B&K ST-140 Check-out Upon Power Up

[Falcon]

Hi:
I recently put a B&K ST-140 back together and powers up. I measure the power rails and records +/-65.78Volts. So I believe it is a 105wpc version ( with only one pair of HITACHI 2SK135/2SJ50 output transistors per channel ).

Each of the PCB per channel has two trimmer pots ( P1 and P2 and both are 200 Ohms trim pots ). I guess both pots are used to adjust the DC offset and Idle Current for each channel.

I would like to know what is the procedures that I need to do with this two trimmers to check out whether each channel is ok before I hook up speakers onto the unit. Each of the two rails per channel has one fuse and the speaker also has one fuse. All the fuses seem to be ok upon power on and no blown fuse.

Each PCB also has two test terminals ( JP and JN ) after a 1k Ohms resistor across both ends of P2. Also what is JV+ and JV- uses for?

Any help is much appreciated and Thanks.


Regards,
Mike

[sakis]

idle is about 100-120 ma per amplifier board ....remove +fuse, install fused Ameter adjust trimmer for about 120 ma ...check for ofset adjust that also ....allow 20 minutes warm up observe bias and ofset again readjust if needed ...repeat procedure again if needed , abort if bias or ofset drifts a lot ...there is something wrong ....

keep in mind that in amplifier if this age there might be a hell of a lot of problems before you reach the point that you need to adjust offset and bias

some of them are here listed depending on the importance
---- dry or leacky electrolytics ( any or all of them )
---- resistors exposed to heat or moisture drifetd by far from original
---- all shorts of mechnical things ( loose contacts , bad solders )
---- warn /broken trimmers
---- leacky transitors


proceed with caution , take one step at the time , read the manual and all will go very smooth

Beyond that there is upgrade otions for this machine froom my point of view ....but this is really secondary ...

regards sakis

[nigelwright7557]

First port of call would be to look for excessive dc bias on output.
A rail voltage will probably point to output transistors gone.
In this case dont connect a speaker !

[wahab]

Be careful, only a pair of these devices for such a supply voltage
can be overkill if the speaker is less than 8 ohm..
With a similar supply, my mosfet amp use three pair of the devices
you mention.