Technics CD-4 Demodulator: Left Channel shows little to no Carrier Signal Strength

[Q-Eight]

My Technics SH-400 Demodulator has never shown good Left Channel Carrier Signal. You depress the left signal tuning button and that's what the dial is supposed to display, but it always shows as low or non-existent. It does however still demodulate properly.

Stupid question, but does anybody else have some good experience with these devices? I'm barely an armchair electronics-type person. Is there anybody out there that might-could service these old things?

 

[scifi]

Here's the service manual and operating instructions.
https://scifiles.opendrive.com/files?OV83Mzc4NDA5NF9KZ0VZVw

http://www.quadraphonic.info/manuals/Technics-SH-400_Operating_Instructions.pdf

You could try cleaning the switches and other controls using DeoxIt D5. With the last project I used it on, it wouldn't clean some of the rotary switches so I had to take the nut off of them to back them out so I could scrape the corrosion off manually.

 

[Q-Eight]

Thanks! This will help a lot!

 

[The Quadfather]

The Technics SH400 is, appearance wise, the most beautiful demodulator out there in my opinion. However, it's performance is not the best available. Find a JVC 4DD5. The 4DD5 does not have all the separate circuit boards that the SH400 has and is not prone to intermittent connection problems. But if you insist on using the SH400, look at the inter board connectors. This type of connector was famous for developing cracks around the pins in the solder where they are soldered to the PC boards, not just on SH400's but on any device that used them. The pins are strong and inflexible, and when the temperature changes, it's the solder that gives way. Often the cracks are hard to see without strong magnification. Resoldering will solve the problem for a good while, but it will eventually return. Also, a good cleaning of all the controls and switches with an electronic control cleaner like Deoxit will definitely be of benefit. Try the cleaner first, and if that doesn't do it, check the solder joints. Also, the SH400 has double sided boards, but these weren't usually a problem unless they used rivets or metal pins to connect one side to the other. Looking at the SH400 I have here in front of me, it doesn't appear that they used either pins or rivets, but used plated through circuit boards which are known to be problem free.

 

[Q-Eight]

Had to update this 2-year old thread of mine. I took The Quadfather's advice and bit the bullet on an AT440MLa. Man, that made a huge difference. Then, I spent $30 on the Mobile Fidelity "Geo-Disc" and yeah, my eyeball alignment was piss-poor in comparison to where the Geodisc wanted me to plant everything. That made a HUGE difference. But, I was still getting low carrier signal on the left side. I was beginning to think my poor little SH-400 that's been with me for so long may have finally demodulated it's last.

I'm pleased to say, that's not the case.

I pulled it apart tonight, sprayed the ever-loving baby Jeebus out of every pot, switch, contact I could find. On a hunch, since I had the thing open, I noticed there's a crazy upright board in the chassis that has a great big pin connector that connects to the board with all the adjustment pots. Like the fuses in old cars, these contacts surely must mildly corrode, or, at the very least accumulate some kind of build-up. So, I gently spread the connector, shot it some Deoxit, worked it back and forth and put the whole kaboodle back together.

After letting it all air-out and dry, it got put back in the stereo stand for one last hurrah.

And what a hurrah it was!


Just got through Side one of Toulouse Street and this little bastard held some serious carrier lock all the way to the runout groove. It used to be when I played a CD-4 LP, I'd lose carrier lock in the inner tracks - not so now!! Just started Side two - had the control out for Left Channel Carrier Signal and as soon as the Radar Light came on, that needle slammed the right hand peg!! Me gots some GREAT signal now! Happy Camper for sure. Now to play all the Quadradiscs that I've collected but never played because I couldn't play them properly!!

So, you want a good CD-4 recipe? Here ya go:

Pioneer PL-530 Turntable.
Audio-Technica AT440MLa Cartridge/Stylus
Technics SH-400 CD-4 Demodulator

....and your favorite 4-channel amplifier to handle the rest!


Yes folks, Jesus IS just allright. :music

Left Carrier Signal


Right Carrier Signal

 

[Doug G.]

That's great! The SH-400 IS the most beautiful CD-4 demodulator ever made. I have one too along with A Panasonic SE-405.

My SH-400 works fantastically for My EPC 450C II cartridge but, like yours, has kind of a shortcoming in the left channel with magnetic cartridges. Same as yours, the carrier level is a bit weak although usable.

I have never fixed it because my SE-405 works fantastically with magnetic cartridges so I just use that for them.

It always bugs me a bit when something doesn't work like it should so I still plan on fixing it some day. I gave up trying to get the Technics alignment record that has the carrier crosstalk tracks on it. I just set the controls mid way (one of us who has the record said that's pretty much where they end up anyway) and go from there.

Here I am almost 65 years old and still fascinated by this stuff - analog four channel sound. I think it's because, being an electronics kind of guy, it almost seems a miracle it works!

