Re: Weird(?) behaviour in tek 492BP power distribution - Please help

Harvey White

I don't know this (and don't have one) particular instrument. All I can do is to give you some general pointers.

Many power supply designs depend on a particular supply to be exact.  Generally you'd expect a zener or a reference to point out what voltage the supply *should* be.

ANY drain on that supply that would cause it to shut down due to overload, or not supply enough current to achieve the proper voltage will throw every other power supply off, if those supplies use "the supply" for a reference.

If that reference supply is ok, then look at each individual supply to see if those are at the proper voltage.  Too much current on any supply can cause it to fold back or drop out of regulation, however, if it's not the reference supply, then look at that supply only.

Typically, supply problems start with (and are not in order of probability, but are working from the line to the equipment):

1) bulk supply errors, the transformer/switcher etc does not give you the right supply voltages.  Generally things like bulk capacitors and bridges in linear supplies, gets more complicated in switching supplies.

2) reference supply errors (see above).

3) individual supply errors.

Many individual supply errors are caused by too much current drawn.  Other than bad semiconductors (less frequent than perhaps thought), your main problem is likely to be extremely leaky or dead shorted tantalum capacitors.

Sometimes bad supply voltages are caused by bad resistors in voltage dividers, or bad transistors.  Perhaps less frequent than you think.  Depends on the individual piece of equipment.

All this does ignore any other faults such as solder splashes, broken wires, etc.


On 5/27/2019 3:39 PM, Roger via Groups.Io wrote:
Dear all,
I bought one of those beasts at a reasonable price and it arrived in a very good overall condition.
The trace was crispy and I could see the peaks of the calibration signal.
Then I realized the the center frequency/marker button was not working and I disassembled the front panel to find the cause.
Later I learned the it was only a burnt mini lamp that was replaced and the front panel was reassembled.
When I turned the instrument on again, there was a flick and no trace was visible.
I checked the voltage test points on the Z-axis board and the +100V was gone and the +300V was low.
So I disassembled the Power Supply Module and checked the caps, and noticed the 100V (F-1035) was burnt.
Following the advice given by John Miles in his notes, I replaced the caps in the +100V and +300V supplies and measured all voltages (with the power module disconnected) and they were all good.
Since there was a blown fuse I checked for shorts on the Z-axis board, deflection boars and the HV board.
The only problem I could find was a shorted Q1073 oscillator transistor that was replaced by a NEC C2333.
Reassembled the board, turned the instrument on and to my surprise, NO TRACE.
Checked the voltages again on the Z-axis board and the +300V dropped to +245V and +100V raised to +145V. Is this normal?
Also, there was no Hi Voltage in any of the terminals of P742,!!!
I bought an HV board from Ebay and a set of extension boards from Norway Labs.
While these items are on their way, I decided to measure the voltages on the male header of the motherboard (just in case...).
To my surprise none of the values make sense!!!
The +15V, and the -15V were totally off, the +100V was +85V and I am not sure the exact value of the +200V.
My Z-axis board has the power status indicator led that is green, and all test points measure ok?
Also, I could not find a diagram with the pinout of my motherboard (12 pin header, one pin not installed)
Are there any components on the power supply rails of the motherboard?
I could not find any reference to them.
Any suggestion would be greatly appreciated.

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