Trying to fix this Samson amp

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Finally got around to this again. I received my replacement driver and output transistors from Ampslab. I went over every resistor, diode and transistor to make sure all were OK. I replaced all of the blown parts with brand new parts. I even bought a decent variac off Ebay for $50.

Hooked my DMM across the circuit breaker to measure current draw and used the variac to turn it on slowly. Took it all the way up to 120VAC with no more than 0.1A draw... IT WORKS!!!

Well, after testing it, 2nd channel works perfect, like before. The 1st channel, which I put new parts in, works but the sound is distorted :(

The amp has three status leds for each channel: -40dB, -20dB, Clip. For some reason, the 1st channel's -40dB lights up with the gain knob all the way down and will flicker or change brightness with no input.

What could be causing this?

The drivers (Q108, 2SC4370; Q111, 2SA1659) were replaced with different transistors that were the closest match I could find (2SC2238, 2SA968) since I couldn't find the originals. Would the difference in specs cause the problem?

http://www.datasheets.org.uk/specsheet.php?part=2SA1659
http://www.datasheets.org.uk/specsheet.php?part=2SC4370
http://www.ampslab.com/PDF/2sa968.pdf
http://www.ampslab.com/PDF/2sc2238.pdf
 
anatech said:
Hi rkc7,
$60 isn't too bad for 10 transistors you know. I've spend more in my day easily. Try to get new insulators and driver transistors while you are at it. Even if the drivers measure okay, change them.

-Chris


Id have to agree with this. I watched a fellow tech that i am friends with work on a carver amp you know them smps ones with no tranformer and they weigh about the same as my shoe. :( he spent over 300$ in transistors for the amp.. well you can't use a variac with the amp.

What you can do is power it up with a external psu for the rail voltages. Well the amp powered up properly but when returning it into the chassis and using the manufactures psu it blew up. I didn't think i would see some one get so mad :eek: :eek: :eek: Till this day he has a policy NO CARVER AMPS..

Take your time post lots of question's Transistors are not cheap. If your in doubt post a question. The time it takes for some one to post it's well worth it.. Saves your wallet too :)
 
Check for excessive DC at the speaker terminals. if it's over 100mV you've got a problem.

As a temporary test, try removing the VI limiting transistors Q127 and Q137 and see if the distortion goes away. If it does, these transistors could be damaged (if your FET's Q109 and Q110 were, it's a fair bet these are too).
 
I tested it and saw no more than 10mV DC at any given time, with no speaker connected and with a speaker connected.

The amp did produce a high-pitched hissing when first turned on, but eventually crackled and went away after about a minute of being on.

I didn't replace the FETs before, they tested OK with comparable measurements to the other channel. Same goes for Q127 and Q137. I retested all again as well.

Could it be something wrong in the small-signal section? I'm not sure how anything would be damaged, but I seriously tested everything on that board with exception to the ICs and capacitors.

Could a bad capacitor cause the problem? And I did test the ICs for shorts, but I guess they could still be damaged.
 
Well, the only variable resistor on the board is what I adjusted (as I said, I guess it's for the bias), and I tried turning it slowly through it's entire range with no noticeable affect on the distortion.

Now that I'm back from work, I'm going to try testing components on the input and control boards seeing if anything's bad there, then maybe start testing capacitors on the amp board.
 
EEK! randomly adjusting the bias trimpot (VR101) is not a good idea as you can blow the output stage!

As you have one working, distortion free channel, the best thing to do is measure the current bias on this channel. To do that, set a multimeter on DC millivolts and measure the voltage over one of the output emitter resistors - R141 for example. Then, on the channel you have replaced the outputs on, connect your multimeter over the same resistor, and adjust the bias pot until you get the same reading. You will need to recheck and readjust this as the amp warms up.

Does the amp hiss on both channels on switch on, or only on the channel youve replaced the outputs on?

You can test if it's the preamp stage by disconnecting pin 1 and 2 of CN101 (pin 3 and 4 power the opamps and are needed), and connect a regular unbalanced line-level signal there.
 
well, I found and fixed the problem and the amp works great now.

It wasn't anything I suspected... I missed soldering one of the legs to one of the drivers :xeye: Noticed it when I removed the board to check the bias pot and I felt kinda dumb. Oh well, glad I found it.

What did I learn? Check everything twice or more! You think I'd of learned that the first or second time :rolleyes:

I appreciate everyone's help on this. Now I can finally use this thing!!
 
You beat me to it, I was going to suggest a driver not doing its job.

I have had good luck ordering semis from MCM. The "Sankens" I have received so far appeared to be genuine. Nobody has blown up on me so far. I use them servicing not only Samson and Hartke, but SWR also uses them, among others.

A hint to Samson servicers. I have had any number of their amps - the Servo series in particular - where small lytic caps dry out in the 7317 protect circuit. It either sticks in protect while the amp channels are working fine. Or it trips into protect extremely easily. They use the chip in a number of models, though the support circuitry for it varies. Check those little caps.

And a couple suggestions for future service. First work without a load until you are fairly sure it has settled down. ANd when you are blowing outputs, until you have found out why, most of the time the amp will run perfectly well with only a single output device on either polarity. Obviously it cannot produce full power into a load that way, but if it is blowing up in the first place, power delivery is not the issue. So put one pair of outputs at risk instead of five. Once the amp is repaired enough to hold a fuse and pass a signal, then you can install the rest of the outputs and run it up under load.

Samsons are not my favorites to work on, but I've gotten used to them.
 
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