my attic sx-3800 repair

8golftango

8golftango

Respect the classics
Hello!!
Some may remember a short lived thread about me finding an sx3800 in my attic. Any ways I have dragged it back out of my closet and am ready to take another look at it. Things I have done/know about it are:

-powered up on a dbt with 40watt bulb. It did flicker but didn't really dim. (edit: definitely failed the dim bulb)

-powers up on mains and comes out of protection. tuner section works. aux input works. headphones work. haven't hooked up speakers yet. Right channel is lower than left thru the headphones and on the flouroscan output meters.

-I have at least one of the infamous transistor sockets cracked. So I guess I will have to fix those.

-there is a hot spot on one of the circuit boards. gets hot very quickly. im not sure what the component is yet.
 
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OK the "hot spot" I was referring to seems to be a resistor r167 (0.47 5w x2) kind of a strange component. (white square resistor) Im trying to find all the threads I have read in the past about this amp where did they all go ?? lol How does one deal with these wire wrap connections? EDIT: maybe im overreacting looks like the amplifier board can be removed completely without messing with those.
 
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reference photos for surety.
 

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ok a little info about me. Im pretty good with a solder iron and even desolder braid. Pretty good at replacing/understandind capacitors and or cap replacement. Ive replaced some capacitors and pots on some of my keyboards, even totally reworked my juno-6. Can read resistors etc.. can use a normal dmm. Im just not very skilled in actual amp circuit diagnosis. And im unsure if I need to totally remove these amp and power circuit pcb from the chassis. Which means desoldering all those wire wrap connections. Which isn't a problem per se... its just that if I forgot or didn't know to test something under power or in-circuit..it would be a huge pita.....make sense?
 
So I guess I need to know if I should go full monty and buy some more wick braid and go crazy? pull the pcb, desolder the wire wraps and replace all/most components? From what I understand irt markthefixer that most components need to be tested out of circuit, which means if they cost then may as well replace them?
 
MarktheFixer has been using a particular plugging device to replace those when they break. Do a little research on AK and see if you can find a thread with product info. The 3800 is a particularly difficult and complicated unit to work with and no one knows them like MTF. Hopefully he will notice your thread and throw in some words of wisdom to assist you.
 
This needs my special attention, which I cannot give until after the first week of march.

look for the word phoenix in regards to those connectors.

the three legged white things are dual resistors, which a study of the manual, board and schematic would reveal.

look for solder connections that have heated and unsoldered themselves and LOOK CAREFULLY at the sx-3900 and sx-d7000 amplifier repair/upgrade threads, particularily with regard to the conversion of 4 transistors that heat up, to TO-126 transistors (ksa1220,ksc2690).

It MAY be revcobverable with just soldering if no parts look toasted.

the amp CAN be powered up with NO output transistors connected for troubleshooting, then when (fixed) the protect clicks in - it's ready for one more test (o/p bases at +/- 0.6v) then the outputs can be connected.

with this the guys can break down what I have said and help you until I am able to.
 
Thank you guys and esp. Mark for your help. My updates on this thread may be a little erratic because of my work schedule but i promise to keep after it. I have researched this stuff quite a bit but sometes hard to find old threads with the site search engine. I guess they are archived. I will start desoldering yhose wire wrap blocks and remove the amp and power boards. But i will be at work for thenext ten days too so may not get to it for awhile...
 
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yessir and thanks again for ur edit and help. Which are the to-220 transistors that should be replaced? power supply or amp board?? obviously the ones with the metallic heatsink but I am behind the curve as I still haven't removed the pcb. too busy at work
 
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NCM_0177.jpg

removing the power supply circuit board with the big capacitors, desoldering the wire connections. in between the two plastic connectors on the right there was another wire wrap connection/bar that had to be desoldered, I did not know abt
 
Something to look at...

I have an SX-3800 that I gave to my son. There was a problem with one channel out or one channel stronger than the other. It was the Tape Monitor switches. They are slide type switches that would not stay in one position. That was causing the erratic sound of one channel.
I originally glued the slides in place, but they kept drifting. Eventually with help from MTF, I removed them and installed jumpers in their place on the PCB.
Since then, my son gets great sound from it.
 
While walking my dog, I found one of these literally lying in a ditch. It had gotten a little wet from dew, but brought it home, and the preamp part worked. Played the tuner section thru other gear, until it eventually laid down.

Would love to get it going again, but it is pretty funky inside.
 
Ok heres my plan on the power supply pcb, suggestions are welcome of course!
1. Replace all electrolytics, except for the big filter caps
2. Replace all the trans with the little heatsinks, q201, q209, q210 and q205
3. Replace the problematic q204, q208 (2sa905 & 2sc1915)
4. Replace the four flameproof resistors
I do not intend on pulling every component and testing everything...bad idea??
Im only concentrating on the power supply board for now..what am i forgetting??
 
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Those little heatsink/trans got really hot and discolored the circuit board u can see from the power supply board picture. I dont know if there is some way to install a impoved heatsink or fan? Or did they only get hot because they failed? Because i know that these have repaired or replaced in the past. Edit. Thanks emark for the heads up on the tape-mon. switches, for sure mine are messed up, i identified that pretty early on. Because of the failed dim bulb test that fix will have to wait.
 
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Question for people in the know..on the power amp board are c101, c102 the amp input coupling caps? And how about c125, c126? Do they have a similar purpose?
 
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