Hi everyone, new here.
This Monster HTS-3500 MKII has an analog voltmeter dial that does not register any voltage and is not lit while the unit is powered.
The PCB at the back of the voltmeter gets 120 Vac from the main board. Traces upstream on that 120Vac path on the main board burned up. Someone bridged them and they burned up again in a different spot. The voltmeter PCB itself showed no signs of overheating and components tested OK, so I bridged the burned traces in that 2nd spot and powered up the unit with the voltmeter PCB installed, but not the meter itself. Everything looked OK - no overheating, suggesting the meter itself is the culprit somehow.
The meter has 4 terminals: posts A and B, and leads C and D. Posts A and B connect to the movement (?), 75 ohms across them. Leads C and D are connected to two pins at the bottom of the dial face. Infinity ohms across them.
My questions:
Elsewhere, this unit has six always-on receptacles that are powered, and four switched receptacles that are not. That issue looks unrelated to the voltmeter though, so that's one for another day 🙂
Any thoughts and ideas would be much appreciated! Thanks in advance everyone.
This Monster HTS-3500 MKII has an analog voltmeter dial that does not register any voltage and is not lit while the unit is powered.
The PCB at the back of the voltmeter gets 120 Vac from the main board. Traces upstream on that 120Vac path on the main board burned up. Someone bridged them and they burned up again in a different spot. The voltmeter PCB itself showed no signs of overheating and components tested OK, so I bridged the burned traces in that 2nd spot and powered up the unit with the voltmeter PCB installed, but not the meter itself. Everything looked OK - no overheating, suggesting the meter itself is the culprit somehow.
The meter has 4 terminals: posts A and B, and leads C and D. Posts A and B connect to the movement (?), 75 ohms across them. Leads C and D are connected to two pins at the bottom of the dial face. Infinity ohms across them.
My questions:
- First and foremost, could anyone point me to schematics for this unit? I searched high and low but came up with nothing.
- On the voltmeter PCB, there is 13.7Vdc across A and B when the unit is powered. However, just 1.5V from a AA battery across A and B on the meter is enough to bury the needle. I don't want to connect this meter until I understand why the circuit is sending 13.7V into an instrument that's overdriven at 1.5V?
- Are C and D just holder pins for a lamp that has been removed?
- If so, what is the black cube-shaped component just upstream of C and D on the control board? Googling WT-EL7 did not yield any results.
Elsewhere, this unit has six always-on receptacles that are powered, and four switched receptacles that are not. That issue looks unrelated to the voltmeter though, so that's one for another day 🙂
Any thoughts and ideas would be much appreciated! Thanks in advance everyone.
13,7V is across the meter terminal holes without the meter connected?
ZD1...4 seem to zeners, they need the circuit (R1 - VR1 - ZD1....4 - meter) to provide some current to operate as designed.
Hi, mystery component WT-EL7 is a voltage regulator for a backlight lamp/LED. The question is what voltage is on the orange and red wire. If the regulator/switcher is fed 120V AC via those burnt PCB tracks while being defective that would explain some things. It would also again confirm already existing doubts on switchers 😀 Like many armchair designers I think a miniature fuse would have been necessary.
The voltage meter probably is as often a current meter with surrounding circuitry. It is not wise to put a battery directly to its terminals. Better draw a schematic and understand how stuff is done.
The voltage meter probably is as often a current meter with surrounding circuitry. It is not wise to put a battery directly to its terminals. Better draw a schematic and understand how stuff is done.
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Can the "real" voltmeter and current meter "camouflaged" as voltmeter be visually distinguished?
Or only the schematic would tell the difference?
Good question! I just heard my brains work as it is buried somewhere in a dusty drawer. Normally the applied symbols tell a few things like moving magnet or moving coil (only DC/current) etc. Since this thingie has none it is just an indicator and not an instrument.
Best is that OP draws a schematic for us to learn.
I save my *** by saying to wait for the reply on the regulator/switcher OK?!
Best is that OP draws a schematic for us to learn.
I save my *** by saying to wait for the reply on the regulator/switcher OK?!
