Help with schematic voltages - Page 1
Forum section: electronics / repair

author: OilsFan
author_rank: Regular Contributor
timestamp: August 27, 2014, 07:07:40 am
content: I'm probing around a malfunctioning digital audio effects unit with my DMM checking voltages and I had a question for you old hands at the game. In the schematic below, you'll see D34 has +15 on one side and D33 shows -15 on the other. When I probe them they are exactly opposite of the schematic. Any ideas why that may be? I'm only basically familiar with diodes.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: August 27, 2014, 07:14:13 am
content: Ask your self what the potential difference would be at that point if the voltage was to swing greater than plus or minus 15V. To me it looks like over voltage protection.

author: mikerj
author_rank: Super Contributor
timestamp: August 27, 2014, 07:50:43 am
content: Quote from: OilsFan on August 27, 2014, 07:07:40 amIf the voltage really were backwards then both diodes would be heavily forward biased and a lot of current would be flowing, possibly enough to kill the diodes depending on the capabilities of the +15v supply and diode rating. Are you sure you aren't getting the polarity of the diodes confused? The striped end of D34 should be at +15v and the plain end of D33 should be at -15v.

author: mij59
author_rank: Frequent Contributor
timestamp: August 27, 2014, 07:55:57 am
content: Hi,Maybe the schematic is of a different version , or someone has messed up the numbers.Pin 8 of opamp 20b should read +15V, pin 7 should be at 0V.

author: OilsFan
author_rank: Regular Contributor
timestamp: August 30, 2014, 08:24:25 am
content: The striped side of D33 is +15, the non striped side is 136mV. The striped side of D34 is 136mV and the NON striped side is -15.Pin 8 of Opamp 20b is +15 and pin 7 is zero.

author: IanB
author_rank: Super Contributor
timestamp: August 30, 2014, 08:36:42 am
content: It looks like you need to ignore the diode labeling for the time being and identify the diodes physically. The diode shown as D34 on the schematic should in the physical circuit be connected between pin 7 of the op amp and the positive +15 V supply rail (the same destination as pin 8 of the op amp). Follow the circuit traces and locate the diode that is physically in that location. That is going to be D34, regardless of whether it is labeled "D33" or "D34" on the PCB.

author: jlmoon
author_rank: Unknown
timestamp: September 03, 2014, 03:40:50 pm
content: How about if in a state of confusion, pull one end of each diode (don't remove them, just lift one end of each.. )and verify that they are not leaky or shorted. A simple test would be to bias each of them with a 10k resistor a 0 to 10V variable power supply and a good dvm, you can even get better results if you measure the current flow through them as you raise the voltage. You should see them start conducting around .6 - .7 volts forward. Don't pay any attention to the board layout or schematic unless you physically draw it yourself from the actual printed circuit board, trace by trace.

author: Precipice
author_rank: Frequent Contributor
timestamp: September 03, 2014, 03:49:37 pm
content: It may just be because I can't see much of the schematic - but is that Q39/Q40 pair an npn/pnp buffer stage with the input shorted to the output? That seems odd. How sure are you of this schematic?

author: jlmoon
author_rank: Unknown
timestamp: September 03, 2014, 03:52:37 pm
content: Quote from: mij59 on August 27, 2014, 07:55:57 amI would think Pin 7 would be following the bias on input pins 5 & 6, probably a AC signal with some potential for dc bias components. Not enough above and below the current schematic to tell what should be happening.

author: jlmoon
author_rank: Unknown
timestamp: September 03, 2014, 03:55:44 pm
content: Just guessing resistors R144 & R150 (4.7k) are tied to their respective rail supply voltages in effect providing current paths for the Emitter / Base junctions of Q39, Q40.

author: mij59
author_rank: Frequent Contributor
timestamp: September 03, 2014, 04:22:23 pm
content: Quote from: jlmoon on September 03, 2014, 03:55:44 pmThe schematic is wrong , pin7 of opamp 20b can't be connected to node R145-R151.Seems that R153 should be between pin7 and the node R145-R151.

author: jlmoon
author_rank: Unknown
timestamp: September 03, 2014, 04:25:57 pm
content: Quote from: mij59 on September 03, 2014, 04:22:23 pmIf I had it in front of me, I would be drawing my own schematic just to be sure. I have had to do this so many times, it isn't even funny. Wonder what 20b IC really is?

author: mij59
author_rank: Frequent Contributor
timestamp: September 03, 2014, 04:33:40 pm
content: Quote from: jlmoon on September 03, 2014, 04:25:57 pmIts probably a buffer, Q39 and Q40 are used to boost the output current of the opamp 20b.

author: Precipice
author_rank: Frequent Contributor
timestamp: September 03, 2014, 04:38:09 pm
content: Quote from: mij59 on September 03, 2014, 04:33:40 pmIf it is, then its input is shorted to its output. That's not a conventional way to build a buffer...Edit: typo

author: mij59
author_rank: Frequent Contributor
timestamp: September 03, 2014, 04:43:26 pm
content: Quote from: Precipice on September 03, 2014, 04:38:09 pmPlease explane, don't see the short.

