Sony C800G PSU Calibration help (AC-MC 800G)
« on: November 21, 2018, 04:23:52 PM »
Hey guys, I'm having an issue with adjusting my C800G's power supply within spec after changing the tube.  According to the service manual on page 9 (attached) the value across resistor R111 is supposed to read at 1.150 V dc after about 40 mins .  And if it is above that you are supposed to use the trim pot on RV101 to reduce the value until it is at 1.150 volts. 

The problem is that with the trim pot wide open I am getting a reading around 4.3v, and after adjusting the trim to the lowest setting the lowest value I can get across R111 is 2.3 v.  This is twice the value that is within spec. 

Does anyone have any idea what might be going on, or how I might go about fixing this issue?  Is there something wrong with the peltier element?



Also, how would an issue like this affect the sound of the mic? Since apparently I've been running it like this for a while.

Thanks.
« Last Edit: November 21, 2018, 04:42:25 PM by tesco_1 »


Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #1 on: November 21, 2018, 04:41:59 PM »
Here is the Service manual for the C800G (for the microphone specifically)

My first post above has the Service manual for the PSU (AC-MC 800G) attached

Khron

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #2 on: November 21, 2018, 05:08:39 PM »
Are you measuring voltages with the microphone (or a dummy load) connected to the power supply? Or are those measurements with the power supply unloaded?
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Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #3 on: November 21, 2018, 05:23:46 PM »
Are you measuring voltages with the microphone (or a dummy load) connected to the power supply? Or are those measurements with the power supply unloaded?

I followed the procedure outlined in the PSU service manual using the microphone connected instead of a dummy load.  This is detailed on the right hand side of page 10:

In case of using C-800G
Caution: The following steps 1 and 2 should be executed
with power switch OFF.

1. Connect C-800G to MIC IN connector with Microphone
cable.

2. Connect digital voltmeter across R111/PS-295 board.

3. Turn the power switch ON.
Adjust after 40 minutes from turning the power switch
on.

4. Adjustment

Specification: dc voltage across R111 = 1.150 V dc
Adjusting point: 1RV101/PS-295 board

5. Check the specification on the item 4 is satisfied 10
minutes later.
When it is not satisfied, adjust again.

6. Confirm that the voltage between pin-3 and pin-4 of
connector CN108/PS-295 board, is in the range of
+4.0 V thorn +5.0 V dc.

7. Turn the power switch OFF.

Khron

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #4 on: November 21, 2018, 05:28:24 PM »
As far as i can figure, that adjustable part of the power supply only deals with the Peltier cooling element.

If you're measuring too high a voltage across R111 (which in this case, is used as a current shunt), that means the Peltier element is drawing about double the current it normally should. That is, barring some shorted wires / connections somewhere.

What voltage are you seeing between the yellow and blue wires of CN108? The schematic says you should be seeing 4.8v. That, plus the drop across R111 should add up to the 6v indicated "before" R111 (ie. between D108's cathode and ground).
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Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #5 on: November 21, 2018, 06:04:42 PM »
As far as i can figure, that adjustable part of the power supply only deals with the Peltier cooling element.

If you're measuring too high a voltage across R111 (which in this case, is used as a current shunt), that means the Peltier element is drawing about double the current it normally should. That is, barring some shorted wires / connections somewhere.

What voltage are you seeing between the yellow and blue wires of CN108? The schematic says you should be seeing 4.8v. That, plus the drop across R111 should add up to the 6v indicated "before" R111 (ie. between D108's cathode and ground).

I'm getting 4.1 v across pins 3 and 4 of CN108.    Is that value within the acceptable range?  I'm not an electronic expert, but I am a tinkerer and can follow along pretty well. 
« Last Edit: November 21, 2018, 06:09:42 PM by tesco_1 »

Khron

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #6 on: November 21, 2018, 07:04:27 PM »
The schematic on page 14 says you should be seeing 4.8v between those two pins.

Note that that's NOT the same measurement (ie. across R111) as the one you claim(?) you couldn't get low enough.

That measurement, between the two ends of R111 (not between either end and ground), is an indication of the current drawn by the Peltier element.

