Relay Chatter

[lowell]

Have a Mesa Boogie Road King Version/Series 1. When the footpedal Ch3 is selected the relays switch rapidly back and forth and cause a loud buzzing sound. It sometimes does it on Ch4 as well. Ch1 and 2 work fine.

The following has been checked and confirmed working:

Another footswitch has been tried, correct footswitch, and still relays buzz.
The rotary switch on back of amp (not using foot pedal) to select channels works fine.

I talked with Mesa Boogie and the tech was kind enough to guide me over the phone a bit. He explained the IC LM3914 controls the Channel switching and this IC is controlled by Pin5. The signal at Pin5 comes from the foot pedal and tells the IC which output to pull down to -3v. That IC is not used when the rotary switch us used. And this is clear when viewing the schematic for the switch matrix.

Pin5 voltages with footswitch:
Ch1 -3v
Ch2 -2.5v
Ch3 -700mv
Ch4 -1.6v

So it's clear Ch3 voltage isn't right and my guess is that it should be -2v.

Of note: pin3/6 state +2v. I have +1.2v there. I tested the 10ohm/5v zener for that supply out of circuit and got about only +3v.

Now that I'm posting this I realize that zener should measure 5v...not 3v. Duh. +2v from a +/-3v supply is 5v. I'm an idiot and am going to replace it...doubt I have any in stock though.

I've replaced the LM3914... No change.

I'm gonna post this any way and see what y'all think of the issue. Maybe the zener is the cause and I'll report back after replacing it.

I'm an authorized MB tech so I have all the schema and I am not able to post it. Hopefully y'all are authorized or have found a way to get a hold of it. Make sure it's series 1 and that the Channel select jacks aren't grounded but instead have -3v on their sleeve.

Thanks for any input!

[Mick Bailey]

I don't have the schematic, but what voltage is actually getting to the relay coil, and is this correct? I've had quite a few amps where there's pulsed DC on the relay. Some Fenders (for example) use half-wave rectification plus a small smoothing cap to power the relay. If the cap goes open the relay chatters. Same thing if the diode shorts and the relay sees AC.

[lowell]

Here's a link to the IC. http://www.ti.com/lit/gpn/lm3914

I thought I fixed it yesterday. The +2v was 1.2v as in my OP. I replaced the 5v zener. I had to use a 6.1v 1w zener to get the +2v up. It ended up being +2.4v. After that... Ch3 no longer chattered. I thought I was outa the woods!!

Came in today to check it again. Now there's a different result. Here's the list:

Ch3 - no chatter but there is a hum and the fan slows down a bit.
Switching from Ch3 to 2 does nothing.
Switching from Ch3 to 4 - chatter on 4.
Selecting Ch3 is very loud for half a second then normal
After messing around with it for a bit, it seems Ch3 is now broke again and chattering again.

Yesterday I tried (per the app notes for the 3914) adding a 4.7uf 16v cap across +/-3v close the IC. The cap immediately exploded!!! ??

I will run some more tests and hopefully have more measurements to post.

Mick...thanks for the post I will check for ripple on the supply and check the filters and rectifier.

[lowell]

Started messing with the progressive linkage on the back. I realized that on Ch3... any one of those switches (there are many!) Could be an issue. I got those relays chattering and realized if I finagled the toggle bat just right the chatter would change and sometimes stop. Removed the bazillion switch and pot nuts to remove that board and removed the toggle for the speaker A/B select. Hard wired two solder pads together as if switch was in that position. Fixed it. Waiting on exact switch from Mesa and will report back on if this is a success or not. This could save someone a week of troubleshooting.

[lowell]

Yep it fixed it . Was a journey. Mesas are very challenging to work on. So many pots, switches and relays etc... IMO every switch or added variable component to any amp is just another headache waiting to happen down the road. That said...this amp is extremely versatile.