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Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 818
Registered: 9-2004
| | Posted on Thursday, November 10, 2005 - 11:22 pm: | 
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i was doing a bit of testing with my 2000gtl hooked up to my kl300p, wattmeter, and then to a dummy load.
after just a few minutes of short key ups, while keyed up, the power out on the wattmeter dropped way way down.
i quickly unkeyed, and hooked the wattmeter to the radio to see if the amp or the radio had caused the problem.
it was the radio. when keyed, the s meter would kick down to nothing, back up to almost normal, and back down to nothing. this happened once every second or so.
since this is a very old radio, and i have not had to replace the driver and final yet, they are my main suspects. i have replaced pretty much all of the elec. caps in the radio if not all of them, so im leaning away from a bad cap.
as for mods, the radio has had the final volted and the carrier turned down to 2 watts. on a RS wattmeter, it keys at 2 and hits almost 20 watts on the PEP scale. (i think its more like 12-15)
there have been audio mods done to the radio, but i run my mic gain at 12 o clock and the gain on the desk mic at 10 o clock.
one more thing; when i let the radio sit and cool for a few minutes, i could key the mic and it would stay normal for a few seconds and then sart the decay thing again. this is another reason why i suspect the final or driver or both.
thanks for any help or advice or experience anyone can lend,
i had to get to work before i could take a good look inside the radio.
matt |
Wildrat
Intermediate Member
Username: Wildrat
Post Number: 258
Registered: 12-2003
| | Posted on Friday, November 11, 2005 - 7:53 am: |
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how do you know it's not keying and unkeying?
Wildrat |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 1860
Registered: 7-2004
| | Posted on Friday, November 11, 2005 - 6:27 pm: |
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hmmmm....my XL has been modded the same way & seen HEAVY use for 10 years now...i did have a final problem due to a defective diamond sx100 wattmeter, but not as yours. i'd suspect predriver, driver or final as well, mainly due to age & any other abuse it may have sustained over the years. if you check out all 3 transistors' specs, you'll see that mA bias, operating voltages & wattage input & outputs are ALL well WITHIN spec limits. therefore, the mods in themselves are tolerable, however, you don't know what's happened in the past.
does the wattage on dead key reduce as well, or is it only when the signal is modulated? outside possibility it's audio related...
check voltages & mA readings as compared to sams, that should give you an idea as to whether or not 1 of those transistors is ready for the trash heap  |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 832
Registered: 9-2004
| | Posted on Sunday, November 13, 2005 - 2:43 am: |
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wildrat, im kind of playing the odds on this one.
i have gone through most of the radio already, but havent replaced the final and driver, and they are definitely the stock parts. your suggestion is in the "could be" category though.
pat, yes it happens on dead key too. thats when i noticed it. i had the dynamike all the way down checking SWR with the amp on.
the reason im guessing amp section is also because of my assumption that since a transistor is a switch; that maybe the driver is switching on and off and thats causing the wiattage to rise and fall from "nothing" to where i had it set for. (2 watts)
i agree, that the power devices should be able to handle the mods just fine. i just figured that since the radio was made in 78 or 79, that the amp section might just be on its last legs.
one good thing is that i do have the 2166 and 1969 on hand, so if i start scratching my head too much, i can just replace them and see what happens.
pre driver too eh? hadnt considered that, but i guess i should.
do you think a faulty cap could cause this phenomenon? i did find a few near the final and driver that i havent replaced yet.
i will check the voltages tonight, i have the sams.
i have been busy bringing a new friend's Grant XL back from the dead.
you should have seen this poor radio!
the PLL was sitting on the board at an angle, and pins 10-18 were connected with short jumper wires to the PC board! looking at the solder side of the board; EVERY TRACE ON THAT SIDE OF THE PLL WAS GONE!!! ALL JUMPERS TO FOIL TRACES!
i cleaned up some soldering here, but left it alone as the thing actually worked! and those chips are hard to find. (he doesnt have that kind of money anyway)
s meter didnt work, all pots maxed, (except VR 3,4,5,)
R131 removed, TR24 removed, some resistor near VR11 cut.
mic plug looked as if it had been chewed off in the past. RF gain turned into RF power control, using wrong impedence and wrong size pot.
there's more but i cant remember.
now its all clean and shiny, new connectors, new pots, and i even gave him an old knob to match the others on the grant.
re repaired his mic repair, and this radio is working good. i wont say great, only because he wouldnt let me put the mod limiters back or on switches. "i like it loud" mentality.
the thing that he's going to trip out on the most is that the S meter works now. he said it never had since he got the radio. i think every CB'er in las vegas has had their hands in this radio.
wait until he sees what nice neat work looks like!
