[Hallicrafters] sr400a

[Walt Cates]

Simon. See red below for a quick response. I'll be back Monday with more on the het osc.  Dear Walt


further to earlier posts, I went back and started from beginning again.


no problem with carrier osc.


the heterodyne osc is ok on 80 and 40 showing over 4vpp at pin 8 V2.


however 20 and above low and waveforms distorted. volts on V12 ok except pin 6. according to mauaul shold be 95v. also resistance shown as 40K. however 150v line has a resistance of 8k and pin 6 is fed through R78, 100R and R 77 10kR. so could not possible get to 40K. the R78 and R77 agreed with circuit diagram. however part lists shows R78 as 1K and R77 as 33K. The schematic is correct, the parts list is incorrect. In the original 400 R78 & R79 were 1K and 33K. The change in value was not updated in the 400A parts list.


Which is correct?


what ealse should I check to get Het Osc giving correct output on higher bands?


With regard to incorrect voltage at tie point cr11 and l20 found that r18o on audio cathode had gone up to over 1K on replacing with 120R volts now 15 on rx but still a little high on tx at 2.5v. pulling v15 leaves volts at 2.5. Ok, this is good now. That should solve some of your problems. It is a good idea to replace both C146 (25uf) and C218 (5uf) if they are original. You may want to try a quick alignment at this point to verify what faults remain.


in your earlier response you said that CR11 should be 7v in both RX and TX. Rx is 7 v but TX neg 1.8v. The anode should be 7v in receive and transmit modes. The cathode should be 14-16vdc in receive and less than 2vdc in transmit mode. To read the bias voltages on this switching network you need to pull V12. The load of a meter in the ckt, with rf present will turn the ckt into a rectifier and produce the negative voltage you are seeing. You can also measure the cathode voltage below L20 where there is no rf from the het osc.


bye the way in your repair & Rest notes para 3-5-2 C you state that if the volts are correct the either CR11 or C47 are bad. I think this should be C107. You got me there, my error it should be C107 you may also add C106 or a shorted L20 to the possible faults.


Simon G8OJ 


Best Regards, Walt Cates, WD0GOF
 
 A generous person will prosper; whoever refreshes others will be refreshed.