Announcement: Technics SU-8600 rebuild

jeromach

With sufficient thrust, ducks fly just fine
I have a weak spot for the Technics SU-8600.

I already did a "refresh" on one once (repair + recap "light") and I know it's not an amp for the fainthearted, nevertheless I got myself one yesterday as a project successor to the Technics
SU-8088 I recently finished.

The 8600 has been produced from 1975 to 1976, so it's one of the earlier Technics integrated amps. Its specs are ok, not outrageous, but delivering 73 Watts at 8 ohm isn't bad.

The one I bought yesterday was a bit of a gamble, the seller couldn't tell me anything about it, not even if it was working, but the price was ok, so I just bought it from a couple of pics.

When I got home and did some quick inspection it appeared to be above expectation. It's missing two of the small black plastic knobs which is not really exceptional, since they all crumble away
getting older. Also the power switches tend to break, although in this one it's still ok. The front plate is quite nice, no scratches, that IS exceptional.

Technically it appeared to be completely original, something I can really appreciate. I fired it up on a dbt initially, but the lamp dimmed immediately and the relay engaged, so no problems here.
A lot of distortion though when engaging the switches and turning the potmeters. It's rather normal for the 8600 unfortunately, all switches need to be taken apart and cleaned, no deoxit here
and the volume pot is particularly nasty to get right. But I knew what I was up to.

Still, this particular amp is even more nasty than what I was used to. Dropping channels and distortion could be evoked with the stereo/mono/reverse switch, the selector switch, the
tone on/defeat switch, the balance pot and - but not even that much - the volume pot.
The tone on/defeat switch was the nastiest source of distortion, so much that it also affected the volume, in fact in a way I was happy to have speakers with overload protection. On top of that
there was another strange phenomena; when selecting phono 1 or 2 there was a very loud noise (nothing connected to the inputs), like the wind through a forest, but then on a storm level.

Apart from this the amp sounded fine :). I did manage to find an equilibrium in all the states the switches can be in, all ok, except the tone switch only operating normally at defeat.

The inner environment had collected a lot of dust, also nothing strange here. Although, maybe just one thing though; the voltage selector was at 240 volt, but no finger prints in the dust.
That's a little awkward, changing the voltage from 220 volt to 240 in these amps is something of a relatively recent fashion, so either the dust was collected recently too, or it somehow was on
240 for a longer time already, that would be strange.

Today I removed most of the dust. Not the final cleaning, that will be more thorough, but it's nice to work in an environment as clean as possible. The phono pre-amp / input selector board had
the most dust on it, apparently even so much that once removed also the phone 1 and 2 inputs stopped making the stormy noise. Problem 1 solved :).

So, the next step will be dismantling to a further stage. One of the other nastities of this amp is the difficulty getting to all parts of the main board at the solder side. There is a latch, but removing
that doesn't give access to all parts of the board. I will try to remove the complete bottom plate, but that is uncharted territory yet, so I need to see how to do that. At the "refresh" I did earlier I
removed the complete board, but that's a "never again", since reconnecting all the wires to the board is a small disaster.

All right, enough of all the blahblah, here's some pics;

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The dusty phono pre amp / input selector board

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And seen from another angle

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Original power outputs, nice!

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And the bottom with the serial number (built 27th of July, 1976, thank you MacPhantom :)) and the small latch

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To be continued....
 
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pretty cool find! Healthy looking puppy.

Damned dust bunnies will get you.. Did you know that spider silk is electrically conductive?

Looks like the output stack convection cooling could use a decent fan..
 
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Hey, mr. fan man :).

Some amps are cool enough without fans if the bias is set correctly, this is one of them :).

They do suffer from bad caps often though, just this one is the exception on the rule.

But indeed I think you're right about these dust bunnies, since the phono problem WAS solved after tidying up the mess inside.
 
Progress

Oh oh, maybe this time I went too far....


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Stage 1 of dismantling;

- power outputs desoldered, large heatsink removed
- front (2 x) and bottom plate removed
- all larger switches removed
- all larger potmeters removed
- bridge rectifier and heat sink removed
- large filter capacitors removed
- head phone socket removed
- relay removed
- just 1 (one) wire desoldered...

Around two, two and a half hours work.