im no expert, but the work i do is clean, and works for a long time. just ask anyone who's ever had me set up and tune their guitar for them. they stay in tune for a long time and they play great!
thanks for the input guys, i very much appreciate it, keep it coming,
matt |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 1874
Registered: 7-2004
| | Posted on Sunday, November 13, 2005 - 10:55 am: |
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since almost everything else has been replaced, go ahead & change the remaining caps before you do the finals-just in case!
last basket case i had that someone wanted looked at...it worked, more or less, so i kept it for parts & gave him a $45 e-auction 148 with a few tweaks. he was happy & i got another 8719 & some other spare parts |
Hatchet
Junior Member
Username: Hatchet
Post Number: 47
Registered: 7-2004
| | Posted on Sunday, November 13, 2005 - 2:09 pm: |
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I would say it is NOT the driver and final.
You said " one more thing; when i let the radio sit and cool for a few minutes, i could key the mic and it would stay normal for a few seconds and then sart the decay thing again. this is another reason why i suspect the final or driver or both."
The driver and final either work or they dont. I have never seen one with an intermittent problem like this. Your problem is elsewhere. Id replace the remaining electrolytic caps and go from there |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 839
Registered: 9-2004
| | Posted on Monday, November 14, 2005 - 12:06 am: |
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it was either the final or the driver.
i replaced them both last night when i got home from work and VOILA! the radio works flawlessly. re biased and tuned up TX and i am very happy.
one thing i did that i shouldnt have was to remove both the final and the driver at the same time.
so now i dont know which one was bad and sooner or later i will use one of them from the junk box and go, "crud! that's the bad one!"
oh well! it sure was nice to have the spares on hand. (i ordered them with some other parts months ago in anticipation of this very problem)
the reason i replaced these before checking and replacing the caps that could have been culprits is; i dont get off of work until 10pm. by that time, even Rad Shak is closed, so unless i wanted to stare at the thing all night, i went with my best guess. and i was right!
pat, did you put an MB8734 in the 148 before selling it? LOL
hatchet, i appreciate the input. im surprised though, that you have never come across a thermally intermittant semiconductor.
i have seen this many times. that's why they make a product called "freeze spray". if you havent heard of this, you spray it on a part that you suspect is overheating and either opening or shorting. if the problem goes away when sprayed, then you've found your culprit.
i was already 80% sure that i knew what the problem was, but i had never seen this exact phenomenon before.
in the past, once the semiconductor reached its thermal breakdown point, it failed and stayed "failed".
the fact that this one pulsed, leads me to believe that it was the driver, and it was failing intermittantly, causing a lack of drive for the final, and thus, dropping the wattage out down to nothing.
of course i cant be sure because i got ahead of myself in my exitement.
mark this one down in the memory banks boys! a first for me, but nonetheless, the same fix!
matt
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Patzerozero
Senior Member
Username: Patzerozero
Post Number: 1894
Registered: 7-2004
| | Posted on Monday, November 14, 2005 - 10:12 am: |
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PHEW! AND GOOD!
looking back at my wattmeter failure last winter, even after the texas star SMOKED, the XL did alternate between output & none once or twice...i did change both transistors & was good to go
actually, i've sold a few 8734 radios intact. just hope the buyer remembers that i DID tell them channels CANNOT be added. they'll still sound GOOD on 1-40. i don't have the time to do what i NEED to do, certainly not enough time to do such a naughty thing  |
Tech833
Moderator
Username: Tech833
Post Number: 1089
Registered: 8-2002
| | Posted on Monday, November 14, 2005 - 10:39 am: |
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If you have exceeded the manufacturer's voltage ratings, then it IS possible to create an intermittant semiconductor.
This is never a good thing, and I have to ask, is that extra 2 dB of output REALLY worth it? Especially when you are running an external amplifier? (I am guessing you are using this on the 10 meter ham band).
After all is said and done, it is your radio, and you are free to do with it as you wish. If you wish to run it in a reckless manner, you are the one that has to replace the dead parts, so enjoy! |
Kid_vicious
Advanced Member
Username: Kid_vicious
Post Number: 842
Registered: 9-2004
| | Posted on Tuesday, November 15, 2005 - 1:04 am: |
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by volting the final, putting 13.8 volts to it; are we exceeding the manufacturers' voltage rating?
my GUESS is that they ran the final and driver at 8 volts because that was all that was needed, not because thats all the part could handle.
i'll have to check the spec sheet when i get home tonite.
dont get me wrong; i appreciate the advice; and im willing to replace parts when they go bad.
im a strange guy, i replace my spark plugs and fuel filters before they need it just because i enjoy the process.
have a fine one,
matt |
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