Batboard

Looking for a little help with some Motorola Motracs. I picked them up for free a few months ago and now I'm trying to test them (at least testing power up and that they transmit). I've wired everything up per the service manual but I'm still not getting any power. I've experimented with wiring combinations between the green and orange wire to power, but am slowly running out of things to troubleshoot. These won't go into use for me, but I'm looking to get rid of them soon and I just want to confirm they're still in working condition at least before I list them and get lucky enough to find someone that wants a working radio.

Anyone got tips or suggestions on what else to check with these dinosaurs?

Just a few things come to mind:

Double check wiring numbers on the head, and that the head matches the radio
Make sure your power supply will source 25 amps or more
Make sure red and black wires are connected
Verify the transmit crystal elements are in place and functional
Check receive functions first then check transmit functions

Pin 19 on the control head, green wire, goes to +12 volts thru a 5 amp fuse. Receiver power.

Pin 20 on the control head, Orange wire, goes to the battery or IGN switch thru a 6 amp fuse and provides power for the transmitter filaments.

Make sure you ground the control head along with the ground on the radio

Fowler

Now we can tell who's from the spark radio era in this board!

Ancient as these radios are, they are easy to work on if you have schematics.

There should be a red pushbutton "netting" switch on the radio chassis. It enables the receiver audio and keys the transmitter. Connect the radio to a dummy load and set the internal power switch to low. Key the radio with the netting switch while on a simplex channel. You should hear your own audio on the speaker if the multiplier circuits in the exciter and the TCXO are working. You can actually tune these up with a DVM, frequent counter, and wattmeter. There are DC test points for most adjustments. If you can find a schematic for your model, it's easy. Get the transmitter tuned up on frequency using your counter. Next, tweak the tuned circuits for max RF out. Then use your transmitter as a signal source. Key the netting switch, and use the metering points to peak the rf signal into the 1st mixer. Don't mess with the IF if you don't have a sweep generator. Tweak the 1st LO frequency by measuring for a null at the discriminator.

As mentioned before, make sure that you have power to the filaments. Assuming that you have the typical inverter plate power supply, you should hear a high pitched whine when you key up. No whine = dead inverter. Good luck fixing those if it's a cap or transformer. Ditto for selenium rectifiers. Carefully check for plate voltage on the PA tube cap with the radio keyed and the TCXO exciter crystal pulled.

Check for a blue glow in the driver and PA tubes when you key down. If the tubes are old and gassy, they will glow blue when you key up, and the power will drop to almost nothing.

My low band Motrac has a very hot and tight receiver. We replaced the 1st LO and mixer PNP germanium transistors with PNP silicon transistors. We got better than 0.1uv sensitivity for 12 db SINAD. The passive front end is very clean, and we got better than 80 db IM rejection. Gobs of audio out. These are great monitor receivers for openings on 52.525.

Good luck. These are oldies but goodies if you can fix them up. No software or computer needed. Just some time and very basic radio tools.

We replaced the 1st LO and mixer PNP germanium transistors with PNP silicon transistors...

Which series receivers did you do that on? I've got VHF Lo and VHF Hi, "L" series receivers. Also, if you don't mind saying, what transistors did you use for replacement. Both of my receivers are pretty darn hot though they're still on the commercial frequencies and I haven't measured them yet but they hear stuff that I wouldn't have expected them to.

Thanks for any help you may offer.

Tom DGN

Also make sure the radio is plugged for negative ground, there is a jumper in the power supply section that is either neg only or can be pos or neg ground. The "L" receiver was the next to last model made, by that time they had an excellent receiver. I still use them in old TAC receiver chassis for monitoring.

IMHO, the MHT series are, pound for pound, the best radios Mother M made. Good luck, though, changing frequency ranges and if the Permakay filter goes you're SOL, as far as I know. Excellent receivers, ultimately repairable, great audio. I was lucky enough to order the replacement helicals when "M" still stocked them and I now proudly own a 144 MHz. MHT receiver. The Mocom 70 is a better transmitter, at least at or below the 50 Watt power level.