CX-600 30-year maintenance; unit working, but would love the advice of the more skilled and experienced

[poorsolder]

Greetings!

Long time reader, short-time lurker, first-time caller.

I have had a CX-600(R)for about 30 years, fronting a Kenwood Basic M1D. In the nineties to early oughts they drove a pair of floorstanding Infinitys from their low-sumer line that sounded just okay but could fill a sizeable warehouse space with impressive volume. I hope the kid I gave them to actually re-did the driver surrounds and enjoyed them. And me-now would like to apologize to all of my angry then-neighbors.

My partner preferred an NAD integrated amp (why are there two things to turn on? what button is for...? it's so LOUD! they are so scruffy-looking!), so the CX-600 has been hardly used for many years, and about 8 or 10 years ago it got wonky: massive drop in volume from disabling "source direct", noisy and unpredictable results from use of tone and balance controls, loudness knob increasingly resistant to being turned, and then loss of one stereo channel.

It now needs to become my daily driver so,

1. I went to school by reading @Bratwurst7s multi-year CX-600 journey (I can't adequately express my thanks for that, but thank you!).
2. I opened the case up for the first time ever the other day, and found that it was remarkably clean inside considering the life it has led. One of the power caps is a bit swollen on top, and there are some suspicious brown spots on the board, but not a crime scene.
3. I cleaned out all of the pots with Kontakt 60, and rinsed them each with 1cc of 99.9 isopropyl in a needle syringe, which i discovered is 80% cheaper than the iso-in-a-spray-can.
   • I still intend to lube them with Kontakt 61, but wanted to test and see if any deserved another cleaning before going there.
4. I let the iso dry out a day, closed the case, connected it to the kenwood, set it to "source direct" (although I forgot to replace that button somehow ) and powered it up.
5. I asked the squeezebox to choose a song, and suddenly The Impressions were singing "Fool for You", and sounding great-- in both speakers!

Status of the unit:
- Providing two stereo channels despite the marshmallowing power cap
- Tone and balance controls seem to be working just fine without a volume drop
- The loudness knob turns easily like the other pots but behaves like a master volume control: at FLAT there is sound, at the other end the amp is silent
- Have not done a full test of all of the ins-and-outs
- I ordered mundorf power caps of identical spec to those @Bratwurst7s used at @avionic 's suggestion.
- The pad on top of the transformer is, well, "crispy" is the best word I have for it. The top of the case is getting warm there, but can it conduct heat effectively in this form?
- Q407 seems of interest on both sides of the board, and Q403/406 on the solder side
- PCB is brown around the chip under the tape-out selector ribbon, although solder side doesn't look suspect to me.

Skills and tools
- I have always owned a soldering iron and been confident in using it--built a good bit of kit as a teen--but that confidence is probably overconfidence at the moment since I haven't soldered anything except appliance repairs in...20 years?
- My cheap mini soldering station is dead, I discovered, so I ordered a smart iron, i'll use the stand on the existing one, and hollow out the works for solder and tip storage.
- I have a solid if older and no-frills fluke meter with impedance and capacitance. theoretically also temperature, but the thermocouple lead spacing is identical to european wall plug spacing and thus my elder child fried it as a curious pre-teen (with no injury to child or property, thankfully) and I never replaced it.
- My electronics knowledge once exceeded simple circuits a bit, but does not any more.

Plan
- Replace the power caps. They are big and friendly to solder
- Clean the loudness pot again with kontakt 60 and isopropyl
- Lube all the pots with Kontakt 61
- Find the dang remote
- Desolder things for testing/replacement only where cost/benefit is high, such as preventing damage, extending usable life, ending world hunger. I am not chasing the moonrock needle.
- Try not to take on any more hobbies no matter how interesting. meaning don't move on to the AKAI AM-2280 next. or the celestion f3s you saw for 50€.
- See how the phono stage behaves when the TT arrives next week. I have an NAD phono stage I can use if needed.

Advice and help requested
- Ideas/suggestions on the loudness knob issue
- Should krusty the thermal pad be replaced with something? Any suggestions? I found 8mm thick alumimum rail in 60cm width, but the minimum purchase is six linear meters, and I need 48mm, so thats...125 preamps worth
- Important warnings about mistakes I am making or about to make
- Given that the cap nearest the transformer is the cooked one, is a heat shield of aluminum flashing worth it?
- (Politely) disabusing me of my ignorance

Kindly,

Mike

 

[Soundscape]

I see a short...The intermittent type.

 

[ghankstef]

Maybe reflow those transistor solder joints. They look like they have been exposed to a lot of heat.

 

[mrehorst]

That thing is 35 years old. You need to replace ALL the electrolytic caps. They aren't made to last that long.

 

[poorsolder]

I see a short...The intermittent type.

