SX-680 Restoration Troubles

[merlynski]

Notice how the dc offset is now very good, -11.0mvdc. The resistors close the feedback loop for the diff amp with the 2SA798 transistors. This tells me the diff amp and other circuitry in the power amp section is in good shape. Anybody would be happy with that dc offset. Looks like the problem is the modules having the 150mV offset you reported. That offset probably won't hurt anything, except maybe the sound, at low levels. Are the Wattmeters sitting at 0? I expect so. The modules did not blow fuses or damage the amp, do you want to try them again and see if it will play music?

 

[NickDeO]

Notice how the dc offset is now very good, -11.0mvdc. The resistors close the feedback loop for the diff amp with the 2SA798 transistors. This tells me the diff amp and other circuitry in the power amp section is in good shape. Anybody would be happy with that dc offset. Looks like the problem is the modules having the 150mV offset you reported. That offset probably won't hurt anything, except maybe the sound, at low levels. Are the Wattmeters sitting at 0? I expect so. The modules did not blow fuses or damage the amp, do you want to try them again and see if it will play music?

I'll plug it in now and see what happens!

 

[merlynski]

I'll plug it in now and see what happens!

It won't play music like it is, without the current amplification of the modules, we have just verified the circuits are good that connect to the modules, and you have to remove the test resistors, and reinstall the modules.

 

[merlynski]

There is another option, there are custom replacement modules you can build that work great to replace STK modules, and they are repairable if they get damaged somehow. Cost is between $75 and $100 for the parts, pcbs and heatsinks. You buy the pcbs and heatsinks, order the components from a reliable distributor like Mouser. Then you build them yourself.
The were designed by guys here on AK, I used them in that repair I gave you the notes for. Here is the link to the thread by the designer with lots of people (including me) contributing to it: STK-0050 replacement for SX-780 and others | Audiokarma Home Audio Stereo Discussion Forums
It is a long thread . . .

 

[NickDeO]

There is another option, there are custom replacement modules you can build that work great to replace STK modules, and they are repairable if they get damaged somehow. Cost is between $75 and $100 for the parts, pcbs and heatsinks. You buy the pcbs and heatsinks, order the components from a reliable distributor like Mouser. Then you build them yourself.
The were designed by guys here on AK, I used them in that repair I gave you the notes for. Here is the link to the thread by the designer with lots of people (including me) contributing to it: STK-0050 replacement for SX-780 and others | Audiokarma Home Audio Stereo Discussion Forums

I removed the resistors and reinstalled the modules. I'm getting humming and popping, so I suppose my $16 power packs were a bad investment. Is a $75 option the cheapest way to fix this? I feel like I'd be losing quite a bit of money on this project if I decide to sell it... I'll check out the thread!

 

[merlynski]

I have never seen an amp that passed the dbt and the resistor test procedure fail to work IF you have good modules. I have seen people use used, new old stock, modules from 'reliable sources' and these custom modules. The $75 to $100 option is not intended to be the cheapest, but is works really well. You might be able to find other modules that are good, but it only takes getting ripped off a couple of times to equal the cost of the custom modules. For me it was a 'no brainer'. But you have to decide, its your time and money.
Here is a couple of pictures of my SX-780 build:

 

[NickDeO]

I have never seen an amp that passed the dbt and the resistor test procedure fail to work IF you have good modules. I have seen people use used, new old stock, modules from 'reliable sources' and these custom modules. The $75 to $100 option is not intended to be the cheapest, but is works really well. You might be able to find other modules that are good, but it only takes getting ripped off a couple of times to equal the cost of the custom modules. For me it was a 'no brainer'. But you have to decide, its your time and money.
Here is a couple of pictures of my SX-780 build:

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I'm reading the thread and this actually looks like a really interesting project. It'll be a great way to learn more about power amps, so I figure I might give it a try. Are there any major differences between the STK-0039 and the STK-0050? Is this project plug and play with the SX-680? It seems so, given the replies in the other thread

