Bad power to IC 161, one last question, i hope - Page 1
Forum section: electronics / repair

author: robsharp1984
author_rank: Newbie
timestamp: January 21, 2015, 01:49:09 am
content: Ive got an amplifier im trying to repair, I was able to get it into diagnostic mode and determine there was incorrect power to IC 161 circuit.I swapped transformers with another of the same model, no problem there.I tested voltage after coming out of rectifier, +20v and -20v, seems correct.I tested voltage coming off of transistors 164 and 165, +7 volts off 164, -200mV or something close coming off 165.(on the other amp which works fine, voltage is +7 and -7)So I assumed either the transistor was bad, or the IC 161 chip itself.I pulled the transistor off the board, it tests fine.So my assumption is that the IC161 chip itself is bad, I just wanted a second opinion that none of the other components involved could be causing the problem.The red marks are my voltage readingsThe green circles are the other components which I suppose maybe could cause this?The two circled yellow things, I wasn't sure what they were, capacitors? not labeled so not sure how i find on the board. (pictured larger in second picture)Please let me know if my conclusions make sense, and if i should pull one or more of these other components and test them first before spending the money to replace IC 161Thanks

author: lowimpedance
author_rank: Super Contributor
timestamp: January 21, 2015, 03:16:32 am
content: 200mA from Q165?, what was the voltage you measured here?. Not -7V then what was it?.The component symbols in yellow are electrolytic capacitors and are required after the rectifier bridge to reduce ripple into the regulators to provide a smoother DC and thus prevent dropout with a load current.Are the +/- 12V outputs from the regulators (in the green circles) good?. If yes and the output of Q165 is not -7V or so then does it get hot indicating too much current and possibly a bad IC161 etc.If isolating the output (if possible) of Q165 allows its output to come back to -7V then it could be okay and IC161 more suspect.BTW the other green circled symbols refer to the circuits common ground (which is particular common copper traces on the PCB for components to connect to the supply 0V,in this case on the power transformer between both AC2).

author: robsharp1984
author_rank: Newbie
timestamp: January 21, 2015, 06:15:28 am
content: I don't remember the exact measurement on it, it was below 500mv for sure, i think below 300mv, but i cant remember if it was positive or negative. I think I said mA in my post but i meant mV. might help huh?the line there is supposed to read -7V which it did on the working model.the thing is kind of taken apart now, but if you think it is essential to reassemble and get exact measurement there I could, I just figured if 7V is going into the IC as its supposed to and nothing is coming out, it had to be the IC, or a component after the IC in the circuit (165 or 236)I added the third picture as a redo of the first one with correct labels

author: poot36
author_rank: Frequent Contributor
timestamp: January 21, 2015, 07:29:07 am
content: I agree that voltage is out of wack. I would set your meter to ohms mode and connect the negative lead to that test point and the positive to chassis ground and see what the reading is. Also try reverseing the probes and test again. You can also try this test on the diode mode on your meter and if it reads 0.5 V or less you most likely have a shorted ic or other semiconductor based device on the board. If your meter has a high digit count ohms mode (ie it can read very low resistance) you can use this to help you find the shorted component without unsoldering stuff.

author: macboy
author_rank: Super Contributor
timestamp: January 21, 2015, 02:25:22 pm
content: You need to measure the -12 V supply. This is essential. Otherwise you can't tell if the fault is after the 12 V regulator, or the 7 V regulator.The 7 V regulator is probably made with a zener and a transistor as a voltage follower. The zener or the resistor biasing it could be bad. You said you tested the transistor. It is also possible that there is a bad decoupling capacitor somewhere on that -7 V rail that is shorted out, dragging the voltage down (old tantalum caps are notorious for this). Of course there is the possibility that a device powered by the rail is bad (i.e. IC161).

author: robsharp1984
author_rank: Newbie
timestamp: January 21, 2015, 09:11:21 pm
content: ok, i reassembled it and tested the voltage after IC 237, it should be -12, I'm getting 1.1v.I guess both caps must be fine, otherwise it would be changing the voltage after coming off the rectifier right?so based on this I think we can be sure that the problem is IC 237 not putting out -12v right?I just wanted to make sure whether or not it would be possible that IC 161 could be malfunctioning and cause these readings.I have updated the picture

author: 22swg
author_rank: Frequent Contributor
timestamp: January 21, 2015, 09:53:26 pm
content: 7912 the IC 237 is a common component (neg 12v reg) should be inexpensive to replace regardless , although its output -12 v may be being made over current and reg will shutdown.

author: lowimpedance
author_rank: Super Contributor
timestamp: January 21, 2015, 11:21:54 pm
content: You have measured the -20V at the input terminal of IC237 ?. With no -12V on the output pin (+1.1V!) does isolating this pin from the circuit restore -12V at the pin itself?.As suggested though IC237 is a cheap common part anyway. If replacing make sure you observe the same mounting techniques, if any, for the TAB.One other thing to note is excessive ripple causing dropout due to a dried out electrolytic cap on the input of IC237 (after the rectifier!).Hard to tell measuring the -20V DC rail with a multimeter (try comparing each 20V rail using AC V on the multimeter), best to use a scope though!.

author: robsharp1984
author_rank: Newbie
timestamp: January 22, 2015, 01:52:41 am
content: ok thanks for the help.ill probably go ahead and pull 237, test it, if it tests good probably 161 is bad, will replace it either way just to be sure before time/cost on 161.

author: robsharp1984
author_rank: Newbie
timestamp: January 23, 2015, 12:16:34 am
content: OK, have an update.so i pulled IC 236 and replaced it, stupid me I forgot on the T-220 chips that when the back tab is metal it is sometimes (or always?) an additional input/output.when i powered the device it shorted out the chip with a nice little puff of smoke since the back was mounted to a grounded heat sink.readings now are about the same as they were before with the old chip, have not yet tested old chip or this one without being connected to the circuit.but something is bugging me on the readings, how is it possible that coming off the rectifier I get -20v, but then on the input of 236 I am getting no voltage, right now the input is still grounded to the heatsink at the chip, but again, readings are about the same as they were with the old chip when it was not grounded at that point. the diagram shows nothing between the rectifier and 236, and if that cap nearby was grounding the circuit out, I wouldn't think I would get a reading of -20v off the - pin of the rectifier.picture has been updated with a green circle around the readings,once again thx for the help.sidenote: why do they make the the metal back of those chips an additional input? are there applications of chips like this which actually use the heatsink to conduct current instead of a trace?

author: 22swg
author_rank: Frequent Contributor
timestamp: January 23, 2015, 10:42:53 am
content: Did u mean 237 ? replaced ? and after the smoke as well ? zero and -20v at ends of one wire cannot be... is there a zero ohm resistor / fuse somewhere ? re your T-220 welding . a picture of board may help

author: robsharp1984
author_rank: Newbie
timestamp: January 23, 2015, 05:46:29 pm
content: yes sorry, 237, that was what i was 99% sure of with the readings, not possible on same line, unless diagrams dont include every single tiny component? like tiny surface mount components?

author: poot36
author_rank: Frequent Contributor
timestamp: January 31, 2015, 11:19:52 pm
content: Every proper schematic that I have seen includes all of the components on the board so they should be there somewhere.