KR-7400 Bad Channel - Am I Reading the Schematic Right?

[ringading]

Picked up a 7400 at the local thrift - a miracle in itself as they usually send these off to fleabay.

It was working well and I replaced the capacitors on the main amplifier board and gave it a once over with DeOxit. On adjusting the board for screwing it back into the brackets, one of the legs on a transistor just snapped off. I assume that it was brittle due to the heat this particular model puts off. It was the 2SA620 on the right side of the amplifier board. Replaced it along with its counterpart on the same side with a KSA992 (Qe1 and Qe3).

Now I very little sound on the right channel. Not sure what is going on. I was going to start pulling the transistors to test starting with the outputs and working my way back with the drivers. However, I assumed that since the new transistors were on the right side of the board that they would be the right channel. However, looking at the schematic, I am not sure. I am still learning to read schematics but it would appear that odd numbered components are actually for the left channel and the even are for the right channel - even though on the board the odd are on the right and even on the left.

Can anyone confirm that even # are right and odd # are left? With the replacement of Qe1 and 3 I assumed that perhaps they were incorrect or something else is bad on that side.

 

[turboyam]

I cannot confirm the numbering but i've seen this numbering from other manufacturers. Are you sure the replacement transistor's pinout is the same as
the originals?

 

[mbz]

Correct, odd numbers left, even numbers right.
Trace the path between the 2 emitter resistors between
Qe21 and Qe23 to the protection relay then speaker
selector ...

KSA992 is ecb, 2SA620 is also ecb. Was the 2SA620 a metal
"can" or was it the "traditional" black plastic TO92???
Maybe you got the orientation wrong on 2SA620???

Maybe give the speaker selector another clean otherwise
basic fault finding, localize problem as pre or power amp...

 

[ringading]

Thanks mbz. Strange that they would make the board that way - kind of opposite of what you would think looking at it since the even #s are on the left side but are for the right channel.

The 2SA620s were the small metal can. Double checked my work but those were Qe1 and 3 so were the left channel which is fine. Also cleaned the selector switch again with no change. I will start to check measurements per the schematic to check the drivers.

 

[ringading]

Also - I noticed that the trace you highlighted is for the left channel/odd # components - my problem is on the right channel so I would assume that I would look at this but on the other side of the board.

 

[ringading]

I checked the DC voltage from the emitters of all of the transistors comparing the left channel (good) to the right channel (bad) and they are all identical. Pulling the outputs to check those.

 

[mbz]

I know it's boring but probably an issue with dirty switches, bad solder joint or wire contacts....
If you can run the KR as seperates with another pre and power amp to localize the fault. This
would help greatly.

You can check some of the switches by,
KR powered off/unplugged. Input selector set to AUX, stereo, volume at 9o'clock, everything else centered.
Measure the resistance from the left AUX RCA(inside) to pin 1 on the tone amp (input for left channel)
Then repeat for right AUX RCA(inside) to pin 2 on the tone amp (input for right channel).
Both resistances should be about the same, +/-20%.

Maybe check the bias and dc offset, unlikely to be cause but general house keeping..

 

[ringading]

Looks like my last post didn't go through. I reflowed the solder on the main amplifier board and we have sound on both channels!

Now to figure out how to set the bias. The manual says + to TP 10 and - to TP 11 for one channel, adjust Vr1 and 2. Then it says to set ammeter (measureing mA) to TP 12 and adjust Vr1 and 2 again. I know these will be on the emitter resistors, just need to figure out which ends to use.

 

[mbz]

Great news on the sound both channels.

Maybe best to wait on someone with first hand knowledge on the bias procedure.
Sadly I've not had the pleasure of the KR-7400, yup, envy. My take on the bias
procedure would be something like,
- connect red probe of MM to TP10, left channel
- Connect black probe to TP11, left channel
- Adjust VRe1 trimmer (hopefully new bourn multi-turn) so voltage reads 30-40mV
- Repeat for right channel adjusting VRe2 to achieve 30-40mV
- allow 5 minutes for the amp to heat up/reach operating temperature
- set bias for left channel at 40mV adjusting VRe1
- set bias for right channel at 40mV adjusting VRe2

This is the coarse adjustment

TP12 is probably a plug type connector on a metal post.
Power off/unplug the amp. Remove the plug from TP12. Set multimeter to milli-amp scale
insert red MM probe into plug and connect black probe to TP12, may need a clip. Need solid
contact. Power on the KR, adjust VRe1 (left) and VRe2(Right) for 40mA.

Add my vote for a recap of the power supply and pre/power amp stages using
quality audio caps(Elna, Nichicon,,,) from a reliable source (Mouser, Digikey,,,)

 

[ringading]

Thanks mbz. I am trying to figure out just where test points are on the actual board. I think I have it figured out. Will report back if successful. The service manual is not real clear as compared to some others.

 

[mbz]

Page 29 shows connection points using volt meter then ammeter, missed this first time around.
Not sure why you would need to do both...