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Author Topic: Rebuilding a Carvin SX-200  (Read 7855 times)

secretweapon

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Rebuilding a Carvin SX-200
« on: November 09, 2010, 06:51:56 PM »
I was given a Carvin SX-200 head for free awhile ago by a friend. Apparently it had 'blown up' while he was using it and wouldn't turn on anymore.

Fast forward to two weeks ago. I take the amp apart and diagnose the problem; the VIPer100A (high speed switcher) and K817P opto coupler isolator were damaged, as well as a few capacitors and a resistor. I drove to the Carvin factory near my house and picked up the parts I needed and brought them home and installed them. Now the amplifier turns on but once it powers up, the Power LED indicator flashes on and off and the signal cuts in and out as if the amp is turning itself on and off. I can also hear the small board-mount transformer kick on and off as well. As far as I can tell I get a full signal and everything works, but the amp is automatically turning itself on and off! The on/off cycle is fast too, turning the amp on and off about twice per second. I dont see any other damaged parts on the circuit board and have triple checked all my connections. I installed sockets and plugged the new parts in to avoid possible damage during soldering.

Here is the SX-200 schematic. http://www.carvinmuseum.com/pdf/amps/sxpowerac.pdf
The parts I replaced are the 'Viper' (VIPer100A), OP2 (K817P opto coupler isolator), C31, C40, C41 and R40.

Any help would be very much appreciated. I am really close to bringing this poor amp back to life...  :-[

J M Fahey

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Re: Rebuilding a Carvin SX-200
« Reply #1 on: November 09, 2010, 09:35:26 PM »
From the datasheet:
Quote
The power supply has low ripple voltage, good transient response, and be able to current
limit by power limiting and cycling on and off during a hard short.
So now you know.
You probably have the power amp shorted.
Diode check Q1 and Q12 (TIP142/147).
Pull both out of the board, and check whether the power supply now stays on, and you have around +/-40V on terminals labelled V+ and V-.

secretweapon

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Re: Rebuilding a Carvin SX-200
« Reply #2 on: November 09, 2010, 10:06:32 PM »
How do I diode check Q1 and Q12? By taking them off the board and running it without them? What should I do if it works without them?

Just to clarify; you're talking about Q1 and Q12 by E1-E6 in the schematic, not the Q1 and Q12 diodes by line out in the schematic right?

J M Fahey

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Re: Rebuilding a Carvin SX-200
« Reply #3 on: November 10, 2010, 09:18:19 AM »
That schematic is somewhat confusing becuse they don't draw some parts properly, but scatter their pins all over the drawing, such as the VIPER IC, the converter transformer, and the output transistors.
I refer to the two big plastic power transistors: TIP142 connected to E1/2/3 and TIP147 connected to E4/5/6. Oh well.
Diode test: with them still in circuit, amplifier OFF and UNPLUGGED FROM MAINS set your multimeter in diode test position.
Each transistor has 3 legs, you have 3 pairs (Left to middle, left to right, middle to right) touch your probes to each pair, first one way, then the opposite . Maybe your multimeter beeps when you short both probes together.
If it beeps continuously between any two pairs, that transistor is shorted and must be replaced.
To confirm. unsolder it CAREFULLY, pamper the board, use a good solder sucker, remove as much solder as possible, so you can pull the transistor without tearing the PCB pads.
Re-measure it "outside".
Your amp "should" start without the power transistors.
I forgot, no speaker connected.
If it starts, and transistor(s) show shorts, replace both (even if only one is damaged, the other has suffered too).
Mound them cerefully, remember thermal grease and mica or the appropriate silpad, diode test from center leg to ground to check you have not shorted the mica.
Solder them, turn amp on, it should work.
Good luck.

Enzo

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Re: Rebuilding a Carvin SX-200
« Reply #4 on: November 11, 2010, 07:00:29 PM »
What JM said.


Your switching supply comes on and tries to power the system.  SOMETHING is shorted and loading the thing down, so like any good switcher, it shuts itself down.  And then of course immediately tries to start up again.   Sorta like a self-resetting circuit breaker with a shorted load.

A shorted output transitor is most likely, but a shorted rectifier on the switcher itself would also do it.  As could any shorted secondary load.

secretweapon

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Re: Rebuilding a Carvin SX-200
« Reply #5 on: November 11, 2010, 10:11:57 PM »
Well I pulled Q1 and Q12 off the board and tried to turn it on, but it wouldn't do anything. I checked them with my DMM but I don't have a diode tester, only resistance and continuity (and of course VDC, VAC, etc.). They rang out fine with no shorts between the legs.

I put them back on the board. I guess i'll check all the other onboard transistors and such for shorts between the legs? I still need to fire it up to make sure i'm back where I started before I took Q1 and Q12 off the board...

J M Fahey

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Re: Rebuilding a Carvin SX-200
« Reply #6 on: November 11, 2010, 11:04:13 PM »
Hi.
If your multimeter has a beeping continuity tester, *that* is your diode tester.
You use the word "rang" which in this context is most confusing.
Did you get any beep/ring across any two legs or not?
Or did you get any resistance reading when using the 200 ohm scale, out of the PCB?
Leave those transistors out, for now, until you get the switching PSU to start.
As Enzo said, the PSU itself might have problems.
Diode check D22and D23, the main suspects, but also D18, D19, D4,  D20.
Anything more complex than that (in the switching supply) is a bit over what I handle comfortably, and becomes a PITA to "remote control".
Being close to the Factory, I would visit them again.
Good luck.

secretweapon

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Re: Rebuilding a Carvin SX-200
« Reply #7 on: November 12, 2010, 02:59:19 AM »
Sorry for the confusion. I work in commercial/industrial electrical so 'rang out' is a general term for doing a continuity test.

There were no shorts (no beeping) between any of the legs of either Q1 or Q12. As I recall the lowest resistance reading I got between legs was something like 3.5 megohms, with all of the other combinations being significantly more.

On a diode test I should get continuity in one direction and high resistance in the other, right?

J M Fahey

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Re: Rebuilding a Carvin SX-200
« Reply #8 on: November 12, 2010, 08:39:40 AM »
Right.
Modern multimeters (as opposed to old, needle type ones), use only 100mV at the test leads to be able to measure resistors in-circuit without turning nearby junctions on ... and by the same token *cannot* check diodes.
So they have just one extra setting, usually combined with a continuity beeper, which applies 2 or 3 volts in series with a unspecified resistor and measure the voltage across test leads.
Try it yourself across a known good regular diode, say, a 1n4002 or similar.
You should see either "open/out of range" (a lonely 1 and all zeros blanked or blinking) or a value around 500/600 (the voltage drop across a junction).
Testing the same good diode on the regular resistance position will show open both ways, which I guess is what you're getting now.

secretweapon

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Re: Rebuilding a Carvin SX-200
« Reply #9 on: November 12, 2010, 10:15:49 AM »
When I checked Q1 and Q12 I used continuity first and then played with the resistance function to see if I could get it to read anything for me.

In resistance mode I got a lot of random measurements as I recall. In continuity mode none of the legs beeped with each other in any combination.

I'm going to diode test the rest of the op amps on the board and those diodes you mentioned next... Maybe i'll get lucky. If not I might call Carvin and tell them whats going on.