Old Midland 151M good power no modulation

[Randy Nix]

This radio receives fine but when keyed the power drops to 8 volts and I cannot get any modulation. Does anyone have any idea what would cause this? I'm a newbie and using this unit top play with.

 

[nomadradio]

Hi Randy,
Can't find any reference to a "151M" model anywhere. Does it have a sticker with a 5-digit model number anywhere on it? Should start with the digits "77-" followed by three more digits, usually.

Is this a base radio with a built-in AC power pack?

73

 

[Randy Nix]

It's a mobile unit. I found the model# 77-151 on a steel plate on the back. It says 151M on the front of the unit. Made in October '83. Made in Korea. Thanks.

 

[Switch Kit]

IM not to sure what dropping to 8 Volts means ? If it has a PA ,did you try that ? at least this way you will know it isn't the audio chip , or did you check the mic plug to make sure all the wires are soldered on correctly ? ...sometimes it can just be a basic fix . Good Luck

 

[nomadradio]

Now that's a model number, '77-151'.

Trouble is, the last volume of the Sams CB series was #293, published in April 1981. A 1983 radio just won't be in there. I checked the index, and that model number's not included. Too new.

Don't have any data here for it at all.

Dang.

73

 

[Randy Nix]

I get a pop then nothing when I try the PA. I'm measuring the voltage inside the case coming from the power plug. Thanks!

 

[Switch Kit]

Just a guess Randy ,but it might be the audio chip . If you can get the number off it / I might have an extra

 

[nomadradio]

I'd try checking the output capacitor between the audio chip and the speaker jack/internal speaker. They tend to short, and will then hammer the TA7205 audio chip. Pretty sure that's the type that radio uses.

Beyond that "generic" description, that part is probably rated at only 10 Volts. A rating of 16, 25 or higher would have been wiser. The positive end of this capacitor will be wired to pin 10, of the TA7205, opposite the end with the "notch" in the black plastic and the negative side will lead to the speaker jacks. May have a capacitance value between 330 and 1000 uF.

73

 

[ButtFuzz]

Once you get the short figured out, here's the peaky-tweaky stuff:


Midland 151M, 202B, 202M, 2001, 3001 Midland 4001: <L304, L305, L306, RV201>