Wednesday, 3 December 2014

Technics SC-CH900

Not really a repair, but possibly useful info for somebody and an excuse to share some love for the SC-CH900, for me the best, the pinnacle of the mini hifi system in the 90's. Im sure a sony MHC-6600 fan would disagree :-)



So the Technics SC-CH900, I lusted after this baby for 23 years ! I always had a fascination and interest in home electronics, hi fi's TV's etc so back in 1991 after I started work I set about saving for and buying my first stereo. I knew all about HiFi, what cable to buy etc but despite it not being 'HiFi' in the seperates and OFC speaker cable sense I was hooked on getting one of these. I think it was the copious use of VFD displays that did it. So every week I would check out the stereos in the local shopping precinct, It was between the Technics and a Sony MHC3600/5600/6600. So the time came to buy, what I actually bought was a SC-CH700, largely the same system but without Dolby surround capability or the brushed metal finish. Crucially though it was from memory about £200-£300 less than the £1000 of the 900, a lot of money for me in 1992. The CH700 did me many years of good service and I loved it but I always regretted not getting the top of the tree 900. So good old eBay delivered the goods and all these years later I have a pristine SC-CH900 and I love it.

The only issue with it was the display of the main amp unit, the other components looked as new but the amp had 2 issues, the cover / filter over the VFD was cloudy / hazy / dirty. Secondly some parts of the display were significantly brighter than other, some looked over bright, others too dim ? As seen in the picture below from just after I bought it.



I looked into getting a new display and considered swapping the display from my 700. Panasonic UK said that they did not support the unit anymore but armed with a part number from the service manual I inquired in the US and found it was still available, would have cost about £40.

I did a bit googling on VFD's and found some referance to the grid wires oxidising due to lack of use. Either manually powering them and flowing a bit more than normal current through them may help or just leaving the unit in for an extended length of time. Although the clock is lit all the time it didnt seem to be getting any better after a week in standby so I left it fully powered on overnight. Success, the next day the display was bright and uniform.

So I then stripped the unit down to clean the inside of the display, as the amp is on all the time in standby and has a fan running when the volume is turned up I suspect that is why it had got dirty and the other components looked like new.

I wasn't dissapointed with the construction, the service manuals are amazing in quality and the unit was too. It came apart very nicely, lots of screws, no forcing or plastic tabs breaking.





A careful gentle wipe and it was like new again.



All set up with my SL1210 MK3D. A video of it in action.


http://youtu.be/_xItB0q_CI0

I knew I had the brochure sat in my loft somewhere so dug it out and scanned it below, ah the memories.



A link to the whole brochure I scanned in.

https://www.flickr.com/photos/120943638@N03/sets/72157649178929430/

The only other issues I have come across with these systems relates to the fan, when the volume goes above a certain level an internal fan kicks in, if this fails the unit shuts down. My SC-CH700 did this and I fitted a new, quiter fan. The 900 seems ok so far (and I am actually just useing hte the whole thing as a pre amp currently).

The fan was my only gripe with the 700 as I found it intrusive the way it would come in and out, almost with the beat at the volume levels I listened to the unit at.


27 comments:

  1. The amp cutting out is due to electrical noise from the (ageing) fan tripping the protection circuit. An easy fix is to solder a 10uF (IIRC) capacitor across the fan terminals on the PCB. Note the polarity markings are wrong - they're the wrong way round. I've got a CH900 too; they're great.

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    1. Hi..Matt need your help..I have same problem like you...how you mean solder the cap 10uF(IIRC)across the terminal fan on PCB.Can you have photo to show me..I really need your help..

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  2. Hi Lowfinger! I have the chance to purchase this system for 215 GBP. Do you this it is a bargain?
    Thanks!

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    1. I have seen a few go around the £100 mark but most look very scruffy, even one defect on the front is going to limit appeal. Collection only on ebay adverts also keeps values of these low. I would say buy on condition alone, if it is in mint condition and you can collect / inspect it first without travelling too far then £215 may be worth it.

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  3. I´ve finally got it! It looks pretty cool despite the years, and it sounds great too!
    May I ask you something? I´m trying to connect my laptot to the system via minijack-RCA cable. The problem is that the sound is very lame, it seems it comes from the laptops sound card instead of the system´s amplifier. Do you know any solution to this? Thanks in advance.

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  4. Good to hear you got it. Although not properly impedance matched, a headphone output with the volume on the laptop turned high should give an acceptable sound. Which input on the Technics are you connected to ?

