Technics SU-8044 repair

[Anolsos_1974]

So, I have found at least one culprit - one of the pins was loose in the power pack, and probably rubbing intermittently against the connecting bit of the circuit board causing the drop in sound from the left channel. I think the other pin came off when I was removing the power packs. But nonetheless, at least one was not attached properly. Replacements seem around €13 (~$10?).

I have a couple of queries though, the 'power pack' units, look similar, but not identical. Am I imagining things? Also, is it really going to be as simple as changing this out? As I mentioned above, when you pushed down on the power pack with the loose pins, it sounded lovely...but any advice would be welcome before I go any further.

 

[Anolsos_1974]

It's fixed! I swapped the good power pack for the bad one and we have lift off! The only thing is a small earth noise, but I connected it back with only one power pack attached so I'm hoping it's that. Thanks for your help again.

 

[Schmidlapper]

Just match the broke side with the working side and you should be fine. I don't know what the original power packs did or didn't look like. The occasional flexing of the circuit board when you move the chassis around shouldn't be an issue once the solder joints are made good. I mean that is how Technics produced it. Not sure about the ground hum, it could be because you only have half of the pair installed. You are making great progress.

 

[Schmidlapper]

Was thinking maybe you could leave a screw out of the circuit board where it is closest to the flexing part of the chassis. Maybe it wouldn't flex along with the chassis as much.

 

[Anolsos_1974]

Just match the broke side with the working side and you should be fine. I don't know what the original power packs did or didn't look like. The occasional flexing of the circuit board when you move the chassis around shouldn't be an issue once the solder joints are made good. I mean that is how Technics produced it. Not sure about the ground hum, it could be because you only have half of the pair installed. You are making great progress.

Just order the module from g

 

[Anolsos_1974]

Just match the broke side with the working side and you should be fine. I don't know what the original power packs did or didn't look like. The occasional flexing of the circuit board when you move the chassis around shouldn't be an issue once the solder joints are made good. I mean that is how Technics produced it. Not sure about the ground hum, it could be because you only have half of the pair installed. You are making great progress.

Just order the module from g

 

[Anolsos_1974]

I'm not sure what is going on with my posts! Anyway Schmidlapper, what I found odd what that there was reinforcement of the right channel power pack but not the left. Close study indicated no obvious difference in structural setup - just puzzled me. Anyway, I don't know if I have a genuine stk00039, but I am running out of ways to stop the thing originally coming from China so I'll just see!

 

[felix_katze]

Don't leave any of the screws out of the pcb near the power packs - very likely that they are using grounding through the screws to the chassis.

 

[Schmidlapper]

Do you have the biasing and DC offset procedure for this? I would assume once repaired they both would need checked and possibly adjusted. I don’t know if they use good matching of the differential amps as the method to set DC offset or provide a pot. If the former it may still be good.

 

[Schmidlapper]

Yes and When I suggested leaving a screw out I didn’t mean any needed as a ground. Once again I have no clue what this thing looks like inside, just that you believe the chassis flexing is harmful to the circuit board. If it doesn’t help or make sense don’t do it.

 

[Anolsos_1974]

Do you have the biasing and DC offset procedure for this? I would assume once repaired they both would need checked and possibly adjusted. I don’t know if they use good matching of the differential amps as the method to set DC offset or provide a pot. If the former it may still be good.

Hi - I don't, but I would very much appreciate a link if it's convenient,

 

[Anolsos_1974]

Yes and When I suggested leaving a screw out I didn’t mean any needed as a ground. Once again I have no clue what this thing looks like inside, just that you believe the chassis flexing is harmful to the circuit board. If it doesn’t help or make sense don’t do it.

Hi - I didn't read it this way - anything to increase earthing is good in my book! Attached is a photo taken before all this started so forgive the fact you cannot see the edges (the amp is in pieces currently). You can clearly see the lack of bar on the right hand end of the cooling array (not sure real of name of array). There are no fittings in the case for a bar either so it isn't missing. I found a youtube video of a guy who was also replacing the same unit (not sure if it breaks rules to post it but I can if requested).