author: Precipice
author_rank: Frequent Contributor
timestamp: September 03, 2014, 04:45:21 pm
content: http://www.ecircuitcenter.com/Circuits/pushpull/pushpull.htmhas a standard npn/pnp buffer stage.OP's schematic has nodes 1 and 2 tied together, or nodes 11 and 12, if you look that the more sanely biased version, further down the page.Without being able to see more of OP's schematic, it's hard to tell what's going on. It's possible that it all makes sense - but I wouldn't bet on it.

author: mij59
author_rank: Frequent Contributor
timestamp: September 03, 2014, 04:51:55 pm
content: Quote from: Precipice on September 03, 2014, 04:45:21 pmYes you right, that why i think that R153 is drawn in the wrong place.

author: Precipice
author_rank: Frequent Contributor
timestamp: September 03, 2014, 04:55:01 pm
content: 100 Ohms would be very, very low for that. I suspect that (unless there's something interesting off-sheet), there should just be no connection at all.It's always possible that the buffer stage is optional, and you cut that track if it's fitted.Basically, insufficient information.

author: mij59
author_rank: Frequent Contributor
timestamp: September 03, 2014, 05:07:00 pm
content: Quote from: Precipice on September 03, 2014, 04:55:01 pm100 Ohm is possible, low currents can be driven by the opamp.Agree, need more input.

author: Precipice
author_rank: Frequent Contributor
timestamp: September 03, 2014, 05:12:31 pm
content: Quote from: mij59 on September 03, 2014, 05:07:00 pm--just thinking aloud--If the opamp output impedance isn't able to overwhelm this 100R (and it probably can't, or you wouldn't want to buffer it), then any (inevitable) buffer stage voltage offset will win, and drive the input. I'm not sure if the buffer stage would end up at one rail or the other, or whether it would sort itself out at some intermediate voltage - but I really, really can't think why you'd want to do this!

author: jlmoon
author_rank: Unknown
timestamp: September 03, 2014, 05:57:01 pm
content: Right to the West of the R145 / R151 junction is a line that goes to the top out of sight.. that could be a feedback - filters/no filters path back to the inverting input on 20b too.Would be a game changer

author: OilsFan
author_rank: Regular Contributor
timestamp: September 04, 2014, 10:01:11 pm
content: OP here interesting discussion, though I don't understand most of it.That was the only place I tested in the whole unit that didn't agree with the schematic so was just wondering if something was not working there. I think I'm going have to put my scope on it and inject a signal and see where it goes awry. If you are interested, the schematic is in this service document,http://www.synfo.nl/servicemanuals/Roland/DEP-5_SERVICE_NOTES.pdfit's a Roland digital reverb circa 1986 and there are a billion resistors, diodes, caps and transistors in it.

author: IanB
author_rank: Super Contributor
timestamp: September 04, 2014, 10:32:02 pm
content: The key thing is don't trust the printed schematic without also verifying it against the actual circuit. It is easy for errors to creep into documentation and not get noticed. What is shown as "D33" on the schematic is not guaranteed to be what is labeled as "D33" on the PCB.

author: mij59
author_rank: Frequent Contributor
timestamp: September 05, 2014, 04:47:51 am
content: Quote from: OilsFan on September 04, 2014, 10:01:11 pmThanks for sharing.Please describe the malfunctioning .Do you know how to operate the unit ?Please post photo's of the boards.You could start by checking the power supply voltages, it think the +15V and -15V are present, how about the +5V rail ?Check the backup battery.Take a look at the block diagram in the manual .

author: OilsFan
author_rank: Regular Contributor
timestamp: September 05, 2014, 06:01:13 am
content: Quote from: mij59 on September 05, 2014, 04:47:51 amI do know how to operate it as I've had about 5 of these units through the years. I have 3 of them now. Two are malfunctioning and one works. I bought this particular unit as NOS. It had been sitting on the shelf NIB since 1986. I turned it on and it ran for 15 minutes then there was a small pop and the audio cut out. I power cycled it and the direct audio came back but no effect sound can be heard.So far I have checked all voltages mentioned in the service manual. They are all correct at the test points. I additionally checked voltages a few places around the circuit and compared them to the schematic. They seem fine except for that mentioned in my OP. The backup battery voltage is fine. I measured a bunch of caps with an ESR meter and they all seem fine so far. Mostly the power supply caps but we know that is working. I haven't actually done a whole lot of trouble shooting yet as I don't have a lot of time. I have hopes that when I hook up my scope and function gen and look around the test points I will narrow down where the anomaly is.I've attached a pic if anybody is interested. These things were very expensive high end studio stuff back in the day. Built really well. I've only ever found one bad cap in any of them yet. I think they are Nippon Chemicon but not sure. Also FYI the gunk you see around the base of some caps is old glue from the factory.

author: mij59
author_rank: Frequent Contributor
timestamp: September 05, 2014, 06:34:05 am
content: Quote from: OilsFan on September 05, 2014, 06:01:13 amSince the unit is quiet old check for mechanical issues like bad solder joints, connectors, switches, etc.