Audio cutting in and out on 2220B

[Yahn]

Hello.

I have a 2220B that I purchased a long time ago. I bought it second hand off someone with too many projects. It needed some love and a recapping, and since then its original problem, which made any sounds coming from it have what sounded like someone was blowing into a microphone overlaid on top of it, subsided. Since then I have had problems with audio stop working completely. It could turn on and work fine, but after 5 minutes all sound stops. All functions of this receiver work at any given time, except that after a while audio stops coming out. Some times it fades in and out before stopping all together.

I know this seems like a long shot, but anyone had this problem?

Jan

 

[rBuckner]

Welcome to AK! I've recently heard of that symptom but don't recall the solution if there was any. I'd start with looking at the power supply voltages to see if they change radically from working to not working. I bet the service manual with the schematics is in our database at www.akdatabase.com and if not, they're likely at www.hifiengine.com. You'll have to register there but I've never experienced any issues from having done that years ago.

 

[Yahn]

Thanks for the service manual I gave it a quick read through. I might have some clipping happening, and the cause of that are the trimming resistors. I did some more searching on here and found a thread with a similar issue here. The problem with the unit was a "garbled sound." I had the same problem initially. Maybe I need to test output transistors on the power amp board to see it those are causing the trouble. It seems that the problem comes before the output transistors because both channels are affected when the audio stops working completely. I have a hunch the problem is in the power amp board.

Regardless, I will test the power amp board and the power supply voltages. rBuckner, do you have any guidance on what those voltages should be on the power supply?

 

[rBuckner]

The schematic in the SM I have is on page 18. Down at the bottom center is the power supply. The voltages of interest are the DC ones on the right side with the exception of the lowest two as that is the 8Vac for the lamps. Only the +35Vdc output is adjustable. Note that off to the right on the schematic that the "+" sign for cap C004 is on the wrong side. It should be at the bottom.

 

[Yahn]

I tested the voltages at the pins J801 thru J808 and found the following:

J801 0V
J802 0V
J803 -29.0V
J804 +29.0V
J805 +19.3
J806 -+19.5
J807 0V
J808 0V

I didn't bother measuring the voltage on any other components. J801 fluctuates from negative to positive when my meter is set to the micro volt setting. Any idea what could be causing the problem?

 

[Yahn]

I also tested the what was supposed to be +35.0V J812 voltage pin and the result was +23.0V

I didn't bother with the bulb pins.

 

[rBuckner]

J801 and J802 are AC voltages and you must have measured them with a meter in DC voltage mode, that's why you saw the varying reading. The AC voltages must be there since the voltages at J803 and J804 are okay.

The interesting reading was the one on J808 which should have been +14Vdc or reasonably close. I suspect that zener diode H807 is shorted out but possibly the resistor to the power source, R805 could have gotten broken. Another possibility is that C809 is shorted. With the power off, measure the resistance between the chassis (or J811) and J808. I'm betting it is very low in ohms.

If someone has worked on the board, maybe they created a solder short so take a look at the bottom side of the board in the area of the zener diode H807.

 

[Yahn]

Quick update. I fixed a solder bridge near the H807 area and tried to see if that fixed the problem. Lo and behold the fading in and out disappears. I also tested J808 and got a reading of 13.5V, and my J812 voltage is about 31-32V. I also tested the resistance of 811 to J808 and this was fairly high. In the Mega Ohm range. C809 was installed correctly and not shorted.

However I ran into a weird problem further testing it. When the input is set to AUX it plays beautifully (haven't tried phono yet) but when I play FM the voltage across J812 decreases to 20V and starts chopping the audio, and eventually stops playing altogether. Then fades back in sporadically. Again, there is no choppy audio when the AUX is the input.

Could this be because I didn't recap the AM/FM board?

 

[Yahn]

Also I tested H807 in the 2M ohm range and got a positive value in one direction, and the same negative value in the other direction. I think this component is fine.