Uniden President Grant Short?

[Robb]

Another test for the real radio repair gurus . . .

Radio has a dead short. Checked and replaced protection diode. Everything on the power plug to switch is fine - including the switch. Removed power feed wire after the switch to the board - and no short.

Removed the choke transformer T1 leads from circuit and re-tested; so I know the short isn't on that side of the circuit. Checked TR41 & 42 too. Replaced C84, 95, 172 & 181 as well. D44 is OK.

Pulled the audio IC and voltage regulator IC pins off the board. Same with the final and driver. No luck.

Can't seem to find TR28 & 35 on the board to check them.

Is there a zener diode in the schematic?
Can't find one of those to check either.

One would think that a dead short wouldn't be that hard to find. I've been trying to hunt it down for the last 6 hours or more. Getting frustrated but not giving up.

Thought I'd toss this problem out to the gurus for some insight and thoughts.

Schematic:
http://www.cbtricks.com/radios/uniden/grant/graphics/grant_sch.pdf

Are there any other parts or areas that I should look for on the board when looking for a dead short that I didn't mention?

Are there any tricks to find a short that I can use?

 

[Robb]

Found TR28 & 35 and checked them. They are OK. Radio still has short, so it is impossible to track down any other problem in it yet - because I cannot turn the radio on without it tripping the breaker in the power supply.

I checked through the schematic along the path of the 13.8v input and cannot find any more points that might be shorted except for the meter bulb and the channel LED supply.

There are no zener diodes on this chassis.

Any ideas or thoughts bob85, ExitThirteen, unit_399, AudioShockwav, tecnicoloco, Jazzsinger, or others?

 

[RatsoW8]

Are you sure it's not the power supply?

 

[Robb]

Are you sure it's not the power supply?

Yep.
I can put another radio on it that doesn't have a problem and it works just fine.

 

[K9GAS]

Robb can you adjust you're power supply, like a variac, and bring the voltage up slowly and see if you're able get some testing done? It may work or it could trip even on low voltage.

 

[AudioShockwav]

You can smoke test it, crude, but effective.

73
Jeff

 

[Robb]

What happens if the short itself is stronger than the board traces?
POOF!

This is a radio that is in otherwise nice shape. It had been modded; put a c945 back into the TR24 spot. There are other mods too. Testing each transistor for Base to Collector/Base to Emitter readings and the leakage vs high resistance test too. All of them seem fine so far. So do the diodes.

When I got it from a Ham Flea Market, the voltage regulator looked fried so I replaced it.

Replaced C84 1000uf/10v with a 25v part. I replaced C95 just because it is C95 with a higher voltage rating. Replaced the power filter cap/C172 with a new one.

Sure would like to get this Grant into working condition again.
It is a real classic.

 

[AudioShockwav]

You already Know that answer.




73
Jeff

 

[Robb]

You already Know that answer.




73
Jeff

:blink::laugh:

 

[jazzsinger]

You can smoke test it, crude, but effective.

73
Jeff

very effective,lol.

i'm not that familiar with US model grants, which pcb is it Robb ?

if the transistors etc aren't showing shorts, maybe you should get a magnifying glass out and start seeing if any of the tracks have been shorted with solder.

a high powered torch shone from the component side whilst you view the tracks can help find shorts/opens,

i found a hairline crack on a pb010 chassis that was missing hi band this way, was right next to the solder pad just where the track from the band selector joined it, because it only affected one diode of the matrix, i was still getting vco lock on a couple of channels that i could get a frequency reading from, subtracting the n code of the frequency i was on (26mhz somewhere) from the n code i should have been getting told me which diode wasn't getting/passing juice, the obvious conclusion was a faulty diode, but a tiny hairline trace break next to it gave the exact same symptoms.

p.s.
i wouldn't recommend a smoke test on a decent board, worth a try on a heap of shit, but not a good board, as it can get messy.

from the shit quality on this netbook the schematic suggests C186 (i think) and C401 (straight across the power plug) might be worth lifting to see if power is restored. finding it very difficult t read the schematic, even at 600 times magnification its blurry, the small screen ain't helping me either.

 

[Robb]

very effective,lol.

i'm not that familiar with US model grants, which pcb is it Robb ?

if the transistors etc aren't showing shorts, maybe you should get a magnifying glass out and start seeing if any of the tracks have been shorted with solder.

a high powered torch shone from the component side whilst you view the tracks can help find shorts/opens,

i found a hairline crack on a pb010 chassis that was missing hi band this way, was right next to the solder pad just where the track from the band selector joined it, because it only affected one diode of the matrix, i was still getting vco lock on a couple of channels that i could get a frequency reading from, subtracting the n code of the frequency i was on (26mhz somewhere) from the n code i should have been getting told me which diode wasn't getting/passing juice, the obvious conclusion was a faulty diode, but a tiny hairline trace break next to it gave the exact same symptoms.

p.s.
i wouldn't recommend a smoke test on a decent board, worth a try on a heap of shit, but not a good board, as it can get messy.

from the shit quality on this netbook the schematic suggests C186 (i think) and C401 (straight across the power plug) might be worth lifting to see if power is restored. finding it very difficult t read the schematic, even at 600 times magnification its blurry, the small screen ain't helping me either.

Didn't find any cracks; but I'll look again . . .

Won't do the 'smoke test'.

The board is a PC-409AC . . .

 

[Robb]

Found the fault after several hours of pulling parts, wires, and hair out today . . .

This radio had the service bulletin #1226 done to it before I got it. That zener diode used in that bulletin was the problem.
http://www.cbtricks.com/radios/cobra/142gtl/graphics/cobra_142gtl_tech_note_1226.pdf

It was blown - literally. I touched it and it fell into pieces. That was the 'short' I'd been trying to locate all day. Put in a new 18v zener in D44 location and installed a new 1N4001 diode across pin 1 & 2 of the new MB3756 regulator IC. Had the diodes already in my parts bin. No more dead short!

The radio works xlnt now.

Also replaced all 34 of the electrolytic caps in it.
Five of them were very weak - and another one was shorted.
At first, I thought that shorted cap was the problem.

Going to change out the TR41/2SC1419 AM power regulator for an NTE-152 and TR43/2SC458 AF amp for a NTE-85 next and then give the radio a full alignment.

This was a $30 radio that I bought at a local hamfest last year for parts.

Parts/Cost to fix:
$6.00 - Electrolytic caps
$10 - MB3756 voltage regulator
$3.50 - NTE-152 & NTE-85

Thanks all . . .

 

[melaza]

Hi,
I'm gonna give you a little trick.
Go to your nearest auto part store and buy a 12v tester (the one that looks like a screwdriver). Next, connect in-line with the (red) 12vdc cable that comes from the power supply. Once the 12v tester light is off, you got it.

Greetings from the Caribbean...:w00t:

 

[k3cfc]

You can smoke test it, crude, but effective.

73
Jeff

I had one like that. took pics of both sides of the mother board. put covers back on add ( 1 ) car battery. bang found a diode blown apart not listed on schematic radio worked fine. FIXED lol.