Voltages and schematics - Page 1
Forum section: electronics / beginners

author: BrianHansell
author_rank: Newbie
timestamp: August 25, 2021, 10:21:22 am
content: Forgive me I'm a new at this. When you start fault finding you start by checking all the voltages from the Power Supply that they within spec. When you then start measuring around the circuit you will see the voltage written and check it against your recorded value of plus or minus.Now my question is, on Tektronics schematics some of the voltages are contained in oval boxes. Would I be right in assuming the these are "must be", as in what's written in the box voltages?I have been from cover to cover and there isn't any explanation. I assume they're saying, "You should know what you're doing before you mess around with our kit".regardsBrian

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2021, 01:44:43 pm
content: It would help if you posted or cited an example.

author: rsjsouza
author_rank: Super Contributor
timestamp: August 25, 2021, 02:30:18 pm
content: Also, tekwiki has several interesting maintenance tips for Tektronix equipment:https://w140.com/tekwiki/wiki/Main_Page#Repair_and_Maintenance_Issues

author: BrianHansell
author_rank: Newbie
timestamp: August 25, 2021, 03:00:03 pm
content: I'll try and make it clearer. From the attached schematic you can see that around Q747 there are three Oval boxes with voltages in, -5.3, -3.6 and -6.0 volts. The supply voltage is the generic -8.6 volts. Around transistor Q753 we have voltages of -0.4, 0 and -0.8. We have +4.9 in the voltage divider, and +5.6 in the path to Q770. These are all encased in Ovals and fed by the generic -8.6 and +8.6 volts.My question was that the generic supply voltages from the power supply can be plus or minus a percentage, but must the enclosed voltages be accurate.Hopefully thats made things clearer.Brian

author: TimFox
author_rank: Super Contributor
timestamp: August 25, 2021, 03:28:07 pm
content: Their convention indicates that the supply voltages (no oval) are reasonably accurate and constant, and that the oval-enclosed voltages result from the supply voltages and operation of the transistors and other components. These operating voltages are typical values to guide troubleshooting and will have broader tolerance than the supply voltages. The troubleshooting or adjustment procedures should give required tolerances on the supply voltages.

author: BrianHansell
author_rank: Newbie
timestamp: August 25, 2021, 06:22:14 pm
content: Thanks Tim, that has now steered me in the right direction. Once again many thanks.Brian

author: Benta
author_rank: Super Contributor
timestamp: August 25, 2021, 07:37:49 pm
content: My take on the ovalled voltages is, that they're all in the signal chain and should be measured with a high-impedance voltmeter. Matter of course today, but not back then.

author: rsjsouza
author_rank: Super Contributor
timestamp: August 25, 2021, 07:52:38 pm
content: Looking at the diagram, the voltages within the oval shapes are measured. The voltages outside simply indicate the net where the nodes are tied.I wonder if the power supply schematics sheet also has oval shapes to indicate the measured voltages in those nets.