5150ii problem with channel switching

[5150ii]

Hi, I'm having a problem with my 5150ii amp. It will not switch to the lead channel via the front channel switch or the foot switch.

I sent it to an amp tech and they spent 3 months trying to get it going.. they finally replaced the main circuit board with no luck...

the rhythm channel works fine there is no humming or funny noises but it will not switch to the lead channel...

any help would be appreciated...

[Enzo]

Replaced the main board? Holy cats. I wouldn't suspect that board anyway. I suspect your problem is on the small breakaway board the footswitch jack is soldered to. I could be wrong though.

Before going further, check the lead channel LED. Does it have a little conector down along the panel? If the LED itself is bad or broken off or the wires or connector are faulty, the switching won;t work. The current for the relay runs THROUGH that LED. That is my best guess for what's wrong.

The system is relay based. They run off the +24VDC power supply. That comes through R37, make sure there is +24v on both ends of that. Now you should find around +22 to +24v on all the relays coils. Easiest way to do that is take readings on either end of the diode next to each relay. If the voltage is there, proceed, if not, there is the problem. All four relays, don;t skip any. Presumably with a new board they'll be OK.

Now off to the jack board. There is a ribbon cable to it, and it must be good also. Check the solder closely under that board. Especially the footswitch jack itself. Note the jack has three resistors next to it, and one alone nearby. There are also three small transistors - little black things with three legs. Ten pin ribbon connector. Ignore the three audio jacks for now, they are not involved.

That lone resistor is wired to the third pin from the end. That third pin needs to have +22v on it. The end of that resistor nearest the ribbon also must have +22 on it. Also, it is critical that the grounds for all those connectors has continuity to ground on the main board. Third pin from the other end of the ribbon connector is ground, I think.

Let me stop to say this. I am not at all concerned if the 24v is 26 or 23 or something, and there are both 22 and 24 on the schematic. The actual voltage is not the issue, the issue is whether the voiltage is missing or not.

That lone resistor pulls up the bases of the three transistors. The +22v goes through it to the three other resistors which branch to the three transistors. Those transistors then are turned on by this, and they conduct, completing the circuit for the relays and the switches when the footswitch is gone. The FS itself has a jumper wire in it to disable this path - turns off the transistors so it can controls switching itself.

[Mars Amp Repair]

yeah,
Yikes, too. I can't imagine replacing a mother board for an issue like that.

Anyway as I recall, there is an FET or switching transistor on the footswitch DIN socket board on the rear panel. I can remember replacing it for channel switching issues. glen

[5150ii]

Hey thanks it worked.. I unplugged the led from the main board and ran a current to it, it never lite up..

So I picked up a pack of leds for 2 bucks and tried one and the channel now switches to the lead...

It was in the store for 3 months and I was billed $167 for an amp that never got fixed.. well they sent my old main board with the amp so I guess I got a spare one for parts...

thanks again I owe you a beer..... lol

[5150ii]

one more quick question i'm using a 2.2v 20 ma - 150 mcd led is that ok???

[Enzo]

Amp now works? Relays now click on and off? SOunds OK? You are done.

Look, I believe the guys who serviced your amp made an honest effort, but swapping out the whole circuit board was a guess. Or a "Hail MAry" pass to the end zone in the closing seconds of the football game.

I would take the repair receipt, print out a copy of this thread, and go to the store manager where the repair was done, and tell him you don;t feel you were well served by their efforts. Tell him you'd like him to make things right.