SX 780 and intermittent operation

Left side is 560Ω resistors, correct? That looks good, about what I expected.
Right side is still the STK installed? I expected pin 1 to be about -1.4Vdc and pin 0 to be about +1.4Vdc as the schematic shows. The right side was making music that sounded good?
 
I have gone back and read through this thread again. Looking at your readings I think the right STK has a problem also. Pull the right STK and put in the 560Ω resistors like the left side has and remeasure. If the measurements are the same as the left side, the STK packs are both bad, but the rest of the amp is probably ok.
EDIT: Need to do a bit more testing (power supply) before final analysis.
 
Last edited:
I have gone back and read through this thread again. Looking at your readings I think the right STK has a problem also. Pull the right STK and put in the 560Ω resistors like the left side has and remeasure. If the measurements are the same as the left side, the STK packs are both bad, but the rest of the amp is probably ok.
EDIT: Need to do a bit more testing (power supply) before final analysis.

Thanks for all your help (and everyone else's as well)! I apologize for my lack of knowledge and having to go over this many times----but really find this all very interesting.

I went back over my notes and found that i made a couple mistakes so I started over on all of it. I've taken both STK's out, installed 560ohm resistors in place and tested all connections. Here is what I come up with. It appears the 0-9 test failed on the left side STK (remember I didn't have any sound on left channel but right was fine)

Left channel readings on PCB at STK location (with stk removed and 560 ohm resistors in place)
1: -03.7
2: -41.4
3: 00.0
8: 00.0
9: 41.3
0: 03.7

Left STK readings (out of receiver on bench)
2-3: Open
8-9: Open
1-0: 1593
1-2: 1087
1-3: Open
0-9: Open
0-8: Open

Right channel readings on PCB at STK location (with stk removed and 560ohm resistors in place)
1: -3.4
2: -41.6
3: 00.0
8: 00.0
9: 41.6
0: 03.5

Right STK readings (out of receiver on bench)
2-3: Open
8-9: Open
1-0: 1369
1-2: 1074
1-3: shows voltage for a second then goes Open
0-9: 1157
0-8: shows voltage for a second then goes Open


If that is correct, that the Left channel STK is gone, my only choices would be to either find an original one (which I read is next to impossible) or the better choice to build (or find someone who could build for me) 2 of the custom built modules?
 
Thanks for all your help (and everyone else's as well)! I apologize for my lack of knowledge and having to go over this many times----but really find this all very interesting.

I went back over my notes and found that i made a couple mistakes so I started over on all of it. I've taken both STK's out, installed 560ohm resistors in place and tested all connections. Here is what I come up with. It appears the 0-9 test failed on the left side STK (remember I didn't have any sound on left channel but right was fine)

Left channel readings on PCB at STK location (with stk removed and 560 ohm resistors in place)
1: -03.7
2: -41.4
3: 00.0
8: 00.0
9: 41.3
0: 03.7

Left STK readings (out of receiver on bench)
2-3: Open
8-9: Open
1-0: 1593
1-2: 1087
1-3: Open
0-9: Open
0-8: Open

Right channel readings on PCB at STK location (with stk removed and 560ohm resistors in place)
1: -3.4
2: -41.6
3: 00.0
8: 00.0
9: 41.6
0: 03.5

Right STK readings (out of receiver on bench)
2-3: Open
8-9: Open
1-0: 1369
1-2: 1074
1-3: shows voltage for a second then goes Open
0-9: 1157
0-8: shows voltage for a second then goes Open


If that is correct, that the Left channel STK is gone, my only choices would be to either find an original one (which I read is next to impossible) or the better choice to build (or find someone who could build for me) 2 of the custom built modules?

No need to apologize. we have all been there before, ourselves. :) Be careful, this hobby is habit forming :p.
If you notice, the 0 and 1 PCB voltages are "balanced and ≈ equal in voltage, but opposite in polarity" (+/-). That indicates to me that the amp STK driver circuits are good, but both STK modules are defective. With a good STK in place pin 1 and 0 will be about +/- 1.4 volts or a little more, but centered above and below 0Vdc. IMHO I would (and have) replaced both with the custom modules. They work great. FWIW I don't test semiconductors with an ohmmeter, I have a different method, so I will not give an opinion whether the ohmmeter tests on the modules are pass or fail.
If the protection relay clicks in now, while off the DBT and on regular AC that is a good indication of progress.
 
Last edited:
No need to apologize. we have all been there before, ourselves. :) Be careful, this hobby is habit forming :p.
If you notice, the 0 and 1 PCB voltages are "balanced and ≈ equal in voltage, but opposite in polarity" (+/-). That indicates to me that the amp STK driver circuits are good, but both STK modules are defective. With a good STK in place pin 1 and 0 will be about +/- 1.4 volts or a little more, but centered above and below 0Vdc. IMHO I would (and have) replace both with the custom modules. They work great. FWIW I don't test semiconductors with an ohmmeter, I have a different method, so I will not give an opinion whether the ohmmeter tests on the modules are pass or fail.If the protection relay clicks in now, while off the DBT and on regular AC that is a good indication of progress.


Does anyone sell the modules complete that are ready to install or are they still kit form? I'll start looking to do that next= Would prefer ready to install units, but if they are simple enough I'd give it a try.
 
