President Lincoln PA Stage question

[Phuture]

Hi all,

Referencing Big Pimp's President Lincoln version chronology here: viewtopic.php?p=536299&hilit=lincoln+MRF477#p536299), if you had the fourth version of the Lincoln in the list ("Bold writing black knobs MRF455 large mic") but someone removed the MRF455 pill sub-board, is installing an MRF477 a simple drop-in replacement or are there modifications needed to restore the PA stage of the newer Lincoln main board "back" to make it compatible with an MRF477?

Thanks.

[cb4ever104]

Good question . Unfortunately I don't know the answer . I've had a couple of late 6 band JACKSONs with that MRF455 pcb fitted . The main pcb was the same , and I don't remember seeing any additional components around it , but I can't be 100% sure , and I don't have those radios any more to check . Hopefully someone else can answer it for you .

[Techquest]

all you need to do is put two 560pf ceramic caps in parallel from the base of the 477 to ground to give you full power

[Phuture]

Thanks very much Sammy and TQ.

TQ, that's a good spot. I assumed the schematic matched the PCB layout drawing on cbtricks.org, but C124 and C125 are not present on the latter. I know there are silk screened printed empty ceramic cap positions around the MRF477/MRF455 solder pads, so presumably a couple of them would be for C124 and C125 where fitted. I'll check.

[Techquest]

they were mounted on the trace side of the board

[Phuture]

Yep, it looks like you're right. I took a look online to see if I could find photo examples and I found a couple that unfortunately aren't close up but you can see the two ceramics soldered on the track side beside the, presumably, MRF477 transistors.

So without these two caps have you seen reduced output power?

MRF477 Lincoln with 560pF caps.jpg

[Phuture]

Found the track side image with edits for the main board in the service manual. Interestingly, it shows a 560pF cap and a 150pF cap in parallel with the base and emitter of the MRF477.

At the end of the SM, the part values for C124 and C125 are both 560pF, just to make it more interesting.

If anyone cares to check their own MRF477 version of the Lincoln or HR2510 feel free to enlighten us.

Lincoln SM Track Side.jpg