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Old 1st March 2006, 02:55 AM   #1
rkc7 is offline rkc7  United States
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Default Trying to fix this Samson amp

So I have this S1000 amplifier that I've been trying to fix on and off for a while now. It's apparently more complicated than I expected. They state it's a "stable bipolar design" yet it has mosfets which I was told in other posts that will do some rail switching because of the delicate output transistors.

Here's the schematic of the amp:
http://www.pitt.edu/~rkc7/s1000.pdf

I replaced Q108 (KEC 2sc4370) with a 2sc2238 and Q111 (KEC 2sa1659) with 2sa968 because both were blown and I couldn't find their replacements anywhere, but those alternatives were almost identical in their properties listed in their datasheets.

I also had to replace most of the output transistors since many were blown (Sanken 2sa1492 and 2sc3856). I purchased all new pieces with consistent datecodes and put them in, but did not do much "matching."

All other parts (resistors, capacitors, transistors, etc) test ok and do not look damaged.

So, the one channel works but the other doesn't. The working channel had some blown parts, but I took all the matching transistors from the other channel that worked and replaced the blown ones with those. The other channel has the new drivers and the non-original output transistors.

The good channel passes signal and can stay on without getting hot, while the other will turn on and pass signal, but will get hot very quickly. I won't leave it on for more than 10-15 seconds because I'm afraid I'll burn up the new transistors.

I tried adjusting the only pot I can find which I assume is the bias (VR101), and turning it down all the way leads to the conditions above and turning it up further causes the symptoms to worsen.

What could be wrong? What should I look for? Are the new drivers inappropriate? Do the output transistors need to be closely matched for proper function? Could some of the other transistors be malfunctioning yet not blown?

Oh, and the new parts are from Audio Lab of Georgia (http://audiolab.home.mindspring.com/), should I be concerned of counterfeits or factory rejects?
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Old 1st March 2006, 08:07 PM   #2
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Default Re: Trying to fix this Samson amp

yet it has mosfets which I was told in other posts that will do some rail switching because of the delicate output transistors.

I don't see a rail switching design based on posted schematic.
I don't see any mention of class H on the Samson website regarding this model. Seems like a simple amp to me.

I replaced Q108 (KEC 2sc4370) with a 2sc2238 and Q111 (KEC 2sa1659) with 2sa968 because both were blown and I couldn't find their replacements anywhere, but those alternatives were almost identical in their properties listed in their datasheets.

That could be a problem, unsure. Are you sure the FET's are
ok since they are in series with the blown transistors.

So, the one channel works but the other doesn't. The working channel had some blown parts, but I took all the matching transistors from the other channel that worked and replaced the blown ones with those. The other channel has the new drivers and the non-original output transistors.

Swap channel parts again to see if the problem moves.

What could be wrong? What should I look for?
When a piece of electronics breaks and someone repairs
it, yet it's still non operational, then that means you haven't
found the gremlins yet. The trick is to find the bad parts.

Are the new drivers inappropriate?
One possiblility

Do the output transistors need to be closely matched for proper function?

This is a production amplifier, I'm 99.9% sure it's not a hand
crafted design where the design is critical on matching, so
the answer would most likely be no. You can match transistors
to improve variables, but the amplifier should be functional
without matching.

Could some of the other transistors be malfunctioning yet not blown?

Semiconductors don't have to be physically destroyed to be
bad, for the most part when a transistor blows up it shorts
and a DMM will read this. If you have an open circuit FET
that is blown, it could throw you for a loop as you may think it's
ok.

The replacement parts you got, verfiy them with a DMM as
a sanity check to make sure they send you what appears to
be good parts.

When you tested the parts, did you remove them or test them
soldered into the PCB? It's best to remove the part from the circuit to do a quick check if it's good.
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Old 1st March 2006, 10:55 PM   #3
rkc7 is offline rkc7  United States
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Well, thank you for clearing up my inaccuraties in the design and giving attention to all of my probably annoying questions. I don't have much knowledge of amplifier circuit design, I just like to tinker and I'm good at fixing things, so that's what I do.

