sx 780 recap - problems

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veritasxxx

New Member
Hey, guys. I've searched and read every sx 780 recap thread I could find. I've had the problem of my 780 turning on, but the source light not turning on. Then (sometimes) it'll eventually decide to turn on then randomly turn off again. Then it will go weeks without turning on. Here are a couple pictures of what I've recapped already. I really don't even know what they're called 'cause I'm pretty new to vintage gear, that's why I'm including the pictures. After these recaps, I'm still having the same problems. If anyone has any ideas of what else I can do (which caps to do), that would be awesome.

IMG_5382.JPG IMG_5383.JPG
 
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The unit plays, but the source lights don't work correct? I would check all the solder joints to the wire wraps that lead to the source lights. Also check the joints under the heat-sinked transistors in your first photo, the rectifiers, and any other location that typically gets hot. Not for the light issue, but for general stability of the receiver.
 
jheu02 said:
The unit plays, but the source lights don't work correct? I would check all the solder joints to the wire wraps that lead to the source lights. Also check the joints under the heat-sinked transistors in your first photo, the rectifiers, and any other location that typically gets hot. Not for the light issue, but for general stability of the receiver.
Click to expand...

Oops, I wasn't very clear on that part. The main lamps turn on right when I turn it on, but nothing else works unless the source light is on (which is pretty random), so it's not just the source light that is the problem.
 
Wiggle/toggle the tape monitor switch.It may be a dirty switch.-----------Disreguard
 
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Check the fuse and fuse clips for FU2 and FU3 first. I'd slap in a couple new fuses. Make sure the clips are securely holding the fuses and the solder connections have not been compromised.
 
I take it the (Stereo and Tape Monitor ) LEDs are not illuminated as well as the (Source)..?
 
Wow, what a chopped up schematic in the hifiengine service manual! Looks like your dial lights run on AC from dedicated taps on the transformer. Your LEDs run on DC from the rectified power supply. The three voltage regulator transistors (with heatsinks) shown in your first picture are known to overheat and cause bad solder joints. Probably a good first step to reflow these joints and see if the problem goes away.

...Edit Oops. Sorry Jheu02, didn't see that you mentioned this in post #4
 
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I'd also check D-20. Looks like either a 1/2w or a 1W 6.5v Zener diode.

All fuses are AGC Type Fast blo. for fractional values go DOWN to the Closest WHOLE VALUE or CLOSEST HALF Value. 3.2a would go to 3.0a. 3.8a would go to 3.5a. Try to keep the replacement slightly below the original value and as close as possible.
 
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two.dogs said:
Wow, what a chopped up schematic in the hifiengine service manual! Looks like your dial lights run on AC from dedicated taps on the transformer. Your LEDs run on DC from the rectified power supply. The three voltage regulator transistors (with heatsinks) shown in your first picture are known to overheat and cause bad solder joints. Probably a good first step to reflow these joints and see if the problem goes away.

...Edit Oops. Sorry Jheu02, didn't see that you mentioned this in post #4
Click to expand...

Are the voltage regulator transistors the three things at the bottom center of the picture?
 
If you mean the black cylindrical things in your pic at the bottom - no , those are the powersupply rectifier diodes.

Voltage regulators are larger (usually black but sometimes green or blue in vintage gear) square 3-legged blocky-looking transistors. They are probably the ones mounted in the silver heatsinks shown about halfway up your first picture.

Someone else can correct me here - I don't have the manual to confirm, but having a look at the schematic - I think the voltage regulators here may be Q15, Q16 & Q19.

Note also that I would be very careful working on this unit - it has small signal FET transistors - a special kind of transistor that doesn't take kindly to things going wrong (ie: anything producing an electrical surge). I have destroyed several technics units working in my usual ham-fisted manner on them, and I think the problem was killing the FETS every time. These are practically irreplaceable today. So if you haven't already - get an anti-static wriststrap to use when working on this.
 
Slime is right, they are the three aluminum C-shaped devices just behind the big blue cap (and about the same height). They are bolted to the three transistors that regulate voltage. Take your best quality photo of the bottom of the board and let us look at the solder joints in that area. If the joints have failed its a really easy fix.
 
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