BMWR75
Journeyman
Got two new ones coming via the @Northwinds coordinated group buy. Do I recall there being a certain way new caps of this size need to be formed? Am hoping this is possible installed in the 700B using a variac.
Forming new 700B power supply caps?
- Thread starter BMWR75
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BMWR75
Journeyman
How many of you are just going to install the new caps in your 700B and turn it on? That is what I've done every time PS caps have been replaced in a bunch of other amps (always with new caps, not NOS). Was I being stupid? Granted, the caps in the 700B are larger than any I've replaced.
Is installing the new caps in the amp, pulling the 5 amp supply fuses, and bringing the amp up slowly on a variac a viable alternative?
Is installing the new caps in the amp, pulling the 5 amp supply fuses, and bringing the amp up slowly on a variac a viable alternative?
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Forming large capacitors is typically recommended by the manufacture and it is best practice to do so. I'll be forming mine.
Pulling the rail fuse isolates the output stage from the power supply but the +/-100 V is still present at the input and VA stages.
I recommend always forming caps of this size with a large amount of liquid in them. Results of a bad cap of this size can be pretty devastating.
I form them with a DC power supply with a 1K 2W resistor in series. Let them charge up from the 100V supply with the resistor in series and monitor the voltage across the 1K resistor. After a long settling time the voltage across the resistor should drop to ~50mV on a new cap. If it does not drop to a value like this or drops and then starts to climb again, I would be suspicious. Climbing forming current usually is accompanied by a slight warming of the caps as well. A good cap at idle should not warm up.
BMWR75
Journeyman
If I was to build Lee's 100V DC power supply and use a variac, will need a full wave bridge rectifier. My local Radio Shack has a 25 amp 50V version. Will that suffice for cap forming?
You won't need a lot of current but you're going to take this up to over 100 volts to form these. I'd say the amperage is good but not the voltage. You could do it the old fashioned way and use four diodes if you have some lying around.
Hi Scott
You should consider using your PL700 for this.
Let old caps fully discharge.
Remove all 4 DC rail fuses
Remove old caps
Install new caps
Connect copper bus bar to both caps observing proper polarity.
Do not connect the plus and minus wiring to the other terminals
Connect one end of a 1000 ohm 2W resistor to each of the open plus and minus cap terminals. This will leave the other end of each resistor unconnected at the moment.
Using a short alligator jumper, connect the B+ lead from the bridge to the open end of the resistor connected to the plus terminal of one cap (usually the upper cap)
Ditto above for the minus terminal resistor.
Bring up to full voltage on your variac.
Using DVM watch the voltage on each resistor drop. Each should drop to around 50mV after several hours, they are formed and not leaky if you get a reading like this.
After forming, disconnect power and carefully discharge using the same 1K resistors.
Clear?
BMWR75
Journeyman
Got it Gepetto!! Thanks much.
Joe - when using that method to form the caps is the time cut down to a few hours or do you recommend forming them for 48 hours? By the way that makes life easier for those that don't access to a 100 volt power supply. Nice explaination - even I followed it. 
Do you form the WO capacitors in pairs?
Do you form the WO capacitors in pairs?
Joe - when using that method to form the caps is the time cut down to a few hours or do you recommend forming them for 48 hours? By the way that makes life easier for those that don't access to a 100 volt power supply. Nice explaination - even I followed it.
Do you form the WO capacitors in pairs?
Forming is done over the first several hours Dennis. I let them go for 24 hours though just to ensure they don't form and then start climbing again after they bottom out (that indicates a leaker). With 160V caps being formed at only 100V, they should definitely bottom out and stabilize nicely.
For the WO PL400 cap kits, I form the stack for 24 hours at 90VDC once assembled into a 2 cap stack. Same limit. Most times they go down to about 35mV after that 24 hour period, that is a fairly typical settling voltage on the resistors.
These 160V 15KuF caps at 100V applied voltage settle in at about 25-30 mV (25uA leakage) after about 2 hours.
