CDM1250 to 1550 head conversion

[Jay]

Has anyone converted a CDM1250 head to a CDM1550 head?

Studying the service manual (see figures 4-7 and 4-8) it seems that there are some jumper and resistor differences on both sides of the board. Just wondering if anyone has done this before and has any thoughts.

Jay

[RADIOMAN2002]

You can use a 1550 head on the 1250, but unless you re-flash the radio to a 1550 it won't do anything. I have converted 1250 heads to 1550's. You have to change some resistor patterns, so the radio understands what head it has. There is also other resistors that you need to install, they are value critical to make the additional rows and colums work, IE: the other buttons.

[Jay]

You can use a 1550 head on the 1250, but unless you re-flash the radio to a 1550 it won't do anything.

Yep, I've seen the WRONG HEAD message when trying to just make random swaps.

I have converted 1250 heads to 1550's. You have to change some resistor patterns, so the radio understands what head it has. There is also other resistors that you need to install, they are value critical to make the additional rows and colums work, IE: the other buttons.

Ok - that answers my question. The resistor patterns seem to be pretty well documented on those pages in the service manual, and it looks like it is fairly easy to make it happen. Too bad they are value critical...I guess I shouldn't try to wing it and should order some parts.

Jay

[RADIOMAN2002]

Telling the radio what type of head is easy, I just relocated resistors r8057 and moved to r8058, then r8053 to r8052. the ones for rows and colums are the critical value ones. BTW those resistors are NLA from Motorola, I tried. I then found this :
Item Number. 0613952R17
CER CHIP RES 47K OHM 5% 0402
Unit of Measure: 1 EA
I am awaiting my delivery of those to see if they work.

[Jay]

That will be interested to see if they work....please keep me posted. There has to be a current source for parts (hopefully replaced by another part number). I hope so - since they are still making the radios!

[RADIOMAN2002]

Actually no, they recommended replacement of the whole board. They also sugusted Mouser or other parts vendors.