Cobra 135XLR Bias?

[357]

Anyone know how to set the bias on this rig?
I'm going to attempt a recap for my friend.
First thing I noticed it the D325 regulator was toast and the radio (believe it or not) was being fed 17.6 volts!! for over a year!

This is one tough radio man wtf!
It seems most of the circuits are regulated separately, but still, that just nuts.
I fired the d325 into the trash and put a D717 in there, it doesn't break a sweat.

It'd be nice to set the bias though if anyone has the info.

Now its happy at 13.6V

 

[loosecannon]

Anyone know how to set the bias on this rig?
I'm going to attempt a recap for my friend.
First thing I noticed it the D325 regulator was toast and the radio (believe it or not) was being fed 17.6 volts!! for over a year!

This is one tough radio man wtf!
It seems most of the circuits are regulated separately, but still, that just nuts.
I fired the d325 into the trash and put a D717 in there, it doesn't break a sweat.

It'd be nice to set the bias though if anyone has the info.

Now its happy at 13.6V

View attachment 17640 View attachment 17641

hey 357, i've got the sams for that one i believe. ill write myself a note to check it tomorrow and get back to you.
Aren't there two versions of the 135XLR?
can't remember right now, but i think there are.

hopefully the bias is the same for both.
the SAMS i have is for the PLL version i believe.
LC

 

[357]

Thanks. Its a PLL TC5080
The board is actually pretty compact for 1977.
I'm not sure who built it, I'm guessing Toshiba.
They give these away on Ebay...

 

[loosecannon]

boy i sure don't envy you having to recap that monster.
talk about component density!

i actually have one of these in a box in the garage, and i thought i would get to recapping it someday, but every time i think about how packed with parts that board is, i always seem to find another project.LOL

so, the SAMS says that the bias pots are on the solder side of the PC board and are labeled R86 and R89 (well, i would be willing to bet that there is no label on them at all)
R86 is for the driver, and R89 is for the final.

the way they tell you to set the bias is weird and ive never seen one like this.

they tell you to inject a 100mV 1000hz signal at the mic input, with the mic gain at max, and adjust the bias pots for maximum.

weird right?

maybe someone here that has done this before can jump in an give some perspective here.

if it were me, i would try it their way, and if "maximum" meant turning the pots all the way up, then i would step back and try to find some more info.
if they seem to "peak" at some point in their tuning range, then i would be more apt to trust the info.
good luck,
LC

 

[357]

I think since this radio was never touched and runs cool, I'm just gonna leave it.
I started pulling caps and the checked as good as the new ones, very low esr.
I changed the big ones, the tantalum by the audio chip, and the powersupply caps and thats it.
They used Marcon caps in here and they were some good caps.

If the caps I pulled were way out of spec, I'd get the rest of them but they were still like new.
Plus my solder sucker blew apart and I'm not gonna try without it.
I got one on order, gonna be 30+ days.
I tried a desolder bulb but it was useless.

 

[loosecannon]

357,
have you ever tried solder wick?
once you do, you'll never go back.
makes doing a recap job much simpler, since it gets ALL the solder off.
LC

 

[357]

357,
have you ever tried solder wick?
once you do, you'll never go back.
makes doing a recap job much simpler, since it gets ALL the solder off.
LC

I'll have to try it I guess.
Radioshack (now "the source") want 7.99 for some super thin stuff.
I'm gonna try and find some on the china site, 7.99 is pretty pricey for my broke ass.
$1 is more like it