Orion. If I disconnect the antenna from the R-4C then I hear no signals. I continued to scratch my head, my tests having seemed to prove my R-4C didn't have a problem but the bc interference telling me firmly that it did. I decided to re-run my earlier test for 3rd order performance. This time with a 9+40 signal on 7010 and another at S9+40 on 7020, I found S9 signals at the 2f1-f2 and 2f2-f1 frequencies of 7000 and 7030. That's only 40db down as indicated on the R4C S-meter which isn't calibrated. It looked like I might well have a 3rd order problem. Why results were so different this time, I don't know. After finding this, I did put a 10db attenuator in the antenna lead to the R-4C and noticed the unwanted bc sigs on 20m dropped by 20 dB. Just maybe, I'm on the right track now? As darkness fell yesterday evening I noticed several unwanted bc signals at the bottom of 40m There was one, particularly strong on 7050. As might be expected of a third order product, this carrier had two sources of modulation. One of these was quite recognisable and in Spanish. By tuning around with my Orion I managed to locate this station on 7105. I tuned around looking for the other station of the pair but failed to find it before I had to leave for dinner. If I hadn't been in such a hurry, I could have calculated that I needed to listen on 7160 for the other station. 2 x 7105 - 7050 = 7160. Ah well, I was and I didn't, at least until this morning. This morning, I set up the 570 & 930 to produce signals on the R-4C at 7160 and 7105 respectively, both at a strength of S9+25 as indicated on the R-4C S-meter. When I adjusted the R-4C to 7050 there was a carrier of S7. This would seem to confirm beyond any reasonable doubt that my problem is a third order problem. Incidentally, the carrier on 7050 peaked with a degree or two clockwise rotation of the preselector control. Just as the unwanted bc signal had. I replaced the RF amp 6BA6 and the 1st mixer 6EJ7 with no benefit. I decided to press another radio into service and as a TS870 was to hand, I used that. I wanted to identify whether the problem was being caused by the rf amp or the 1st mixer. I set up the TS870 to share the signal received by the R-4C. I located the carrier on 7105 and the one on 7160, both indicated S9+20 on the 870 S-meter. I tuned the 870 to 7050 and found no sign at all of the third order product still S7 on the R-4C. With the knowledge that the 870 third order response was acceptably good, I coupled the 870 input to the output of the 6BA6 RF amp in the R-4C via a 10:1 scope probe and a 10pF series cap. The signals at 7105 and 7160 were still about S9+20 on the 870 and there was still no sign of the third order product on 7050, despite it remaining firmly in evidence at S7 on the R-4C. Unless there is something wrong with my reasoning here, I believe this confirms the 6BA6 rf amp is not the cause of the problem. Next. I coupled the 870 into the output of the 6EJ7 1st mixer and I checked for the signals on 7105 & 7160. They were both present at reasonable strength. I then checked for the product at 7050 and it was there too. Of course, the strength of all of these was a little suppressed but that is to be expected as the output of the 1st mixer is tuned to 5645. Next, I coupled the 870 to the input side of the roofing filter and set it to ~5645. I adjusted the R4-C to 7105 the 870 S-meter indicated S9+20. Next, I adjusted the R4C to 7160, the 870 S-meter indicated S9+20 there too. Finally, I adjusted the R-4C to 7050 and the 870 S-meter indicated S7. Unless there is a glaring hole in my methodology here, the above would seem to confirm that my problem is being caused by the 6EJ7 1st mixer. I did the resistance checks set down in the R-4C manual on the 1st mixer and these all seemed to be in order. I did the voltage checks and found anode and screen voltages a little low but not obviously bad. I'm wondering if the problem might be caused by poor LO injection level. I don't know what the correct level is but I have measured it at the junction of C31 & T4 and find it to be around 2.2V p-p and a very clean sine wave. I then disconnected my scope probe from the scope and attached it to the input of the TS870. I found the premix LO on 12645 indicating S9+40 on the TS870. I tuned 1 MHz either side and found absolutely nothing audible, let alone capable of moving the S-meter. I think that gives the premix chain a clean bill of health. So that's as far as I've got. If I am not mistaken, whilst I am someway short of actually fixing this fault, I at least now know it's the 1st mixer that's causing it. As I am sure is obvious from my postings, my status as a radio technician is strictly amateur :-)) So if any of you fellas can spot a gaf in my methodology or conclusions, I'll be obliged if you'll let me know. Otherwise all advice on how to nail the 1st mixer problem will be gratefully received. 73 Bob, 5B4AGN ----- Original Message ----- From: "Geoffrey S. Mendelson" <[EMAIL PROTECTED]> To: "Bob Henderson" <[EMAIL PROTECTED]> Cc: "Drakelist e-mail reflector" <[email protected]> Sent: Tuesday, July 12, 2005 3:10 PM Subject: Re: [drakelist] R-4C image problem > On Tue, Jul 12, 2005 at 11:22:36AM -0000, Bob Henderson wrote: > > > I returned the R-4C to the shack last night expecting the interference from > > BC signals on 20 & 40m to be thing of the past BUT they were as loud as > > ever. On 20m still peaking within a few degrees counter-clockwise rotation > > of the preselector and on 40m peaking within a few degrees clockwise > > roatation. My heart sank. I left the receiver set to 40m with the > > preselector peaking the interfering signals i.e that few degrees clockwise > > from the 40m peak and I retired to bed. > > Bob, I've been following this since the beginning, but am hazy on a few > details. My first question is there an antenna connected to the radio > when you hear the signals? Ignore for now the '570 and '930. > > Disconnect the antenna and if you can short out the antenna connector. > > I ask to make sure they are not sneaking in from somewhere else. > > Then connect a good DC ground and check again. Still quiet? > > If not, you have either leakage from an external source, or a bad connection > acting as a rectifier and detecting the AM signals. If the receiver is in > a metal case, as IMHO it should be, make sure the case is properly grounded > and if the chassis is not hot (I'm not familiar with the receiver in question, > so I thought it best to ask) make sure the case is grounded to the chassis. > > If it is, then connect up your antenna.Come back? Hopefully they did > not. but they probably did. If you have an antenna tuner in a metal box, > then connect the antenna to it and a good DC ground to it. Tune for > maximum noise on 7020. At this point if you have another receiver > connected instead, it should hear almost nothing of the AM stations on > 7020 or their actual frequencies. If it does, then there is probably a > SW relay of them near you and you are picking a reflection of it up and > confusing it with the same stations AM broadcast. > > Assuming there is nothing, Connect up the R4C without the preselector. > What do you get? Please report. > > BTW, since you have a '570, you probably should do the AM broadcast > mod on it. This involves moving a jumper (like on a computer card) > and gives you great medium wave sensitivity. > > Remember that ground waves travel long distances over water. If I had a > decent antenna (I don't) in the summer near sunset I could probably work > you on 2m. :-) > > Geoff. > > > -- > Geoffrey S. Mendelson, Jerusalem, Israel [EMAIL PROTECTED] N3OWJ/4X1GM > IL Voice: (077)-424-1667 IL Fax: 972-2-648-1443 U.S. Voice: 1-215-821-1838 > VoN Skype: mendelsonfamily. Looking for work as a CTO or consultant in > handheld gaming, large systems development, handheld device construction, etc. > Support amateur (ham) radio, boycott Google!!! > ---------------------------------------------------------------------- On Behalf of "Bob Henderson" <[EMAIL PROTECTED]> Submissions: [email protected] Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: http://www.zerobeat.net Brought to you courtesy of TLCHost.net http://www.tlchost.net/ ----------------------------------------------------------------------
