Grant PC999AC rx issue AM/FM

[tcom75]

Hi everyone,
I just found a new old stock CB President Grant with PC999AC board. The radio has been sleeping for more than 3 decades till few days ago when I found a dealer with this new radio (box never opened).
I unfortunately did not follow the process to apply low DC voltage at the 1st power on.. so I powered the radio with 13,8V directly.
Grant has been working 100% fine for a couple of hrs, then an issue came out. No RX at all on both AM/FM. SSB Rx is fine and also TX is 100% fine all modes.

I checked TR8 and it seems to be good.

Issue: no RX at all on AM/FM

Any suggestion to fix this is very welcome. What specific component /scection should I check?

Many thanks, Steven

[tcom75]

Additional info:
- Squelch doesn't work on AM/FM, works 100% fine on SSB
- AGC works 100% on SSB

73 Steven

[ch25]

Check AM/FM path: http://malzev.tripod.com/cb-radio/block_gr.gif
Measure voltage on output pin of FL1 filter. Should be circa 1V. If 0V replace filter.
Chris

[KlondikeMike]

After all the time and effort that will go into chasing that bad electrolytic, you will find another problem will probably rear its head. Its a game created in your country called “Pop! Goes the Weasel” with 30 year old electrolytics doing the Popping. In North American (AUS as well) we like to replace ALL the electrolytics first in vintage gear and then start the troubleshooting process. I just happen to have a complete kit of electrolytics already built and ready to ship that will get you started;

https://www.ebay.ca/itm/142505776247

[ch25]

When car is broken you replace everything before diagnosis? That's interesting

[KlondikeMike]

When car is broken you replace everything before diagnosis? That's interesting :)

If your car (or whatever else) is 30 years old and has electrolytics in the failed area, YES!

[lbcomms]

Based on experience, that's the way we treat those old unidens with the 10V electrolytics. Replacing just the bad one means there is another one of its friends just about to die, and the worst thing for a business charging for repairs is to have a radio fail a few days or weeks after it leaves us - that can hurt reputations quickly.

So our approach with any old Uniden SSB with the grey 10V electrolytics is replace the lot, then worry about any other fault afterwards. 95% of the time, there is no other fault, and the caps have to be replaced anyway to make the radio reliable.

Less likely, but seen before is a bad crystal (7.345 MHz, sometimes held down with a corrosive brown glue that eats the crystals legs). That will also kill receive on AM and FM but leave everything else functional.

Correction edit - Ignore that line, that only applies to the 40 channel AM/SSB version.

[cb4ever104]

Agree that electrolytics might be an issue, but that seems to be your solution to every fault . And it just so happens that you sell and advertise recap kits on here...? Might be a good idea to actually look at the schematic first and perhaps come up with an action plan that doesn't involve just changing the caps.

[ch25]

"Made with pride in North America" in Shenzhen.
@OP measure first output voltage on 455KHz filter before you start butchering your radio.

Less likely, but seen before is a bad crystal (7.345 MHz, sometimes held down with a corrosive brown glue that eats the crystals legs). That will also kill receive on AM and FM but leave everything else functional.

Just wondering, where is that crystal located on PC999 board?
Chris

[tcom75]

Tks for all the comments. I will start checking the 455Khz filter first. I will report outcomes tomorrow.
By the way I cannot locate the 7.345 Mhz filter on my board..

73 Steven

[lbcomms]

Sorry, my bad, the crystal is only on the FCC version of the Grant of the same era (MB87xx PLL). The version with multiple bands and FM (PC999) has a 10.695 first IF and uses some spare 10.24 signal from the PLL for 455KHz conversion.

I've edited my original post to correct it - the perils of doing stuff from memory...

[dt307]

i will keep it simple ....ish
@chris i think sue is refering to the older version grant that has that xtal the 7.8 if version and a valid point as i have seen 2x pc999 grants with the 455 filter had glue under it and was shorting out the pins, removed the filter cleaned all up and refited the filter and rx came back.
@op what i do and has never failed me yet get the service manual and the voltage chart for the ic/tranies
have a real close look for a loose wire around the mode switch
press and prod and tap on the board both sides for bad joints(use your finger not a metal object).
check the voltages on all the ic/trannies and try to do a by the letter factory service alignment
it may well be a cap/resistor/filter/loose wire from mode switch,
but 100% you will not be able to do a full alignment and a voltage will be a miss.
once you have found a wrong voltage on a trannie ect post here and chris/sue/nick/sammy or i or lots of other people on here can pin point the area of issue and help you further.
changing the 10v caps is fine if you find a faulty one and it be rude not to change all the 10v caps.
i personaly dont do a full recap on radios nor do i change channel leds or meter bulbs or change the thermal compound on ic/trannies if its still juicy(they will all fail in the end at some point).
if its working i dont fix it.
thats the best way i can explain with limited equitment.
if you got some test gear and a brain its a different way i would go and its acactly as chris said.
armed with sig gen and the radio on am following the diagram chris posted starting from the vol pot start injecting a signal and work through the blocks.
every one may be right here ....
mikes cap kit could well sort your prob or not (lot of money and work for a spray and pray aproach and unless your soldering skills are ok you could make things a lot lot worse)
again sue could be on the money with caps or glue like ive seen under the filter.(and sue is one of the most talanted and clever techs ive ever seen).
but im with chris on this do some non invasive checks first,dont reach for the soldering yet.
plan b..........send it to nick get him to add zeros on a switch and add uk while he got the lid off haha

[tcom75]

I checked FL1, I have the following values assuming:
Pin 1 = IN
Pin 2,3,4= GND
Pin 5 = OUT

Testing between filter GND and PIN 5: 0,8V
Testing between filter GND and PIN 1: 8,20V

I'm kind of lost now...what would be the next step?

73 Steven

[ch25]

When you touch filter pins with finger or DMM cable can you hear broadcast stations? Noise, stronger hiss, whatever sound?
Take out FL2, in and out jumper with ceramic capacitor a few pF will do the trick. That way we will verify if it is OK or not.
Chris

[tcom75]

Yes when touching pin 5 of the 455Khz filter I can hear stronger hiss. Ok let me take action on FL2 and will report outcomes.