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jamesmafyew 23rd September 2009 05:48 PM

Trying to repair Samson SX1200 w/ blown outputs on one channel
I've got a Samson SX1200 power amp with one blown channel that was sitting in protect mode. This was a "gift" from a friend who trashed it by running it bridged into a 2-ohm load. :mad: I pulled the blown channel board entirely and the amp works fine with just the other channel. No power supply issues, the other channel is perfectly ok (I was really surprised by this).

All 6 output transistors (3 pairs 2SA1943/2SC5200) were shorted on the blown channel, all emitter resistors open circuit, and two other resistors I can't identify were visibly burned. The drivers are testing ok (but I know I should probably replace them).

1. Does anyone have a schematic for this amp? Samson isn't returning my emails or phone calls. Seems like a common amp that people would have worked on, but a search of Google and this forum didn't turn up much.

2. The drivers are labeled K A1659 and K C4370A. There is one of the former and three of the latter. Maybe I'm clueless, but I can't seem to find replacements for these. Where can I get them? What are some suitable replacements?

3. Some small-signal transistors, labeled "A970" and "K 400 C3200" are near the site of the two burned resistors. They test ok with my diode check (out of circuit), but I may want to replace them anyway. Any idea what they are and who makes them?

As I'm working, I'm checking resistance measurements at different points in the circuit against the other, working channel.


llwhtt 24th September 2009 02:22 AM

Don't forget to add the "2S" prefix to the transistor numbers you can't seem to find. "2SA1659", "2SC4370A" , etc.


jamesmafyew 24th September 2009 04:41 AM

Well, here's what I've found after spending the whole evening on Google:

The drivers are made by KEC, they are KTA1659A and KTC4370A. No one sells them, so the closest nicer option is Toshiba's 2SA1930 and 2SC5171, from looking over the datasheets. The Toshibas are identical except for a higher current handling.

The small signal pieces are a mix of KEC and Toshiba --- 2SA970BL and KTC3200GR. The KEC is a clone of the 2SC2240GR (why didn't the manufacturer just use these?). I want to have some replacements for these on hand, but can't find them at the usual outlets... Fairchild's cross reference comes up with KSA992F and KSC1845P for these.

I've ordered a bunch of this stuff from Mouser, so we'll see what happens Still no schematic.

east electronics 25th September 2009 07:12 AM

schematic not really needed since you have a working module on the other side .....

a couple of things ....

can you id if this is a a class AB amp or class G ??? you may understand that from the windings of the trafo or even from the psu board ....

class G has two seperated voltage to power up the amp .... class AB only one thus one set of caps or two respectivelly ..... class G will be more difficult to repair ...

it will be probably very importand to find related threads about "trouble free start up " and use tricks like installing only a couple of outs to make sure that amp is working .... but not at full power ....

proceed with caution

jamesmafyew 25th September 2009 03:25 PM

Hmm... not sure. There are two sets of +/- 59V taps on the transformer, and two separate bridge rectifiers going to two sets of caps --- but it's one for each channel, like a dual monoblock-ish arrangement. It's symmetrical. Check the photos to see what I mean. I don't see evidence of multiple rails at different voltages... but I may just be clueless. The only class G amp I've worked on is my NAD 2600.

I've checked over the power supply board and tested the rectifier and caps --- as far as I could tell, it was working properly. The connector between the PS board and the channel board has four connected pins: B+, GND, speaker output, and B-. The B+ and B- measured 90VDC and -90VDC, respectively.

On another note, since the power PCB is listed as "SX1800" and this is an SX1200, I'm assuming they use all of the same boards. The channel boards have spaces for two extra pairs of 2SA1943/2SC5200 per channel (there are spaces for five pairs per, but only three pairs are installed).

Makes me wonder if I could turn it into an 1800 just by installing the extra transistors + associated resistors. The heatsink is only big enough for one extra pair, though.

wg_ski 25th September 2009 04:29 PM

+/-90 on 3 pairs with no rail switch is crazy. I'd go ahead and add the 4th and maybe 5th pair.

It looks like two independent power supplies - one for each channel. There are some circuit topologoes that require it.

llwhtt 25th September 2009 05:06 PM

Just pulled an S1000 out of my junk pile to try a lend a hand but it's totally different animal. Came in with both channels blown and was deemed not worth the effort to repair. I tried.


jamesmafyew 25th September 2009 07:59 PM

Am I correct in believing that adding pairs won't change the gain of the amp, just its ability to handle lower-impedance loads?

wg_ski 25th September 2009 09:08 PM

Correct. And obviously there was a version of it designed to handle lower Z loads, higher supply voltage, or both.

jamesmafyew 26th September 2009 01:56 AM

So if I were to add the extra pairs, is lot matching important, or do the emitter resistors make that moot? I.e., if I add an extra pair to the channel that still works, can I just add the pair, or do I need all-new transistors from the same lot?

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