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Old 2nd October 2012, 01:42 AM   #1
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Default Vintage preamplifier power supply problem

Hi there. I am upgrading a wonderful old Technics SU-9011 preamp. This amp has a lot of potential. The person who had it before me blew up the power supply. I have gotten stuck with replacement of two voltage stabiliser transistors (fairly large) which were originally 2SC1226A-Q (TR503) and 2SA699A-R (TR504). A replacement site reckoned these could be exchanged with BD507 & BD516 respectively, altho it doesnt look like these are still available (at least not in poxy new zealand). Can anyone recommend good subs for these original transistors?

Another question - the guy before me replaced TR501 & TR502. These were 2SA666AI and 2SC1328 respectively. It appears these can be subbed with BC560 and BC550 repectively - any issues with doing this?

I also have two 20V zener diodes in the PS circuit, can these simply be subbed with replacement zener diodes of the same voltage rating, as it appears to be a bit of a mission to test these, and a drop-in replacement could be simpler?

Please see circuit diagram excerpt attached. cheers for your help!
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File Type: pdf schematic.pdf (77.0 KB, 45 views)
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Old 2nd October 2012, 01:58 AM   #2
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You'd be better off changing them and the surrounding circuit components to support LM 317/337 regulators and forget about the hard to find original devices and questionable substitues
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Old 2nd October 2012, 02:25 AM   #3
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The important characteristics of the original devices on the schematic are

2SA564: PNP, 45 volts, 50 mA, Beta>50, TO92
2SC828: NPN, 45 volts, 50 mA, Beta>50, TO92
2SA699: PNP, 50 volts, 3000 mA, 10 Watts, Beta>50, TO202
2SC1226A: NPN, 50 volts, 3000 mA, 10 Watts, Beta>50, TO202

Since these have "blown up" twice, it seems prudent to replace them with more modern devices having better specs (higher breakdown voltage, higher max current, higher max power). Dunno about NZ but the four transistor types below are extremely common in the US:

1. 2SA564 ---> 2N5401
2. 2SC828 ---> 2N5551
3. 2SA699 ---> D45H11
4. 2SC1226A ---> D44H11

I have deliberately omitted the specifications of the suggested replacements, so that OP can have the pleasure of hunting down their datasheets and extracting the electrical specifications. Hint: they're all sold by Digikey and by Mouser, who each provide links to the transistor datasheets. They're shockingly inexpensive too.

OP will also want to check the pinouts of the existing transitors (by tracing the PCB tracks) and verify that the suggested replacements can be fitted into the existing drill holes / heatsinks.

Last edited by Mark Johnson; 2nd October 2012 at 02:28 AM. Reason: mention heatsinks!
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Old 2nd October 2012, 04:55 AM   #4
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Hey thanks for that Mr Transistor - just to clarify - the last person replaced 3 transistors with 3 compeltely different ones - I assume the originals also blew. His workmanship was shoddy was wnat to start again.... great advice and thanks a heap....
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Old 3rd October 2012, 01:16 AM   #5
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Hi Mr Transistormarkj - I'm actually not sure where you got a couple of your original transistor values from? I originally posted that I needed to replace a 2SA666A & 2SC1328 - however you have recommended replacements for 2SA564 and 2SC828?

Cheers however for your recommendations for the D45H11 and D44H11 to replace the 2SA699 and 2SC1226A respectively - these so far look pretty good for the job.

So anyway, any recommendations to replace the 2SA666A & 2SC1328? It appears that the BC560 could potentially replace the former. But I want to confirm this. Cheers for your help!
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Old 3rd October 2012, 03:17 AM   #6
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Sorry that it was not clear when I wrote

> The important characteristics of the original devices on the schematic are

I was trying to convey the idea that the original circuit designer (a consultant hired by Technics?) certainly knew what she was doing, whereas the skills of the person who made the previous repairs/modifications are, uh, not known to us.

So it may be prudent to disregard the decisions made by the previous modifier (the modern word is "deprecate"), and instead focus upon the decisions made by the original circuit designer.

But of course it is YOUR preamplifier and, after seeking advice, you are free to follow it or disregard it, as you please.

I am also sorry you are not sure where I obtained performance characteristics of the original devices on the schematic. Let me clarify my procedure so that your present "not sure" status, does not persist indefinitely: I used Google's search engine to seek out old datasheets and old catalog entries. Since Google is widely available, others can perform these kinds of searches too.

Last edited by Mark Johnson; 3rd October 2012 at 03:19 AM. Reason: colon
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