Discharging Filter Capacitors in Mesa Boogie Single Rectifier

theimageofall

Hi All.

Looking for some advice from those who may have worked inside amps.

I have a Mesa Boogie Single Rectifier 50 watt head, which features an effects loop in parallel. This can cause issues with some time based effects creating a helicopter/whomping sound as it feedback into itself (my MXR Carbon Copy delay does just that). An easy fix is to change the effects loop into series, which requires desoldering and moving a couple of wires which I am comfortable with doing.

What I'm a little nervous about is death. That doesn't sound ideal to me and is a risk when working on tube amps!

I've read lots about discharging filter capacitors in amps, but have been unable to find anything specifically on the Mesa Boogie Rectifier. Does anyone have any experience on methods to discharge the voltage so that I don't shock myself and worse, die?!

Appreciate any advice.

Thanks

ecc83

ICBM will no doubt give us the benefit soon but, just don't turn the amp on tomorrow! Just fix the FX wires. If you want to be DBSure there is no residual voltage, check pin one of any triode to chassis.

I am assuming you have a meter? If not get one PD bloody Q! NOBODY should contemplate ANY kind of work on any electronic gear without a meter.

Have to say, unless you have a dicky ticker (like me!) the discharge of say 50mfds at 400V is rarely lethal*. BLOODY hurts! I grant you but not likely to kill and you would have to get it hand to hand, across chest.

*Never work on HV kit alone, you really need another adult around because even if you were shocked, basic CPR (+a 999 call) would almost certainly save you. Other vital precautions. ..No kids. No pets. No booze.

Of course, many amp techs here have no choice but to go solo but they and I have learned (OH! How we learned!) and lasted. Old but no longer bold.

Just to add. Some kit is really dangerous. Microwave ovens run 1.5-2kV at about an amp. No chance if you copped that. There are some serious HT voltages in some big amps. SVT 600V+ The early Backstar S1 200 700V later reduced to a pussy cat 600 or so.

Dave.

theimageofall

Thanks for that Dave.

Yes, I have a meter, alligator clips, one hand in my pocket as I type as practice for when I get in to the chassis!

Just looking for some comfort I suppose. I've read all about pulling the plug out with the amp and standby switch set to the "on" position to help drain the caps (also strumming a guitar whilst the sound fades away), but thought I'd ask the friendly community here for their experiences!

ICBM

Really simple on this one - just turn the amp straight off without using the standby switch (leave it set to 'play') and wait a minute or two. The first filter cap is a stacked pair with divider resistors which will discharge all the caps.

theimageofall

Ah that's great! I was hoping that would be the case, but didn't want to risk it before asking!

Thanks both.

ICBM

theimageofall said:

I've read all about pulling the plug out with the amp and standby switch set to the "on" position to help drain the caps

Bear in mind that 'rule' is not always correct - it depends on the filter cap arrangement and the position of the standby switch. For example, a Fender Hotrod Deluxe is the exact opposite, you need to leave the amp *on* standby for a minute or two, then turn off. 

Ossyrocks

Good that you’re aware of the dangers.

An old Fender bandmaster once gave me a hell of a jolt when putting the chassis back in the head cab. My thumb slipped inside the chassis onto the switches. Bang! I ended up about six feet back from where I started. 450v DC straight out of the caps is rather unpleasant. 

It was a good lesson, but it could have been my last.

theimageofall

Thanks ICBM. Good to know it isn't always the case, but ok with the Rectifier.

I really don't want to feel the voltage passing through my arm if I can help it. I'd rather put up with a problematic effects loop!

ICBM

Ossyrocks said:

Good that you’re aware of the dangers.

An old Fender bandmaster once gave me a hell of a jolt when putting the chassis back in the head cab. My thumb slipped inside the chassis onto the switches. Bang! I ended up about six feet back from where I started. 450v DC straight out of the caps is rather unpleasant. 

It was a good lesson, but it could have been my last.

Right at the very beginning of my repair career I found that out too, on an old Marshall 50W amp. That's another one that doesn't automatically discharge its own caps when turned off - but being inexperienced, I assumed that since I'd been sure to unplug it from the wall, I was safe. There was still over 200V on the caps when I measured it *after* it had given me quite a nasty belt.

HHwarner

On my Marshalls, with the guitar still plugged into the input, I usually switch the amp off but keep strumming the guitar. You can hear the volume fade as the caps discharge. Not sure if that would work on the Mesa though. I do that and check with a meter after. usually less than 10v residual after this. Just to make sure I then crock clip a cable to ground to drain the residual and stop them re charging. Just remember to un clip the crock when done (yes i once powered the amp back on with the crock still connected) 

ICBM

HHwarner said:

On my Marshalls, with the guitar still plugged into the input, I usually switch the amp off but keep strumming the guitar. You can hear the volume fade as the caps discharge. Not sure if that would work on the Mesa though.

Yes, but it isn't necessary because of the divider resistors.

ecc83

There is a nemonic in the heavy electrical industry. Switch off. Isolate. Dump. Earth.

Best to "dump" by crocking a pre amp anode, pin 1 or 6 to chassis then if you DO forget the cable, no harm done assuming the anode loads are decently rated.

Dave.

theimageofall

Did the mod and still alive!

I did a quick test last night and it appeared to clear up the effects loop actually (which is what I was hoping).

Going to do a proper test today, but so far so good.

I have to be honest that there was a little fear when diving into a Mesa Boogie with a soldering iron!

Many thanks for your input everyone.

lespaulcustom68

Can you run me through what you did - I have had a series 2 for almost 10 years and never got around to it.

theimageofall

lespaulcustom68 said:

Can you run me through what you did - I have had a series 2 for almost 10 years and never got around to it.

Sure - run through the draining caps process or the wiring?

lespaulcustom68

Sorry been away. The wiring if possible.

theimageofall

lespaulcustom68 said:

Sorry been away. The wiring if possible.

No worries.

Looking at the picture below, desolder wire 1 and tape up (attach securely to other wires with cable tie). Solder wires 2 and 3 together (these are the wires attached to to the send level knob).

This leaves nothing soldered to the pot, so the pot will not work (obviously) and makes the effect loop series (from parallel) and means I can hear my MXR carbon copy delay properly!

ICBM

You can actually do it more simply - just disconnect 1 and tape it, then bend terminals 2 and 3 slightly so they’re touching, and solder them together - you don’t need to disconnect those wires.

theimageofall

ICBM said:

You can actually do it more simply - just disconnect 1 and tape it, then bend terminals 2 and 3 slightly so they’re touching, and solder them together - you don’t need to disconnect those wires.

Yep, that would do the same thing. 

Either works though, so good luck if you decide to do it. Loads of info out there on the mod and the benefits of it in certain situations.