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Old 19th June 2011, 10:21 AM   #511
johnm is offline johnm  United Kingdom
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Morning - hmmm very interesting. Sony really pulled out all the stops in designing this amp to get the best out of the devices available at the time. I know in more modern amps people often change to a more modern transistor without needing to worry about the rest of the circuit, but as you pointed out it really does seem to have been designed to fit these two like a glove, as it were.

I've got to do some work today so won't have a chance to try those changes for a day or so, but I'll give them a whirl sometime next week. It's all working fine today still and sounding OK, if a little cold and brittle sounding. Always seems to be that way after fitting new caps, which seems to disappear after a few hours use - the infamous 'burning in' thing perhaps? Noticed it with several DIY projects over the years.

Thanks for all your help/suggestions again Mooly - I really would be stuck with a nice looking (and rather huge!) paper weight if it wasn't for your posts/help!

Last edited by johnm; 19th June 2011 at 10:25 AM.
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Old 19th June 2011, 11:02 AM   #512
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P.S. I'm thinking is it worth coming up with all these fixes to get the existing output transistors working comfortably, or would it be better to take a more ground up approach and find a transistor which is better suited to the amp? (you mentioned the low gain of the MJs vs the higher gain of the Hitachi's a few pages back).

Ideally I'd love to get two pairs of the original Hitachi outputs, but I guess these are pretty much unobtainium now (aside from a few possible fakes on Ebay!). Even if I did find four I'd have no guarantee they were matched either as from what you said they had a wide gain window.

So, perhaps I should push the boat out (or at least blow up the dingy!) and get a better pair of outputs better suited to the characteristics of this design?

Having said all that it's sounding VERY good again this morning after spinning some vinyl for a couple hours (no longer 'frigid' sounding), but if I can get any extra performance out of it I might as well do it.
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Old 19th June 2011, 12:05 PM   #513
Mooly is online now Mooly  United Kingdom
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I still think I would try slightly raising the resistor values first perhaps... I really think that could be the key to this.

You may have to experiment somewhat and in small steps increasing both resistors and perhaps upward to nearer 33 or 47 ohm, but in small steps. Its a case of getting a feel for whats going on.
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Old 19th June 2011, 12:17 PM   #514
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I dont think the gain of the MJ21193/4's is lower. The Hitachi originals specify their minimum gain at Vce = 5v and at Ic = 1A. The OnSemi's are specced with Ic = 8A. If you look at the graph for gain vs Ic it would suggest the gain is more like 60 at 1A.
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Old 19th June 2011, 12:29 PM   #515
Mooly is online now Mooly  United Kingdom
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Hi Jaycee. Its certainly a bit of a puzzle and a bit more difficult not having the unit to actually work on and test. Its very odd for an output stage to be so dependant on device type, particularly DC characteristics.

Other than MJ15003/4 I can't really come up with much that is readily available. I was thinking MJ4502 and MJ802 but these have dropped off the radar from many suppliers.

Turn on volts of silicon junctions is pretty much an absolute but are these MJ's as fitted doped differently to try and make them more linear and complementary.
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Old 19th June 2011, 03:29 PM   #516
jaycee is offline jaycee  United Kingdom
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Those old MJ's would be even worse at any rate. The MJ21193/4 is a TO-3 version of the MJL21193/4 (TO264) transistor and these are excellent audio transistors. Onsemi have better still, but they are in modern packages like TO-3P or TO-247. Faster fT might also cause a big problem in such an old circuit.

Better driver transistors might help in this circuit... but then we are getting further away from the original design. I would say live with the small performance difference TBH!
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Old 20th June 2011, 10:40 AM   #517
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Thanks for your comments Mooly/Jaycee.

After giving it some thought over the weekend, I think I'll stick with the output transistors that are in there now - the unit sounds very nice so I think I should quit whilst ahead there rather than make a sideways move. I will however restore the 4k7s to stock, and try increasing R317 and see what affect that has on the voltages.

Spent most of yesterday evening replacing every electrolytic on the EQ and the control boards. Very fiddly job - glad that's done now. There were at least 5 caps that looked like they'd leaked slightly so the unit was obviously on its last legs with some of the originals caps. I also desoldered that huge diecast 4-gang ALPS volume pot and sprayed some contact cleaner into that to get rid of the crackle when using it. All back together again now and working fine - definitely sounding clearer now and the volume pot is working as it should.

Whilst going through my notes I remembered that I'd replaced the following electrolytic polarised caps with MMK non polarised types:

C311/312/361/362

Was it OK to do this, or should I have replaced with polarised types like the original design?

Thanks.

- John
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Old 20th June 2011, 11:29 AM   #518
Mooly is online now Mooly  United Kingdom
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Those four caps are probably better specced than the originals. A small value like across a rail is used to provide a low impedance path to HF noise and hash. Large electroylitics in theory are better... larger the cap the lower its reactance (resistance to AC)... in practice it doesn't quite work like that due to real world electroylitics having significant self inductance which negates the desired effect. So a small cap is actually better and a film type probably best of all.

Be very interesting to see if the resistors can be tweaked to bring the current up. I think that would have real benefts in bringing the voltage up on the IC too.
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Old 20th June 2011, 02:04 PM   #519
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Thanks Mooly - figured they'd be OK but thought I'd best check with an expert first

Definitely - so only start with R317 first, right?

Do you think I'd get any mileage in upping the value of the following PSU caps (or does it make less difference with an SMPS?):

C801/805 (1000uF / 200v) - immediately before the SMPS from what I can see.
C802/803 (2200uF / 50v) - immediately after the SMPS.

- John
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Old 20th June 2011, 04:54 PM   #520
Mooly is online now Mooly  United Kingdom
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Yes start with R317, just working on the one channel. Be prepared to experiment a little increasing the value in small steps and always retesting with the bias on minimum after each change.

I would tend to leave the PSU caps as designed, changing the values changes time constants and with some SPSU's that can be critical for correct and safe start up.
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