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I'm having trouble locating the test points on a circuit board. I see them on the schematic, but can't for the life of me find them on the actual board. My question is; Can a TP in the circuit not always be a pad/terminal and instead be a lead from a component or something else entirely?
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My gut says yes, but, what are you working on, and can you put up the schematic and test point layout, etc? You'll get more info if you give more info...
Justin"Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
"Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
"All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister - -
Yes. A Test Point is just a designated node in the circuit where the designer has chosen to specify a certain voltage or waveform that should exist under certain operating conditions. In fact, many other points in the circuit can be considered TPs as you troubleshoot such as following a signal through the circuit to isolate a problem. These are not even marked on the schematic. I find that TPs which have a dedicated terminal are more commonly found in circuits that are expected to undergo regular calibration. In that case they can be a great a convenience for the technician. For common consumer equipment you are often on your own to find the circuit node that is shown as a TP on the schematic.Originally posted by HNKTNK View PostComment
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Thanks for the reply. Page 3 of 4 is what I'm looking at here. I have an Ampeg b5r, which apparently LOUD doesn't have a schematic for and I wasn't able to find it anywhere online. I was told that the schematic for a b2r would work, so that's what I'm using. The amp I have has a bad relay, I believe. I was wanting to remove it and then jump tp11 and tp12 to see if I get any output out of this thing (preamp works fine, but no output).Originally posted by Justin Thomas View PostComment
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Test points are places in the schematic. Only rarely is there an actual "test point" on the circuit board. The schematic test points are what they appear to be: the end of a resistor, the emitter of a transistor, the cathode of a zener, etc etc.
Once in a while for setting bias, you might find a power amp with two little posts sticking up to measure a voltage - rather than getting right on an emitter resistor.Education is what you're left with after you have forgotten what you have learned.Comment
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Thanks for the replies Tom and Enzo. So on page 3 of the schematic i posted, would tp11 be on the r52 and tp12 be on the output jack?Comment
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Not the way to do it.I was wanting to remove it and then jump tp11 and tp12 to see if I get any output out of this thing
First, listen to the relay or put your finger on it. Does it click on after a few seconds at power sup? If not, there is the basic issue. But there will be a reason the relay does not click on, you want to find that out before cross connecting the output. Is ther DC voltage sitting at TP11? Is there signal at TP11?
If the relay does click on, then look for signal at TP11. Also do the basics, plug a guitar or other signal into the power amp in jack, get sound? Likewise plug a cord from preamp out to power amp in and plug guitar into the regular input jack. Sound?
If the relay does not click on, it COULD be a bad relay, but far more common for the relay to not be turning on electrically, the relay doing its job in other words.Education is what you're left with after you have forgotten what you have learned.Comment
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TP11 would be that resistor or any of R47,48,50,37,39,41.Education is what you're left with after you have forgotten what you have learned.Comment
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The relay does not click on, which is what made me suspect that it was faulty. The problem with testing at TP11 is that I don't exactly understand where that is. My guess would be that it's on r52, but I'm not trying to prod around in an amp based off of guesses. And yes, I tried all of the aforementioned procedures that I could before disassembling the amp.Originally posted by Enzo View PostComment
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Thanks for the insight, Enzo. I've been a long time lurker and gotten a lot of valuable information from you over the years.Comment
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No relay click:
Then we need to isolate the problem to either the relay itself, the power supply to the relay, or the circuit that controls the relay.
You want to find if the relay coil has power. The terminals are under it and hard to get to, but diode D19 is parallel the relay coil, so measure there. DO you see about 60v on the cathode? Note TP18 is the line controlling the relay. Is it OK? All those little transistors along the bottom of the page are the relay control, I see also TP17. All that has to work.Education is what you're left with after you have forgotten what you have learned.Comment
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I'll check when I can and then get back to you. Thanks for the help!Originally posted by Enzo View PostComment
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If you have a blown output section there will be abnormal amounts of dc voltage on TP11.
THAT will keep the relay in an off state.Comment
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Update
First off, I'm an idiot. I'm not sure how or why I ended up with the b2r schematic, because the schematic I needed was for the b4r (b4r and b5r share the same power amp schematic). This alleviated all of my confusion, because well, now everything is matching up from the schematic to the circuit itself. I was able to remove the cover on the relay, which enabled me to get a reading on the terminal. 15vdc like it should be. I checked all the test points in that circuit and got readings that pretty much aligned with what they're supposed to be. So does this pretty much point to the relay being bad?Comment
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How did you measure that 15Vdc?
That circuit is a little different than the B2R.
This one employs +15Vdc on the coil at all times.
When it is engaged, Q120 will provide -15Vdc to the coil.
Thus the -11.5 Vdc of TP12.
You really should check that TP11 does NOT have gross amounts of volts dc on it.Comment
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