I am troubleshooting a system 1, as mentioned in the title. The generator stopped making sine waves. The overload warning comes on when you try to switch the generator on. So far, I have traced the problem to the oscillator section, which is quite a tricky circuit. Anyone have any experience with this beast? I have more details, I'll have to get the notes and post them later.
Dave,
Check for physically leaking Caps - especially any 5mm and 6.3mm Dia parts which seem more prone to failure.
Check for physically leaking Caps - especially any 5mm and 6.3mm Dia parts which seem more prone to failure.
Good idea, I had to replace a bunch of those on another board a few years ago. Didn't notice any and not many around the oscillator, but I wasn't specifically looking for them either.
Hi Dave
There are no Electrolytics within the oscillator circuit. The MDAC (switched resistors) and 4 fixed caps for the 4 ranges. You can download the service manual from the AP web (56MB pdf). The MDAC are not in the schematics only a block network.
I service the systems as AP no longer service several of the older products.
Duke🙂
There are no Electrolytics within the oscillator circuit. The MDAC (switched resistors) and 4 fixed caps for the 4 ranges. You can download the service manual from the AP web (56MB pdf). The MDAC are not in the schematics only a block network.
I service the systems as AP no longer service several of the older products.
Duke🙂
That's what I was thinking. I have the service manual, bought it years ago, when they still supported them. I began isolating stages to try to isolate which one is making it not work. Form my experience working with op amps, the DC offset should not be much, but I am finding 5 or so volts on the outputs, and no AC. Since they are wired in a loop, I cut a trace and pulled one amp, and found that at least part of the circuit will pass a signal, with a more normal looking offset of .5V or less. I don't have my notes here, on which stage, etc, I will be able to add that Monday. I noticed they list the MDAC modules as not repairable. Due to matched resistors, or calibrated ICs?
Ok. I cut the trace at pin1 of A161. I removed op amp [and installed a high quality socket] U171. I fed a sine wave into pin 6 of the socket, which makes it all the way back around the oscillator loop to pin 6 of U252 - but there is -4.5V of offset there also. The other stages are under .5V offset. The signal I injected was same frequency as the generator settings on the control panel.
If I inject a .36V sinewave at pin 1 of the A161 MDAC, with U171 installed, it has 4V offset, .34Vac. U271 has -.22VDC and 0AC. U361 has 3VDC and .24VAC. U252 has -5.7VDC and .13Vac.
Ddave,
I'll try to scan the output stage for you if it helps?
BTW. I just sent you a PM
I'll try to scan the output stage for you if it helps?
BTW. I just sent you a PM
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Thank you. I may be confusing something? I do have a full copy of the service manual and schematics. The problem is in the oscillator section, [sheet 2 of 8 of the generator schematic] no audio signals of any kind [just dc, which triggers the overload warning] are getting to the output stage [sheets 5,6,7]. To enable testing, I pulled U331 from it's socket, which is the output stage driver. this stops the overload warning.
Oh, I forgot to mention - when I drive a sine wave into pin 6 of U171, I can get a reading with GENMON setting on the panel.
Ok. I made some measurements on a working unit. Very little DC offset in the oscillator stages. In the one I am attempting to troubleshoot, 4 & 5 & 7 volts [wired back up as original]. Point being, the DC isn't supposed to be there, so off to hunt down where it's coming from.
Hi Dave
I can help you with the Generator for the S1, Some of the files are to large so I will need to send direct.
Here is the basic GENERATOR BLOCK. You need to download the System One Service manual form ap.com 56mb
The MDAC is a digital controlled 13 Bit resistor, Most failures in this are AD7523 /MX7523
I built an MDAC tester 10+years ago to debug them.
If you read the "LinkedIn, Classic AP Audio Systems LA" I showed a FREQ RANGE problem. It had DC offset in a single range. A silver mica had a broken lead seal and was leaking 120k ohms witch caused the generator to not work.
Duke🙂
Email me duke.aguiar@ieee.org
I can help you with the Generator for the S1, Some of the files are to large so I will need to send direct.
Here is the basic GENERATOR BLOCK. You need to download the System One Service manual form ap.com 56mb
The MDAC is a digital controlled 13 Bit resistor, Most failures in this are AD7523 /MX7523
I built an MDAC tester 10+years ago to debug them.
If you read the "LinkedIn, Classic AP Audio Systems LA" I showed a FREQ RANGE problem. It had DC offset in a single range. A silver mica had a broken lead seal and was leaking 120k ohms witch caused the generator to not work.
Duke🙂
Email me duke.aguiar@ieee.org
Attachments
I have installed sockets for A161 & A162. I put a 1.00K resistor from pin 1 to 2 on each. All other ICs are in place. I get -6.4VDC at U252, +5.4VDC at U171, -.25VDC at U271 and +4.2VDC at U361. Measured on the output pins. No AC present. So the offset problem isn't coming from the MDAC modules.
Hi Dave
After the talk today some of the questions have been answered. When you are measuring voltages tell me the pin#. U252 is a inverting stage and the 6k R3504 pin #2-6 & the 5k R2501 should be close to Zero. You need to measure the other voltages. Some current is coming from some source like U251 or? that is causing the problem.
Measure all of the PS volts and IC pins on the schematic and then scan the schematic and send it to me.
Duke
After the talk today some of the questions have been answered. When you are measuring voltages tell me the pin#. U252 is a inverting stage and the 6k R3504 pin #2-6 & the 5k R2501 should be close to Zero. You need to measure the other voltages. Some current is coming from some source like U251 or? that is causing the problem.
Measure all of the PS volts and IC pins on the schematic and then scan the schematic and send it to me.
Duke
Ok, here is the list: -6.4VDC at U252 pin 6 +5.4VDC at U171 pin 6 -.25VDC at U271 pin 6 +4.2VDC at U361 pin 6 Today additional readings: U252 pin 2 is -.007V U252 pin 6 is -6.6V R2501 -.27V on pin away from U252 pin 2, which is at -.007V -- the .27V end is wired to U271 pin 6. U251 pin 16 is at -.016V I am going to start writing the readings on a schematic to scan now.
I had to compress this a lot to get it to upload. Some voltages. Power rails are +14.4 & -14.15.
Taken with 1.00K resistors in place of A161 and A162. Panel set to 1KHz at 0dB. No AC being generated.
Taken with 1.00K resistors in place of A161 and A162. Panel set to 1KHz at 0dB. No AC being generated.
I found that 7V is coming into R3611 from BPF In from sheet 2. U252 pin 6 has 4.2V just as I would expect looking at the R values. So I have to go after that 7 V. It is only connected to J23 pin 8.
Hi Dave
OOPS I always remove any IMD & or BUR module before troubling any GEN OSC problem. I hope that you can fix the BUR or call me
Duke🙂
OOPS I always remove any IMD & or BUR module before troubling any GEN OSC problem. I hope that you can fix the BUR or call me
Duke🙂
OK. Found the -15V rail was dead on the Burst Gen board. The trace had gotten eaten away under the header for P251 until it went completely open.
I had to replace the row of electrolytic capacitors on that PCB a long while ago, as they had leaked and corroded some traces. I fixed that, and all was well. For a good long time. There must have been some electrolyte left under the plastic part.
So, first thing to do if you have a dead oscillator, is pull the Burst gen if you have that option.
Thank you for the help,
Dave
I had to replace the row of electrolytic capacitors on that PCB a long while ago, as they had leaked and corroded some traces. I fixed that, and all was well. For a good long time. There must have been some electrolyte left under the plastic part.
So, first thing to do if you have a dead oscillator, is pull the Burst gen if you have that option.
Thank you for the help,
Dave
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