CrossFire VR1000D

[Dr Zeus]

I've got this one with a bad blown PS. This amp uses IRF3205 with 22ohm gate resistors. The drivers are A1266 on a rising driver board. All three of these devices are bad, so I've replaced the drivers and am now troubleshooting the amp with 1x IRF3205 in each bank. This amp look very familiar, almost like a design from a Memphis 16-1000D, where there are IRF640/9640 and what appears to be /very similar output driver cards like the old chrome memphis amps. The power supply looks very similar as well, but of course all these components are mounted to what looks like a genericly designed board.

The power supply I beleive is fixed, but it runs hotter than I would prefer. I think this supply needs some modifications so it will run with more stability. I replaced two A1266 drivers with BD140. From BD140 the IRF3205, there is only the 22ohm gate resistor; however there is also a 1k resistor connected on one side to ground, and the other to the base of the BD140. Would changing this resistor's value make the PS more stable?

Also, I'm not sure if this amp is supposed to run un-regulated. When it first starts up, the waveform is regulated, but after about 2 seconds its running unregulated.

[timwebb]

ive wondered that too.but i know playing with those will make the parts of the amp come on to fast or too slow. i think u might have to change the timing cap if u use a higher resistor. im not sure on this though.

[MOER]

The 1K resistor from the base of the driver is just a hold off resistor and the value of 1K is fine. A lower value will do nothing.

If the power supply has no secondary inductor, then it is not regulated. Observe the waveform of the power supply on one of the drains of the IRF3205s. If it is just a square wave with a 50% duty cycle, it is just a driven supply with no regulation.

The only way to check the supply is to disconnect the amplifier section from the supply and this can be done by cutting the +/- traces to the amps.

Do the amplifiers pass a sinewave?

Always and I mean always have a limiting resistor of between 2 and 4 ohms in the 12v line when powering up an amplifier.

I of course use 6 dual beam scopes when checking say a 6 channel amplifier.

I always have one scope dedicated to the power supply. By looking at the drain waveforms it is relatively easy to gauge the health of the supply BUT the 2-4 ohm resistor must be in place. All amps will swing sinewave with this resistor but of course it must be bypassed when attempting to drive current


Steve Mantz
Zed Audio

[Perry Babin]

How do you define 'regulated'? If you're referring to a reduced pulse width that slowly increases to nearly a 50% duty cycle, that's considered a soft-start. A regulated power supply would be one that would maintain a target rail voltage (by varying the pulse width) if you vary the DC input voltage from 12-14v (some can maintain over a wider range).

Post photos of the waveform on the gate leg of two power supply FETs (one from each bank). If the drivers aren't pulling the voltage down quickly enough, that could cause the overheating.

You should also check the operating frequency. It's not a common problem but if the operating frequency is too high or too low, it could cause heating of the FETs.

Reducing the value of the 1k pulldown resistors may help but it's best to check all possible faults before you start modifying an amp.

What is the difference in the current draw from the time that you see the pulses start to the point where the duty cycle becomes constant (before the outputs begin oscillating)?

[Dr Zeus]

Thanks guys, I appreciate the responses. I'll try to see if I can answer some of the questions. Please ask them again if I didnt quite understand of missed something.

Do the amplifiers pass a sinewave?

The speaker terminals present an amplified rca input (sinewave or otherwise).

How do you define regulated

Regulated waves going to the gate and coming off the drain are not clear on the scope. They have a lot of variation but still maintain the maximum defined voltage requirements. Unregulated shows a clean square wave on gate and drains.

Here is waveform on the gates of PS fets. Both banks look identical.

[Dr Zeus]

I'm kinda starting to think this amplifier may be alright. Really, the PS only starts to get slightly warm at idle after about 30 seconds. I bet/know if I installed the other 6x IRF3205 then any heat would average and be reduced between all 8 of them.

Nothing else but a couple of higher temp resistors get warm in this amp, and it plays.

Maybe I had too late of a night last night to be fixing amps. LOL

[Perry Babin]

The strange waveform on the drains is related to duty cycle, not necessarily regulation. Regulation relates to maintaining a target rail voltage.