Trying to repair Samson SX1200 w/ blown outputs on one channel

[jamesmafyew]

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...

[jamesmafyew]

And one other thing --- I checked the DC offset of both channels, and they are both at 3.5 mV, totally fine. That was part of what led me to believe that it was an earlier stage that is causing the problem.

[jamesmafyew]

Turned out it was an open-circuit resistor, which I had apparently missed before. Replaced it and the amp works fine now.

[wg_ski]

Originally Posted by jamesmafyew

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?

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%.

[King Mustang]

I would like to know what part # you used to repair this amp I have the same amp in protect mode it had 1 resistor 3.3 k ohm 1/4 watt burnt that I replaced location 171 on board and amp is still in protect mode.

[JPM14]

Originally Posted by wg_ski

+/-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.

+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!

[sakis]

Originally Posted by JPM14

+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

[wg_ski]

Originally Posted by sakis

Sorry ...I think you are wrong and please ...Welcome to now days Vi limiters

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.

[sakis]

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 ????

[JPM14]

Originally Posted by sakis

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 ????

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!