Peavey 400BH not working first 30 seconds, then stops after30 minutes.

[PfeifferElectronics]

I have a strange situation with this powered mixer. When first powered on, i get sound but with in a few seconds i get nothing. then i wait awhile more while plugging input into different channels and suddenly it starts working again. now it doesn't matter what channel I plug into, it works fine. Then after playing for awhile; maybe 30 minutes, all sound stops. The power lite stays on but no sound. At first I thought it was the thermal switch but nothing is hot except the 5W resistors next to the power caps. nothing to make the thermal switch open. I tested the thermal switch and it opens very hot, nothing near what the NPN transistor heat is at.

Thanks for your help!

[Enzo]

400BH is a power amp board, what is the product model name?


I think you may be making assumptions.

You refer to the 400BH, which makes me think you are assuming the problem is there rather than in the front panel. It may be, but we don't know that yet. Also, you suspect a thermal switch, but the switch isn;t getting hot. I don;t know what makes you suspect the thermal switch, but they can be defective, they don't necessarily ONLY open when hot. They can open when they fail too. But an ohm meter or a clcip wire will tell you in an instant if that is the case.


I will assume it is an XR600 series for discussion, but please tell us exactly what model you have.

When I hear a complaint such as yours, my first reaction is almost always the patch jacks lower right front. The power amp in jack has cutout contacts, just like an FX loop in a guitar head. And unless you have patched it otherwise, there is also a graphic in jack which also has cutouts.


Whateever you have, if it goes quiet, plug a signal into the power amp in jack. Get sound?

In fact, before that we need to make sure the speaker and speaker cables are good.

This is also a place for my whack test. Ball up your fist and whack the top of the amp hard. Does that brinh the sound back? Or if it is working, does whacking it make the sound quit? Any reaction to the whacking means a loose connection inside. Turn the reverb down before whacking. If your fist is too dainty, use a rubber mallet.


But the essence of troubleshooting is to isolate the problem. Is it the speaker/cord? The power amp? The mixer/preamp? The power supply? Plug a signal into the power amp jack, and try into the graphic in jack. ALso, plug a signal into one of the channels and set the controls as normal, then run a cord from MAIN OUT on the front over to some other power amp and speakers. Any signal coming out that main out? If so, patch a cord from MAIN out to POWER AMP in. Each channel has a monitor feed and a monitor master. COnnect the monitor out on the front to some other amp. ANything come out that?

You could have a bad ribbon, so even if the channels 1-6 don't work, try feeding the AUX in and turn up the AUX volume, sound?

If your 400BH section is indeed the trouble, then start at the start, are all power supplies present? You have the two main 52v rails, then the power amp ICs use +/-15vDC. Remember the two 7815/7915 Vreg ICs are ONLY for the preamp panel, the powr amp has its own 15v rails from zeners. Right by where the speaker jack wires plug onto the board is a big ugly coil of wire, your output inductor. Grasp it and pull up on it to see if one end is broken free.

[PfeifferElectronics]

Thanks enzo! Loose ground wire next to pwr amp input.

[PfeifferElectronics]

I replaced the far lower right 1/4" jack for the pwr amp in. then I noticed that the post soldered to the board was really loose, so I re-flowed and added some more solder to that ground post connection. Working great now. The whack test revealed the problem because when I whacked, sound stopped. Then I plugged signal into pwr amp in and sound returned. Hope this helps.