Mocom 70 Base Troubleshooting

tuba · Post by **tuba** » Tue Jan 29, 2002 5:25 am

<font size=-1>[ This Message was edited by: tuba on 2002-01-31 21:34 ]</font>

<font size=-1>[ This Message was edited by: tuba on 2002-01-31 21:34 ]</font>

perthcom · Post by **perthcom** » Tue Jan 29, 2002 6:13 am

Ah.. a good ole Mocom 70..
Well if you hear the transformer hum, then the fuse by the power cord is good for sure.

There is a 20A fuse on the top side of the power supply chassis, I suspect this fuse is blown, this is the fuse for the unreg. DC voltage.
What happens is there is a pot that adjusts the voltage that is now dirty from sitting around for 25 odd years. When you moved the radio, the jossling caused the pot lost its wiper setting. This caused voltage from the power supply to exceeded the overvoltage protection setting and the crowbar circuit tipped blowing that fuse.
You can try cleaning the pot, but it would be better to remove it, measure the pot and replace it with 2 fixed resisters.

Bruce

Glen W Christen · Post by **Glen W Christen** » Wed Jan 30, 2002 9:52 am

On some of the PS's the twenty amp fuse is on the underside of the chassis. Also, early models had a protection circuit that would shut down with power fluctuations - powering down would reset it. Motorola got rid of that with an SRN that replaced it with another fuse.

kens · Post by **kens** » Wed Jan 30, 2002 10:19 am

I remember having to replace diodes in the Consolette power supply. You might want to first test your radio by bypassing the supply and powering it up by connecting a known good supply or battery through the molex connector on the chassis. I used to do this a temporay fix until we could get the power supply back in service.

R F Burns · Post by **R F Burns** » Fri Feb 01, 2002 5:41 am

If you have DC at the output of the rectifiers (diodes), check the electrolytic filter capacitors (large cans at the front end, opposite the heat sink). I had a supply that blew two caps (they were old and probably dried out) open and it caused the regulator to stop regulating. Got no current out of the supply. Oddly, the receiver worked, but transmit barely moved the wattmeter. Kinda drove me nuts 'cause the voltages weren't far our of line -- 'til I looked at it on a scope and saw ripple.

-RF Burns