Pix Request, 148 GTL / Grant XL

[LeapFrog]

Simply put, my solder remover/sucker is broken.

C67 (under the wax) was charred, burned to a crisp!
Need to replace it and continue testing, I have 13 volts at the test point in RX.
(The Rick Jackson 148 schematic)

I think it's a 1uF tant, hard to tell the "soot" is melted plastic and will not wipe off.
C67 on the Cobra 148 GTL schematic is a 1uF tant cap, so that's what i'm going to try next.

update: threw a 1uF electrolylitic just to test, no tx still.
one of the three green caps (near c67) look slightly burnt also.
tr39 has a signal under tx, tr37 might be the problem I haven't probed this one yet.

Checking C95 next

 

[LeapFrog]

Does this Cobra update need to be done on the Grant XL?
i think there is a small chance that I could look at the board and figure it out.


I haven't recapped the whole radio yet, because I am out of radio funds at the moment.

 

[sonoma]

I do not think this update is for the grant XL. at least it should have to do with some of the very 1st radios put out with this style of board. I have never seen the XL or LT radios needing this mod. they were all built after the 140/142/ and 1st 148's that were put out.

I was going to tell you I use the new VR's to replace the old ones in the radios. you just have to bend the legs of the new VR's if the holes do not line up. more than likely you have the all metal VR's in your radio. the new ones have a white top on them and stand out from the old ones. not sure just what ones you have in your radio. the new ones if you buy the kit runs about 10 cents a piece in the kit 's.

 

[LeapFrog]

I do not think this update is for the grant XL. at least it should have to do with some of the very 1st radios put out with this style of board. I have never seen the XL or LT radios needing this mod. they were all built after the 140/142/ and 1st 148's that were put out.

I was going to tell you I use the new VR's to replace the old ones in the radios. you just have to bend the legs of the new VR's if the holes do not line up. more than likely you have the all metal VR's in your radio. the new ones have a white top on them and stand out from the old ones. not sure just what ones you have in your radio. the new ones if you buy the kit runs about 10 cents a piece in the kit 's.

All metal ones in here, thanks for the service update info btw!
I will have to find some of the metal VR's, these might not be out of tolerance, I haven't done enough testing yet.

I am going to get serious about this radio, I'm pulling my hair out.
I may need a voltage table chart for RX/TX at various TR's, if a simple re-cap doesn't fix it.

 

[sonoma]

here is the vrs I use and just bend the legs gently to fit the holes.

http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=variable+resistors&_sop=15

 

[LeapFrog]

Thank you for the link Sonoma, this radio beat me today so bad that I'm tired of looking at it, so I'm using a 29 LTD for now.

I will post an update when/if I know more.
Going to continue to change the original caps one by one.

 

[Robb]

The update is to give the voltage regulator a zener diode to help its regulation. If the newer chassis have that zener in there - instead of just a regular diode - then it is already upgraded.

 

[LeapFrog]

I'm going to have to look and see what's installed, this radio may be a 1992, i'll check for some date codes on the ic's.

It probably already has the corrected zenner, never hurts to make sure.

Thanks Robb

 

[loosecannon]

skimmed through the thread a bit, so if i say something that's already been covered, then pardone' moi'.

i see you have no TX power on AM or SSB, but the RX/TX light does change from green to red. this means that the 8 volts on pin 8 of the voltage reg is there on TX like it should be.

check TP7 or TP8 (they are the same) for 13.8 volts in SSB or somewhere between 3-8 volts for AM.
is this voltage there?

my first thought is that one of your bias diodes is shorted.
you have just learned a valuable lesson in turning VR's without the test equipment connected. how do you know what you had the bias set to?

unsolder the leads of each bias diode from the PC board and test them just like you would any other diode using your DMM.
are either of them open or shorted?

you are at a point now that you need to start scouring the schematics. you CANNOT progress to the next level without gaining the skill of reading a schematic on your own.
print out four or five copies of the schematic, and draw colored lines on it to represent voltages that you are trying to follow. this will make it easier for you to check for shorted caps and the like along your path.
remember to always start your troubleshooting at either the source of the voltage and move toward the load, or start at the load of the voltage and work backwards to the source, never start in the middle.

good luck,
LC

 

[LeapFrog]

skimmed through the thread a bit, so if i say something that's already been covered, then pardone' moi'.

i see you have no TX power on AM or SSB, but the RX/TX light does change from green to red. this means that the 8 volts on pin 8 of the voltage reg is there on TX like it should be.

check TP7 or TP8 (they are the same) for 13.8 volts in SSB or somewhere between 3-8 volts for AM.
is this voltage there?