 

[merlynski]

I'm reading the thread and this actually looks like a really interesting project. It'll be a great way to learn more about power amps, so I figure I might give it a try. Are there any major differences between the STK-0039 and the STK-0050? Is this project plug and play with the SX-680? It seems so, given the replies in the other thread

There are some differences internally but one of the posts in that thread states they were installed and working great in a Pioneer SX-680. You may have to drill a couple of holes to mount the custom heatsinks to your SX-680 heatsink. I don't think the STK-0039 mounting holes are the same as the STK-0050, not sure. Other than that you should be good to go.

 

[NickDeO]

There are some differences internally but one of the posts in that thread states they were installed and working great in a Pioneer SX-680. You may have to drill a couple of holes to mount the custom heatsinks to your SX-680 heatsink. I don't think the STK-0039 mounting holes are the same as the STK-0050, not sure. Other than that you should be good to go.

I just bought the ebay pcb and adapter plate set you mentioned on the other thread. I'll have to keep reading up on this, but I think it'll be a fun project. There's no way I'll ever make any money on the 680, but I'm hoping it'll sound good enough for me to want to hold on to

 

[merlynski]

I just bought the ebay pcb and adapter plate set you mentioned on the other thread. I'll have to keep reading up on this, but I think it'll be a fun project. There's no way I'll ever make any money on the 680, but I'm hoping it'll sound good enough for me to want to hold on to

I thoroughly enjoyed building it myself. FYI, The one thing (minor) I slipped up on during assembly is recognizing there are two different wattages of 47Ω resistors, and put 4 lower wattage resistors in each build, instead of two of each wattage in each build in their appropriate places. Since it is well engineered it didn't hurt anything and is still working perfectly.
I have looked at the costs of trying to refurbish vintage stereos for profit and could not come up with any way to do so. I really enjoy fixing vintage stereo stuff, but I am way too picky and slow to make a profit from it. So I help on AK, I get to have fun fixing stuff, and the person I am helping doesn't end up with a bill from me larger that the value of their unit.

 

[NickDeO]

I thoroughly enjoyed building it myself. FYI, The one thing (minor) I slipped up on during assembly is recognizing there are two different wattages of 47Ω resistors, and put 4 lower wattage resistors in each build, instead of two of each wattage in each build in their appropriate places. Since it is well engineered it didn't hurt anything and is still working perfectly.
I have looked at the costs of trying to refurbish vintage stereos for profit and could not come up with any way to do so. I really enjoy fixing vintage stereo stuff, but I am way too picky and slow to make a profit from it. So I help on AK, I get to have fun fixing stuff, and the person I am helping doesn't end up with a bill from me larger that the value of their unit.

That’s a really good way to put it, as it’s a lot of fun to keep these old stereos going - regardless of profits or losses. I wish I had more to offer on AK other than music suggestions and other little opinions here and there... This is by far the most productive and rewarding form of social media I’ve seen.

I’d really like to build my own stereo preamp/power amp at some point in the near future, but I have a lot of learning to do in the meantime. I’m looking forward to putting these modules together! Would you say there’s any advantages to these over the original stock STK0039s? Any difference in WPC?

 

[merlynski]

That’s a really good way to put it, as it’s a lot of fun to keep these old stereos going - regardless of profits or losses. I wish I had more to offer on AK other than music suggestions and other little opinions here and there... This is by far the most productive and rewarding form of social media I’ve seen.

I’d really like to build my own stereo preamp/power amp at some point in the near future, but I have a lot of learning to do in the meantime. I’m looking forward to putting these modules together! Would you say there’s any advantages to these over the original stock STK0039s? Any difference in WPC?

