When Volume up - left channel sound prob ?

[Krokotam_]

Hello,
as i said i`ve got problems with my left channel ( everything looks good on the pcb ) the right one is perfect, but the left one is not amplifiyng proper the signal - and from that we have distortioned left channel, on low volume levels it sound normally but when the volume is up everything goes to hell ...
I don`t have osciloscope to check on, so any practical suggestions are wellcomed. If that helps the amp is Marantz PM 440 , based on Nec output transistors.

Hmm very strange

 

[unclejed613]

it is difficult to diagnose distortion without a scope. first thing i would try is to check all of the B-E juncion voltages. in an amp functioning normally, all of the B-E junctions in the amp, except for the protection transistors should be forward biased at about 0.6V it varies for different transistors between 0.5 to 0.8V. this test is done with no load and no signal.

 

[Krokotam_]

Other helpfull ideas ?

 

[AndrewT]

You need to identify the collector lead of each of the transistors.
Then measure the Vbe. +ve for NPN and -ve for PNP.

 

[unclejed613]

on about 99% of all japanese transistors, identifying the collector lead is simple, it's the one in the middle. measureing Vbe is easy, measure across the two outside leads. that goes for all TO-92, TO-220, and TO-3P, as well as most japanese TO-214 devices. which one is the emitter? that's what changes between different packages. on TO-92, it's ECB, and on TO-220 and TO-3P it's BCE. TO-214 transistors are usually BCE, but sometimes are EBC(like american transistors)

 

[Krokotam_]

We have at the start 51 Volts from the out , but only from the single channel. After some seconds it drops to normal values 0.2 mV. That`s very strange , any suggestions ?

 

[Mooly]

Without a scope and circuit diagram you are limited as to what you can do. Try comparing one channel against another

 

[unclejed613]

there's an electrolytic cap next to or near one of the input transistors. it's usually between 33uf and 470uf and most often rated about 20 to 100% above the rail voltage (for 51V rails this would be maybe 63, 75 or 100V), and it may even be a non-polarized cap. this cap being bad would also cause gain problems and distortion. try replacing this cap with a new one. it could be that the one you have has developed high ESR which in this case makes it take a long time to settle because it's charging current is severely limited. or another (not quite as likely to make such a huge offset, but possible) possibility is one of the current source caps has developed a high ESR problem and so the output latches to the opposite rail until the current source has a chance to catch up.