Over protective Yamaha M-70

[avionic]

BTW
FYI

There are glaring differences in the C-50 Vs C-70 driver board circuitry. Different P/N's as well. C-70 is and upgraded C-50 sorta.

 

[T3SAM]

Perfect choice would have been KSC1815 & KSA1015. IMHO.

Meanwhile. Back at the ranch.


Measure the output transistor collector voltages. Mini grabbers recommended!! Monitor each for fluctuations. Both on the positive and negative rails. Any fluctuation could/will result in + or - DC offset.

Again thanks lot Avionic. As I am not an expert I did not understand "Both on the positive and negative rails". I do have mini grabber. If you please tell me where to measure the voltage that will help.
I think the collector on all the output tr is in the middle in this amp. I have measured emitter voltage on all of them. R side unstable <1v but L side stable <1V.
Really appreciate.

 

[avionic]

I did not understand "Both on the positive and negative rails".

Output transistors have their collector pins connected directly to the unregulated power supply rails. The PNP (2SA) outputs are connected to the negative rail and the NPN's (2SC) are connected to the positive rail.
I need you to measure these two rails. The easiest place to measure is at the transistor pins.

I have measured emitter voltage on all of them. R side unstable <1v but L side stable <1V.

I need exact voltages, stable or unstable. Measured in millivolts. I'm trying to figure out the reason for 400 mv of DC offset.

Ideally the collectors should be the same absolute value. Around +39 and -39 vdc.

For example.

+ 39.65 and - 39.65

If you have over a 400mv difference between the two. You could potentially have a bad/ leaking output transistor. That may or may not be detected by a multimeter test or component checker like a PEAK DCA55. The DCA75 still may not be able to find it but definitely provide you more information.

 

[avionic]

I think the collector on all the output tr is in the middle in this amp.

Yes, they are BCE. collector is in the middle.

 

[T3SAM]

Yes, they are BCE. collector is in the middle.

Thanks a lot Avionic. I have measured voltage with Fluke 116 as attached.
The E voltage I have noticed changes time to time. I measured yesterday it was different to this. The E volt was negative yesterday. Anyway E volt for ch R jump around a lot, fast for few Tr, thus I gave a range for E volt. For C voltage both ch does not jump.
Sorry my English is not so good. The DC offset I can adjust to 0 but it keep change a lot and very fast sometime (sometime slow, seems like stable) and sometime spikes to 400+mV then also come down quickly.
I also measured the B voltage.
I have measured DC volt in speaker terminals and found about 1V AC, if I select DC it shows 0, if I select mV it shows OL.
I am annoying you I know, but I am struggling with this and appreciate your help.

 

[T3SAM]

Hi All,
I have swapped all the output Tr but still getting the same issue. Please help me if you can. Thanks