Does anyone sell the modules complete that are ready to install or are they still kit form? I'll start looking to do that next= Would prefer ready to install units, but if they are simple enough I'd give it a try.
I have never heard of anyone selling complete modules, they would probably be cost prohibitive to buy, and/or unprofitable to sell. It cost me less than $100 US to buy the kits and all the parts. The kit is on ebay, and consists of the PCB and the mounting/heatsink plates, plus the correct screws for mounting the transistors to the heatsink. The BOM is on the 'bay page, and fully discussed, with alternatives, here on AK (see link in earlier post). If you can solder and turn a screwdriver they are easy to build. A little bit of sanding the plates smooth on a flat surface is a good idea. The guys that designed them are AKers, and the guy that sources them is also on AK. Modern components to build them are available from mainstream parts vendors like Mouser, Digikey, Arrow, etc. As far as I can tell nobody makes any money off them, they are available because there was need to provide replacement options for unobtainable originals better than the fake modules from china. It is just enthusiasts helping enthusiasts keep vintage gear singing. I did see one person on ebay offering to provide 'build and install' as a service, but you had to ship your unit to him, and it was not cheap.
 
I have never heard of anyone selling complete modules, they would probably be cost prohibitive to buy, and/or unprofitable to sell. It cost me less than $100 US to buy the kits and all the parts. The kit is on ebay, and consists of the PCB and the mounting/heatsink plates, plus the correct screws for mounting the transistors to the heatsink. The BOM is on the 'bay page, and fully discussed, with alternatives, here on AK (see link in earlier post). If you can solder and turn a screwdriver they are easy to build. A little bit of sanding the plates smooth on a flat surface is a good idea. The guys that designed them are AKers, and the guy that sources them is also on AK. Modern components to build them are available from mainstream parts vendors like Mouser, Digikey, Arrow, etc. As far as I can tell nobody makes any money off them, they are available because there was need to provide replacement options for unobtainable originals better than the fake modules from china. It is just enthusiasts helping enthusiasts keep vintage gear singing. I did see one person on ebay offering to provide 'build and install' as a service, but you had to ship your unit to him, and it was not cheap.

Great ! Thanks for your help and I’ll start reading up on them. I’ll post back here once I get a kit ordered and start building.
 
Thanks Merlynski for helping this member out, you have it all figured out. I should use your posts for a document on the module. I'll start it right now.
I have them listed, that is all I can say about that. If it was easy to do in barter town, I'd do it too, but it is not setup for this sort of commerce.
Getting the Al plate mfg setup was always the issue and still is today. They all have to be hand tapped as well.
It is labour/time intensive, low volume, so understand why things are the way they are. It is setup to make it as easy as possible for someone trying to learn to do it themselves.
There is even a utube video too, watch that, it is fun to see the guy operate, i got a chuckle out of his understanding and explanation of what we did, it was like a replay of me showing my wife how to build one up the first time.
I guess I can say, about soldering, let the solder flow, with the heat applied to the opposite side of the lead, using a pre-tinned iron tip( solder pre-applied), as I explained to my wife, do not treat it like glue.
Good Luck
Rick
 
Thanks Merlynski for helping this member out, you have it all figured out. I should use your posts for a document on the module. I'll start it right now.
I have them listed, that is all I can say about that. If it was easy to do in barter town, I'd do it too, but it is not setup for this sort of commerce.
Getting the Al plate mfg setup was always the issue and still is today. They all have to be hand tapped as well.
It is labour/time intensive, low volume, so understand why things are the way they are. It is setup to make it as easy as possible for someone trying to learn to do it themselves.
There is even a utube video too, watch that, it is fun to see the guy operate, i got a chuckle out of his understanding and explanation of what we did, it was like a replay of me showing my wife how to build one up the first time.
I guess I can say, about soldering, let the solder flow, with the heat applied to the opposite side of the lead, using a pre-tinned iron tip( solder pre-applied), as I explained to my wife, do not treat it like glue.
Good Luck
Rick
You are most welcome, it is my pleasure to help. Please do use my posts.
 
Thanks Merlynski for helping this member out, you have it all figured out. I should use your posts for a document on the module. I'll start it right now.
I have them listed, that is all I can say about that. If it was easy to do in barter town, I'd do it too, but it is not setup for this sort of commerce.
Getting the Al plate mfg setup was always the issue and still is today. They all have to be hand tapped as well.
It is labour/time intensive, low volume, so understand why things are the way they are. It is setup to make it as easy as possible for someone trying to learn to do it themselves.
There is even a utube video too, watch that, it is fun to see the guy operate, i got a chuckle out of his understanding and explanation of what we did, it was like a replay of me showing my wife how to build one up the first time.
I guess I can say, about soldering, let the solder flow, with the heat applied to the opposite side of the lead, using a pre-tinned iron tip( solder pre-applied), as I explained to my wife, do not treat it like glue.
Good Luck
Rick


Thank you-I'll search for the 'tube video and also read the main thread on these modules. I'm excited to try it out!
 
It all started with one member asking me if it was possible, now we are working to rid the world of sub par STK' s :)
And the list of amps and modules (and Brands) this one has replaced successfully grows almost daily :biggrin::thumbsup:.
With an International impact, no less!
:eek2:
 
Last edited:
If you want the original, full-size versions, they are here (linking them seems to be shrinking them):

PioneerSX-780SchematicWide.jpg


SX-780PCBoardLayout.jpg

Hi Watthour, the links to your version of the sx-780 schematics seem to be broken. Would mind relinking them? From the smaller jpg of them, the color coding shown will be useful. Thanks.
 
The link works fine for me. I've seen this before. Try clearing your cache and forcing your browser to re-read the file.
 
Last edited:
Back
Top Bottom