 

[felix_katze]

Having a bar on one side is normal. In the centre of the other piece, you can see a screw in the middle to the heatsink, I'd bet on the other side of the heatsink, opposite that screw, but out of shot of that photo, is another screw - that is the earthing screw I was referring too - you can see some fairly beefy traces running to it. If you do voltage tests without that earthed, you'll get crazy results. If screwing the PCB down causes crackling or static then likely a sign of bad solder joints, so best to deal with that.

I don't think there is any requirement for biasing these power packs - I believe they are mostly plug and play.

 

[Anolsos_1974]

Having a bar on one side is normal. In the centre of the other piece, you can see a screw in the middle to the heatsink, I'd bet on the other side of the heatsink, opposite that screw, but out of shot of that photo, is another screw - that is the earthing screw I was referring too - you can see some fairly beefy traces running to it. If you do voltage tests without that earthed, you'll get crazy results. If screwing the PCB down causes crackling or static then likely a sign of bad solder joints, so best to deal with that.

I don't think there is any requirement for biasing these power packs - I believe they are mostly plug and play.

- that sounds like good news - assuming that I have actually been able to order a genuine part!

My perspective on the reinforcement is that (with my engineering head on - I'm not an electrically trained person) - when you push down at one end of the cooling array it experiences more flex than the other end - I would consider this a failure, because - if there is a reason for putting reinforcement at at one end, then that reason shouldn't be limited to that end only. Unless of course I'm missing some redundant strength elsewhere in the case which compensates for the lack of bar. I might be thinking too much about this also!

 

[Anolsos_1974]

...I just got an email from my supplier responding to my question saying - "this is not an original IC. It has left our place yesterday"

please imagine the laughing emoji here

 

[felix_katze]

Some of these STK modules are very hard to get. I have a supplier in Tokyo that has some, but not others. There are some compatibilities - they were designed to allow manufacturers to offer different specs off the same basic board - but nothing available that fits yours. I've got some STK0029s that would fit in that board, and as long as you paid attention to supply voltages, pretty good chance that would work.

Generally only the very early ones were manufactured in Japan, after that it was moved out to factories in Korea, Taiwan, Malaysia, or where-ever. There is the possibility of stock in small pockets here and there. If you buy from AliExpress, in my experience, stay away from the cheapest items, and sellers in Hong Kong/Singapore tend to be more reliable. Anyway, give it a go when it arrives, you might be OK.

 

[Anolsos_1974]

Some of these STK modules are very hard to get. I have a supplier in Tokyo that has some, but not others. There are some compatibilities - they were designed to allow manufacturers to offer different specs off the same basic board - but nothing available that fits yours. I've got some STK0029s that would fit in that board, and as long as you paid attention to supply voltages, pretty good chance that would work.

Generally only the very early ones were manufactured in Japan, after that it was moved out to factories in Korea, Taiwan, Malaysia, or where-ever. There is the possibility of stock in small pockets here and there. If you buy from AliExpress, in my experience, stay away from the cheapest items, and sellers in Hong Kong/Singapore tend to be more reliable. Anyway, give it a go when it arrives, you might be OK.

This is what I had hoped actually. With some luck I have ordered an ok one. I purposefully avoid aliexpress.

 

[Anolsos_1974]

It's fixed! Incredible! Sounds lovely too. There is still a small earth noise however, it's not really noticeable but I would like to diagnose it. Any first suggestions would be warmly welcomed.

But thanks for everyone who helped me get it fixed. It would have gone back online if you guys weren't here.

 

[PioneerPro]

I am feeling really really stupid right here, but I need to know what this is all about. I recently bought this Technics SU-8044 and the previous person who worked on it had no reason to work on Electronics.

The crazy thing is it all works. Can someone tell me what the hell is going on here, because I have never seen anything like this. Two legs of the transistors are soldered together.

The Service Manual on HIFI Engine is just crap and very hard to read.