Help needed with Old President Grant PC409AB Issues

[CSA1863]

Here are the Voltages for the MB8719 PLL on Channel 19
Pin 1 - 3.4v
Pin 2 - 4.28v
Pin 3 - 5.15v
Pin 4 - 3.44v
Pin 5 - 3.44v
Pin 6 - 8.09v
Pin 7 - 0v
Pin 8 - 4.45v
Pin 9 - 8.05v
Pin 10 - 8.05v
Pin 11 - 8.05v
Pin 12 - 0v
Pin 13 - 0v
Pin 14 - 8.05v
Pin 15 - 0v
Pin 16 - 8.05v
Pin 17 - 3.65v
Pin 18 - Gnd

Pin 6 is the loop detect and if it doesn't have voltage it means the PLL is not locked and will not transmit or receive.
Pins 10 -16 are the programmable inputs and will either be high or low depending on the channel selected

The 2 three pronged devices that are mounted on the back (rear) of the chassis are the driver and final transistors.

The voltage regulator is the MB3756 which is an 8 pin IC mounted on the right side of the chassis

 

[Dr. Frankenstein]

The ORANGE wire on the Trace side of the radio tells me that the Clarifier has been clipped (UNLOCKED), which is good for the rest of us.

 

[MKwrench]

I have a hunch that the regulator is being bogged down by one of the grey 10v caps on the board. You've already checked the usual suspects, but even if they test "good" a recap can make everything run way smoother in the end. I use nothing less than 25v caps everywhere. See if pin 1 and pin 3 of the regulator stays about 8v while acting up, that would rule out the bog down.
I've seen the glue you saw around C84 had corroded L53 and/or a nearby diode on a few of my 409s (double check these). Almost all of the mods for the 148 and Grant XL are the same with this board.
I think these are really cool radios, and I hope you get it going. Let us know your progress.

 

[bigtoyboy01]

Here are the voltages from the regulator:

1: .280
2: 13.6
3: 8.18
4: 0
5: 13.6
6: 8.17
7: Not connected
8: 0.038

I read somewhere else that Pin 8 of the PLL should always be 8v...mine reads basically nothing.

 

[MKwrench]

Here are the voltages from the regulator:

1: .280
2: 13.6
3: 8.18
4: 0
5: 13.6
6: 8.17
7: Not connected
8: 0.038

I read somewhere else that Pin 8 of the PLL should always be 8v...mine reads basically nothing.

8 should be 8v during TX (6 and 8 should toggle, RX/TX). Pin 1 is being dragged down, it should always have 8v on it. Here is a schematic from CB tricks.org. It's for a 148, the regulator circuit is nearly identical. I chose it because it's very clear and searchable.
The 1st thing I would try is disconnecting the added wire on pin 1, something in that "clarifier mod" might be causing the short. Search "Easy Grant PC409 clarifier unlock" and try that after repairing the existing mod.
Other than that follow the trace and the schematic from pin1 and go from there.
I think you got this!!!

 

[bigtoyboy01]

What is this component that is attached to pin 1? While picking at the glue I accidentally shorted the 2 sides of it together, and my 8v came back to pin 1 of the VR and the channel display came alive. Still no tx or rx, but channel display is on...for now.

 

[MKwrench]

What is this component that is attached to pin 1? While picking at the glue I accidentally shorted the 2 sides of it together, and my 8v came back to pin 1 of the VR and the channel display came alive. Still no tx or rx, but channel display is on...for now.View attachment 74146

There was a TSB on this board that added a Zener diode in that spot as a safety limiter. That one looks pretty darn corroded to me, It should operate just fine without it.
I would unsolder that wire and the diode and try to clean that area up with a little wire brush. Then see if the 8 volts comes back to pin 1. Well they're disconnected you can check to see if either one was shorted with a meter.
Edit: I'm at work on my phone right now and didn't see your whole last message.... I think you found the issue. I don't think that that is the TSB diode I mentioned above. Once that area is cleaned up, I'll get another peek at this page when I get home from work. The no RX/TX is another issue, not sure if it's related or not. We'll get into that after we know this part is all good.

 

[bigtoyboy01]

There was a TSB on this board that added a Zener diode in that spot as a safety limiter. That one looks pretty darn corroded to me, It should operate just fine without it.
I would unsolder that wire and the diode and try to clean that area up with a little wire brush. Then see if the 8 volts comes back to pin 1. Well they're disconnected you can check to see if either one was shorted with a meter.
Edit: I'm at work on my phone right now and didn't see your whole last message.... I think you found the issue. I don't think that that is the TSB diode I mentioned above. Once that area is cleaned up, I'll get another peek at this page when I get home from work. The no RX/TX is another issue, not sure if it's related or not. We'll get into that after we know this part is all good.

