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Old 20th September 2011, 04:23 AM   #1
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Default Dirty connections in Technics su-v7

Hello!
I'm brand new to this forum and somewhat green when it comes to reading schematics, which I have for the amplifier I'm trying to fix/modify.
My problem is with my Technics su-v7 stereo amplifier. I've used this amp for a few years now after finding it next to a dumpster. At the time, it was fully functioning with the exception of the switches for mono/stereo, viatone/straight dc, low filter, high filter, and loudness. These switches were dirty and crackling a bit when I found it, but after switching them on and off a bunch they cleaned up well enough to not notice any loss of fidelity.
Now they are either dirty again, or the switches are wearing out enough to not provide a good connection.
I never use the amplifier with any of these options on (straight dc operation with no tone or filter in stereo mode), but now the switches are causing a bad connection even in the off position, especially the loudness switch. If I wiggle the switches while they are in the off position I can eventually achieve clean sound, but it's pretty irritating having to do this every couple of uses.
My question is: would it be simple enough to just bypass all of these switches by soldering the pcb so that all of these options are permanently in the off position, or would it be better to try some kind of cleaning solution, and if so, what? I would like to just get rid of these options all together as they are useless to me and only causing problems.
Obviously I'm being cheap (actually, completely free), but I'd like to keep it that way if possible.
I love this amp and would be happy with it for a long time to come if I can fix this issue.
Thanks very much in advance for advice!
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Old 20th September 2011, 04:40 AM   #2
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Hi and welcome to diyAudio

I think that bypassing the switches if you never use them is the most sensible approach. Just identify which pins get switched to which (input and output) when in the position you use it and jumper only those. Some tinned copper wire should probably do the trick.

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Old 20th September 2011, 07:17 AM   #3
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Thanks for the quick response Tony!
I attached a pic of the schematic zoomed in on the loudness switch in hopes that you or someone else can make sense of it. I can provide more pics if this one doesn't provide enough info.
With the amplifier casing removed top and bottom, I can see exactly where the switch is soldered to the pcb, but the switch itself is encased in plastic and I can't see which connection is being made (if any) on the pcb to put the loudness in the off position. My concern with leaving the switch soldered to the board and bypassing it is the possibility that while it's in the off position, the switch is making a necessary connection. Also, I worry that if I simply bridge the points on the board to make the loudness off, and if while in the off position a connection is still being made by the switch, it would cause a damaging or hazardous short if the switch were to be pressed to the on position.

Thanks again for bearing with the newbee!
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Old 20th September 2011, 01:59 PM   #4
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Just a hint from another SUV Technics user - yes mine did the same with the tape monitor and tone bypass switches dying all the time until I realised that the problem was silver plated contacts on the tiny open backed alternate action switches.

This is bad in urban environments, as the silver forms insulating compounds with various components of the very air we breathe. I used contact cleaner/lubricant with lots of "working" of the switch with great success but this needs a re-clean every couple of years in my coastal environment.
Hey, it's dead easy and it works, ok?
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Old 20th September 2011, 11:47 PM   #5
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Thanks again guys.
I spent the $8 on some CRC contact cleaner and everything works perfectly. Thanks for the tip Ian! I didn't get the lubricating kind because a local vintage audio shop advised against it. Not sure why he recommended not using the lubricating kind though. Anyone know?
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Old 21st September 2011, 03:45 AM   #6
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It's a tradition to avoid lubricants that may attract dirt and produce breakdown products which are also bad news with relays and contact switching. However, at signal levels there won't be breakdown products like with arcing contacts and the dirt build up is, well, minuscule with the front panel PCB in most SUV models. It's broad brush thinking that ignores positive experiences with careful application. IOW - BS!

In, fact, ornery 'ol WD40 has been used for audio with good results too! Problem is, it's not an overpriced specialist product and it's not marked "for audio use" This makes audio guys and engineers in particular, really jumpy but a friend's SUV40 has been chugging away, sounding as good as always on just one thorough dose about 5 years ago. I don't recommend the stuff but lubricated types last a lot longer than plain cleaner, so there you go - make sure you mask the PCB and nearby parts as best you can and clean all residues from the PCB in that case. - enjoy.
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Old 21st September 2011, 03:54 AM   #7
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good to hear that it's working again I use the CRC CO myself when I need it. completely residue free.

I once had a can of the crc contact cleaner with lubricant, it is horrid stuff, the guy gave you the right advice. It gets onto the circuit board, if you try and wipe it off, it takes the silkscreening with it, and generally makes a sticky mess.

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Old 21st September 2011, 12:58 PM   #8
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damn!
Very happy with the response and solid advice on this forum!

Maybe this should be a thread of it's own, but I'm curious about others opinions on the su-v7 or whatever model of suv you're using. It seems that enough of you have used them, so I can't help but ask.
When I found this amplifier I was using a Technics sa500 receiver which I believe is a B class amp. The su-v7 is a "new class A" amplifier, which is basically an AB amplifier, right?
Anyway, the difference in sound quality was astoundingly apparent from the get go, even with the dirty connections.
Are all B class amps harsh, or is the sa500 just a bad sounding B class amplifier?
The su-v7 when compared to the sa500 has a much, much smoother and more controlled sound than the sa500, and I believe this is an observation that anyone could pick up on, not just audio nuts with a "trained" ear.
I'm a bit confused by this because most things I read about "upgrading" your sound system have said that the amplifier has the least effect on over all sound quality, putting speakers first, room second, and source third.
Upon finding this amp next to the dumpster, I definitely did not expect it to be such an obvious and audible upgrade to the sa500 I had been using for so many years.
Now that I've seen first hand what a difference the amplifier can make in sound quality, I wonder how this amplifier sounds compared to other types of amplifiers.
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Old 21st September 2011, 07:45 PM   #9
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Comments on the model/family won't be out of place.
AFAIK, the SUV series is a long running family developed from the SEA or New Class A series (Sandman's class S) from around 1980. The principle of these all differ in the details but broadly speaking, there is a tiny class A amplifier attempting to drive the load in a bridge sensing circuit which allows it to be buffered by a large, class B amp. which supplies the actual muscle yet almost has the subtle crossoverless amplification of class A, as it emulates that source.

Many 'pooh-pooh' the scheme but it definitely sounds better than your average class AB amplifier in smooth detail and imaging IMO. It runs dead cool most of the time and otherwise has a long, trouble free life. Yes, I like even my later, budget SUV50.
That is from a series labelled "Super Class AAA+ , whatever that meant.

'Very clever designs and awful to adjust and stabilise. The design details are not in the schematics. These are buried in the proprietary chips or hybrid power modules in the Technics SUV models, so I can't say what the actual details are but there have been threads here which you can search from the top of the page.
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Old 23rd September 2011, 08:44 AM   #10
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I have another issue with this amplifier that I didn't mention initially because I thought it was related to the dirty switches and pots, but after cleaning everything and powering up the amp in a quiet environment, I can still hear a hum (more like a buzz) that was always present at about 60hz coming from the amplifier itself, and after the relay switch kicks in, the noise is sent to the speakers. Adjusting the volume does not effect the volume of the noise getting to the speaker, or the volume of the noise within the amplifier. Upon closer examination, I noticed a component on the speaker selection circuit that looks like a plastic rectangular box marked with "5W 220ohm J, 10 362" that looks a bit melted and blackened around the contact points with it's pcb.
With my limited understanding of circuitry within amplifiers, I'd assume that this problem is with the power supply since the sound is being made by the amplifier even with no speaker connected, and also that the sound is coming from the side of the casing with the power supply. dur?
How do I fix this???
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