I take it you've done one of the new ones. Thanks for the input Joe.
BMWR75
Journeyman
I think I just fried something on the WO board.
Had installed the new caps and connected the copper bus bar. The other positive and negative terminal wires not connected. Nor were the resistors on these same posts.
Just wanted to check and make sure I had my positive jumper on the + DC side of the bridge rectifier and - DC on the negative. Brought the variac up a little and hear a faint arc and saw a little puff of smoke on the WO board.
All I can figure is the negative terminal wires I disconnected from the PS caps were touching the chassis. Did this do it?
Had installed the new caps and connected the copper bus bar. The other positive and negative terminal wires not connected. Nor were the resistors on these same posts.
Just wanted to check and make sure I had my positive jumper on the + DC side of the bridge rectifier and - DC on the negative. Brought the variac up a little and hear a faint arc and saw a little puff of smoke on the WO board.
All I can figure is the negative terminal wires I disconnected from the PS caps were touching the chassis. Did this do it?
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If you followed the instructions, there should have been no power in the chassis except the 2 connections from the bridge rectifier outputs to the caps via a 1K resistor, that's it. All else was supposed to be disconnected.
BMWR75
Journeyman
I have the circuit hooked up per the instructions. I have Watts Abundant's DC protect circuit installed in this amp and there are two white wires connected to the positive terminal of the lower capacitor. One runs to the WO board (chassis ground I think) and one to DC protect board (I'm sure). Is one of these white wires the culprit? Being an idiot, it never occurred to me to not reconnect the double white wire connection to the lower cap positive terminal.
The smoke appeared to have originated in the area circled in red.
The smoke appeared to have originated in the area circled in red.
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Both of those wires are ground Scott so as long as every other connection was removed from the bulk capacitors it should not matter. Nothing looks suspicious in the area you circled. Have you measured the resistors in that section to see if any are off from their marked value?
Damn Scott - I hope things didn't get too messed up. Did you follow Joe's instructions for forming the caps in the amp? It was important to make sure nothing down stream of the power supply was hooked up. Maybe a photo or two of how you have it configured will help here.
I'm assuming the caps made it unscathed. They should have as an ass ton of bubble wrap was used. I took it from the Easter bunny.
BMWR75
Journeyman
Here are some pictures of how the caps are connected for forming. Both are reading below 50mV now and have been forming for ~27 hours. The red jumper is connected to the B+ side of the bridge rectifier, the black jumper to the B- side. The other end of the red jumper is connected to two 470 ohm 3 watt resistors in series. The green jumper is connected to the other end of these resistors and then to the positive terminal of the top capacitor. The black jumper from the B- is connected to two more 470 resistors in series. The other end of these resistors is connected to the negative terminal of the lower capacitor.
This last picture shows the two black wires that are normally connected to the cap negative terminal in routine operation. One of these wires was touching the metal amp chassis when the spark/smoke event occurred. It is not touching the chassis now. There are two white wires connected to the positive terminal of the bottom cap. One of these is the ground for the WO board, the other is the ground for the Watts Abundant DC protect board.
I am not going to do any trouble shooting until these caps are done forming. Then will let them discharge. After that will reconnect the amp wiring to the caps and start troubleshooting. First thing I plan to do is power it up on a DBT and see if the bulb dims or not. If it doesn't, will then start checking for burned up components.
This last picture shows the two black wires that are normally connected to the cap negative terminal in routine operation. One of these wires was touching the metal amp chassis when the spark/smoke event occurred. It is not touching the chassis now. There are two white wires connected to the positive terminal of the bottom cap. One of these is the ground for the WO board, the other is the ground for the Watts Abundant DC protect board.
I am not going to do any trouble shooting until these caps are done forming. Then will let them discharge. After that will reconnect the amp wiring to the caps and start troubleshooting. First thing I plan to do is power it up on a DBT and see if the bulb dims or not. If it doesn't, will then start checking for burned up components.
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