Help with MA Audio HK1000D

[shawnk]

Looking at this amp for a buddy. The amp has good output up untill ~15vac at the speaker terminals. At which point you can hear the muting relay click 'off' and no more output. Protection led does not light. From reading a few of the other threads I gather that at times these amps have open filter caps. I have already removed and checked B+ and Rail caps, all are within tolerance. Any suggestions? Thanks.

[Perry Babin]

Can you post a photo of the inside of the amp?

[shawnk]

Hope this worked.

[Perry Babin]

I've seen open filter caps (with absolutely no visible signs of damage) in some class D amps but they were Samcon. These appear to be Samyoung. In amps with open primary capacitors, you can connect a large electrolytic capacitor (2200uf or greater) across the B+ and ground terminals and the amplifiers will generally work properly. You should try that before you replace the original caps.

It also appears that someone has replaced a resistor. It may not be the correct value or may have opened again. This may simply be part of a snubber and not a critical component.

It appears that some of the mica insulators are missing. Were they missing when the transistors were clamped to the heatsink?

[shawnk]

Thanks for the reply Perry.

It's getting a little late to test the amp with my sub right now so I'll try the capacitor trick in the morning and I'll let you know how it goes.

I myself have replaced a couple of resistors. You may notice some black heatshrink bundled up near the largest blue capacitor. The factory 1/2 watt 330ohm (metal oxide?) resistor in this location appeared to get so hot that it was melting the solder and discoloring the board. I'm not sure if this is due to a defect or a poor design. Inside the heatshrink is 4 330ohm 1/2 watt resistors in a series/parrallel config to try and disipate heat better. This is just a temporary fix untill I order a larger replacement. You may also see a missing resistor to the left of the blue cap beside the relay. I essentially did the same thing here as the factory resistor, 2.2 ohm 1watt, was reading .5 ohm (out of circuit). The new resistor is under the board (temporary). I still have the insulators for the fets. I simply had the board set into the base of the amp just for the picture.

I'll keep in touch. Thanks

[shawnk]

Ok. So I'm a bit confused here. I installed the cap (50v 3300uf) across B+ & B- and seemingly no more issues. I can easily get ~25v of output and the amp's relay stays engaged. GREAT! However, I removed the cap, just for kicks, and got the same results (no problems). The amp isn't acting the same as it was last night but nothing has changed!

[Perry Babin]

When this problem is caused by open capacitors, the problem can easily be seen as excessive ripple on the B+ terminal. Without a scope, it will be more difficult to determine if open capacitors are the problem. Do you have a scope yet?

Does the problem return if you push on the filter capacitors? There could be an intermittent connection (either inside the cap or on the circuit board).

[shawnk]

Yes sir. I do have a scope now, and I did look for ripple on the B+ but it appeared smooth (flat). I only did this because I read in one of the other threads that this is what you had suggested to do. I just tried to push on the caps but it didn't change anything. Odd.

[shawnk]

Would it be wise just to replace the rail & B+ caps anyway?

[Perry Babin]

Did you check for ripple when it was shutting down?

If it does it again and there is no ripple, monitor the voltage on pins 1 and 7 of the NJM2904 op-amp on the power supply. Does the voltage change when the amp goes into protect (or shuts down)?

Also confirm that the voltage across the diode D8 (near the relay) is not dropping far below 12v. It should be approximately the same as the voltage across the B+ terminals.

Did you confirm that the B+ voltage (measured directly across the B+ and ground terminals on the amplifier) isn't dropping far enough to cause the amp to shut down?