New 722 Cooling Mod

[TheKrell]

I've been silent here because I concede the points on the previous page.

But I bemoan the use of "high voltage" for 25V and above. Good grief, what do we call 1000V?

I'm also surprised to hear that the can of that filter capacitor is charged; usually (ha!) it's grounded, and with good reason. 1000 microfarad is a common (large) size. I have a 10,000 microfarad capacitor lying around somewhere, which when charged up to 250 volts really makes sparks fly when bridged with something you don't care about, such as an old screwdriver. Don't try this at home! Ah, the things we used to do at college...

Note that energy stored is W=CV^2, so you get more bang when using a capacitor rated at higher voltages, such as hundreds of V, rather than one of those feeble 25V jobs.

 

[Smith P.]

It's related to that level when it could create deadly current in your body, not tied to round numbers like the 1000 V or 12KV.

 

[bebop]

It's related to that level when it could create deadly current in your body, not tied to round numbers like the 1000 V or 12KV.

Indeed! You could have 1000V at 1 microamp and have no problems.

 

[cparker]

I had a boss in a plant I worked at that always felt this need to check on my work whenever I told him something was busted. Even though he knew virtually nothing about the equipment I worked on.

One day I got tired of it so I charged up a big capacitor with a loose terminal and then set it on his desk. I had taken a note that said, "Loose terminal" and simply spiked it down over a terminal. When he picked it up he had the entire can in his hand and then had to touch the terminal to see if it was really loose.

That cap bit him so hard I think he wet himself.

He never felt the need to check my work again.

 

[HansenFarm]

The large capacitor being referred to is to filter the DC power being supplied to the components on the 722 circuit board, the highest DC voltage being developed is 24 volts. Besides that the case of the capacitor is insulated and the only place to contact live voltage is on the under side where the leads go through the circuit board.
The 120 AC voltage seems to be limited to the little fenced of area in the rear most left-hand corner of the unit and it would hard to contact the AC from the top side of the circuit boards.
Best to not operate unit with cover off with children around or if something could be dropped into the components that could cause a short.
I thought my 722 hard drive was done for until I laid it on edge on top of the unit with a small fan blowing on what would be the lower side of the drive and also cooling the the inside of the case with the top off.
It is evident that there was a lack of thought given to the cooling situation on these units and engineers thought the components could be operated at their max allowable temps with no margin left for over heating situations.
One would think it might have been been better to have considered better engineering from a physical standpoint versus all of the returns of units and unhappy customers.
One of the many reasons I would likely make a poor designer because I usually go for stout.

 

[digidss]

In alot of switching power supplies I've worked on, the negitive side of the cap is below ground potential ! This will give you a nice poke if your touching the chassie ground and happen to touch the top of the capacitor.

 

[HansenFarm]

Not at 24 volts DC and not if the case is insulated as it is on the 722

 

[bebop]

Besides that the case of the capacitor is insulated and the only place to contact live voltage is on the under side where the leads go through the circuit board.

I don't know if I understand what you wrote, but it seems like you are saying you can ONLY contact live voltage under the circuit board where the leads of that capacitor are located. This is not true, so please clarify. If you don't believe me and you want to get shocked, try touching the top center of the capacitor with it plugged it. I guarantee you'll get zapped. I don't recommend doing it, but we ought to be giving clear warning to others who read these posts to NOT be touching components, especially when the power is on.

You may need to also be touching the case or some grounding point to receive the shock, but I didn't want to test it out to say for sure.

 

[8arrett]

I don't know if I understand what you wrote, but it seems like you are saying you can ONLY contact live voltage under the circuit board where the leads of that capacitor are located. This is not true, so please clarify. If you don't believe me and you want to get shocked, try touching the top center of the capacitor with it plugged it. I guarantee you'll get zapped. I don't recommend doing it, but we ought to be giving clear warning to others who read these posts to NOT be touching components, especially when the power is on.

You may need to also be touching the case or some grounding point to receive the shock, but I didn't want to test it out to say for sure.

Isn't this what they make meters for? --Grin&Barrett




Remember: DON'T BE A BONEHEAD!!

 

[bebop]

Don't hands work as meters if you're lacking a meter?

 

[8arrett]

Don't hands work as meters if you're lacking a meter?

Very true! In my younger years, I was thrown against a wall by the HV housing of a console television! I was drumming on the housing with a screwdriver or a hex driver. That was a surprise! As you stated, I didn't need a meter to tell me there was HV there! I'll have to try this on my 622 and report back - OR NOT! With age comes wisdom. --Grin&Barrett




Remember: DON'T BE A BONEHEAD!!

 

[HansenFarm]

The large filter capacitor I referred to does not have any exposed metal on my 722 and it measures 0 volts to chassis ground. The many smaller capacitors on that same board have metallic centers showing but also measure 0 volts to ground. There are some component ends above the board that measure 5 volts but 120 AC does come into the little box in the corner.
I was not trying to make a case for leaving the cover off the case but when the conversation went to all of the dangerous and painful voltages under the cover, I felt the need to respond.
I am probably jaded by the fact I grew up working on B&W television in the 50's when sometimes you had to work within several inches of 15K volts supplying a bare metal bell of a cathode ray tube. Where was OSHA then?
Still would like to see an upgrade for the 722 that would do a good job of cooling the way it should be. Bet it would sure slow down the receiver failures.

 

[Smith P.]

http://www.satelliteguys.us/1541984-post568.html

 

[bebop]

http://www.satelliteguys.us/1541984-post568.html

Capacitor 'C31' when touched on top and probably grounded or touching the housing, will cause a shock to a human hand (or other body part ). Thanks for the good pics - as it shows up best in photo 7 of 9 there.