Actually, that's also what the RCA engineers thought when they first gazed upon a color TV screen, even though they were sure their calculations and all that were correct.

Hey, I just noticed your CC controls are at center too!

Doug

 

[Q-Eight]

Hey, I just noticed your CC controls are at center too!
Doug

They aren't any more. This was just during initial setup and I wanted documentation that I actually had left channel carrier signal. Previously, the left side was between -30 and -20 on the scale. So this was an insane improvement. Not to mention actually getting demodulation on the left side.

As of right now, I still seem to be experiencing weak Left carrier.... although weak in comparison to the right channel. After some tinkering, I have insane Right Channel Carrier with the control set half-way. The left side is cranked right up and at the low side of the mark. But, I've noticed this is also related to which disc I'm playing. No two discs show the same amount of carrier level so I'm going to say my setup is good, it's the software that ain't.

Oh, and currently, my CCC controls are Right: Dot #3 and Left: Dot #5. The Owners manual says to set the CCC for lowest reading on the dial when playing the CCC band on the setup disc. I just play a record and adjust for lowest CCC. It does seem to make some audible difference in the amount of background noise.

It's funny though on some discs, the left back channel distorts (Elvis- Aloha) but only when the horns kick in hard. Other discs distort the back right (Doobies- Toulouse) but only when the lead guitar is hitting loud in back right. I suppose I could turn down the separation a bit but.... that's just not how I roll.

 

[The Quadfather]

I have a Technics SH400 also, I just don't use it. It is a beautiful demodulator. I just wish it performed as well as my JVC 4DD5. I like the sound of the JVC better. I guess you could say it's a bit more well defined, clearer. However, if you want to keep using the Technics, and this is true with any quad era equipment, including the JVC 4DD5, you might have to replace a few electrolytic capacitors. Most electronic devices of that era uses them for coupling from one stage of processing to the next, and they tend to fail more often than other components, especially if the equipment sat on a shelf in a closet somewhere for a few years which is often the case with quad gear. Any competent component level technician should be able to handle repairs, especially older ones who have actually serviced quad gear. Just make sure whoever you might take it to knows what they're doing in a general sense. Good Luck,

The Quadfather

P.S. I don't even think about carrier level anymore. I set it when I installed my AT440MLa and haven't touched it since. I get excellent play from the beginning of the record to the end on just about any record I play. Expect good performance, it's achievable.

 

[Q-Eight]

Man, I give great advice. I'd forgotten about this thread and recently, that same demodulator was losing left carrier signal again. I googled the issue, and this thread came up.
I took the little beast right apart tonight, sprayed all the contacts with good cleaner, re-assembled and now I have stronger carrier on the left than I do on the right.

What a crazy hobby. I'm giving myself advice from the past.

 

[Doug G.]

Bringing this back. Recently, my SH-400 radar light was constantly on which means, of course, 30 kHz was getting in there somehow or being internally generated. I used my SE-405 until I decided to troubleshoot the SH-400.

The other day, when I decided to check it out, the radar was no longer staying on all the time but there was an indication on the meter all the time of some frequency there and the carrier controls basically had no effect. This was with both channels although the right channel seemed to be a little responsive to the control. Shorting out the inputs and outputs made no difference.

Now, from experience, I know weird things like spurious oscillation with ultrasonic or radio frequencies are usually caused by poor grounds or loose connections. So, the first thing I did was pull up on the vertical board near the front which plugs into the main board with three 6-pin large Molex connectors and also into the two horizontal boards, at the front with the controls on them, with two 6-pin large Molex connectors apiece.

The board cannot be completely removed without disassembling the whole front of the unit to get the two control boards out since you are trying to pull it up and back at the same time. There isn't enough room or give in the connectors to do this. However, there IS enough give to get the connectors all partly out of the sockets to "disturb" each connection and "scrape" off any oxidation formed over the many years.

Viola!...er, I mean...Voila" It's back to normal and working great! Lou Dorren's (RIP) chips are doing their thing! Does anybody else think he was a genius?

Doug

 

[sjcorne]

Recently, my SH-400 radar light was constantly on which means, of course, 30 kHz was getting in there somehow or being internally generated.

Interesting - my unit was doing this intermittently in MM mode (the radar light would stay on after I lifted the tonearm), but it seems to have stopped since I switched to an SC cart.

 

[Doug G.]

And, as far as the notion that the SH-400 was tuned to reduce noise and is thus, dull sounding- Nonsense! The high end is just fine.

Doug

 

[Q-Eight]

And, as far as the notion that the SH-400 was tuned to reduce noise and is thus, dull sounding- Nonsense! The high end is just fine.

Doug

Agreed. Mine has plenty of high end, even if you use the Hi Blend switch, plenty nice cymbal crashes and percussion. I think I need to come up with a name for my method of rejuvenating the SH-400!