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  5. I´m using the VCR input interface of the sound processor, ´cause direct connection to the RCA interfaces of the amplifier didin´t work. Do you think another pair of RCA cables from VCR output to the amplifier will work?

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  6. Whoever is suffering from the dreaded amp cut-off, you can still order the original replacement motor for the fan. Just search for this part number MDN4RB4MRC. I found a few in stock from a shop in Poland.
    Yes, you will see it's an electronically actuated volume controller. Fear not. You can take the casing apart very easily and just extract the motor from the volume control assembly. Worked perfectly for me and I can crank the volume to max without issue. Before it would cut-off in 2 seconds at -28db.

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    1. Please send me the email id of the shop in Poland. I have the same problem with my Technics scch900. My email id is dbhube@gmail.com. Thanks.

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    2. Please send me the email id of the shop in Poland. I have the same problem with my Technics scch900. My email id is dbhube@gmail.com. Thanks.

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  7. Impel Poland? Just ordered :) Thanks for the info! Been hunting for a year!

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    1. Yes sir. It's Impel. I'm glad I helped and I'm glad I found these guys, because it's a simple fix with the original part and it works great. No soldering, no PC cooling fan, none of that nonsense.

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  8. I am from India . I am also having the same problem of fan motor. Kindly provided me the email address of the shop in poland.This will be a great help. My email is dbhube@gmail.com

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    1. Here you go: http://e-electronicspareparts.com/panasonic-variable-resistor-with-motor-50kohm-mdn4rb4mrc,9569.html
      You can look for the part number on google, there are more shops, maybe closer to India, that can send it to you.

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  9. My motor arrived from Poland today, and as described came attached to a worm gear driven potentiometer. It was easy to lift the small metal tabs on the housing, remove the potentiometer then disconnect the worm gear by removing two small screws. You're then left with the motor with a thread adaptor on its shaft. This pulls of with a pair of pliers. I made a note of which way around the brown and grey wires went in relation to the motor before de-soldering them. I soldered the wires on to the new motor and re-assembled the fan and its housing. The amp is now working perfectly and doesn't shut down as previously with the slightest hint of bass!! I found the fan vibrated when I first reassembled everything and this was down to the tension of the metal fan holder once screwed back down. A little bit of bending and adjustment and the fan is now pretty silent. Thank you to mowgli for the tip on this! A cheap fix (think I paid about £10 inc delivery) and saves buying a new amp :D

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    1. I'm glad it worked. I ordered 2 more of those motors, just in case. I plan on keeping my Ch900 for a long time ;) Cheers

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    2. I've just purchased my long awaited (23 years!) SC-CH900 and it has the cut off problem.
      Been reading the useful info on how to fix the problem but a little confused as mowgli says no soldering required which is exactly what I need as I haven't got one and useless with one anyway.
      But in Shane's comment it seems it requires soldering?

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    3. Mowgli, I think means no soldering onto the PCB, but the motor does sound like it needs modifying, wires soldering onto it etc.

      My advice would be to fix with the 10nf capacitor, you could buy a cheap soldering iron for a small job like this.

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    4. Thanks lowfinger I'll try that.
      Although a video of how to on YouTube would be of great help ;-)

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  10. Friends, I applied the solution described replacing the engine with a new one, but my result was the same as before replacing the amplifier keeps shutting off. What I notice is that when starting the fan cut occurs. If by chance someone knows another solution to the problem is appreciated and from any information.

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    1. Hi, solder a 10nf ceramic capacitor across the fan connector terminals on the PCB, this will fix your problem.

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  11. Can you please link and appropriate capacitor please Lowfinger? there's all sorts on ebay uk!

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  12. Hi, sure, here is a ebay item id 182330299305, select 10nf from the drop down on he listing.

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  13. Hi, Lowfinger!

    I have a problem regarding my equipment.

    I opened it becuase the light dislay of the cd player kept going in and out every x minutes. I couldnt find a way to reach to the conections on the side of the display itself. So I reconnected and closed it.

    Damn me now the buttons from the cd player doesn´t respond at all. Do you know how can I fix this? I´ve been careful and the buses and cables seem to be in good condition.

    Any advice you can give me I´ll be very thankfull.

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    1. The display going out is not caused by the display cable or the display itself, but rather the main cable plug in the back. At least that's what I found. The plug where the ribbon cable goes in wiggles a bit so moving it might toggle your display on and off. as for the buttons, you probably messed something up. You can find the service manual online. Just check the panel cable.

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  14. Many thanks for the advice, dodgy amp all working normally again

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  15. Hi I'm looking for a ribbon cable for a technics su-ch700 do you know where I can get one thanks

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