Ekeb4KA.jpg


Next stages;

Tonight;
- Collecting new parts from stock and sort them for quick assembly

Later;
- Dismantling, cleaning and putting back together all switches
- Reconditioning potmeters
- Removing all electrolytic caps, all transistors and all diodes + check
- Thorough cleaning of all other components + pcb's
- Putting in new components
- Reassambly, adjusting, measurements
- Enjoying

Parts!!

k8OVGpk.jpg
 
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Wow,that's exactly what i wanted to see!
Desoldered power outputs?! My god,i was thinking they were held by the screws (ignorance will kill me)!
Furhtermore, all electrolytic caps seem (seem!) still good: a great achievement compared to those present in the su 8088!
Maybe in 1976 technics used better parts compared to those they used in 1978 for the 8088?
And the filter caps seem way easier to remove,maybe one solder iron was enough this time!
 
Don't worry thes, the 8088 doesn't have soldered power outputs, but in the 8600 they are soldered AND held by screws :). A terrible system btw with these screws, washers nuts and additional little heatsinks,
removing them is no issue, but putting it back together is a small disaster.

Yes, the electrolytic caps indeed seem good. The amp was working ok too and I haven't noticed other broken parts, although R509 has cracks. As far as I know this is not unusual, I have seen the same
at other 8600's, even a burned one on the 8600 I did before. That amp had several earlier replaced caps and apparently broke down again, since that's how I got it. I needed to replace several burned transistors
and gave it a recap (without the large PS filter caps though).

Anyway, I will measure all caps once I take them out, just out of curiousity. I already measured all new caps, to be sure they are ok before I solder them in. It's not that these 8600 caps are better than those of
the 8088, the general thought is even that they are worse than later ones, although I myself am not sure about that. The issue with the 8088 (and 8099) is just that in fact a design fault has been made by placing
the pre- and main amp pcb's vertically, causing heat problems. And electrolytic caps just don't like heat, it ages them a lot faster.

It's true these main ps filter caps can be removed more easily, that's because there are no additional metal bars here. It also uses regular snap-in caps, so no need for any additional diy'ing here.

However, I thought I had suitable ps main filter caps in stock, but that wasn't so. It's because the odd size of these caps, 35 mm diameter x 80 mm high. I only have 40 diameter in stock, so I had to order news ones.
That will delay this project a little bit, it'll probably take until Wednesday to get the new caps delivered (sorry janikphoto :)). But there's enough other work to do meanwhile.

Another odd thing is that these caps in this amp were 18.000 uF, while the service manual mentions 15.000 uF. Regular is grey caps and even higher than 80 mm, but this amp had black 80 mm caps. It must have been
factory original though, they are Matsushita/Panasonic and from that time.

The cracked R509;

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The odd ps main filter cap;

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PS: I guess your ignorance is diminishing quickly :)
 
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Cleaning day;

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And desoldering all components to be exchanged, checking the old ones too.

Nothing broken, but a number of caps out of spec, several 100 uF / 50 Volt caps measuring around 75 uF and the one below being the worst, a 100 uF / 6,3 volt cap measuring just 28 uF. Like I
mentioned earlier, often the cap looks fine, this one too, as if it just came out of the factory.

3rFyzyC.jpg


Another oddity; damage at the board, like some one has been scratching it with a sharp object. Perhaps it has something to do with the non original main filter caps (this is where they are soldered
to the board), like maybe the original caps have been leaking and were replaced with the 18kuF caps. But it must have been a while ago, since the caps I found weren't particularly young.

rxLzObj.jpg


Next step; cleaning the boards and putting in the new parts.
 
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So those big filter caps weren't originals..that's why they have different specs from those published in the service manual :scratch2:
Hope the board near the big filter caps isn't compromised by those scratches,can't wait to see the pcb cleaned!
What is the best way to clean pcb's?
Also,there's a thing that i always fear when it comes to vintage (misthreated) amps (like mine or those to be found on the ebay):is it possible that they can kill the speakers?And what can be done to avoid it? Checking bias and dc offset would be enough?
I've also noticed that flux (glue?) where the big filter caps used to be (a little can be seen near the relay's soldering points) ...looks hard to remove.
I must say i'm still fighting with my ignorance but i don't complain: at least it keeps me busy so i don't get bored!:D
 
Hi Jeroen, wonder how you clean the pots, do you use an ultrasonic cleaner and what do you put in?

Myself I tried at work, using 25% ethanol in an ultrasonic cleaner heated to 50 degrees celcius, but I did not want to let it run for ages, and the result was not good (pots still dirty....)
 
Awesome cleaning job! Looking forward to seeing more progress to this beauty! :banana:

Just one thing: I see that the main filter caps have been replaced (the original ones were gray), but those black things are nevertheless good replacements (original Matsushita). I have smaller versions of the same ones in my SU-7700 (originally, with 10'000µF each), and though I have not tested them, I bet they still meet all factory specs. Who knows; maybe these caps were put in by a Matsushita-certified partner who did not pay enough attention not to damage the PCB?