I'm getting 4.1 v across pins 3 and 4 of CN108.    Is that value within the acceptable range?  I'm not an electronic expert, but I am a tinkerer and can follow along pretty well. 
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Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #7 on: November 21, 2018, 07:42:41 PM »
The schematic on page 14 says you should be seeing 4.8v between those two pins.

Note that that's NOT the same measurement (ie. across R111) as the one you claim(?) you couldn't get low enough.

That measurement, between the two ends of R111 (not between either end and ground), is an indication of the current drawn by the Peltier element.

Correct.  The pins on CN108 are reading 4.1 volts, and R111 is reading 2.3 volts with RV101 turned all the way down.

Khron

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #8 on: November 21, 2018, 08:09:47 PM »
In that case, assuming the Peltier element is still ok, are you sure there are no shorted wires anywhere? Or perhaps pinched somewhere in the casing when reassembling things?

If R111 is indeed (still) at its nominal 1ohm value, 2.3v across it would mean there's 2.3A flowing through it, so double what it should be.

Although that still doesn't make much sense. If you're seeing 4.1v on CN108, and 2.3v are dropped across R111 (which will then be dissipating 2.3W - that's gonna get toasty!), that would mean the voltage "upstream" of R111 (ie. from the cathode of D108 to ground) is 6.4v minimum.

Assuming all the passive components and connections are in spec and solid respectively, at the minimum travel of RV101 you should get about 4.9v, while at the maximum, about 7.4v.

Assuming the LM317 regulator is still alive (it certainly won't be too happy with a 2.3A load), you'll want to double-check R109, R112 and RV101.

PS: In order to eliminate the Peltier variable, i don't suppose you can get your hands on the three resistors you'd need to make that dummy-load, can you? Or even "just" the 3.9ohm / 8W (or higher) that's the test-load for the Peltier supply.
« Last Edit: November 21, 2018, 08:12:55 PM by Khron »
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Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #9 on: November 22, 2018, 12:14:30 AM »
In that case, assuming the Peltier element is still ok, are you sure there are no shorted wires anywhere? Or perhaps pinched somewhere in the casing when reassembling things?

If R111 is indeed (still) at its nominal 1ohm value, 2.3v across it would mean there's 2.3A flowing through it, so double what it should be.

Although that still doesn't make much sense. If you're seeing 4.1v on CN108, and 2.3v are dropped across R111 (which will then be dissipating 2.3W - that's gonna get toasty!), that would mean the voltage "upstream" of R111 (ie. from the cathode of D108 to ground) is 6.4v minimum.

Assuming all the passive components and connections are in spec and solid respectively, at the minimum travel of RV101 you should get about 4.9v, while at the maximum, about 7.4v.

Assuming the LM317 regulator is still alive (it certainly won't be too happy with a 2.3A load), you'll want to double-check R109, R112 and RV101.

PS: In order to eliminate the Peltier variable, i don't suppose you can get your hands on the three resistors you'd need to make that dummy-load, can you? Or even "just" the 3.9ohm / 8W (or higher) that's the test-load for the Peltier supply.

Ok, so I went to the store got all the resistors and rigged up a dummy load.  With the dummy load plugged in to the PSU, I now get a reading of 2.15 volts across R111.   Again this is with RV101 at its lowest setting.   Any thoughts?


Khron

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #10 on: November 22, 2018, 05:20:59 AM »
What voltage are you getting between the yellow and blue wires of CN109 (ie. cathode / band-end of D108, and ground)?

And have you double-checked / measured the values of the passive components i indicated?

Something's really weird there, because what you say you've measured totally contradicts Ohm's law.

If you have that 3.9 ohm dummy load replacing the Peltier element, and you're still seeing 2.15A through it (a 2.15v drop across that 1 ohm R111), that would mean the voltage across that resistor should be 3.9 x 2.15 = 8.385 volts. But you say you're measuring only 4.1v.

Are you 1000000% sure that R111 is indeed 1 ohm in value? If it was more like 2 ohm (or the more standard 2.2 ohm), THEN that voltage drop reading across it would make a lot more sense.

