TU 9900 Problems

I just finished the test in post #53. It had a reading of DC -.38
It had a larger negative reading and started falling until it got to the -.38

The AC was 0.00

I unhooked everything and the 1A fuse in 02 had blown.
 
Ok well the next step is to remove some parts …BUT FIRST how is your de soldering progressing? Do you have an idea how big the DC voltage was prior to the fuse failure?
 
I haven't done any de soldering practice yet but will this weekend. My solder sucker should be here Saturday.

I didn't think to catch the highest reading but I can perform the test again if you think it will help.

Let me know which parts to remove and how to test them and I'll do that as soon as I think my de soldering skills are adequate.
 
Lift 1 leg of R-04 the one that attaches to TR-02.
Put DIGITAL meter ( RED + ) on the R-02/R-04 test point from last test.
Meter ( Black -) to ground.
( you did re hook up the ground wire only out of the 5 you removed earlier , right? )
Replace fuse
Test again
 
And lifting R-04 you can do with the solder wick..
Make sure your tip is CLEAN and TINNED..EVERY TIME
Put the tip of wick down ( still attached to the reel~3 to 4 inches ) hold the REEL with your left hand ( assuming your a righty ) on top of the joint you want to un solder.
Place Iron on top of wick ( start mental clock of count to 3~5 ) when solder is molten it will flow into the wick, like water on a paper towel ,lift iron & wick together.

If another pass is needed, leave time ALLOWING TO JOINT TO COOL INBETWEEN.
Cut end of wick so it's just copper again.. and re do…

The ideal is the MAJORITY of the solder will lift into the wick, but the wick , like a paper towel will only hold so much.
To much heat and the copper trace will lift ( bad but not un recoverable ). Less (heat) is MORE ( productive work )

You MIGHT have to bend the resistors lead up to free it.
I use a small xacto knife and a LITTLE heat from the iron if needed.
WARNING if the resistor lead has ~30% solder still on it do not PRY if it 's COLD.. You have to liquify to pry!
 
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Will do. I'll practice de soldering tomorrow and then lift 1 leg of R-04 and test and report back.

The ground wire was the only one that I hooked up.

Edit; My solder sucker just arrived but I have to go to work. I'm going to order a better digital multimeter. The one I have might be ok but for some reason I don't trust it.
 
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Tested again. DC volts climbed to 1.13 and then started falling. I was concentrating on the reading and neglected to notice whether it was - or + volts.

AC volts 0

F02 popped
I removed F01 and it had popped also. I don't know if it did it during this test or an earlier test. I'm going to start checking F02 and F01 before each test to make sure.

Heading to RS to get some more 1A fuses and a couple more soldering tips.
 
Standing by.....
So I have a though....( well more like an offer ) ....
IF you (we) get to a point that you don't have the tech needed to fix this short of just tossing parts at it & crossing your fingers... Take the board out and ship it to me...
Just putting that out there....as a backup plan...
Mike
 
Resistor R-04 leg that bridges to R-02 was lifted. I had the digital meter positive hooked to the lifted R-04 leg and the negative hooked to the ground 07.

If at any point you think that I won't be able to fix it from this end I'll ship it to you.
 
Ok try again with F-01 & 02 in ... My GUESS is the positive 13 volt will be fine
and the Negative will work ( at this point ) BUT it will be high...

OR

The fuses pop again and the problem is in the 2 rectifiers & caps...

So waiting on you boss
 
If the fuses pop again... lift 1 leg of C-01 .047 disc cap next to the diode's.
It is RARE that ceramic caps go poof but there is a chance....
 
DC volts climbed to 4.34 and then fell. AC volts climbed to 8.4 then fell.

F-01 good F-02 popped

I'll go ahead and lift 1 leg of C-01 and tune in for farther instructions.

I'm out of 1 amp fuses right now but my son picked up some this evening and will be over tomorrow for a family get together.

EDIT; 1 leg of C-01 lifted. I did not know that was a disc cap but now I do.
 
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Yes BK, could have saved … 5 fuses if I count correctly .Last time I built a DBT from store bought parts it was $22.65 all in all new from Lowe's ( but I did get a outside metal rain proof 5S box ) .. $ 2.75 for a 4 pack of fuses at radio shack….
 
AH HAH this was the info I wanted to capture…
DC volts climbed to 4.34 and then fell. AC volts climbed to 8.4 then fell.

So IF this test was with R-04 lifted.. the problem is with one of the 2 dual diodes
Repeat the test IF C-01 was not lifted ( just to be 150% sure )

Now decision time…. probably easier for you to pull this board & send it to me …
OR
You can pull both the diode packs and we can proceed from there.

I'm going to hit up another AK member that MIGHT be willing to part with some OE diodes.. If not I have the subs in stock and can emulate your power transformer & load….
 
I already lifted one leg of C-01. Should I solder it back and repeat the test?

If you think it's best to send it to you I will or I can continue on with the diodes. Do dual diodes only have 2 legs or are they the 2 diodes right next to C-01?

I don't have any more fuses so I"ll have to wait until tomorrow.
 
The Diodes have 3 legs there are 2 diodes in each plastic casing, they are either common ANODE or common CATHODE. The 2 pairs together make a full wave bridge rectifier ( converting AC to DC ) now in a proper circuit the bridge will have a tiny bit of ripple about .25 of a volt or less BUT YOU HAVE 8.4 So a diode isn't doing it's thang.
the question is out of the 4 which one?? PLUS putting AC on a filter cap ( the gray tall cans ) is not so gouda.. What it did downstream is anybodies guess….

My thinking is.. we can keep going BUT in order to do a bit more effective testing a DBT will be handy… your homework is to google that !

Mike
 
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