Edit: whoops, you have mentioned that already. My fault.
 
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Done a lot today, still, a lot to do :).

Of course there are always setbacks in a project like this, smaller or larger. In this case a considerate one. Upon closer inspection of the board it appeared that there were a lot of dry solder joints.
Not terrible and as we know the amp was just working, but still enough to decide to give all boards a full solder reflow. And yep, that's some work.

Meanwhile all new caps, transistors and diodes are in, except of course for the large main filter caps that probably are in a plane at the moment. A couple of resistors also need to be replaced,
after cleaning they appeared to be damaged, these were all near places with heat marks on the board. I do not yet have pictures of this stage, but I'll shoot them tomorrow (when I have better
light, it's evening here already).

This is what the main board looks like after cleaning, meanwhile also all parts to be replaced removed;

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And the solder side;

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Parts that came out;

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Parts that want to get in;

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Next step;

Dismantling, cleaning and remounting the switches.
 
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So those big filter caps weren't originals..that's why they have different specs from those published in the service manual :scratch2:
Hope the board near the big filter caps isn't compromised by those scratches,can't wait to see the pcb cleaned!
What is the best way to clean pcb's?
The board and the big filter caps were functioning before, so I'm not afraid it will stop working now.

As for cleaning the pcb, it's something you need to experiment a little with. In fact water and a degreaser is ok, as long as it doesn't damage the printings on the pcb (something to test first before applying to the whole board). Of course not all electrical things like water, so it should be done with a little common sense. For instance, I put the transformer in a plastic bag before holding the main board under the tap. And I don't think any of the boards would like it to be submersed in water for an extended time. After cleaning I thoroughly rinse the boards with hot water (rather, not too), this takes off the remains of the degreaser and since it's warm the board will dry quicker. Ah, before I forget, I use the hair dryer much more often than my wife does :).

Also,there's a thing that i always fear when it comes to vintage (misthreated) amps (like mine or those to be found on the ebay):is it possible that they can kill the speakers?And what can be done to avoid it? Checking bias and dc offset would be enough?

Yes, no, maybe and I don't know. In the perfect world your 8088 and a lot of other amps have a speaker protection relay. Speakers don't like (too much) DC, for instance the DC that wants to kill them once a power output blows. This will fry the coils of the speakers. So, that's where the relay should interfere, but the question is wheather it will always be quick enough to avoid the damage. I once had issues with an 8099 that only sometimes put DC on the speaker outputs, then while listening to nice music it suddenly gave an enormous bang before the relay kicked in. Even knowing it would happen once in a while it still scared the hell out of me when it occurred.

Another way of speaker protection is fuses. The question here is; what will give up quicker, the speaker or the fuse. I'm afraid the speaker is the weaker part here quite often.

Despite all this I think your speakers will be safe with your 8088, the relay most likely will indeed protect them when something goes wrong. But checking the bias wouldn't hurt, if it's too high it might damage the power outputs.

However, since I of course work rather often with amps of dubious qualtity I'm quite happy with the Technics SB-R4's and their double built-in overload protectors.

I've also noticed that flux (glue?) where the big filter caps used to be (a little can be seen near the relay's soldering points) ...looks hard to remove.
I must say i'm still fighting with my ignorance but i don't complain: at least it keeps me busy so i don't get bored!:D

Flux is not really a problem to remove, althought today - despite using special flux cleaner - the pcb got a white residue on it. I'll need to clean it again with the degreaser to get it looking nice again. So, the flux you noticed is gone, still there is some other brownish goo on it, I think this is glue residue.
 
Why did you toss out those TO-220 transistors? Were they any bad?

SU-7700, nice!

No, they were not bad, it's just to be thorough. It's why I call it a rebuild, not a recap. Some people disagree - ain't fix it if it ain't broke - and to a certain extent I agree with them, still, the people I rebuilt amps for were very (some very very :)) happy with the result.
 
Will you show us some pictures of how you disassemble the switches? And what they look like inside, before and after? :scratch2:

Sure, I hope these are good examples. Probably, because they caused a lot of trouble :).
 
Right, some pics of the partially rebuilt boards.

New components and boards cleaned an additional time;

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Main amp board (right side);

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Main amp board (left side);

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And the phono pre-amp / input selector board;

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It's good to look at things from a different angle once in a while, the perspective can be quite nice;

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Sea of colors;

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Sky scraping caps;

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