It's supposed to be a 5W resistor, so pretty "fat", and as such, the markings on it should be quite clear.
« Last Edit: November 22, 2018, 05:28:30 AM by Khron »
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Gus

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #11 on: November 22, 2018, 08:34:27 AM »
Look for "slow turn on regulator" at page 17 of the link http://www.ti.com/lit/ds/symlink/lm117.pdf

It looks like the circuit is a slow turn on with and output DC voltage range of about 2.45VDC to 6.13VDC  measured at the R109,R111, D108 node to the - of C107

Do you  know if your DMM is good and not damaged? Have you checked a AA and 9VDC battery to see if the voltages are reasonable. I am posting this because DMMs can give wrong readings if damaged.

If you disconnect the 3.9ohm at pins 5 and 6 of the dummy load and keep the other resistor loads you should be able to check the DC out voltage range of the LM317 based regulator circuit.


I would turn power off and unplug the unit let the caps discharge.

Check the connector CN109

I would check the resistor values (R112 RV101)R2 in app note R109 R1 in app note and the other parts in the circuit with a meter

I have read if the pelter is run without a heatsink affixed to the hot side even for a very short time  you can ruin it.

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #12 on: November 22, 2018, 02:08:31 PM »
What voltage are you getting between the yellow and blue wires of CN109 (ie. cathode / band-end of D108, and ground)?

And have you double-checked / measured the values of the passive components i indicated?

Something's really weird there, because what you say you've measured totally contradicts Ohm's law.

If you have that 3.9 ohm dummy load replacing the Peltier element, and you're still seeing 2.15A through it (a 2.15v drop across that 1 ohm R111), that would mean the voltage across that resistor should be 3.9 x 2.15 = 8.385 volts. But you say you're measuring only 4.1v.

Are you 1000000% sure that R111 is indeed 1 ohm in value? If it was more like 2 ohm (or the more standard 2.2 ohm), THEN that voltage drop reading across it would make a lot more sense.

It's supposed to be a 5W resistor, so pretty "fat", and as such, the markings on it should be quite clear.


Ok Gentlemen... so I made a stupid mistake.  Was measuring the wrong resistor, R110 instead of R111.  Now the inconsistencies might make more sense.  Will report back with correct measurements.  Thanks again for taking the time to help me out.

Khron

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #13 on: November 22, 2018, 02:12:26 PM »
Well, there's "RTFM", and then there's this... ;D

I had pointed out that R111 should be quite chubby, a few posts ago...   ::)


Ok Gentlemen... so I made a stupid mistake.  Was measuring the wrong resistor, R110 instead of R111.  Now the inconsistencies might make more sense.  Will report back with correct measurements.  Thanks again for taking the time to help me out.
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Gus

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #14 on: November 22, 2018, 02:41:06 PM »
A little of topic

I would have used a constant current supply for the pelter and  a constant current supply for the heater.
I do like R104 and R103

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #15 on: November 22, 2018, 08:14:28 PM »
A little of topic

I would have used a constant current supply for the pelter and  a constant current supply for the heater.
I do like R104 and R103

Well, there's "RTFM", and then there's this... ;D

I had pointed out that R111 should be quite chubby, a few posts ago...   ::)


Ok, so with the dummy load I was able to get R111 to read 1.150 v.  When using the microphone for calibration I was able to get a reading hovering around 1.13 - 1.14 v.  This is with RV101 all the way open.    I figure a few ohms isn't a big deal, but to be on the safe side I have to ask what you guys think.  Is 1.14 volts an acceptable tolerance in this kind of circuit?

I've seen quite a few of your post Gus, you seem to be very familiar with the C800G's design.

But again serious thanks to both you and Khron for helping me sort this out.

Khron

Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #16 on: November 23, 2018, 04:49:58 AM »
You're measuring volts (and consequently amperes) there, not ohms ;)

1.14 is 99.13% of the nominal 1.15v, so within 1%. Close enough for jazz ;D
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Re: Sony C800G PSU Calibration help (AC-MC 800G)
« Reply #17 on: November 26, 2018, 05:05:26 PM »
You're measuring volts (and consequently amperes) there, not ohms ;)

1.14 is 99.13% of the nominal 1.15v, so within 1%. Close enough for jazz ;D

Thanks guys for the help!

-Cheers


 

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