Crate PA-8 repairs

[Enzo]

Wait, WHICH transistors went in backwards?  How did the pinout become different?

Please tell me you are not using NTE parts.  Real parts should all fit the same way.

[shinychrome0]

I don't know what NTE parts are.  But i replaced the MPSA56 and MPSA06 (Q3 and   One was shorted and the other lost a leg in removal.  But the new ones were just the reverse of the originals.  ECB instead of BCE.  so they basically just had the flat side of the case on the opposite side.

[bry melvin]

Interesting... I have fairchild and a couple of other MPSA56 datasheets on hand from something I had done...everything I have shows the BASE on the center pin.

If you're facing the flat EBC left to right.

[shinychrome0]

Sorry i typed it wrong.  I have selective dyslexia lol.  It was supposed to be EBC on some when looking at the flat side, and others CBE on the same side.

[J M Fahey]

Yes, I have also found small signal transistors made by different manufacturers with differing pinouts.

[Enzo]

NTE is a brand of general replacement semiconductors.  They have a big book that you look up your transistor type in and it shows their "crossreference" type.

I for one have never seen an MPSA06/56 that was not EBC across the front.

[J M Fahey]

Can't speak about MPSxxx parts, must agree with you, because we hardly use "American" types but European ones , which nevertheless are also made by non-European companies (such as Fairchild , ON and TI) and there I find variations for the nominally same part.
My general-purpose do-everything transistors are BC546/556 and I have bought them in EBC seen from the bottom, flat side up, 3 pins in line, which I call an "American or Motorola pinout" or in the "original classic" EBC "triangle", flat side down, or in the "modern" , "fake triangle" , with all 3 leads in line when leaving the plastic case but the middle one bent as to make a triangle so it can be inserted in a triangle footprint.
My high voltage pair, 2N5401/5551 yes, has always been 3 in line, EBC flat side up, being originally a Motorola design AFAIK.
My main supplier (Elko/Arrow) nowadays carries the Fairchild/Vishay lines mainly, although they are historically the Motorola/ON official distributor in Argentina.
I have also had, sometimes, EBC in line, flat side down, so when I buy a new lot,  usually snap one in the Hfe/Beta meter, both to confirm real Beta but aldo to confirm pinout, much faster and "real world" than searching for a datasheet from that particular manufacturer. Beta and actual pinout get written into the transistor bag.
NTE Parts? Thanks God we have been spared that curse; it pays more to dig some old D.A.T.A. book for the really obscure ones and search for equivalent parts.

[shinychrome0]

So i've LITERALLY checked every single resistor on the entire board, and nothing is giving a bad measurement.  Everything i check appears to be absolutely perfect, except for the oscillation.

[J M Fahey]

hi shinychrome.
Is that power amp posted anywhere?
Or some link to it?
Seeing it I might suggest some "normal" voltage readings.
Another question: you post readings such as "70", "1100", etc., which mean nothing to me.
I imagine you are using the diode test function; in that case, what does a regular diode (1N4002, etc.) show?
Because if it shows around 650 to 700 and those are millivolts, either 70 or 1100 are very bad readings.

[shinychrome0]

A 1n4001 tests as 850.  And i've just sent you an email with the schematics for each section of the mixer.  As enzo mentioned earlier though, the low reading of 70 ohms falls within the range of that transistor for an in-circuit measurement.  its got a really low resistance between those two legs.

[Enzo]

This should be the power amp drawing.

[shinychrome0]

Any other ideas?  I've got enough other projects going on right now that i'm close to just scrapping it.

[expanoncolin]

Hi - I just bought one of these and am about to start repairing it, so I may be able to help soon - though I was hoping to get the schematic from you.  My email is colin AT experimentalistsanonymous DOT com (with AT = @ and DOT = . of course).  Let me know!

-Colin

[shinychrome0]

Great!  I'll send you those schematics right away.

[shinychrome0]

I'm thinking this is a lost cause.  I'll probably just scrap it out for parts.