Channel Master 6518 Need replacement for 2SA52  - Page 1
Forum section: electronics / repair

author: Bargainhunter
author_rank: Regular Contributor
timestamp: September 10, 2022, 01:15:56 pm
content: I'm repairing this radio. The FM works fine but it won't tune any AM stations. The AM if section works fine. I'm assuming that the problem is with transistor X9 (see attached schematic). Can someone explain the function of X9. It's listed as AM Conv. Where is the Oscillator and mixer transistors? I searched and found the audodyne converter circuit. Could this be an autodyne circuit also?I replaced all the electrolytic caps but discovered I made a mistake. I assumed all negatives went to ground. After looking at the schematic, I saw the caps had the positive lead going to ground. Looking at the power supply on the schematic, I see plus is ground. The original caps only had a small black dot on the side to indicate the negative lead. So I'm assuming cap C3 if it is reversed is the problem or transistor X9 is bad.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 10, 2022, 01:46:53 pm
content: X9 is oscillator and mixer. (L10 and one gang of the tuning capacitor is providing the tuned feedback to determine the oscillator frequency.) There is a good chance that any small signal silicon PNP transistor can be used as a replacement.It was quite usual for those early sets with germanium PNP transistors to have battery positive as ground.

author: Bargainhunter
author_rank: Regular Contributor
timestamp: September 11, 2022, 01:43:03 am
content: I found a few electrolytic caps installed backwards. I fixed that issue. The X9 transistor checked good on my component tester.I still can't tune a station. I tried adjusting my signal generator from one end of the am band to the other but no signal on the radio. The 455khz IF frequency is heard on the radio but nothing before the first IF. Do you think installing C3 backwards caused X9 or other component to fail?

author: wasedadoc
author_rank: Super Contributor
timestamp: September 11, 2022, 01:20:13 pm
content: No, installing C3 with incorrect polarity will not damage Q9 or any other component except possibly C3 itself.Are the voltages on Q9 in line with those on the schematic?If you don't have a scope to see if it is oscillating you could try this.1. Get a working radio with AM band (550 to 1600 kHz).2. Turn off all other electronic apparatus in the vicinity. Especially all switched mode PSUs.3. On the working radio find a quiet frequency anywhere between 1100 and 1600 kHz. Note the frequency. Turn up its volume so that you can easily hear the background static.4. Place the two radios as close together as possible. Turn on the faulty radio, switch it to AM band and turn its tuning knob around the frequency noted above minus 455 kHz. If you hear the static of the working radio decrease and increase as you tune the faulty radio then its oscillator is working. If so, check that minimum static occurs when the frequency difference on the two tuning dials is approximately 455 kHz.

author: Bargainhunter
author_rank: Regular Contributor
timestamp: September 14, 2022, 05:23:57 pm
content: Thanks for your help. The AM is now working.I tracked down the problem to one of the wires from the rod antenna numbered 3. The center conductor had broken inside the outer sleeve and was not making contact with the base of X9.I appreciate your help.