The custom STK units themselves will handle at least 80W per channel, maybe even 100W (designers' opinion) BUT the SX-680 itself does not have the capability to support that. The power supply is not designed to support much more than the original STK units, it does not have enough voltage or current capability, nor is the gain in the voltage amplification sections great enough, or have the headroom (voltage) to make a large enough signal to drive the power amp to that level. They will handle higher power than the originals for longer times, sound at least as good, if not better, and are repairable. In other words you can turn it up more with less risk of damage, but you are not going to get a difference like night and day. As an analogy, you can put a bigger engine in a car and get more performance, put if you put too big an engine in you will break the transmission, twist off the drive shaft, break axles, or even twist your frame.
They custom ones were designed to replace/upgrade the STK-0050 in an SX-780, which is rated for 45 WPC. The manufacturer of the STK-0050 rated it for 50 WPC use. If you read that whole thread you saw there were lots of different amps that have successfully used that design to replace similar original STKs including (I think) 80W modules.

 

[fromnowon]

So I help on AK, I get to have fun fixing stuff, and the person I am helping doesn't end up with a bill from me larger that the value of their unit.

It's appreciated!
Thanks Merlynski, you've helped me that's for sure.

 

[NickDeO]

The custom STK units themselves will handle at least 80W per channel, maybe even 100W (designers' opinion) BUT the SX-680 itself does not have the capability to support that. The power supply is not designed to support much more than the original STK units, it does not have enough voltage or current capability, nor is the gain in the voltage amplification sections great enough, or have the headroom (voltage) to make a large enough signal to drive the power amp to that level. They will handle higher power than the originals for longer times, sound at least as good, if not better, and are repairable. In other words you can turn it up more with less risk of damage, but you are not going to get a difference like night and day. As an analogy, you can put a bigger engine in a car and get more performance, put if you put too big an engine in you will break the transmission, twist off the drive shaft, break axles, or even twist your frame.
They custom ones were designed to replace/upgrade the STK-0050 in an SX-780, which is rated for 45 WPC. The manufacturer of the STK-0050 rated it for 50 WPC use. If you read that whole thread you saw there were lots of different amps that have successfully used that design to replace similar original STKs including (I think) 80W modules.

Just got all of the parts in the mail yesterday and getting ready to dig into this now! I'll continue posting updates on this thread. If I can complete this project without too many issues, I'll be thrilled! I can't wait to hear the 680 sing!

 

[NickDeO]

Just finished soldering in the resistors, diodes and capacitors. I also wet-sanded the aluminum down into flat mounting surfaces. Hopefully tonight after work I’ll have the rest of the transistors installed. Luckily, I don’t believe I’ll need to do any drilling to install this into an SX-680! When I know for sure, I’ll be sure to contribute to the other thread

 

[NickDeO]

Here are the finished modules — Now to install and adjust them...

 

[mykidsbutler]

Check out post 450 to 453 on this thread for the adjustment portion of your question.
https://audiokarma.org/forums/index...x-780-and-others.721181/page-23#post-11873979
Two items to pay attention to pre power up. Use your DBT and adjust you trimmer resistor on your modules per the instructions.

 

[merlynski]

Here are the finished modules — Now to install and adjust them...

Beautifully done. Note there are two versions of the pcb I am aware of, in the version you have minimum bias is with the trimmer turned all the way clockwise. Verify it is at minimum by checking for 0Ω across it before you power up. If you test bias function on dbt be sure to go back to minimum before going to full mains, then set bias correctly per listed instructions.

 

[NickDeO]

I must say, fitting these modules in the 680 is pretty tight. I wish they were mounted underneath like they are in the 780! They powered up wonderfully with no issues and the sound is phenomenal! Not only was this a fun project, but I think it made the end result feel a lot more ‘rewarding’ than a simple recap job. I’ll probably be redoing the STK packs in my JVC JA-S22 when they decide to go. So far, I can’t get enough of this hobby! My next project (starting in a few minutes) will be attempting a similar resurrection on a Lafayette LR-2500 with two dead channels. Thanks again, @merlynski!

 

[merlynski]

Congratulations on a job well done . And Thank You for posting your successful results in the module's origination thread .