Voltage reference point- Chassis or Circuit?

Thread Starter

Sp@ceR

Joined Mar 11, 2018
50
I have a Meguro MK-668F on the bench going for recap. While referring to the flowchart on one of the boards, one that says: Voltage between TP14 and ground should be -14.5V to -15.5V.
Meguro.png

TP14 on board (circled)
Meg.jpg

To measure the voltage at TP14. The positive probe from multimeter to place at TP14, but the 'ground' negative should refer to chassis ground or there is a circuit ground?
 

AlbertHall

Joined Jun 4, 2014
10,366
Perhaps the first thing to do, if you don't know that, is to measure the resistance between the two (with the power off).
If this reads as short circuit then it doesn't matter which you use for measuring the voltage.
If not short circuit, then the voltage should be measured to the circuit ground.
 

Thread Starter

Sp@ceR

Joined Mar 11, 2018
50
Albert,

Multimeter shows OL (Open Loop) on TP14 and chassis ground (connected to mains earth). I spotted the A-GND terminal (just above TP14). Resistance reading is 5.73kOhm between both. What's next?

Multimeter is Kyoritsu KEW1021R

When you say 'short circuit', do you mean by 0 Ohm?
 
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Thread Starter

Sp@ceR

Joined Mar 11, 2018
50
Albert,

I started to think A-GND means analog ground and should be different from the actual power supply ground?
 

Ylli

Joined Nov 13, 2015
978
Is this board out of the chassis? Often, there are multiple grounds (Signal/power/digital) and these ground get tied together when the board is mounted in the chassis.
 

Thread Starter

Sp@ceR

Joined Mar 11, 2018
50
If you know where the power supply ground is you can measure the resistance between there and A_GND.
Short circuit reading between TP14 and ground? There's nothing else on board that is labelled as GND other than A-GND.

Your flowchart also references a schematic. Therein lies all of your questions and gives the answers. Schematics are a luxury. Manna from heaven!!
Schematic once existed... I email-ed Meguro Corp and they could only supply me with the instruction manual. For whatever reason they told me all the schematics are no longer exist.

Is this board out of the chassis? Often, there are multiple grounds (Signal/power/digital) and these ground get tied together when the board is mounted in the chassis.
All the connectors(3 molex, 2 gold-plated pins) have been unplugged. Unscrew 4 screws at the corners and it's out.
K-0465.jpg
 

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Ylli

Joined Nov 13, 2015
978
All the connectors(3 molex, 2 gold-plated pins) have been unplugged. Unscrew 4 screws at the corners and it's out.
The board ground (including A-GND) may be on one of those connectors that are no longer plugged in. Plug them back in and then check resistance between A-GND and the chassis.

Can I ask why you are re-capping this unit, and if there are any particular problems you are trying to solve?
 

Thread Starter

Sp@ceR

Joined Mar 11, 2018
50
The board ground (including A-GND) may be on one of those connectors that are no longer plugged in. Plug them back in and then check resistance between A-GND and the chassis.

Can I ask why you are re-capping this unit, and if there are any particular problems you are trying to solve?
You mean there is a possibility that one of boards have a connection to chassis?

Ps: I see there's no physical wire connection from any boards to the chassis. Still, worth a try.

Regarding the recap:
No, there's nothing wrong with it. This kind of equipment are usually extensively used in service lab/ factory. The original caps are Elna RE(general purpose) 85C 2000hrs caps. Just a precaution I take in case one one going to short and take away a bunch of obsolete ICs. Some might disagree with me anyway.
 
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