Why do I do this to me

"MM54" (mm54mk2) 07/20/2020 at 20:35 • Filed to: HIFILOPNIK

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Day 1: “Minty hi-fi equipment for a great price? It doesn’t work and you’ve tried nothing but checking fuses ? That’s fine I can fix it! ”

Pristine Pioneer SPEC-4 is mine... it just doesn’t work

Day 3: “What do you mean, +7 volts you bastard there should be -68 there... and they haven’t made that transistor in 30 years... ”

I haven’t even gotten into the ma tching-ish C-21 with one dead channel...

Nibby > MM54 07/20/2020 at 20:39

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you know you can trust someone with electronics repair when you see them using 91% isopropyl alcohol

good luck!

MM54 > Nibby 07/20/2020 at 20:41

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Thanks, I think!

For Sweden > MM54 07/20/2020 at 20:44

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You know it is good because it is rack mount

SBA Thanks You For All The Fish > Nibby 07/20/2020 at 20:46

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Goes great with Tang.

VincentMalamute-Kim > MM54 07/20/2020 at 20:47

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ooohhh..... drool.... So pretty!

I have this one:

AestheticsInMotion > MM54 07/20/2020 at 20:53

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Check the elextricicals

MM54 > AestheticsInMotion 07/20/2020 at 20:54

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It thinks it has too many but in reality it doesn’t have enough

MM54 > For Sweden 07/20/2020 at 20:55

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At 55lb you need a good rack. The ears show zero sign of ever having been in a rack though which is nice

MM54 > VincentMalamute-Kim 07/20/2020 at 20:56

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Very nice! I was on the lookout for an SA-9500II for a while until I finally figured out the intermittent issue in my Technics SU-v909 ( that’s my primary amp) and got it being cooperative again. I’d still probably pick one up if I found it for a good deal , though.

Demon-Xanth knows how to operate a street. > MM54 07/20/2020 at 20:58

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What is the transistor? I've had to find enough crosses in my career that I probably have a good shot.

arl > MM54 07/20/2020 at 20:59

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Yea for old amps (I paid someone to fix mine up....I be electrically dumb)!

AestheticsInMotion > MM54 07/20/2020 at 21:00

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Hmm. Have you tried some percussive persuasion? Maybe a few minutes with the healing hammer?

Once you get this working I say you should build a hot-swappable rack mount setup in the Fiata dash. 

VincentMalamute-Kim > MM54 07/20/2020 at 21:03

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Where do you find good deals on these? The usual (ebay, CL, etc) looking for “doesn’t work”? I shouldn’t ask because I don’t need more.

Googled the Technics. The black version is very pretty even though it’s black. Nice!!

Curious - what was the cause of the intermittent issue you fixed?

EngineerWithTools > arl 07/20/2020 at 21:30

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Is that a Stereo 70? I have an old Stereo 35 that needs some magic smoke put back inside. (Or, you know, caps and some carbon trace cleanup.)

Anyway, that’s nice!

EngineerWithTools > MM54 07/20/2020 at 21:31

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Like any good power amp, it’s 98% power supply, 2% amplifier when measuring  by weight!

MM54 > EngineerWithTools 07/20/2020 at 21:35

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That’s for sure! I have the original box (like I said, minty) and I believe it’s listed as net 55lb, about 60 gross! Those transformers have some meat to them.

MM54 > Demon-Xanth knows how to operate a street. 07/20/2020 at 21:42

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I suspect the problem is in a 2SB536 and it’s possible in its failure it may have taken out a 2SA839A, and a slight change of cascaded damages though a second 2SA839A and a 2SA628A. I’ve not done much research into these yet (will pull them and test tomorrow if I have time) but none of them are still in production ;)

Here’s the excerpt, I have the -90V coming in on 21 but about +7V where the -68 ought to come out on 18/19 (thus my suspicion of Q2 not conducting anymore) . I’m also a little suspicious of D15 (XZ-235 which is a 23.5V zener maybe?) since I do not have the expected -22v on its anode, but that could also be Q4 saying “nah”

MM54 > arl 07/20/2020 at 21:45

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Very nice! My main hobby- electronics work experience is in tubes (design and repair, guitar and hi-fi ) , so I’m a bit out of my element working on solid state stuff like above but getting used to it as I do it more and more . At the end of the day though you can’t beat some hot glass.

MM54 > AestheticsInMotion 07/20/2020 at 21:46

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Unfortunately that doesn’t work as well with solid-state electronics.

This power amp not only is larger than the seat / footwell in the Fiata but I think it also weighs as much as the engine. I’m pretty confident it would be a very tight fit to get it into the trunk.

MM54 > VincentMalamute-Kim 07/20/2020 at 21:51

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This one was via family so kind of cheating. Otherwise, keep an eye on ebay but especially craigslist. I try not to buy stuff that doesn’t work but now and then with the right find, it’s worth taking on the project (and I’ve been kind of bored lately).

