SAE Mark Four C replacement Pwr transistors

Unfortunately, I do not have a schematic for the SAE XXXI amplifier. I was using the schematic for the XXXIb in the hopes I could use it to help figure out some things in repairing the XXXI, but came to the realization that it wouldn't. If anyone out there has an actual schematic for a XXXI, please let me know (free source or not). I can see the change of philosophy in SAE's going towards darlington pairs in the evolution from the XXXI to the XXXIb.
 
My 4C schematic has MJ802/4502, SAE101/202, SAE0063/0069 for the drivers and SAE601 for the outputs. My personal 4C has those drivers with SAE701 outputs. When I rebuilt my MKXXXIM (mono) it originally had SAE601 outputs and I used the 2N3773s as replacements. All of these transistors are relatively slow as are all of the ON Semi TO-3s so shouldn't be a problem to use the latest and greatest MJ211xx types.

Just going thru my SAE schematics and found the schematic that covers the MKIIB, MKIII, MKIIIA and MKIVB. In the notes section it says SAE601 was 2N3773.

Craig

What do you mean by "all of these transistors are relatively slow"? Are you referring to low hFE values?
 
What do you mean by "all of these transistors are relatively slow"? Are you referring to low hFE values?

No, the fT of the MJ211xxs is only 4MHz which is fairly slow compared to the Japanese TO-3 transistors. But since only ON semi seems to be making TO-3s nowadays you don't have much choice. I think the original 2N3773s are slower yet but can't find the fT. The fT of the MJ802/4502 pair is 2MHz. Anyway my point is that none of them are super fast and shouldn't cause any problems with having to re-compensate for them.

I have the schematic for the MKXXXI and the service manual for the XXXIB.

The change in philosophy was James Bongiorno, he designed the XXXIB, IVD and the IIIC, all use the Darlington outputs in one form or another.

Craig
 
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I just noticed and read your long post from yesterday. I usually use Nichicon axials in the SAEs that I work on. In just about every one of these early SAEs there's always one discolored axial capacitor per channel, it's the boot strap capacitor. Why the plastic casing looks funny is beyond me, maybe heat. There is a note in the service manual about replacement of certain diodes so maybe that's why you think some prior work was done. Also the only carbon comp resistors, 1.5k and 2.7k, are in the boot strap circuit, they are usually out of tolerance.

Craig
 
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Assuming the clear blue tinted plastic used in the manufacture of the "holland" electrolytic capacitors is Polyvinyl Chloride (PVC), it is likely those 64 mF 64 VDC capacitors overheated, probably in the range of 212 to 600 degrees Fahrenheit. It would amaze me if all the transistors checked out okay.
 
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It's always just those caps in the boot strap circuit that are funny looking, none of the others in any of the MKII, II, or IV series.
 
I finally checked all the resistors. I will probably replace just five, maybe 7 total. For the most part the resistors seem to be carbon film type. The only carbon composition resistors are the 12 ohms 1 watt in both channels. The bootstrap resistors (1.5 K and 2.7K) are carbon film type and are gold banded tolerance, actual values from DMM are 1.527K, 1.524K, 2.723K, and 2.661K. They will be left alone (except for the one that is a different type as opposed to carbon film).The ones I am replacing are 27K (2), 10K (2) and 2.2k (1). There are two resistors on the A channel that I may replace as they are different types of resistors (having 5 bands and different color indicating a type different from carbon film). They are 100 and 2.7k valuewise.

With respect to the 0.2 ohms, 5 watts resistors, in the aggregate the minimum values are at least 0.2 ohms with the maximums anywhere from 0.3 to 0.5 ohms. My personal opinion is that at an output of 50 watts RMS per channel, the variations are insignificant although out of tolerance. Please comment if you disagree.

With respect to the service bulletin, the diodes (currently glass bodies) on the A channel will go back to original gray bodied diodes.

Last step will be to unsolder the TO-66 sized transistors from the PCB and test with PEAK Atlas DC Pro.

Then the actual repair will start (once all the parts are available)!

There are two questions about the schematic, i.e for the main circuitry (both A and B channels) the electrolytic capacitor marked as 250 mF (typo???) on the schematic but is a 220 mF mounted on the PCB and the other is a 0.47mF capacitor wired in parallel with the Bridge DC outputs as designated on the schematic but not wired at all on the present amp.
 
I don't think the 250uf is a typo just an old school value, in some of the other SAE amps of this vintage there is a 15uf cap coupling the input stage to the diff. pair, again just an old value. I usually use 220uf in the F/B circuit possibly giving up some very low frequencies.

Not sure about the .47uf cap as my amp is out on loan.
 
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