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. 
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).
So:
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.
Thanks!
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).
So:
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.
Thanks!
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.
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.
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
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
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
After receiving the parts, I replaced:
-all outputs
-all drivers (with Toshiba equivalents)
-all feed resistors
-all emitter resistors
-two resistors that had looked visibly burnt
Now the amp powers up and comes out of protect. HOWEVER, the "repaired" channel sounds grossly distorted. The input stage is fine because I can run the amp in "parallel" mode where input 1 feeds both power amps. Channel 2 sounds fine, channel 1 (the one in question) still sounds distorted.
Any ideas what could be wrong here? The only thing I can think of is that a couple of small signal transistors that I didn't replace might be dead...
-all outputs
-all drivers (with Toshiba equivalents)
-all feed resistors
-all emitter resistors
-two resistors that had looked visibly burnt
Now the amp powers up and comes out of protect. HOWEVER, the "repaired" channel sounds grossly distorted. The input stage is fine because I can run the amp in "parallel" mode where input 1 feeds both power amps. Channel 2 sounds fine, channel 1 (the one in question) still sounds distorted.
Any ideas what could be wrong here? The only thing I can think of is that a couple of small signal transistors that I didn't replace might be dead...
Parts from the same lot are usually close enough. If they're from different lots, check to see if they are the same beta group, R, O or Y for example, *and* the vbe (used diode test on your DMM) are within the spread of the originals (about 20mV). If so you're close enough. With another lot your chances are about 50/50. In the same lot it will be near 100%.
+1 on the 90v rails and 6 devices. lot of strain there. Got a Pro-sound 1000 here which uses 8 of these devices on +/- 76v which is a good choice by the maker - Carlsbro. He can't add 5th pair here due to heatsink length, but the 4th pair, yes definately add them. The PSU caps look to be the most feeble I've seen in an amp!
Sorry ...I think you are wrong and please ...Welcome to now days Vi limiters
Simply when you measure performance In a classic way = resistive load of 8 Ohms and 1Khz of input signal expect to find everything according to specs Power will be exactly the same as stated in the user manual .
In real life though Vi limiter will kick in with inductive speaker or any other load than resistive and reduce the drive to a safe level .
Add to the above that the trafo and the PSU is marginal and PSU voltage will ""dive"" to almost 10- or 15 (or more) volts under load which also will provide a better SOA for the amplifier and horrible sound .
This is exactly the approach of how many amplifiers are made in our days and yes at 1khz and resistive load will produce exactly what is said on the manual .
Kind chinese regards
sakis
Yeah, in order to keep it safe, even with the supply dropping to 75V under load that VI limiter had better kick in the instant the load drops under 3.99999 ohms. With any real world speaker that's going to sound like ****.
And those are the most woosy caps I've ever seen. Let's see if we can't exceed the ripple current rating by a factor of 10, and blow little pieces of capacitor all over the place inside the case and make a stinky mess.
On the subject of "sorry - I think you're wrong", (whether you mean the number of O/P devices or the little capacitors), plus you making sarcastic comments about the amp in question, I really have no idea what point you are trying to make!OMG !!!! i never thought that i will find so many uneducated people in this forum ....Make the math ::: It can never be 3.99999 !!!! It will kick in at 3.77777
Cant you see its a professional amplifier ????
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