Pin 1 has been holding at 8v since I shorted the ends of that part. Even when I turn the radio off and back on, and even though it's no longer shorted.

I still have no TX or RX, but I did notice that I can faintly hear the PA Audio if I hold the PA speaker on my ear.

Also, the PLL voltages are half right now...the Pin 17 side voltages are all pretty much right on, but the pin 1 side are not what they should be on Ch 19.

 

[AudioShockwav]

All of that old sonybond (glue) needs to go, it becomes conductive and corrosive.
It will short things out and eat the leads away at the same time.

73
Jeff

 

[MKwrench]

Pin 1 has been holding at 8v since I shorted the ends of that part. Even when I turn the radio off and back on, and even though it's no longer shorted.

I still have no TX or RX, but I did notice that I can faintly hear the PA Audio if I hold the PA speaker on my ear.

Also, the PLL voltages are half right now...the Pin 17 side voltages are all pretty much right on, but the pin 1 side are not what they should be on Ch 19.

If pin 3 of the *audio chip* has 1 or more volts on it, it's a telltale that the PLL loop is not locked. It also mutes outgoing audio, including the PA (pulling C232 is a hack that keeps the audio unmuted by disabling pin 3, even between channels). It's probably due to the pin voltages at the PLL, They all have to jive for the loop to lock. Double check on that soldering and possible split traces around the patched up channel mods.
It looks like we're dealing with some failing older mods, and that corrosion caused by that "crazy glue" that Shockwave is mentioning above.

 

[bigtoyboy01]

Cleaned glue off. Is this a diode? How did it start working when shorted? Did I ruin it by shorting? Is it now backfeeding the VR? When I test it I get same reading no matter which way I hook up to my multi meter. Does it need to be removed for testing?

 

[MKwrench]

Pin 1 has been holding at 8v since I shorted the ends of that part. Even when I turn the radio off and back on, and even though it's no longer shorted.

I still have no TX or RX, but I did notice that I can faintly hear the PA Audio if I hold the PA speaker on my ear.

Also, the PLL voltages are half right now...the Pin 17 side voltages are all pretty much right on, but the pin 1 side are not what they should be on Ch 19.

If pin 3 of the *audio chip* has 1 or more volts on it, it's a telltale that the PLL loop is not locked. It also mutes outgoing audio, including the PA (pulling C232 is a hack that keeps the audio unmuted by disabling pin 3, even between channels). It's probably due to the pin voltages at the PLL, They all have to jive for the loop to lock. Double check on that soldering and possible split traces around the patched up channel mods.
It looks like we're dealing with some failing older mods and that corrosion caused by that "crazy glue" that Shockwave is mentioning above.

View attachment 74148

Cleaned glue off. Is this a diode? How did it start working when shorted? Did I ruin it by shorting? Is it now backfeeding the VR? When I test it I get same reading no matter which way I hook up to my multi meter. Does it need to be removed for testing?

Looks great, Yes I'd take that out to test it. Look for TSB1226 for the Cobra 148, I think that's what this diode is. I'd like to just check this one up under "luck", I have a feeling it was shorted internally, and when shorted externally It opened up.

 

[bigtoyboy01]

I took the diode out between pins 1 and 2 of the PLL, this made no difference in the radio function...I believe it tested good when disconnected. I soldered it back in. Where is the audio chip?

 

[MKwrench]

I took the diode out between pins 1 and 2 of the PLL, this made no difference in the radio function...I believe it tested good when disconnected. I soldered it back in. Where is the audio chip?

Audio chip is the 10 pin next to the regulator on the same sidewall. We want to check VCO voltage before we continue. Find Test point 9 it should be it should be somewhere to the left of center towards the front half, to the left of L21. Put the radio on channel 40 AM Hook your meter to the ground at the power jack and measure voltage at TP9. We should see about 4.25v then about 3.25v on channel 1. If not, use L19 to tweek the voltage in. If there's no voltage there, we'll have to find out why.

 

[bigtoyboy01]

Audio chip is the 10 pin next to the regulator on the same sidewall. We want to check VCO voltage before we continue. Find Test point 9 it should be it should be somewhere to the left of center towards the front half, to the left of L21. Put the radio on channel 40 AM Hook your meter to the ground at the power jack and measure voltage at TP9. We should see about 4.25v then about 3.25v on channel 1. If not, use L19 to tweek the voltage in. If there's no voltage there, we'll have to find out why.

At TP9, I have 6.76v on ch 1 AND ch 40. Also, when I first turned the radio on, the channel display faded out like it had been doing in the past, but came back at some point while I was probing the VR...probably when I probed each side of the diode...like it needs the 13v from pin 2 to jump start the 8v on pin 1. I'm still wondering if there is a failed cap somewhere.