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Old 7th February 2014, 07:54 PM   #1
tmas is offline tmas  United States
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Default NAD T955 Output Devices

I have an NAD T955 amp.

One of the five output channels is dead and has 2 shorted output transistors. One 2SA1943 PNP on the positive rail and one 2SC5200 NPN on the negative rail. The shorted transistors are Q128 and Q131 on the schematic.

They took out the 4 fuses for the bridge rectifiers for the +/- 15V. These are Toshiba Devices. Do the replacements need to matched before being installed?

I have uploaded the T955 service manual pages that contain the schematics for the amp boards and power supply.
Attached Files
File Type: pdf NADT955.pdf (390.4 KB, 31 views)
File Type: pdf NADT955PS.pdf (355.9 KB, 24 views)
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Old 8th February 2014, 07:54 AM   #2
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It's a CFP design output stage with no current sharing resistors - only 2 pairs with shared 0.068R ballast resistors. I would match them for Hfe or only use examples known to come from the same tube. Check what genuine product your supplier offers.
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Last edited by Ian Finch; 8th February 2014 at 08:00 AM.
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Old 8th February 2014, 11:13 PM   #3
tmas is offline tmas  United States
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Quote:
Originally Posted by Ian Finch View Post
It's a CFP design output stage with no current sharing resistors - only 2 pairs with shared 0.068R ballast resistors. I would match them for Hfe or only use examples known to come from the same tube. Check what genuine product your supplier offers.

Do you know of a supplier in the US that offers these devices for sale in matched pairs?
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Old 20th February 2014, 09:52 AM   #4
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Sorry for the late reply - missed your mail!
Without knowing the distributors and service networks in the US that offer parts matching, I can't advise you on your supply. Usually, matched sets would be held by the manufacturer's service network, if any. Pro audio techs usually hold matched pair sets of On-Semi parts but these are large sets for large amps and I have no idea what to expect for Toshiba parts.

However, a matching requirement is unlikely for NAD, who's philosophy is all about high volumes and designing to avoid the need for specialised part sets. NAD's matching would be based on the "same silicon wafer" principle, like the "same tube" for adequate matching. Obviously, unless you or your supplier buys or selects from tube lots (25-30 ea.), you can't be certain what you have.

I would suggest going to a stockist/distributor/service tech who sells genuine Toshiba audio power transistors (these are very common). Just ask for them from the same packing tube if they'll break it down. I know that if I approach Newark's agency here, this is possible through their distribution store. They're cheap parts and any extra cost for only a pair should be minimal. I don't fancy asking a tech to match them for you unless it's a favor though - it could get quite expensive if you also have to buy a lot to sort from and pay a minimum labour charge for just 1 pair. BTW, you don't have to match the PNP with NPN types, just same type.

Buy the same Hfe grade R or O as original and if it's your only option, match this much closer, at least within 5%. Perhaps other DIYaudio members have suitable quantities and can help you out with matched parts on hand. Here are some useful comments and a datasheet if you want to DIY too:
Parallel output transistor matching
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