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DC Loads for Testing 2465B and 7000 Series SMPS
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Hello to the group,
I have been thinking about building a DC load box for use in troubleshooting the switchmode power supplies in the Tektronix 2465B and 7000 series scopes that I own. All of my scopes are currently working, but I suspect it is only a matter of time before I experience the dreaded ticking SMPS problem and need to troubleshoot one of these power supplies. Has anyone determined the proper values of DC load resistance(s) that should be used to load the output(s) of the switching power supplies in these scopes when troubleshooting the supplies on the bench, disconnected from the scope? Which supply voltages need to be loaded in order for these SMPS supplies to operate properly and what value of load resistance should be used on each power supply output that needs to be loaded? Thanks in advance for any guidance that this esteemed group can provide me. Charlie |
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G?ran Krusell
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Hi Charlie,
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You may check??
Using link??
From there you can calculate resistance values and power levels. Switch mode power supplies
require a minimum load for operating properly. It is best if all supply voltages are loaded. Beware of high current levels and very hot resistors in this load.
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G?ran
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Hi,
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I recently used Alberts load value to build a load for 3 7704a power supplies that were not working. I added a 3k load for the hv drive lines. I found that one of the power supplies would not start reliably with that load but started easily in it's 7704A. I am building a prototype 7k extender with a switchable 47watt load. My plan is that it can be used in a mainframe with not plug ins or just the Aquistion (bottom half) of a 7704A. I am hoping that it will work with the non plug in loads to insure start up. I was planning to try and use a 7603 rectifier to build a test bed for the lv regulator of the 7704a as it has input voltages for the regulator that do not cycle off. When troubleshooting a switching power supply, it is helpful to use any tricks you can find. On 7704a power supplies i found it helpful to drive the power supply with a variac. At about 70vac in the supply starts ticking and you can use a scope to see how the secondaries are doing. You look for voltage pulses that proportionally higher or lower than the others. For example pulses on the -50V of -40v with 5v pulses on the +50v line suggest a problem on the +50v circuit. Dave on some others have a far better understanding of the controller ic in these supplies than I do. Jerry Massengale
-----Original Message----- From: Albert To: TekScopes Sent: Tue, Feb 12, 2013 5:42 am Subject: [TekScopes] Re: DC Loads for Testing 2465B and 7000 Series SMPS
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Hi Charlie,
In addition also have a look in the thread of message #83499. Albert |
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Albert
Hi Jerry,
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Nice idea to feed the LV board with another power supply. You plan to use the rectifier outputs from a 7603 so you probably mean the unregulated outputs. Then the 7603 has no protection. I would use the regulated 7603 outputs. That requires slight modifications in divider chains in order to increase all regulated outputs by a few volts. I think there is still enough voltage difference between unreg and reg then to prevent drop-out. (For a first test the LV board under test doesn't need heavy loads, so the unreg voltages can remain quite high.) The 7704A display unit (HV, vert and hor amp) can be replaced by about 11 W load at the 54 V square wave (see message #48756). I'm not sure whether the 7704A PS should work then or that still some plugin load is needed. Albert --- In TekScopes@..., jerry massengale wrote:
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Hi Albert,
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I have a couple of 7603 rectifiers laying around that have good unregulated outputs very similar to the 7704A unregulated voltages. I will not have a 7603 connected. The connection will be from the 7603 output connectors to the 7704a low voltage regulator board to a min load fixture. No mainframes involved. I may power the 7603 rectifier with a variac if needed. I only have one bad 7704a power supply left. The others have been fixed and are running so I am not in a big hurry. I have plenty of cables to make the connections. Jerry Massengale
-----Original Message----- From: Albert To: TekScopes Sent: Tue, Feb 12, 2013 12:17 pm Subject: [TekScopes] Re: DC Loads for Testing 2465B and 7000 Series SMPS
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Hi Jerry,
Nice idea to feed the LV board with another power supply. You plan to use the rectifier outputs from a 7603 so you probably mean the unregulated outputs. Then the 7603 has no protection. I would use the regulated 7603 outputs. That requires slight modifications in divider chains in order to increase all regulated outputs by a few volts. I think there is still enough voltage difference between unreg and reg then to prevent drop-out. (For a first test the LV board under test doesn't need heavy loads, so the unreg voltages can remain quite high.) The 7704A display unit (HV, vert and hor amp) can be replaced by about 11 W load at the 54 V square wave (see message #48756). I'm not sure whether the 7704A PS should work then or that still some plugin load is needed. Albert --- In TekScopes@..., jerry massengale wrote: > > Hi, > > I recently used Alberts load value to build a load for 3 7704a power supplies that were not working. I added a 3k load for the hv drive lines. I found that one of the power supplies would not start reliably with that load but started easily in it's 7704A. > > I am building a prototype 7k extender with a switchable 47watt load. My plan is that it can be used in a mainframe with not plug ins or just the Aquistion (bottom half) of a 7704A. I am hoping that it will work with the non plug in loads to insure start up. > > I was planning to try and use a 7603 rectifier to build a test bed for the lv regulator of the 7704a as it has input voltages for the regulator that do not cycle off. > > When troubleshooting a switching power supply, it is helpful to use any tricks you can find. > > On 7704a power supplies i found it helpful to drive the power supply with a variac. At about 70vac in the supply starts ticking and you can use a scope to see how the secondaries are doing. You look for voltage pulses that proportionally higher or lower than the others. For example pulses on the -50V of -40v with 5v pulses on the +50v line suggest a problem on the +50v circuit. > > Dave on some others have a far better understanding of the controller ic in these supplies than I do. > > > > > > Jerry Massengale > > > > > > > -----Original Message----- > From: Albert > To: TekScopes > Sent: Tue, Feb 12, 2013 5:42 am > Subject: [TekScopes] Re: DC Loads for Testing 2465B and 7000 Series SMPS > > > > > > Hi Charlie, > > In addition also have a look in the thread of message #83499. > > Albert > |
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