SX-626 AM Tuner Issues

[Motnick]

I have a nice sx-626 that I have been working on but I can’t seem to get the AM tuner working properly. The output is very low. I only pick up one or two stations. The signal meter stays on 0.

While probing the ha1138 chip the AM started working and stayed working long enough for me to follow the AM alignment adjustments and get a nice strong signal from both ends of the AM band. Unfortunately the AM only worked for a few minutes before dropping back out. I suspected a solder connection so I have reflowed the AM section of the tuner board. I have also replaced the electrolytic capacitors in the AM section of the tuner board. Since some of the voltages are off on the ha1138 I replaced it with another chip that I had on hand but the voltages are the same after replacing the chip. None of this work has been able to get the AM working. Hopefully someone here can point me in the right direction.

HA1138 Voltages:
1. 11.0 volts
2. 2.75 volts
3. 11.0 volts
4. 11.0 volts
5. 4.83 volts
6. 9.0 volts
7. 1.98 volts
8. 0.88 volts
9. 0 volts
10. 10 mv
11. 0.235 volts
12. 12.1 volts
13. 2.71 volts
14. 0 volts
15. 2.67 volts
16. 0 volts

 

[Motnick]

 

[eizner23]

Pin 14 should see a little voltage, work the two pots back and forth a little, mark original position first.

 

[zebulon1]

Is it completely dead?
Do you hear anything?
You have 12v at pin 12, check the 47uf cap or the 47K trimmer as "eizner23" pointed out. Don't rule out a broken trace. Use a chopstick to push on the board in and around the IC.
C51 is a .1uf Ecap (Replace it? A film will be better). Remove that and see if you can get pin ten some voltage?
The only way to TS this is by checking the surrounding components making sure there are no breaks in the circuit. Check for values if you can.
Pin one as an example, shows 11v's. You know the circuit is good through the primary of F4.
Pin 6, is showing the 8v's but it has a coupling cap C40. You might see an AC signal at T3.

 

[Motnick]

I have very little output and can only catch one station. I need to crank the volume way up to hear anything coming from the AM station.

Probing some more I still have 12.1 volts at Pin 12, but 0 volts at the positive side of C47 and 0 volts at R38. I tried jumping pin 12 to R38 but nothing changed.

 

[Motnick]

I am looking at the board and I don't see any junction connecting pin 12 with pin 14. I think the junction mark on the schematic is wrong.

 

[Motnick]

Got it! C49 10000pf mylar was dead shorted. First time I have encountered a bad green mylar cap.

 

[eizner23]

Got it! C49 10000pf mylar was dead shorted. First time I have encountered a bad green mylar cap.

Good catch, probably the last part anyone would suspect.
I had one go bad on an Atari computer once.

 

[zebulon1]

Great news!
The Mylar caps are moving up on the list of suspects. The tube amp guys replace the ones from the 70’s, mostly without hesitation. I had a feeling it was something drawing down the voltage somewhere. The data sheet did have some clues but checking components is usually faster.