Matching Transistors for Replacement in a Circuit - Page 1
Forum section: electronics / projects

author: MrAureliusR
author_rank: Unknown
timestamp: November 14, 2013, 02:15:29 am
content: I have an old Sony STR-V3 stereo from the 70's that I've been working on (not the same one from my YouTube channel) and after fixing it successfully once, it then crapped out again. I've traced the problem (which is only in the right channel) to a pair of transistors on the power amp board that are clearly crapping out.The problem is, this board was made in the late 70's and while I have (luckily) found datasheets for these transistors, I'm having a hard time finding replacements. The power amp circuit is based around +/-40VDC. So one is rated for Vcbo -120, Vceo -100 and the other Vcbo 120, Vceo 100V. Collector peak current (Icpeak) 100mA, junction temp 150C. However I'm not super educated on transistor specs (no time like the present to learn though!) and the one that's getting me is the Gain bandwidth product or fT. Both are spec'd to 140MHz, however, how crucial is this spec? I've been able to find a transistor that is very close on most, but not all specs. The other one that's getting my mind in a knot is hFE or DC current transfer ratio. The datasheet for the original ranges from 60 to 200 depending on current and the replacement I've found is rated at 2000 (also 160MHz fT), which seems like a huge gap that can't work. Or can it?All advice is greatly appreciated! I've added the datasheets in PDF for the originals that need replacing, plus here's the links for the ones I've been looking at.Negative rail:http://canada.newark.com/diodes-inc/ztx705/darlington-transistor-pnp-120v/dp/98K4082Possibly this one instead for negative?:http://canada.newark.com/nte-electronics/nte383/bipolar-transistor-pnp-100v/dp/15M1922Positive rail:http://canada.newark.com/nte-electronics/nte382/bipolar-transistor-npn-100v/dp/29C4568EDIT: Just realized the negative rail one is Darlington, which pretty much strikes it from the list, no?And now the two I've found seem to be a matched pair, just like the originals, which is probably good?

author: Paul Rose
author_rank: Regular Contributor
timestamp: November 14, 2013, 05:39:02 pm
content: Matched pair is good.Voltage and current meets or exceeds original is good.Double check dissipation vs original.hFE should match or exceed original. Assuming good design and feedback, having higher hFE in the new device should be OK. Unusually high gain (like 2000) is a sign of a Darlington, which you don't want here.You want fT to match or exceed original, but depending on the design, you may not want to exceed by too much ( possibly you may get oscillation at a frequency that exceeds the original fT ).

author: MrAureliusR
author_rank: Unknown
timestamp: November 17, 2013, 07:00:04 am
content: Quote from: Paul Rose on November 14, 2013, 05:39:02 pmThanks for the feedback! (No pun intended) I think I'm going to order the matched pair above. There's another matched pair further along toward the power transistors and I'm thinking about replacing those too so the circuit is all fresh transistors and everything balances out. I actually have the service manual for this stereo but it doesn't contain much beyond schematics and a couple small maintenance tips. However the schematics are excellent. I just need to learn more about power amp and analog circuitry and I could potentially overhaul the whole power amp board.If anyone knows of any resources for fixing/maintaining old stereos (Sony in particular) I'd love to have more info.Thanks!!Typed using Hacker's Keyboard for Android

author: kingkamfa101
author_rank: Newbie
timestamp: December 19, 2013, 12:24:50 am
content: Quote from: MrAureliusR on November 14, 2013, 02:15:29 amHi MrAureliusR:I have the same model STR-V3 that has the same problem. Would you be kind enough to let me know the specs of the amplifier transistors? I am planning to replace all four of them as they appear to be in bad shape. If you have a service manual and specs sheet, I would appreciate it. Thanks.