SX-737 power amp AWH-033 issue, blown transistors

[pa_200]

Clipped two jumpers from pin 28 and 32 to ground, but no change. I suspect it must be some external influence that I can hear now with the receiver on. Maybe one of the LED or energy saving bulb in the living room. Will give it a try it in another room.

Merlynski one other question regarding the power supply. I didn’t change both zener diodes so far because I have good voltages there. The WZ-140 measures +14vdc and the WZ-130 shows -13.3vdc.
Any need to replace them? Are they prone for failures?

 

[merlynski]

Clipped two jumpers from pin 28 and 32 to ground, but no change. I suspect it must be some external influence that I can hear now with the receiver on. Maybe one of the LED or energy saving bulb in the living room. Will give it a try it in another room.

Merlynski one other question regarding the power supply. I didn’t change both zener diodes so far because I have good voltages there. The WZ-140 measures +14vdc and the WZ-130 shows -13.3vdc.
Any need to replace them? Are they prone for failures?

Those voltages are in spec (±5%), I would not change them unless they fail.

 

[pa_200]

Sounds good to me,

 

[merlynski]

Grounding the input to the Control amp shows the hiss is not coming in from before it. Next step ground the output of the Control Amp, pins 1 and 3 (AWG-030). Still hissing?

 

[pa_200]

One of my jumper was broken, so I rechecked the pins again and used a headphone with the speaker switched of.
When connecting at AWX-071 pin 28 and 32 to ground, this time always the left channel goes quite. Doing the same on pins 1 and 3 (AWG-030) is different. Pin 1 to ground cuts of the right channel and Pin 3 to ground cuts of the left channel.
Then switched to Phono and AUX with nothing connected, if you turn the volume up one can hear the tuner as well in AUX. Testing the same procedure, pin 1, 3 and 32 behave like on tuner selected, but connecting pin 28 to ground hat no effect on both channels at all in the AUX or Phono function.

 

[pa_200]

Merlynski I believe there is a mistake on the main schematic printed on page 27/28 with the pin numbers for the AWX-071. The number 28 can be found on the left and right side and I'm quite sure, the 28 printed on the right side below the 30 should read 29!!

Will check the pins again in the afternoon.

 

[merlynski]

Merlynski I believe there is a mistake on the main schematic printed on page 27/28 with the pin numbers for the AWX-071. The number 28 can be found on the left and right side and I'm quite sure, the 28 printed on the right side below the 30 should read 29!!

Will check the pins again in the afternoon.

The individual schematic of AWX-071-0 on SM page 60 clearly shows that you are correct, it is pin 29.
Good catch!

 

[merlynski]

One of my jumper was broken, so I rechecked the pins again and used a headphone with the speaker switched of.
When connecting at AWX-071 pin 28 and 32 to ground, this time always the left channel goes quite. Doing the same on pins 1 and 3 (AWG-030) is different. Pin 1 to ground cuts of the right channel and Pin 3 to ground cuts of the left channel.
Then switched to Phono and AUX with nothing connected, if you turn the volume up one can hear the tuner as well in AUX. Testing the same procedure, pin 1, 3 and 32 behave like on tuner selected, but connecting pin 28 to ground hat no effect on both channels at all in the AUX or Phono function.

After noting the correction, when grounding pin 32 and pin 29 of AWX-071 does the hiss go away in both channels?
What position is the Rec Selector switch in? If it is on FM you may get some cross-talk from the amp output with AUX or PHONO positions on the Function switch. Move Rec Selector to Source and listen for cross-talk.

 

[pa_200]

The SX-737 is moved to another room and now the hiss is gone completely. No clue so far who the culprit is. Must be some external source like Wifi repeater, LED-lights or what so ever. There is a lot of different electronic stuff in the living room especially in the time before Christmas.