I have the silver su-v909 - the black on is nice but you can’t beat the silver equipment (for me at least).

The intermittent issue was random clipping/distortion/being-very-loud on one channel. The pain in the ass thing was it would rarely do it with the case open, but set it back up in the living room and it would act up. Banging on it would sometimes make it stop so I knew it was a mechanical issue . I eventually was able to chase it down to where a ribbon cable meets the preamp board (I think it brought in both the signal and bias supply but could be mistaken) - I cleaned and tightened all the pin connections and reflowed the header to the board and it’s been well-behaved since.

This is what we'll show whenever you publish anything on Kinja: > MM54 07/20/2020 at 21:51

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Pretty sure it was /s, the remaining 9% being water which remains after the alcohol has evaporated and has the potential oxididdlyize your compiddlyonents.

I believe Nibby prefers non-chlorinated brake cleaner.

Also /s. Maybe.

MM54 > This is what we'll show whenever you publish anything on Kinja: 07/20/2020 at 21:52

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I’ve used enough 91% to know the remaining 9% water doesn’t stick around! I also don’t usually use it on the boards, I had that out from taking some weird sticky spot off the front (looked like splashed soda).

This is what we'll show whenever you publish anything on Kinja: > MM54 07/20/2020 at 21:56

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:)

gmctavish needs more space > arl 07/20/2020 at 22:09

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Very nice. I have an ST-70 chassis with transformers that’s waiting to be built up one of these days....

Thomas Donohue > MM54 07/20/2020 at 22:15

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It’s been sitting on a repair bench for most of it’s life.  The rack ears are pristine!

Thomas Donohue > MM54 07/20/2020 at 22:16

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Posts like this are why I don’t have a retro Carver.

Yet....

Demon-Xanth knows how to operate a street. > MM54 07/20/2020 at 23:21

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2SA1859 isn't too far off.

Demon-Xanth knows how to operate a street. > MM54 07/20/2020 at 23:25

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Check to make sure it isn't Q6 conducting and pulling 18 high. You're getting a lot of positive voltage so you should start looking high for it. Measure the diode drops with a meter as a quick check.

arl > EngineerWithTools 07/21/2020 at 07:00

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Yep, an older ST70 with the upgraded driver board from tubes4hifi.com

I had it recapped and put in a a vintage NOS Mullard 5ar rectifer and 4 new Gold Lion KT77's.

It’s sweet!

arl > gmctavish needs more space 07/21/2020 at 07:00

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Thanks! They’re sweet sounding amps for sure. 

arl > MM54 07/21/2020 at 07:03

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Thanks - I love the amp, but admit I know next to nothing about how the insides work. Sort of like how I understand my cars. I understand them in a g eneral theory sort of way - but I couldn’t fix them  to save my life. I can change the oil (or tubes), and polish them up to look good, but if they need fixing I have to send them out to the pros.

MM54 > Thomas Donohue 07/21/2020 at 12:16

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Came through a family connection, has been in on-and-off use until a couple years ago I believe

MM54 > Demon-Xanth knows how to operate a street. 07/21/2020 at 17:52

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A fair consideration, however the -13 from the emitter of Q6 is sourced from the -68 and runs the panel meters (which are currently pegged, presumably because they’re biased all wrong and hopefully not because more stuff is burned out). The bold line in the schematic above (ties to pin 15) is actually ground. I don’t remember the exact reading but I did measure for the -13v and found it slightly positive, which makes sense if Q6 is open.

I tend to get mixed up working in negative voltages (because yeah) so you could be completely right.

This evening I’m going to pull all 5 transistors and possibly both diodes in that circuit and test them (I do have a transistor tester, but can’t use it in-circuit of course).

Appreciate the input! Once I know exactly which parts I need, the hunt for replacements will begin. I had to rebuild the horizontal driver circuit in my (CRT) oscilloscope from 1969 a few years back, and amazingly about 2/3 of the silicon was available NOS on ebay - I doubt I’ll have such luck this time.

MM54 > Demon-Xanth knows how to operate a street. 07/21/2020 at 20:28

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Q2 and Q27 are bad, Q2 is open and Q27 is shorted, go figure. Having a hell of a time finding a replacement for a 2sa893a

Demon-Xanth knows how to operate a street. > MM54 07/21/2020 at 23:00

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KsA992FBTA looks pretty close.

MM54 > Demon-Xanth knows how to operate a street. 07/22/2020 at 12:05

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Ordered a couple KSA992FATA last night after giving up on any sort of method other than pull up the 893's datasheet and start punching it in as minimums in online catalogs. I feel better knowing that you independently came to the same result (with the other gain range) haha

(Also MJE15033 to replace the 2SB536)