my 8080DB screw up

benindavis

benindavis

Active Member
Hi gang,
So my VietNam vet neighbor gave me his 8080DB that had been hanging around in his garage for a few years collecting dust. Good for me, right? So I opened it up and blew out the dust ( remarkably little by the way) plugged it in and everything fired up. Some crackling on the volume and balance but otherwise all systems go. I figured Id leave it alone til I could get some DeOxit and some of the new WD 40 Contact Cleaner. Got the WD and gave a few things a spray, waited a bit and powered up. much improved! I was pleased, and walked away from it and decided to do a serious clean on a day when I had time to do it right. A few days later I gleefully powered it up and ... Speaker protection mode. ok.. did some research and replaced the R33 and R34 180 Ohm resistors. Plugged it in powered up ! sweet!!! decided to do the bias and DC offset. Did the bias adjustment no prob. Then..... I began to try the offset. ... I accidently touched something I shouldnt have while attemtping to measure off of the fuse holder and a tiny spark and a pop on my speaker occured . oops....now. the speaker protection is on again andthe dc offset is WAY high, and it wont adjust....
Any ideas on where I should start to look or what I have done to this fantastic gift I was given???
Thanks in advance

Ben

:confused:
 
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You didn't blow one of the DC fuses did you? If one channel is missing a DC rail, it could cause a big offset.
 
Are you perhaps confusing offset and bias? If the unit was out of protection, you check the offset at the speaker terminals.

Regardless, you may have blown an output transistor or two.

Rob
 
I just tried it and it works. It's something at your end that's not allowing you to link up.
 
Don't know what's wrong at your end. Here's a screen print of the link when I open it.

Screenshot (843).png
 
Just go up to the search button in the upper right corner and search for the 8080DB.

You guys are using a phone and I use a laptop. It's your phones.
 
Ok, turns out I was mistaken about the offset.
:rolleyes: I managed to get the offset to read correctly and if I pull the driver board it comes out of protection. Im gonna have to figure out how to check all the capacitors and resistors etc. gonna try to dig up a service manual I can get for cheap I guess.
 
benindavis said:
thanks!!
Click to expand...
While looking at the schematic for driver board I noticed that a number of the resistors inparticular have no data listed for them. The parts list shows ohm data for R33 to R50, but nothing for higher or lower numbered resistors... ie R56 , R17 ETC.
Am I missing something? Or looking in the wrong place??
 
Sansui didn't put complete parts lists in their manuals for a lot of equipment from that era. It kinda sucks. If you look at the full schematic (not the parts list or board diagram) they may be there, otherwise you can always look at the resistors and read from the color codes.

That said, if you did what you said you did, it's not going to be a resistor issue. What you need to do:
1. Pull and test all output transistors.
2. If you find ANY shorted outputs, that's your bad channel. Test all transistors on that channel of the driver board.
3. If none of the outputs are shorted, test all transistors on the driver board. You can cheat and just test them for shorts while they're soldered in, that is likely to work.
4. Replace all fuse resistors on the driver board. It's just necessary maintenance.
 
EastPoint said:
Sansui didn't put complete parts lists in their manuals for a lot of equipment from that era. It kinda sucks. If you look at the full schematic (not the parts list or board diagram) they may be there, otherwise you can always look at the resistors and read from the color codes.

That said, if you did what you said you did, it's not going to be a resistor issue. What you need to do:
1. Pull and test all output transistors.
2. If you find ANY shorted outputs, that's your bad channel. Test all transistors on that channel of the driver board.
3. If none of the outputs are shorted, test all transistors on the driver board. You can cheat and just test them for shorts while they're soldered in, that is likely to work.
4. Replace all fuse resistors on the driver board. It's just necessary maintenance.
Click to expand...
thanks for the good advice. ill have to learn how to test the transistors. Im gonna replace caps too. I checked all the resistors i could find specs for. 41 and 42, 43, and 44 all test over 150 ohms . 168, 156, 173, and 199 respectively. guess ill hit up some Youtube for transistor testing lesson.
 
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