Carvin B2000 bass amp won't power on, need schematic.

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** That is bad sign of other problems in the amp.

Those are high power IGBTs and they do not blow without a reason.

..... Phil

Thank you Phil and RL. After much poking and prodding trying to reverse engineer which circuit was going where, the previous 105 ohm low readings across D809 and D810 are now normal. Maybe a spec of solder took up residence in the area when I was desoldering the adjacent transistors, Q800 and Q802?

Next (but still without a new set of IGBT's installed) I slowly powered the unit on with the Variac. Relay K1 now energizes right after power up and the internal green LED glows normally. I will now place an order for the IGBT's and NTC thermistor. Phil brought up the point, what would cause them to blow in the first place? After much checking, I could not find any other faults in the circuitry. I'll update you after the new parts arrive and are installed.

I do have one other question regarding the 25A line fuse. What the heck is that supposed to protect? If the power supply fails/shorts, the thermistor bypass relay de-energizes and then the line voltage is sent through the thermistor. I think the 25A fuse will win that battle and not protect anything. The specs on the back of the amp say, 2050W into 2 ohms so I guess a large fuse is necessary however wouldn't a smaller fuse, maybe a slo-blo type, make more sense?

One last comment about the schematic. This B2000 model is a mono amp so anything in the schematic that says CH2, does not exist though about one-third of the circuit board has empty spots for those components.

Thanks for your replies.

--
David Farber 
Los Osos, CA

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** I have you have an isolation transformer or a scope that can be safety floated above earth. You need to inspect the drive waves to each IGBT.

As you Variac up, the wave should snap on staring with a very low duty cycle, then fill out to a perfectly symmetrical square wave.

** At full power with 2ohms load, the AC will be 25amps RMS.

( 2100 /120 = 17.5 with a PF of about 0.7 = 25. )

If you know the speaker being used is 4 or 8 ohms and the player does not thrash the heck out of it - then a 15amp slo-blo will do OK.

.... Phil

Thanks for the tips. I'll check out the drive to each IGBT while powering it up slowly.

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David Farber 
Los Osos, CA

The parts finally arrived. I'm a bit confused as to how the schematic of the IGBT in the datasheet compares to the resistance readings on the component itself.

Placing the positive probe on pin 2 and the negative probe on pin 3 reads open circuit. If I then put the positive probe on pin 1 and the negative probe on pin 3 it reads open circuit. Then going back to my original test of positive on pin 2 and negative on pin 3, I get a reading of a good diode check around 475 ohms. Subsequently reversing the probes from pins 1 and 3 and then going back to pins 2 and 3, that will revert to an open circuit. These readings are what I expect when testing an FET. According to the datasheet, there is a diode across pins

2 and 3. That would mean if I put positive on pin 3 and negative on pin 2, I should get a good diode check reading but I get an open circuit. Can you make sense out of these tests?

Thanks for your reply.

--
David Farber 
Los Osos, CA

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** From where ????

** Shows a a reverse diode HAS been added to the device.

If it is genuine, you should see diode conduction ( 0.8 to 1V) with a DMM set to "diode check" range.

.... Phil

Parts were from Digi-Key. I took another look at the original, shorted parts. They are, STGB30NC60WD. I somehow managed to omit the "D" at the end of the part number when I ordered the replacements (and also omitted it when I reported to the group what the part number was). That would explain why the reverse diode is absent. I guess it's a very good thing I checked all of this out before powering it up again. I will order the correct parts and check back with you in a week or so.

By the way, I noticed a potential problem with checking the input waveforms to the gates of Q800 and Q802. If I put one scope probe across one transisor, from G to E, that should be OK however if I want to see both waveforms together while powering up the unit and then connect a second probe to the scope on the other transistor, the common ground of the probe will short out Q802. Should I just float the ground on one of the probes? Will that work? The amp will be powered by my Sencore isolation transformer/Variac.

Thanks for your reply.

--
David Farber 
Los Osos, CA

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** Both signals come from the same transformer - out of phase.

You can monitor the outputs of the 3525 IC or the primary side of the tranny without the iso in use.

Plus check continuity of secondary windings and their connections to the IGBTS.

.... Phil

Thanks for following up with the details Phil.

--
David Farber 
Los Osos, CA

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Note: The tests I performed below were without the control board connected. The control board is responsible for connecting the earth ground to the audio ground. The schematic of the control board is here:

?2161

The schematic of the main board is here:

?2161

I received the correct parts and installed them. I checked the waveform at the primary side of T2. As I powered it up slowly with the Variac, there was no gradual change in the duty cycle. It was either flat line or full on. I checked the signal across Q802 from drain to source and it

was a very clean square wave and measured approximately 400 Vpp. At this

point I could reassemble everything and see if it actually will perform properly. But before I do that, I would like to figure out the operation

of the power supply circuit that is rectified by diodes D805-D808. It's easy to see that when pin 20 of T3 goes positive, D805 conducts and becomes the V+ line. From that point, it feeds the load and then I imagine it ends up going through ground via the terminal marked HI RAIL on T3 and also through the other ground circuit which is in series with the array of capacitors, C811-812, C849-850, and C34, C36, and C51. Apparently there is also a connection to the bridge circuit, D809-D812. Is there some voltage adding going on there with the secondary transformer windings interacting with the two bridge circuits? This is not a configuration that I have come across before. At the very least I'd like to know where ground is for the V+ supply coming from the cathodes of D805 and D806. I am measuring 90 volts individually across capacitors C850 C852.

Thanks for your reply.

--
David Farber 
Los Osos, CA