[Kenwood] TS 940 Problem

[Simon Watt-Wyness]

Hello,
	I am trying to fix a TS940 for a friend of mine. It has some problems attributed to the 22 volt PSU. This is a simple series pass regulator  supplied from about a 30 volt unregulated rail but the driver and Zener reference is derived from the +28v regulated supply.

My problem is this:-
	In RX, the heatsink above where the power cord goes in, IE - the PSU heat sink, get s very hot after about 5 mins of running on RX. The voltage is a bit low at 18v and rises when heating.

The current drawn by the downstream circuits (control and digital boards) are about 540mA on RX and 750mA TX. Now the series pass transistor - a TO220p package is disspating about 7 watts.

Can anybody tell me what current should be drwan by the down stream boards? I measured by popping the pin on the emitter lead coming back into the PSU board. Some connectors look slightly yellow, indicating some heating.

Isolating the supply on the cont bord shows a rise in voltage as expected and cooler running PSU.

The fault is lots of ripple on C3 in PSU as a result of excessive load. The display is dim and its inverter supply is derived from the 22v. Also hum on TX which in some cases may lock VOX in TX due to the hum being picked up by VOX. All seems to be caused by the 22 volt supply.

Any help would be good.

Even better, a fix. :)

Thanks in advance.

Simon.  ZL1SWW



> Simon Watt-Wyness 
> Technical  Consultant (Communications Specialist) CCNA / CCDA / NZCE / Ham Call ZL1SWW
> Eagle Technology Group
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