JVC amp - Right ch. not working

[dmx2020]

Hi all,

I have a JVC AX550bk (90wrmsx2) late 80's model amp with a troubled right channel. When the right speaker is connected there is extremely low volume audio which it gets distorted as the volume is turned up. Left channel is working as normal.

It has a toshiba output pair 2sb755/2sd845 (i.e.: Q761-Q764 in the schematic {around the centre-top-right of diag.}). I haven't pulled these (or any of the output resistors) out to check yet but is there any obvious signs of what might be the problem from the above.

A schematic of the power amp section is attached below.

Please let me know if need a full schematic!

[bob91343]

With this symptom, I'd say an oscilloscope and signal generator are called for. Having said that, I'd first clean the switches to make sure the problem is in the active circuitry.

[Mooly]

Check the volt drop across the output emitter resistors, and compare with good channel.

Check supplies to drivers are OK (the 10ohm safety resistors !)

Have no speakers connected during testing.

[sasmit]

Originally Posted by dmx2020

Hi all,

I have a JVC AX550bk (90wrmsx2) late 80's model amp with a troubled right channel. When the right speaker is connected there is extremely low volume audio which it gets distorted as the volume is turned up. Left channel is working as normal.

It has a toshiba output pair 2sb755/2sd845 (i.e.: Q761-Q764 in the schematic {around the centre-top-right of diag.}). I haven't pulled these (or any of the output resistors) out to check yet but is there any obvious signs of what might be the problem from the above.

A schematic of the power amp section is attached below.

Please let me know if need a full schematic!

seems like a driver / pre driver failure, the outputs are underbiased i.e. reason for low volume , starts distorting as the signal level increases.

[dmx2020]

Thanks for the replies!

@Mooly: I will have the voltage across the E.resistors (R773, R767, R765) checked soon and post. What do you refer to when you say 'supply to the drivers'. Is it the raw power supply to the amp board?

@sasmit: By driver and predriver do you mean the output pair 2sb755/2sd845? sorry about the confusion.

Thanks

[dmx2020]

Another thing I found was when speakers are disconnected and when listening off the headphones only, both channels seem to work as normal (although r.ch is at slightly less output than L.ch). But as soon as the right speaker is conneted, both the R.speaker and headphone right channel are both low volume and highly distorted. Hope this gives a clue as to what the problem might be.

[sasmit]

Originally Posted by dmx2020

Another thing I found was when speakers are disconnected and when listening off the headphones only, both channels seem to work as normal (although r.ch is at slightly less output than L.ch). But as soon as the right speaker is conneted, both the R.speaker and headphone right channel are both low volume and highly distorted. Hope this gives a clue as to what the problem might be.

This is typical of what I suggested earlier, when you're loading up the output transistors it is not able to drive the load because of very little base current. The same isn't a problem when driving headphones. This means the pair of transistors or may be one of the pair that drives the output transistors is not supplying enough current to the output transistors. Let me have a thorough look at the schematic and find out the driver pair.

[sasmit]

Originally Posted by dmx2020

Thanks for the replies!

@Mooly: I will have the voltage across the E.resistors (R773, R767, R765) checked soon and post. What do you refer to when you say 'supply to the drivers'. Is it the raw power supply to the amp board?

@sasmit: By driver and predriver do you mean the output pair 2sb755/2sd845? sorry about the confusion.

Thanks

No the drivers / pre drivers would be 2sc2235 and 2sc2240.

[sasmit]

Originally Posted by sasmit

No the drivers / pre drivers would be 2sc2235 and 2sc2240.

have a look at this

http://www.toshiba.com/taec/componen...c//66/7723.pdf

[Mooly]

Supply to drivers means supply on collectors of Q753 and Q757, and Q755 and Q759.

I would expect around 40 to 60 millivolts across R773 measured between the two end leads (Im assuming it's a 3 legged component and not two individual resistors)

Until that's confirmed you can't go any further.