RX V1700 Yamaha Receiver - transistor short (with diagram!)

[chode]

Hey all!

I have a Yamaha RX-V1700 Receiver that was auto-powering off due to abnormal DC Voltage.

I ventured into the unknown using the repair manual and have finally discovered the problem in the Main board. This is the manual http://elektrotanya.com/?q=showresult&what=rx-v1700&kategoria=&kat2=schematics

I found +46 volts at the surround sound L output post at the back and followed this back through to the main amplifier board. I found a short on two pins of transistor Q370 (see attached). I have now removed both the Q370 transistor (2S C5291) and the C321 capacitor (100pF 630V) and the amp is again working perfectly.

As its working, can anyone tell me what these two parts actually do? I'm not sure if I should bother ordering replacements and soldering these in?

Thanks!

[fubar.gr]

This looks like a push-pull arrangement (actually several stages of push-pull connected in series), where Q370 "pulls" and Q371 "pushes" (or is it the other way around?)

With Q370 removed, it "pushes" like before, but it only "pulls" weakly.

It is still going to work but with some distortion and non-linearity, so the audio quality won't be the same.

[cvanc]

What fubar said.

It may work now but you have lost the symmetry of a push-pull circuit relationship.  Distortion (and maybe headroom) must be compromised.  Definitely replace the parts.

[chode]

This is great, thank you both.

I assumed it was just additional signal amplification as it seemed to play ok through the speaker. I'll get the parts ordered.

[chode]

Also - my guess then is that Q370 "pushes" more voltage when it gets a signal at the base as its on the +ve branch of the amp with an npn transistor.

That would suggest Q371 "pulls" more voltage across to the -ve branch when it gets a signal at the base of the pnp transistor?

I could be massively wrong - I'm a complete novice at this!

[macboy]

The transistor is labelled "C5291", but this is short form for "2SC5291".  The initial "2S" is sometimes implied, it refers to the device having 2 junctions (transistors).