Question about discharging filter caps

[SgtThump]

I recently bought a first version Friedman Smallbox and sent pics to Dave. He confirmed that my amp has two faulty caps from a "bad batch" back in 2015. I am not experiencing any issues, but he says they will fail and should be replaced. I'm very tempted to do this myself.

Does this indicate that the caps are discharged and safe? I have the negative side of the DMM attached to the amp chassis and the other side touching the positive terminal of the cap. Is my DMM set correctly? This shows 1.5 volts? If so, that's completely safe correct?

 

[SgtThump]

I know @harddriver will know!

 

[harddriver]

Hey Thump, Yeah that will work but there is a better spot..... you can also check for B+ DC voltage off of V1 preamp tube socket Pin 1(one) that has the blue wire going to it from the plate resistor. That's where i usually discharge my caps from since it is in series with all the power and preamp tubes and discharges at a nice controlled rate unlike ground out the B+ rail at the 10K dropping resistor in the B+ supply rail. The B+ in that smallbox is probably anywhere from 400 to 450DCV is my guess.

I also recommend the grounding probe to have 1K 10W ceramic power resistor on one end both ends with alligator clips. This allows for a more gradual and controlled rate of discharge instead of just grounding the B+ rail to ground.... that instant discharge isn't really great on electrolytic caps.

Which caps are bad in the smallbox? Is it those 500pf black mica caps, I remember those being an issue years ago...... or is it something else?

Having the DCV range set to 500 or 1000 DCV should be sufficient. That 200m setting I think is 200 Millivolts and too low to measure B+. I'd say 500DCV is all that multimeter is rated for. Turn the selector to 500 your last setting next the off detent. When they are drained you read like 1-5 volts or less and that's safely discharged. Just make sure you remove your B+ drain device when you power things back up otherwise there will be sparks and you don't want sparks................

 

[Geo]

I just usually use my tongue.

 

[SgtThump]

Hey Thump, Yeah that will work but there is a better spot..... you can also check for B+ DC voltage off of V1 preamp tube socket Pin 1(one) that has the blue wire going to it from the plate resistor. That's where i usually discharge my caps from since it is in series with all the power and preamp tubes and discharges at a nice controlled rate unlike ground out the B+ rail at the 10K dropping resistor in the B+ supply rail. The B+ in that smallbox is probably anywhere from 400 to 450DCV is my guess.

I also recommend the grounding probe to have 1K 10W ceramic power resistor on one end both ends with alligator clips. This allows for a more gradual and controlled rate of discharge instead of just grounding the B+ rail to ground.... that instant discharge isn't really great on electrolytic caps.

Which caps are bad in the smallbox? Is it those 500pf black mica caps, I remember those being an issue years ago...... or is it something else?

Having the DCV range set to 500 or 1000 DCV should be sufficient. That 200m setting I think is 200 Microvolts and too low to measure B+. I'd say 500DCV is all that multimeter is rated for. Turn the selector to 500 your last setting next the off detent. When they are drained you read like 1-5 volts or less and that's safely discharged. Just make sure you remove your B+ drain device when you power things back up otherwise there will be sparks and you don't want sparks................

Thanks man! Most of that was over-my-head. LOL

I didn't do anything to drain the caps, except letting some juice run out during shutdown by leaving the standby switch in the "on" position (or is it "off"? Whatever...) I've watched videos about how to use a resistor to drain the caps and stuff, but I didn't do that. I'm just measuring and asking if they are already drained judging by the reading? And am I measuring correctly?

It's the blue caps on the board. I uploaded pics. I'm also having a hell of a time finding the 33uF 500V cap. I see some on eBay, but none at Mouser, Mojotone, Anitique Electronics Supply, etc... Not sure if the few on eBay are cheap and crappy or what.

 

[Geo]

I'm also having a hell of a time finding the 33uF 500V cap. I see some on eBay, but none at Mouser, Mojotone, Anitique Electronics Supply, etc... Not sure if the few on eBay are cheap and crappy or what.

