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Old 22nd February 2012, 01:21 PM   #1
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Default Need help with transistors for JVC VN-700

jvcHi there, I am currently working on an old JVC VN-700 integrated amplifier and it's having problems.
When I received the amp, a couple of the output transistors had been replaced. The original ones are 2SD188 and 2SA627, and a couple had been replaced with 2N3055 and MJ2955 transistors. Would these work okay as replacements?
Also, a couple of the driver transistors are dodgy and I would like to replace them all due to age (amp is from mid 70's). The two part numbers are:

2SC959
2SA606

If anyone can help me find some suitable replacements that are cheap and readily available I would be most grateful. Would it be possible to use the common BD139/BD140 transistors? I have done some searching but didn't have much luck. The datasheets are easy to find but I'm not sure on what figures and characteristics are more important to match up and what else to look for etc.

Last edited by flyingtele; 22nd February 2012 at 09:08 PM.
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Old 22nd February 2012, 06:20 PM   #2
mjf is offline mjf  Austria
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what is your psu voltage ? (Vdc?)
are you shure that both output transistors are sd? isn't it one of them a sb?
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Old 22nd February 2012, 06:50 PM   #3
wahab is offline wahab  Algeria
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BD139/140 are a good replacement for the drivers.

The 3055/2955 pair , albeit working is less adequate as it s significantly
slower than the original parts , and the usual TO3 replacements as
MJ15015/15016 have Ft at 2MHZ , while the higher voltage MJ15023/15024
have a Ft of 4MHZ , still less than the 10MHZ of the original parts but the latter
would surely work well enough.
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Old 22nd February 2012, 08:23 PM   #4
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Japanese TO3 transistors are going to be very expensive or unobtanium here in OZ. Wahab is right, unfortunately, that Ft is low in older US audio designated power transistors. I think though, MJ21193/4 would be as good as they get, though still at only 4MHz Ft. and around $3.50 ea. at Mouser. Locally, you'll be asked for a lot more via E14 or RS.

At the least, use MJ15003/4 which are widely available but often poor second-sourced brands and many fakes abound. 'Careful with that one, but should be a lot better, typically, than using 2N3055 family parts.

Gosh, how many 70's amplifiers have I seen butchered with 2N3055/MJ2955 substitutes? I once thought that was all we could buy here but on reflection, it was just careless, unprofessional work.
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Old 22nd February 2012, 09:06 PM   #5
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2SD627 listed in WES Components (Ashfield, Sydney) catalog @$6.60 trade price.
No luck with the others.
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Old 22nd February 2012, 09:10 PM   #6
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Hi there, sorry I made a mistake, the other output is 2SA627, not 2SD (well spotted)
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Old 22nd February 2012, 09:52 PM   #7
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I have also noticed the the STV-3 bias diodes need replacing. Can I just use three 1N4141's in series? Or does anyone know a better substitute?
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Old 22nd February 2012, 10:14 PM   #8
wahab is offline wahab  Algeria
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It should work with the 1N4148 although it would be better to start
with three 1N4004 instead.

If theses diodes do not allow the quiescent current to be set to about 60mA
by triming the relevant 500 ohms (R420 in the svhematic) variable resistor ,
keep two 1N4004 and replace the third one by a 1N4148 and so on.

Theses diodes should in principle be on thermal contact with the heatspreader,
just be careful to adequatly insulate them from the heatsink electricaly wise.
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Old 23rd February 2012, 01:13 AM   #9
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Thank you for your input. I have ordered a pair each of the MJ15003/4, bought a bunch of diodes, and had the BD139/40 already so hopefully this will stop the amp blowing output transistors and get it running. Also going to replace all the electros in there while I'm at it.
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