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That is the full resistance or in the adjusted position?
If R1 is 15k then 120V AC gets rectified to 120V DC (there is no smoothing capacitor) - can you verify this?
If so then 120V DC results in 8 mA through 15k resistance which seems to be in the correct range for the zener wattage.
VR1 would bring about some reduction of that current depending on its adjusted position.
I would personally connect the meter and power the schematic via some step down transformer or variac and see what is happening.
Never first do things that might be destructive. A schematic is needed.
Maybe the indicator is fine and only the surrounding circuitry and/or regulator/switcher cause things. Switchers are fantastic, green, cheap, beautiful, friendly, successful etc. but in reality they are like borderline girlfriends. And… have you ever experienced an analog meter in relatively high ohmic environment causing burnt PCB tracks? A simple connection diagram and then resistance measurement would be my advice.
Maybe the indicator is fine and only the surrounding circuitry and/or regulator/switcher cause things. Switchers are fantastic, green, cheap, beautiful, friendly, successful etc. but in reality they are like borderline girlfriends. And… have you ever experienced an analog meter in relatively high ohmic environment causing burnt PCB tracks? A simple connection diagram and then resistance measurement would be my advice.
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It is also possible to start from the DC side of the diodes with adjustable small voltage power supply 🙂
Judging from the photos in first post AC gets first rectified into DC and then the R1, VR1, meter and zeners are all in series. Zener values are unknown but probably can be found out when meter is replaced with a resistor.
The values are written on them so OP can write them down while making a schematic.
BTW the backlight can also be a fluorescent lamp or film type LED sheet covering the dial. It can be flaky too. What is the resistance value between C and D?
Assumptions for a higher interest: let's suppose it is a short or low ohmic. Then the regulator/switcher/PSU would also draw overcurrent. Apparently no adequate fuse is used on either primary/input or secondary/output side causing PCB tracks to melt.
Probably UL listed and all that blah blah but no protection. But it sure looked good when it worked OK 🙂
BTW the backlight can also be a fluorescent lamp or film type LED sheet covering the dial. It can be flaky too. What is the resistance value between C and D?
Assumptions for a higher interest: let's suppose it is a short or low ohmic. Then the regulator/switcher/PSU would also draw overcurrent. Apparently no adequate fuse is used on either primary/input or secondary/output side causing PCB tracks to melt.
Probably UL listed and all that blah blah but no protection. But it sure looked good when it worked OK 🙂
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Devices of a magnetoelectric system with an inside frame magnet, as a rule, with a current through the winding of 1.1 mA, have a maximum pointer deviation. Judging by the results of your measurements, the current in the A-B circuit will be 13.7V/15000Ohm=0.9mA. So, if you have a device designed for a current of 1 mA, if it is in good working order, the circuit will be working.
There is some conflict of data here. 120V AC (?) is passed through a 4 diode bridge rectifier. In an open circuit (no meter connected) it cannot measure DC 13,7V - can it? Provided that the zeners will not reduce the voltage in some way without any current passing through them except DMM current in voltage measuring mode.
The input resistance of a digital multimeter is usually 1 MΩ, if the digital multimeter shows 13.7 V, then the zener diodes are not broken and are performing their task, i.e. reduce the voltage from 120V to 13.7V.
Sorry, I couldn't stop laughing while reading Jean-Paul's comments; the irony of circuitry in a fancy line conditioner somehow having inadequate protection is hilarious!🤣
But I absolutely agree with getting some idea of the schematic 1st. Is this one of the units Dick Marsh designed for Monster? I don't know if he still pops in anymore ...
But I absolutely agree with getting some idea of the schematic 1st. Is this one of the units Dick Marsh designed for Monster? I don't know if he still pops in anymore ...
The two circuits on the PCB are entirely separate from one another
- The A-B circuit gets 120Vac and outputs 13.7 Vdc across the meter terminal holes (without the meter connected)
- The C-D circuit has only one component on it, the voltage regulator at U1
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1420 Ω is the resistance at the adjusted position. The adjustment range is from 34 Ω to 10 kΩ