Anyway, the drivers and output transistors I tested out of circuit, all other transistors (and fets) I checked in circuit and none shorted or were close to. The fets seemed to act "weird" when I tested them making the diode checker beep strangely, so maybe they could be the problem or I just had bad contact.

I think I'm going to switch the fets (Q109, Q110) from the good channel to the bad channel and see what that does. I don't think it will hurt anything considering the amp doesn't blow up when I turn it on now, lol.
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Old 2nd March 2006, 01:56 AM   #4
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Check for oscillation. i have fixed a number of Hartke Bass amps that use a circuit that is very similar. i found they will oscillate easily. they are very sensitive to input capacitance and input grounding.
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Old 2nd March 2006, 02:37 AM   #5
anatech is offline anatech  Canada
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Hi rkc7,
Never swap parts from side to side. Ever. Your choice (russian roulette).

If some outputs are blown, replace them all in that channel. BTW, manufacturing quantities of transistors yields decent matching without trying by hand. It is helpful to match these parts.

You should have a 'scope for this work. Check for oscillation. Also check that D119 (1N4148) and R133 (15R flameproof - metal oxide) are not open. Q105 may be okay, maybe not.

Parts can be leaky and not shorted yet. You need a real transistor tester for this. The Heathkit IT-18 is great for this work. You need to measure gain and leakage C-E and C-B.

Don't know your supplier.

-Chris
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Old 3rd March 2006, 01:37 AM   #6
rkc7 is offline rkc7  United States
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yeah, I didn't get a chance to swap parts before I read this, some of the output transistors blew from before.

The diode tested OK, but the resistor did not. The reason I missed it was because it actually was reading a resistance, 260Kohms... which is waaaay off from 15ohms. I guess it's my fault because I just checked all the resistors to see if they had a resistance at all. Guess that means I'll be going over all of them to check for the actual proper values.

I replaced R133 and checked the other resistors for accurate values, replaced the blown output transistors and a few other parts (resistors) which went with the outputs.

So, get it all back together, plug it in and try it. As soon as I hit the switch PO501 in the power supply went up in smoke... so I gave up for the night. And I think another one of the output transistors (one of the 2SC3856) blew because they test short in-circuit.

What is this one part? What would I replace it with? I'm guessing there's something still wrong in the circuit... I'll look another day.

I'm about to give up on this thing and either sell it on ebay or scrap the parts, it has a nice beefy transformer, 4 huge filter caps and some nice heatsinks

Thank you for humoring me in this, I appreciate all of your patience and advice.
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Old 3rd March 2006, 02:02 AM   #7
anatech is offline anatech  Canada
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Hi rkc7,
You know. You are almost there. Don't give up, just approach things calmly and take your time.

Let us know.

-Chris
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Old 3rd March 2006, 06:48 AM   #8
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Quote:
As soon as I hit the switch PO501 in the power supply went up in smoke

Haven't you considered a safer way of applying power? Ideally a variac or at least some resistors in the PS lines?
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Old 3rd March 2006, 11:07 AM   #9
rkc7 is offline rkc7  United States
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heh, yeah, a variac would be a good idea, but I don't have one.... what value/rating and how would you apply resistors to power supply line?

I'm a soon to be medical student with not much money to spare, so I don't really have much equipment, I just make due with what I have.

I occassionaly check out ebay for a good deal on a variac or other things, but I haven't found a great deal on one yet.
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Old 3rd March 2006, 12:43 PM   #10
anatech is offline anatech  Canada
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Hi rkc7,
You only need a 2 amp model. They are less than $100 new as long as you wire it up and put it in a box. They have cheap ones on Eeekbay too.

Failing that, wire a lightbulb in series with your test outlet. Crude, but it works.

-Chris
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