Mocom 70 Install

[Gocart878]

hello, i am trying to place a mocom 70 radio in my vehicle, it was pre-wired into the vehicle in 1989 when my brother was fire-chief, now, i need to use the radio, i the basic radio is installed, with connection to the batter and a good ground, now, i installed the control head, but i am tring to locate a decent source for the control unit power, Fused lead #19. But when i take a wire from #19 on the control head, and hook it into the fuse box, i have no luck with the radio powering up. the radio works well, when hooked to a battery in my basement. What could i use as a source for the fused #19 power lead, and to get the unit to power up only when the key is in the on position. my vehicle is an 89 buick lesabre.

[EC-7]

You can use the fus from the windshield wipers, heater or turn signals. They are on a switched circuit. If I remember correctly, I think there were 2 positive wires from the controll head. Maybe pin 21, 22, or 23. I'm not positive on that though.

[Gocart878]

ok, i determined a bad groud, thankyou for your help. now what can i use to make a speaker?

[wa2zdy]

Any old speaker will work long as you connect it to the right place. Just be careful about using too good a speaker or the PL hum will buzz you to insanity. You don't want to use a good stereo speaker. Cheap speaker for communication quality audio will keep the voice clearer and odd noises to a minimum.

[Glen W Christen]

#19 is designed for unswitched input
#20 is PTT ignition control
Both Motrac and Mocom 70 used the same scheme - you wanted to turn off the filiments in the Motrac or it would kill the battery - so #20 provided that function.

[Gocart878]

ok, the radio problem was fixed, now i have 2 good speakers, and when i hook them up, all i can get out of them is a very low audio sound when the volume is to the max. This is when i have the radio on squelch. when its in normal operating mode, i get nothing. Do the speakers need to be grounded to the vehicle to poduce sound? what could be wrong

[k2hz]

Check your wiring to be sure the speakers are properly connected to the control head. DO NOT ground the speakers.

Audio problems, especially audio output transistor failures were very common on Mocom 70s. They were a great radio but when they did come in for repairs it was almost always receiver audio trouble.

[Gocart878]

well, are these transistors located in the central radio in the trunk, i do have the wiring connections just bare wires sticking inside the back ports for the speaker, thats all i can come up with, is there a special type of end which fits in the speaker ports

[k2hz]

You do need to be sure the speaker wires are making a good connection to the control head sockets. Each wire should be terminated with a pin plug of the appropriate diameter. If you don't have pins from a junk speaker available, you can sometimes make a temporary fix by folding over and twisting the bare wire end until is approximately the diameter to fit the socket and then tin the wire end with solder so it will fit firmly in the socket.

If the speaker connections are OK, the next best step would be to try the radio connected to a known good control head, cable and speaker or use a Motorola test set connected to the radio's receiver metering plug and see if the audio works. If the radio was operated with no speaker connected and the volume turned up high with the squelch open, this increases the possibility of blown audio output transistors in the radio.

In an old radio, there is also the possiblity of corroded connctions on the control cable plug and/or a broken wire in the control cable.

I don't have a schematic for a Mocom 70 anymore but I believe it does have an audio output transformer. You can try an ohm meter test from the control head speaker terminals looking back into the radio. Turn off the radio and unplug your speaker wires. Connect an ohm meter to the control head speaker terminals and see what resistance you read. You should see a fairly low resistance (a few ohms) if the control wires, plugs and the radio output transformer are good. If it is high resistance or open, suspect a cable or plug problem.

If you don't see anything obvious, and don't have access to a test set, the next step would be to have the radio checked at a service shop.

[Glen W Christen]

The Mocom 70 does use an audio output transformer. One side is grounded via pin 5 and the other goes to the control head, pin 3, via front plug, pin 3. You might check for audio at the yellow and yellow/blue wires at t he transformer to eliminate cabling and control head. If you have audio there, short pins 11 & 15 together at the control head - this will bypass the intenal components of the CH.

[Gocart878]

well, i have no clue where the audio transformer is, can this part be interchanged from another 70? what does bypass the internal components of the CH mean? thankyou

[k2hz]

well, i have no clue where the audio transformer is, can this part be interchanged from another 70? what does bypass the internal components of the CH mean? thankyou

What he is suggesting is that if you temporarily short pins 11 & 15 at the control head, it will bypass any problems within the control head by routing the audio from the radio cable directly to the speaker terminals.

If this is not a simple broken wire or dirty connector in the control head or cable and you are not experienced in servicing electronic equipment, you will probably not be able to identify or properly replace the components that may be defective in the radio. It will need to go to a repair shop.

[Gocart878]

all i really want to know is where the transformer is located. this will help me accomplish my task. i have another mocom which i have available to me, i can take parts off of it, or use it for the entire unit, but i would rather only use it for parts.

[k2hz]

I would think it is very unlikely that the transformer is bad. If you do not know how to identify it after the information provided in Glen's response, I doubt you would know how to properly replace it. We are not dealing with plug in components.

If the problem is not external wiring, it is a major service bench troubleshooting and repair issue.

If you have another Mocom 70 have you tried to substitute it for this one to see if it works?

[Gocart878]

i went out and took a look at it again, keeping what you said in mind, i looked at the back of the control head, and noticed that pin 9, 10, and 5 were not connected. i re connected, soldiered the wire ends, and have beautiful sound. thanks for your help and support guys.

mike