Unstable voltage President Grant XL

[paulietech]

Hello,

New to this arena and just trying to learn!

I am working on a grant that I bought on ebay, was trying to change the final to an ERF-2030+ because the output on their mod (MRF477) was only 9 watts peak, no matter what. Now with the ERF-2030+ it's only 6 watts peak. I was following guide: https://www.worldwidedx.com/threads/mosfet-conversions-for-upd858-and-mb8719-boards.95853/ which had links to cbtricks that had bogus info on the companion part as per https://www.worldwidedx.com/threads...ors-in-the-drawing-and-the-correction.157713/ and I smoked a L39 as a result (had another grant around so borrowed that to replace it). Once i got the biasing right, it seemed okay, keys up and swings on transmit.

Here's the problem:
I was trying to check the voltage at gate and noticed that it would start at like 3.4 and drop rapidly according to my multimeter. I tried different parts of the radio and noticed that it was like that just about anywhere I tried to test positive voltage.

I suspect a bad voltage regulator, but is there anything else that might cause this symptom?

It's hard to tell if the voltage is right on the gate if it isn't stable.

I tried the other grant and the voltage was stable.

I swapped the two VRs not realize how static sensitive they might be and now both radios do the same behavior (oops).

I'll have to wear my wrist strap when I get VRs back in (are they really that sensitive?)

I'm also going to try a different multimeter, maybe mine has a low battery or something.

 

[paulietech]

It's something to do with the power supply - in the truck it shows a stable gate voltage.

 

[paulietech]

Grounded it to the negative pin of the powersupply and it sems to be showing a steady ground. Previously was grounding it to the chassis. Strange how this works in the truck but not on a power supply.

 

[NZ8N]

Like most CB radios, the case ground is floating, so the radio can be mounted in either negative ground or positive ground vehicles. Case ground is capacitively coupled to board ground in various places, especially the audio and Transmit areas of the board.
Always use board ground, or an IF "can" (transformer) if from the parts side of the radio, for your ground reference.

 

[paulietech]

Thanks NZ8N! Yea, not sure if there is something else wrong with this radio, it does exactly the same watts max when it had a MRF477 (9 or so) in it (Driver is still C2166). I have more ERF2030+ but hoping I didn't damage this one - waiting for the correct parts to get in and I'll try to swap out the driver and final with fresh ERF2030+s. And now I know how to properly test for gate voltage. I had scraped together resistors that were close (22000ohms) for the EN369FN circuit - it's 3.36v and it needs to be 3.55-3.75v from my understanding.

Someone went mod crazy on this thing, something with the clarifier range from what I can tell, it's supposed to be variable output - deadkey changes but never swings past 9 watts or so, some weird stuff like the black wire in the audio circuit (I think) is cut, and a 10 puff electrolytic capacitor where it is supposed to be 2200 puff ceramic, who knows what bugs might be in it, but hey, it's a good learning box!

 

[paulietech]

Like most CB radios, the case ground is floating, so the radio can be mounted in either negative ground or positive ground vehicles. Case ground is capacitively coupled to board ground in various places, especially the audio and Transmit areas of the board.
Always use board ground, or an IF "can" (transformer) if from the parts side of the radio, for your ground reference.

Might I have damaged the radio by doing this? The receive seems to waver in and out now, it did not do that before.

 

[paulietech]

Like most CB radios, the case ground is floating, so the radio can be mounted in either negative ground or positive ground vehicles. Case ground is capacitively coupled to board ground in various places, especially the audio and Transmit areas of the board.
Always use board ground, or an IF "can" (transformer) if from the parts side of the radio, for your ground reference.

So I got the driver and final swapped out, ran into an issue where I had to trace down a blown diode, seemed like it was averaging 7-8 watts at first, peaking at 25, then when I brought it out to test it in the truck it peaked at 25 on a 3 count and was avg 1-2 watts, then it just turned off, and then noticed that diode burned up (D44), and a capacitor trace had fallen off (c141 grounding). I replaced the diode and resoldered the capacitor and now it only does about .5 watts avg but peaks at 15. The receive no longer wavers (I almost wonder if someone was running a testing device for something). Audio is coming out of it on the PA and to another radio, any idea why it's only avg .5 watts?

I did also replace C48 which was a 10uF to the stock 223 (22000pf) and the resistor R114 which was 10kohms with the stock 27kohms as well.
Do i need to retune something by changing those values back to stock?