Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => MonAmour => Topic started by: jikolptijah on February 20, 2021, 01:19:56 PM
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Hello, Thanks in advance for the assistance. Is this close enough? If not, any thoughts on where I should begin looking?
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You can fine tune the meter needle with the black flat head screw on the front of the meter.
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Thanks for the reply. I haven't been able to get back to these for several weeks now.
I'm able to adjust one meter so that the pointer is at the 6 of 60mA and stable. The other meter goes up to 56 or 58mA but when left on for several hours starts to drift down and settles around 40mA. I saw a previous thread where someone described fluctuating meter readings and checked over all connections that might be contributing recommended in that thread.
As I've been going over everything a third and fourth time, I found that when performing the steps described in "Final Voltage Test and Setting Driver Bias" on the amp with the lower mA readings my voltage reading slowly drifts down (~0.1V every 10-20 sec) when I test the voltage of the MJE5731A at Q2 on the B side after adjusting to 175VDC. I don't know if this is related.
Despite the above, both amps sound amazing.
I would greatly appreciate any advice in troubleshooting this issue.
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The 175V is on the 5670 plate and shouldn't influence the meter. If the meter drifts down and you give it a hard tap with one of your knuckles, does it pop back up? The DC voltage at pins 1 and 4 on the 4 pin socket is responsible for what the meter reads.
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Have you swapped the 2A3s to see if the drifting reading follows one?
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No change with tapping the meter or swapping 2A3s. What is the best way to check DC voltage at pins 1 and 4? Please forgive my ignorance.
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You had to do this when you built the amp. You can put the black meter probe in the black speaker post then set the meter to DC voltage and measure pins 1 and 4 on the 4 pin socket.
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The voltages on both amps are as follows:
Amp with meter that reads close to 60mA
pin1 59.17V
pin4 56.63V
Amp with meter that reads low (40-50mA)
pin1 60.49V
pin4 58.27V
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Are those voltages with the meter reading low?
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Yes. I included pictures. Pin 1- 59.76V and pin 4- 57.36V
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OK, on the meter itself there are the two inboard terminals where there are all those 9.09 ohm resistors and wires coming and going. Can you lift all the connections on the + terminal on the meter, then measure the DC resistance of the meter? (whatever you do, don't turn the amp on like this)
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Please don't laugh. Either of these what I'm supposed to be doing?
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You are measuring between the outer pairs of lugs.
The two outer terminals where the red/black wires go is the light for the meter, so ignore those.
The other two terminals are for the actual meter mechanism. You need to lift all the connections to one of those two terminals (doesn't matter which one), then you can check the impedance of the meter itself.
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When you say "lift all the connections," does that mean desolder the resistors and wire from one of these terminals? If so, I should then measure resistance between the two inner terminals? I can't thank you enough for your help and patience.
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Yes, what you just wrote is the intended result.
What I've noticed is that for the last 5 or so years that I have been using these meters, they have come without compensation resistors installed. What this means is that the 100mA meter will read 100mA when 1mA of current passes through it. To scale this down so the meter works properly, we put those three 9.09 ohm resistors across the terminals. This is rather odd though, as the 200mA version that I've used for even longer has said resistors installed.
Well, the last batch of 100mA meters I got were already compensated, and I didn't notice that, so with the resistors across the terminals they were measuring low. It's possible that you got one of those newer meters, and we would be able to determine that by either measuring the DC resistance across the terminals with no resistors installed, or you could lift the three resistors off of one of the meter terminals but leave both the wires, then fire it up and see if the meter instantly pegs to 100+mA or if it pops up to where it's supposed to be.
For future MonAmour runs we will be measuring the DCR of the meters to be sure this doesn't cause any problems.
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I lifted the three resistors off one terminal, plugged it in and the meter went to 60mA! Thanks again.
After leaving the amp on for several hours, the meter still drifts down 4-6mA. Is this normal?
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You'd want to see if the DC voltages at pins 1 and 4 on the 4 pin socket also drift down, as well as measuring the DC voltage between pins 1 and 4 on the 4 pin socket to see if that is dropping down over time.
In essence it could be the meter having an issue, or it could be an electrical issue with the amp. If all of the DC voltages are 100% stable, then it's the meter. If you find that the DC voltages on pins 1/4 drop to around 55V or so after a few hours, then you would have an issue in the amp that would require additional investigation.
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I got 59.03V at pin1 and 56.66V at pin4. The meter read 50mA at the time these readings were taken. The amp had been on for ~5hrs. These seem pretty close to the previous readings when the amp had not been running for hours. I plan to repeat measurements.
Do I clip one probe of the multimeter to pin1 and put the other on pin4 to measure the DC voltage between pins 1 and 4? I don't want to do anything stupid.
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Do I clip one probe of the multimeter to pin1 and put the other on pin4 to measure the DC voltage between pins 1 and 4? I don't want to do anything stupid.
Yes, that will tell you the filament voltage. If that is not quite right, 2A3 operating current will drop.
The 59V and 56.6V would indicate proper 60mA draw provided your switch is on the higher current setting.
It's entirely possible that you may just need to replace the meter.
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I get 2.388V between pins 1 and 4 with the meter reading 50mA.
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That and the residual AC ripple will be adequate for full emission.
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Does that mean the problem is the meter?
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Yes, this is likely the meter not working 100%.
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Installed the new meter and everything is working perfectly. Thanks again Paul and thanks to everyone at Bottlehead!