Friedman should send you them.

 

[SgtThump]

Friedman should send you them.

I totally agree. He didn't offer and I didn't ask. But I will ask now that I can't find them.

 

[harddriver]

Thanks man! Most of that was over-my-head. LOL

I didn't do anything to drain the caps, except letting some juice run out during shutdown by leaving the standby switch in the "on" position (or is it "off"? Whatever...) I've watched videos about how to use a resistor to drain the caps and stuff, but I didn't do that. I'm just measuring and asking if they are already drained judging by the reading? And am I measuring correctly?

It's the blue caps on the board. I uploaded pics. I'm also having a hell of a time finding the 33uF 500V cap. I see some on eBay, but none at Mouser, Mojotone, Anitique Electronics Supply, etc... Not sure if the few on eBay are cheap and crappy or what.

Just put your positive lead of your multimeter(negative lead to ground) on the V1 preamp tube socket pin 1 that has the blue wire going to it. That blue wire is coming from your B+ DCV rail powering the preamp tube and will have residual B+ DCV on it if not drained down. Set your multimeter to 500DCV not 200m volts and you should be good to go. Yes checking the + side of those filter caps will show whether or not the DCV B+ is drained down but I recommend checking it where I referred you on the V1 preamp tube PIN 1 with the blue wire going to it.

 

[harddriver]

Here's ARS's main page.

https://www.arselectronics.com/ARSWEB/CapPage2.html

 

[JohnnyGtar]

From the photos, it looks like your hand may be too close to the chassis. I'm not tryin' to be a busybody. Don't want ya ta git zapped is all I'm a sayin'. LOL

 

[SgtThump]

Just put your positive lead of your multimeter(negative lead to ground) on the V1 preamp tube socket pic that has the blue wire going to it. That blue wire is coming from your B+ DCV rail powering the preamp tube and will have residual B+ DCV on it if not drained down. Set your multimeter to 500DCV not 200m volts and you should be good to go. Yes checking the + side of those filter caps will show whether or not the DCV B+ is drained down but I recommend checking it where I referred you on the V1 preamp tube PIN 1 with the blue wire going to it.

Thanks man! That was easier to understand. ha ha

 

[SgtThump]

Just put your positive lead of your multimeter(negative lead to ground) on the V1 preamp tube socket pin 1 that has the blue wire going to it. That blue wire is coming from your B+ DCV rail powering the preamp tube and will have residual B+ DCV on it if not drained down. Set your multimeter to 500DCV not 200m volts and you should be good to go. Yes checking the + side of those filter caps will show whether or not the DCV B+ is drained down but I recommend checking it where I referred you on the V1 preamp tube PIN 1 with the blue wire going to it.

When I change the DMM to 500 volts, I get 000 at pin 1 of the v1 preamp tube socket, as well as on the cap positive legs. So it says 000. Seems safe to me.

I won't attempt this until I get the replacement caps. But if I die, you guys can use me as an example of what not to do.

 

[70 Mach 1]

Caps can build up static charges.
Probably wont hurt you but why take chances
I keep them grounded after discharged with alligator clips

Just remember to remove the ground when you fire it back up

 

[harddriver]

When I change the DMM to 500 volts, I get 000 at pin 1 of the v1 preamp tube socket, as well as on the cap positive legs. So it says 000. Seems safe to me.

I won't attempt this until I get the replacement caps. But if I die, you guys can use me as an example of what not to do.

You can set it down to 20 DC volts just to see what that setting shows.

 

[harddriver]

When I change the DMM to 500 volts, I get 000 at pin 1 of the v1 preamp tube socket, as well as on the cap positive legs. So it says 000. Seems safe to me.

I won't attempt this until I get the replacement caps. But if I die, you guys can use me as an example of what not to do.

If you are not comfortable doing it take it to someone.