unknown symbol on schematic - Page 1
Forum section: electronics / beginners

author: sQNdagsGarN
author_rank: Newbie
timestamp: August 14, 2020, 09:24:13 am
content: Hi,Im quite new in the electronics world and was given an old Philips PM3200 scope to mess around with.So studying the schematic i came across a symbol that i dont know what means and cant seem to find any explanation in the service manual.can any of you inlighten me?

author: firehopper
author_rank: Frequent Contributor
timestamp: August 14, 2020, 01:52:22 pm
content: 2K ohm trim pot maybe?

author: TimFox
author_rank: Super Contributor
timestamp: August 14, 2020, 03:03:33 pm
content: I think he is referring to the five locations, away from the wires, to which he points with blue arrows. Maybe some indication of test points referred to in the accompanying instructions? There seem to be waveforms or voltages indicated for most of these points.

author: jonovid
author_rank: Super Contributor
timestamp: August 14, 2020, 03:11:24 pm
content: ferrite beads or a 5 or 10pf thru-hole if your running RF on point to point wiring on a metal case? through hole as in "a hole in the metal case that a wire go's through as an insulator sleeve acting as a 5 or 10pf capacitor." very often seen in vacuum tube circuits in old TV sets,

author: Benta
author_rank: Super Contributor
timestamp: August 14, 2020, 04:22:52 pm
content: Haven't seen it before, but my guess is that it tells you to use a high-impedance voltmeter. It's probably from the days of analog voltmeters that were pretty low resistance.It tells you to use a FET-voltmeter instead (thus the gate-like symbol).With modern DVMs it's no longer an issue, they normally have 10 mohms input resistance.

author: TimFox
author_rank: Super Contributor
timestamp: August 14, 2020, 04:29:43 pm
content: The waveforms do not seem to use the same symbol; the strange symbol seems to be associated only with voltage values. The strange symbol includes what looks like a probe tip.One advantage of an old-fashioned VTVM is that 1 megohm of the 11 megohm input resistance is in the probe, close to the tip. This reduces the effect of the probe cable and meter on the AC character of the node being measured; the worst case is that the probing capacitance could induce oscillation at a high-Z node.You can get the same effect with a 1 megohm inside the probe with a modern DMM, but you need to calculate the change in voltage due to the resistor, which was included in the calibration of the VTVM.

author: Mp3
author_rank: Frequent Contributor
timestamp: August 15, 2020, 03:14:15 pm
content: I have seen similar marking on a pcb to indicate test point locations. Note i chose my words carefully, i literally only saw it once ever, so don't necessarily take my statement as fact.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 15, 2020, 07:18:26 pm
content: It looks like a solder bridge where you can open the circuit without cutting the pcb trace:

author: Benta
author_rank: Super Contributor
timestamp: August 15, 2020, 07:43:06 pm
content: Quote from: PA0PBZ on August 15, 2020, 07:18:26 pmHighly doubtful. The fact that it's in red (like the 'scope traces) and always has a voltage associated with it makes me pretty certain that it's a test point for servicing.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 15, 2020, 07:55:42 pm
content: Quote from: Benta on August 15, 2020, 07:43:06 pmThat was my first thought too, maybe a test point that should be AC coupled because the picture resembles a capacitor, but look at the left part of what I posted above, you can clearly see the trace with the -||- which will prevent the thing from working unless bridged with solder. It would be nice to see a picture of the real pcb. Also, there is no associated voltage for the example I posted.

author: Benta
author_rank: Super Contributor
timestamp: August 15, 2020, 08:17:18 pm
content: Quote from: PA0PBZ on August 15, 2020, 07:55:42 pmInteresting. I didn't realize at first that the PCB picture you posted is in fact the relevant one. I'll go with your hypothesis.BTW, the point does have a voltage associated with it, but it's not well placed: 0 V.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 15, 2020, 08:25:38 pm
content: Ah yes, I didn't associate the 0V with that particular symbol, all the other values have arrows to show where the were taken. There is however a pcb 'blob' on the lower left of the capacitor symbol that could be a test point.

author: jonovid
author_rank: Super Contributor
timestamp: August 16, 2020, 12:37:49 pm
content: show the ground wire or ground bus lines in the circuit, what is the wire to cases or ground symbol used here?

author: sQNdagsGarN
author_rank: Newbie
timestamp: August 17, 2020, 05:56:02 am
content: Thx for all the ideas. unfortunately i havnt had time to look at PCB.I have attached the full cuircut and component schematic. I dont think the marks have any relation to voltages.

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 17, 2020, 03:27:14 pm
content: Quote from: sQNdagsGarN on August 17, 2020, 05:56:02 amthey ARE definitely manufacturer-indicated values for the "static point of polarisation", literally the DC value read with multimeter.I'm old enough to confirm this.Search no moreregards,pierre

author: whirligig
author_rank: Contributor
timestamp: October 02, 2020, 10:07:03 pm
content: After scouring the internet for repair info on my PM3200, I found your post. Indeed, the symbol shows an open solder pad underneath the solder. See my attached photos to verify.Now that I know what they are, maybe I won't have to unsolder this row of transistors to test. Regardless, I will be submitting a new post seeking guidance and suggestions in fixing my PM3200.