Maybe reflow those transistor solder joints. They look like they have been exposed to a lot of heat.

Thanks for your sharp eyes and suggestions. Those will go on my cleanup list

That thing is 35 years old. You need to replace ALL the electrolytic caps. They aren't made to last that long.

Thanks, what you say is sensible. I want to as an eventual objective for accuracy, and as cheap as caps are compared to the cost of shipping one package I may order the whole part list of electrolytics from the get. In the immediate term, I will probably prioritize the ones that are easiest to replace since I see one of my biggest risks as borking traces or components with sloppy work due to hurrying or carelessness, I'll see how it goes with hand-eye-iron-board coordination and go from there. While I am at it I should do the crossovers on the pair of similar-vintage KEF c40s i found for 30 bucks that only needed partial regluing of cones to the meh-but-durable surrounds to rock out again.


Any opinions on the value of replacing the pad atop the transformer?

 

[Bratwurst7s]

It has been a few years since I worked on that CX-600. I'll have to go through my photos and refresh my memory. I suppose that one could call it an "educational experience".

One thing that stands out in my memory is that the removable base plate is smaller than the main PCB and you can't easily get to much of the power supply area without removing the board. It is best to de-solder the wire posts to do that, do not unwrap the wire wraps.

I don't think that the transformer gets warm enough to worry about replacing the pad on the top of it, I would remove and discard it.

I agree strongly with:
Re-solder each and every single solder connection. The one that you didn't do just became the next point of failure.
Replace every elco. I will have to look, but in general I replace small (10µF and under) elcos with film caps where their size will allow. Caps in the signal path I replace with Nichicon UES Muse bi-polar.

I will have to look about that cap next to the heat sink, but a quick thought is that I would probably replace it with a 125° part. The aluminum heat shield might be a good idea if it can be done without shorting anything.

,
James

 

[Mike_S]

Do you have a full schematic with parts list?
As others have mentioned, the electrolytic caps should be replaced. If they are not out of spec yet, ESR has to be very high.
At the very lest do all the caps in the signal path. Trust me, it will make a huge difference!

Good luck, keep us updated!

 

[poorsolder]

One thing that stands out in my memory is that the removable base plate is smaller than the main PCB and you can't easily get to much of the power supply area without removing the board. It is best to de-solder the wire posts to do that, do not unwrap the wire wraps.

That kind of advice from experience is priceless and I shall heed it.

I will have to look about that cap next to the heat sink, but a quick thought is that I would probably replace it with a 125° part. The aluminum heat shield might be a good idea if it can be done without shorting anything.

I ordered the 3330uf Mundorfs from your build which are rated @125

I don't think that the transformer gets warm enough to worry about replacing the pad on the top of it, I would remove and discard it.

Not hot-hot, but It gets warm enough to cook the nearer cap quite a bit more than the front one and sure wouldn't mind spilling that heat outside the if possible. I have an 8mm thick finned aluminum heatsink from some ?PU that i might try and see how it works.

Do you have a full schematic with parts list?

Thanks for asking, I have a full Service Manual from audiocircuit.dk but the schematic scans are chopped and illegible, plus the nice schematics from the AK archive, and between the two I should have what I need.

Probably as important are the three VCRs my buddy who moved to Brazil left me, with which I will play solder surgeon until i get my mojo back or give in and buy something like a zd-8965 for all those caps. Not important but hilarious is the the tape in the box underneath the lowermost unit about which i can only say rhymes with "Camel a hand her nun" .

To mark the arrival of the tiniest iron I have ever owned (which must weigh 1/100th of the very first, a yard-sale Weller gun in a grey plastic box), and the generosity of this community with its knowledge, I present the boot splash for the TS101:

 

[avionic]

Welcome aboard poorsolder Mike..

 

[Bratwurst7s]

I often wondered what it took to destroy the 2SK105 N-channel J-Fet transistors on the inputs in my pre.
I kind of envisioned someone plugging a in portable device through it's headphone output jack at full volume or something like that and moving from input to input trying to figure out why they couldn't turn off the input afterwards, or something like that.

Be careful of what you plug in to the inputs, those transistors are only 250mW devices so are not all that robust. I was lucky enough to find some NOS replacements, so did not peruse a substitute, but it would be better to not have to do that.

That was my first experience with a n-channel j-fet and I really fooled myself more than once trying to figure out why almost all of my inputs were always on regardless of what was switched on. Seems that when they short they short closed and let the signal through.

,
James

 

[Bratwurst7s]

I don't recall what replacement relay I used or if it is still available, but I would replace the relay while you are in there.

,
James

 

[mrehorst]

You don't really need a schematic or parts list- you can make your own. It looks like almost everything in that preamp is on one or two PCBs. You should be able to read the cap values and make a list. Mark each with a dot on top with a permanent marker as you add them to the list. It should be an easy recap job.