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Toshiba SD-3950 / SD-430V distorting at peaks
Toshiba SD-3950 / SD-430V distorting at peaks
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Old 7th September 2014, 02:33 AM   #1
df audio is offline df audio  Canada
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Exclamation Toshiba SD-3950 / SD-430V distorting at peaks

Hi,

I have a Toshiba SD-430V DVD player (which basically has the same pcb and layout as the SD-3950) with a strange problem that just developed recently. The player performs as normal when playing music, but when the sound reaches the peaks (0 db) the sound suddenly begins to clip and distort. This happens only on a single channel.

So far, I have identified that the TI 1795 DAC sends its output through a 100uF capacitor in series (DC blocking cap) and out into a JRC 2115 opamp. The opamp then outputs the signal through a 47uF capacitor (blocking again) before going out into the RCA jacks. While there are some ceramics and film caps connected in parallel along the signal path, as well as a few resistors in series, those are generally more reliable and shouldn't fail as quickly.

I tested the opamp supply voltages (all good) and replaced the four electrolyctic caps mentioned earlier. All the output solder joints have been reheated to ensure a solid connection. The sound had a different "colour" after the replacement, but however the clipping remained. Without a schematic for the player, I have no idea how to continue forwards with my repair.

The electrolytics looked like the most likely point of failure, but turns out they aren't. Are there any other places I should look at?

Last edited by df audio; 7th September 2014 at 10:41 PM.
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Old 7th September 2014, 10:44 PM   #2
df audio is offline df audio  Canada
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Well I've just replaced the film caps and tried it again, but the noise remained. I've also jumpered across the resistors in series (and caused even more distortion) as well. My next step might be to replace the opamp - but there's no guarantee that it's the culprit. I've also thought of doing the Swenson mods and bypass the output stage altogether, which I'll probably try if no one says otherwise.

Last edited by df audio; 7th September 2014 at 10:47 PM.
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Old 8th September 2014, 01:01 PM   #3
Mooly is offline Mooly  United Kingdom
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Toshiba SD-3950 / SD-430V distorting at peaks
Muting transistors (could be leaky).
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Old 9th September 2014, 03:15 AM   #4
df audio is offline df audio  Canada
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I think I've identified the transistor, circled in the image below. However, a quick Google of the part markings (C 3:B) told me nothing about the model number needed for a replacement. Someone here (May 1 2004) has removed the transistor altogether and states that it performs fine - but space is tight and the job won't exactly be easy.

Maybe I'll just perform the Swenson after all.

Click the image to open in full size.
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Old 9th September 2014, 08:37 AM   #5
Mooly is offline Mooly  United Kingdom
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Toshiba SD-3950 / SD-430V distorting at peaks
I don't know that device either but it looks reasonable as a muting transistor and seems as if it could be a dual device from the print and components.

That should come out in seconds with an ordinary iron and some braid.
http://www.diyaudio.com/forums/parts...sed-tools.html
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Old 13th September 2014, 08:32 PM   #6
df audio is offline df audio  Canada
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I've removed the transistor and it now sounds great - the clipping has completely disappeared. Thanks!

By the way, are there any concerns (safety, electrical, etc.) that I should be aware of when operating the CD player without a muting transistor?
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Old 14th September 2014, 07:17 AM   #7
Mooly is offline Mooly  United Kingdom
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Toshiba SD-3950 / SD-430V distorting at peaks
As long as you don't get annoying thumps and pops or strange noises in search etc then its fine.

The transistors also allow little used options built into the format to be used such as multilingual recordings and so on where you can mute each channel individually. Rarely used and not all players have that facility anyway.
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Old 4th December 2018, 08:09 PM   #8
wlowes is online now wlowes  Canada
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Default Belated thank you!

I realize this thread is 4yrs old... Big thanks to you both posting this, however.

I had the same issue with the muting transistor. Would never have sorted it out without this old thread.

My main digital source is down for some repairs. In search of a temporary CD source to fill in for temporary backup I recruited an old Toshiba DVD player...

Soon I was engrossed in the old Toshiba 3950 threads. I started this hobby moding old CD players. I've had great fun over the past few days replacing first the PS caps, then around the DAC chip and clock, adding an IEC outlet and finally bypassing the output stage. Used a heap of previously enjoyed caps from Panasonic, OSCON and BlackGate that were chilling out in the cap supply drawer. In order of impact, better caps in the switching PS made a huge impact. Then some BG's feeding the BB 1751 chip really took off the digital edge. Power cord improvement still defies understanding but it's there. Finally bypassing the output stage opp amps, resistors and filter caps with a pair of BG N coupling caps resulted in a surprising good sound. It's detailed with good sound stage and indiscernible digital fatigue. Not in the same league as my main digital source, but frankly quite acceptable.
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Old 5th December 2018, 08:15 AM   #9
Mooly is offline Mooly  United Kingdom
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Toshiba SD-3950 / SD-430V distorting at peaks
Sounds like you are having fun and interesting to hear you suffered a similar failure of the mute transistor.

I suspect 'hot swapping' of leads can be a contributing factor to failure of these (generally in any model) and particularly so when one unit in the chain has an SMPS. The problem would only occur when connecting leads with the power on.
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Old 5th December 2018, 06:55 PM   #10
wlowes is online now wlowes  Canada
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Hi Mooly,
Again, glad to see I could pass on that your insight proved valuable so long after you posted.

Interesting comment about hot swapping. I did disconnect and reconnect RCA cables with everything powered up. Maybe I fried it. Hard to recall, but the right channel distortion on peak output seemed to be there from the beginning. Removing the muting transistor immediately drove that demon out. As promising as it was stock, the distortion would have made it a non starter. Now after a couple of days break in with new caps it is stupid good. And this from a player that my cable company gave me for free as a promotion. At Ebay prices, the BlackGate caps might have fetched more than the original retail price of the DVD player, but I far rather see them create another good player. LOL
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