No worries, Loosecannon I appreciate you taking the time.
edit: voltages below
Yes the LED still changes. (it's got to be close, the final & driver are good)

my first thought is that one of your bias diodes is shorted.
you have just learned a valuable lesson in turning VR's without the test equipment connected. how do you know what you had the bias set to?

unsolder the leads of each bias diode from the PC board and test them just like you would any other diode using your DMM.
are either of them open or shorted?

I will have to unsolder the diodes, test them & then report back.
I got a little buggered and walked away from it for now.

You are at a point now that you need to start scouring the schematics. you CANNOT progress to the next level without gaining the skill of reading a schematic on your own.
print out four or five copies of the schematic, and draw colored lines on it to represent voltages that you are trying to follow. this will make it easier for you to check for shorted caps and the like along your path.
remember to always start your troubleshooting at either the source of the voltage and move toward the load, or start at the load of the voltage and work backwards to the source, never start in the middle.

good luck,
LC

Thank You Sir, I will follow this advice.

 

[Robb]

Walking away from it can be very useful. Whenever I get really stumped, I walk away from it until I can get distracted by something completely different - like a movie or something - long enough to clear my head. Then I go back to it and take a 'stand back' attitude and work towards the problem from there. It helps me - anyway.

EDIT:
" . . .remember to always start your troubleshooting at either the source of the voltage and move toward the load, or start at the load of the voltage and work backwards to the source, never start in the middle." -loosecannon
Good point.

 

[LeapFrog]

check TP7 or TP8 (they are the same) for 13.8 volts in SSB or somewhere between 3-8 volts for AM.
is this voltage there?

Input voltage 13.67

RX Voltages @ t.p.
AM: 13.03 (noted as steady)
USB: 13.62 " "
LSB: 13.62 " "

TX Voltages @ t.p.
AM: 4.86 (noted as steady)
USB: 13.3 (drops off slowly the longer keyed)
LSB: 13.3 " "

Do these look about right?
I will need to study the digital schematic (Rick Jackson), this will require some time for me.

 

[LeapFrog]

Followup: Radio is RX/TX fine now, going to perform an alignment to finish it.
Problem: A wire from the CB/PA switch was loose, almost not connected!

I see what Loosecannon meant by "Remember to always start your troubleshooting at either the source of the voltage and move toward the load, or start at the load of the voltage and work backwards to the source, never start in the middle.."
Thanks guys for all the help, if I have an issue setting the bias, I'll try another potentiometer.

I still have a 29 LTD that needs an actual repair, perhaps I can sharpen my schematic reading teeth on that radio.

 

[nomadradio]

The reason they don't test the same is that they are two different type parts.

The driver uses a "MV1Y". A silicon diode with the normal 6/10 of a Volt forward drop. Like any silicon diode, this voltage will fall as it heats up. And that's what makes it useful. That same voltage between base and emitter on the driver transistor will also fall with temperature rise. Makes it draw more current with temperature rise. Using this diode to set the bias voltage we feed to that base terminal gets us a falling bias voltage feeding the drive's base terminal as it gets hotter. Tends to keep the driver's idle current from running away at high operating temperature.

The final bias diode type MV13Y is a bit different. It has TWO diodes in series inside it. Forward voltage drop is about twice what the driver's diode will be. The reason for this is that the diode is not connected directly to the final's base circuit. The MV13Y sets the voltage on the base of a NPN transistor TR37. The emitter voltage coming out of TR37 is what feeds bias current into the base leg of the final transistor. TR37 provides a current boost, and isolates the MV13Y bias diode from the final's drive signal. The voltage drop feeding into the base of TR37 and out from its emitter leg is the same 6/10 of a Volt as a normal single diode, and gets us the same remaining 6/10 of a Volt feeding into the final's base terminal.

The MV13Y on the final ALMOST never goes bad, unless one of the wire leads falls off. TR37 also isolates it from transient voltages that could happen if the final breaks down. Other types of radio that connect the bias diode directly to the final will frequently blow out the bias diode when the final fails. THIS radio almost never does that.

Ahh, you ARE taking the test point wire loose and putting a current meter in line with it for each, driver and final stage to set those trimpots, right?

Maybe it's time someone posted a YT how-to video on setting bias for these radios.

73

 

[LeapFrog]

Thanks for the reply Nomad, I think the two bias diodes are okay. I appreciate the info on the MV1Y, & MV13Y though.

It was a loose wire, is all i'm thinking. Now the radio will TX and RX just fine.
I plan on taking the test point wire loose and putting a current meter in line with it for each, driver and final stage to set those trimpots, correct. I just had to get the radio to TX again before I could finish the alignment.

I may do a YT how-to video on setting bias for these radios, just need the time and energy for the video editing. I have the idea down, and I've done it 2-3 times on various radios.