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R/E/P => R/E/P Archives => Bruno Putzeys (Designer) - Dave Hecht (Master Tech) => Topic started by: oldgearguy on March 17, 2009, 09:05:41 PM

Title: power supply and NTE part weirdness
Post by: oldgearguy on March 17, 2009, 09:05:41 PM
OK, if someone can explain why this happened and what I can do to prevent this headache in the future, I'd very much appreciate it.

First - I'm repairing a CS-80 here.  Not a huge deal, just a lot of fiddly work getting to boards, etc.  A guy I know has a CS-80 with a bad power supply.  I told him to send it my way and I'll take a look at it.  He has 2 SVU boards (contains most of the PS circuitry) - one is old, complete, but doesn't match any set of schematics we've ever seen.  The other is in better condition, matches the schematics, but is missing some parts (looks like it was cannabalized some time in the past).

So, I decide to repopulate the newer board and go with that since the other one has definite problems.  Also, since I have a working supply, I can crosscheck voltages, etc.

I put in new electrolytics, and use some NTE parts based on the cross reference guides (original parts were 2SA490 and 2SC828).

Everything powers up fine except the -6.5v is reading -12.5v.

I check the entire circuit and to make a long story a bit shorter, trace it down to the NTE transistors.  I swap the power amplifier transistor (NTE 153) out for an original part from the old SVU board.  The voltage goes from -12.5 down to -8.5.  I swap out the transistor (NTE 199) for an original and the voltage drops to -6.35.  A quick bump of the trimpot and it's a steady -6.5v

So, why would what NTE considers a direct replacement (no notes about swapping legs, etc) for both parts not work in the circuit?

For grins, I attached a section of the schematic showing the PS.  The parts in question are Tr713 and Tr714.

Title: Re: power supply and NTE part weirdness
Post by: Bill_Urick on March 17, 2009, 10:49:48 PM
oldgearguy wrote on Tue, 17 March 2009 21:05

(no notes about swapping legs, etc)

Just to be clear, does this mean that NTE did not identify the leads of the transistor or that you're sure that the NTE's were installed correctly so don't suggest that as a possibility?

I haven't bought any NTE semi's in a while but I'm trying to use up some old stock when I feel it's safe. The NTE-85's I used last week did have the leads ID'd on the package.

As far as being a direct substitute, I would never count on that for any NTE transistor. Each NTE transistor crosses to a pile of different Japanese, US and other transistors, few having the exact same specs.
Title: Re: power supply and NTE part weirdness
Post by: oldgearguy on March 18, 2009, 06:25:52 AM
Bill - the package matched the PDF doc which matched the original...

see now you have me second-guessing myself.  I know the PDF matched the original spec.  Time to dig the packages out of the trash.

As a side note, I typically use some old cross reference books that I have from years ago to match up replacement parts, but lately I've gotten into the habit of working a Mouser order and if they don't have the original, I bring up the NTE website and find the cross.  Other than this one particular circuit, I haven't had any problems.