7854 LV power supply question


GeorgeP
 

For 7854 gurus:

Can you please confirm that I can operate a 7854 LV power supply module on the bench, removed from a scope and not connected to any load?
For troubleshooting purposes. I've made a discharge tool and re-read the manual carefully several times on this module. I am being very careful.
The power switch is ON and power is plugged in to a variac.
I have two which operating in pulse mode on the bench (and in their scopes), but they should work correctly by themselves?
If the LV power supply pulses on the bench, it has an internal fault?
In other words, it is not because I don't have it connected to the scope that it pulses.

Thanks
George


Göran Krusell
 

Hi,
No you can not. You need a load of some 50W. There is a document in the archive describing the various loads you need on the different outputs.
Pulsing may be caused by a load too heavy or too light. There are several conditions that causes the control circuit to turn off and then on again. It is a bit tricky.
Göran


 

No it needs a load - see Dennis Tillman's design for a load:

<https://groups.io/g/TekScopes/files/7104%20-%207854%20SMPS%20Dummy%20Load.docx>

David

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of GeorgeP
Sent: 23 June 2021 16:22
To: TekScopes@groups.io
Subject: [TekScopes] 7854 LV power supply question

For 7854 gurus:

Can you please confirm that I can operate a 7854 LV power supply module on the bench, removed from a scope and not connected to any load?
For troubleshooting purposes. I've made a discharge tool and re-read the manual carefully several times on this module. I am being very careful.
The power switch is ON and power is plugged in to a variac.
I have two which operating in pulse mode on the bench (and in their scopes), but they should work correctly by themselves?
If the LV power supply pulses on the bench, it has an internal fault?
In other words, it is not because I don't have it connected to the scope that it pulses.

Thanks
George


GeorgeP
 

Perfect. Thank you David (and Dennis!)

George


GeorgeP
 

Thank you!

George


Karin Johnson
 

I think the real answer is, "It depends".
I recently had to troubleshoot my 7934 power supply on the bench, completely disconnected from the scope mainframe. I use a variac to adjust the input line voltage to around 60 volts AC. I'm in the US just for clarification. That line voltage is just below where the starting circuit kicks in and depending on the actual fault, you can successfully check some, not all, of the power supply. Mostly the front end, and a bit of the actual high voltage switcher. It worked for me because my problem area was in the front end switcher. Just a thought.

Karin


GeorgeP
 

There is a part in the Tek manual troubleshooting where the variac is set at a lower voltage just below where some part of the circuit kicks in. Thanks.

George


Mark Vincent
 

George,

The dummy load is great to test the supply out of circuit. You say the three you have all do the same thing makes me believe all three power supplies need work. If the prereg. adjust is not right, that will cause the pulsing. Check VR112 on the regulator board and VR88 control rectifier board. I have found these to be open on my supplies. I replaced them with BZX85B9V1. If any supplies are not right, there will be voltage at the fault sense pin on the 0067 IC.

The 120V zener, VR45, can sometimes be bad. C43 could also be bad. The two 1mfd from T30 to the bases of the oscillator transistors can be bad. Those I have seen from 1 to 10mfd. The value is not critical. I put ULD types in mine. The damper diodes across the oscillator transistors could be bad.

I replaced all the condensers in mine. I used UHE types for the secondary filtering.

I said what I did and things to check. Let us know what you find.

Mark