Tweet
The General Mgr at CenterStaging, LLC brought me his Demeter VTMP-2a with the initial complaint of the front panel variable Gain pots being way off with respect to each other, to get both channels equal. He’s using it for sweetening digital tracks in re-record, I presume.
After opening it up, finding garden variety 100k Audio Taper potentiometers installed, and knowing their tracking when you’re attenuating around 30dB on them, their relative settings to each other will be very different. Well over a marker-line difference on the FP silkscreen. Even if I bought 10 or 20 pots and checked their relative tracking, it wouldn’t be much better. I looked around to see if I could find any affordable 100k Nobel or Alps 22-detent step potentiometers, knowing those DO track quite well, being laser-trimmed discrete resistance steps on their PCB-mtg substrates. We used them on a number of BGW power amps for the front panel pots when I worked there decades ago. I know I kept a handful of them, but where they are now, I couldn’t guess.
My search on the internet, and on ebay found their prices varying from $15 to well beyond $100 each. I also came across some precision step attenuators built from 24-position M-B-B rotary switches with precision resistors, yielding 2dB/step down to -34dB, then increases in attenuation steps down to -75dB. I ordered a pair, which will show up sometime before December 7.
Meanwhile, in checking out the preamp in general, I found a number of problems. I’m hearing transformer hum in Ch. A, though not in Ch. B. When I turned the Variac down, to see if it drops relative to Mains voltage, at 112VAC, one of the power supply regulators drops out of regulation, and a new obnoxious background noise appears. Listening to the noise floor at the three gain settings of 30dB, 40dB and 50dB, I also find the chassis is very sensitive to vibration….meaning the tube PCB is vibration sensitive. Microphonics, perhaps. I swapped out tubes, not finding that problem change, so perhaps there’s solder joint issues on the bottom of the preamp PCB.
Getting it off the chassis floor requires releasing the front panel from the chassis to get the wiring harness to move with the PCB. Top left front screw was frozen, with it’s Philips head pattern already buggered by someone in the past attempting the same thing. Getting it to loosen took application of WD 40 into the threads from the rear, hammering on it from both front and rear with a Drive Pin Punch, heating the screw, then hitting it with Freeze mist followed by quick screwdriver action finally broke the locked-up screw bond.
I pondered shock-mounting the tube PCB assembly. I searched for tiny male-female, or male-male shock-mount fasteners, came upon one company that made them for PCB assemblies. Looking at their design, I went to my drawers to find some #6-32 Socket Cap screws, and a short length of the 3/16” ID/5/16” OD Silicone Rubber Tubing that I use for shock-mount protection of power tube hold-down clamps, and fashioned a simple shock mount fastener. I picked up more socket cap screws from my local surplus hardware store on the way home yesterday. When I opened the unit this morning to check the screw size, I found them being #4-40, NOT #6-32. But, my #4-40 Jack Screws (male-female stud-standoffs) work there too, being firmly held by the tubing.
I then thought about the 12BH7 output tubes in the circuit, they being taller than the 12AX7’s and the 12AT7 in use. Using the top cover sitting on the side flanges revealed the 12BH7 tube's height wouldn't allow installing shock-mounts under the PCB. I could use 12AU7’s there in place of the 12BH7’s, which would work and solve the height problem.
Then, inspecting the rest of the chassis, I see the output transformers mounted over between the left chassis wall and the edge of the tube PCB, it’s coil form hangs over the PCB assy. So, I’d have to install standoffs to raise the transformers. Lots of obstacles, but this DIY shock mount fastener solution may work.
I still have to get onto the bottom side of the PCB to see if there are solder joint fractures, as that could be involved in the vibration-sensitive noise pickup.
Not sure what I’ll find in the power supplies and who is dropping out of regulation to cause that nasty background noise. 112VAC is too close to nominal line voltage to be ‘trivial’.
So, more than just replacing the front panel gain pots to get this unit solid again.
After opening it up, finding garden variety 100k Audio Taper potentiometers installed, and knowing their tracking when you’re attenuating around 30dB on them, their relative settings to each other will be very different. Well over a marker-line difference on the FP silkscreen. Even if I bought 10 or 20 pots and checked their relative tracking, it wouldn’t be much better. I looked around to see if I could find any affordable 100k Nobel or Alps 22-detent step potentiometers, knowing those DO track quite well, being laser-trimmed discrete resistance steps on their PCB-mtg substrates. We used them on a number of BGW power amps for the front panel pots when I worked there decades ago. I know I kept a handful of them, but where they are now, I couldn’t guess.
My search on the internet, and on ebay found their prices varying from $15 to well beyond $100 each. I also came across some precision step attenuators built from 24-position M-B-B rotary switches with precision resistors, yielding 2dB/step down to -34dB, then increases in attenuation steps down to -75dB. I ordered a pair, which will show up sometime before December 7.
Meanwhile, in checking out the preamp in general, I found a number of problems. I’m hearing transformer hum in Ch. A, though not in Ch. B. When I turned the Variac down, to see if it drops relative to Mains voltage, at 112VAC, one of the power supply regulators drops out of regulation, and a new obnoxious background noise appears. Listening to the noise floor at the three gain settings of 30dB, 40dB and 50dB, I also find the chassis is very sensitive to vibration….meaning the tube PCB is vibration sensitive. Microphonics, perhaps. I swapped out tubes, not finding that problem change, so perhaps there’s solder joint issues on the bottom of the preamp PCB.
Getting it off the chassis floor requires releasing the front panel from the chassis to get the wiring harness to move with the PCB. Top left front screw was frozen, with it’s Philips head pattern already buggered by someone in the past attempting the same thing. Getting it to loosen took application of WD 40 into the threads from the rear, hammering on it from both front and rear with a Drive Pin Punch, heating the screw, then hitting it with Freeze mist followed by quick screwdriver action finally broke the locked-up screw bond.
I pondered shock-mounting the tube PCB assembly. I searched for tiny male-female, or male-male shock-mount fasteners, came upon one company that made them for PCB assemblies. Looking at their design, I went to my drawers to find some #6-32 Socket Cap screws, and a short length of the 3/16” ID/5/16” OD Silicone Rubber Tubing that I use for shock-mount protection of power tube hold-down clamps, and fashioned a simple shock mount fastener. I picked up more socket cap screws from my local surplus hardware store on the way home yesterday. When I opened the unit this morning to check the screw size, I found them being #4-40, NOT #6-32. But, my #4-40 Jack Screws (male-female stud-standoffs) work there too, being firmly held by the tubing.
I then thought about the 12BH7 output tubes in the circuit, they being taller than the 12AX7’s and the 12AT7 in use. Using the top cover sitting on the side flanges revealed the 12BH7 tube's height wouldn't allow installing shock-mounts under the PCB. I could use 12AU7’s there in place of the 12BH7’s, which would work and solve the height problem.
Then, inspecting the rest of the chassis, I see the output transformers mounted over between the left chassis wall and the edge of the tube PCB, it’s coil form hangs over the PCB assy. So, I’d have to install standoffs to raise the transformers. Lots of obstacles, but this DIY shock mount fastener solution may work.
I still have to get onto the bottom side of the PCB to see if there are solder joint fractures, as that could be involved in the vibration-sensitive noise pickup.
Not sure what I’ll find in the power supplies and who is dropping out of regulation to cause that nasty background noise. 112VAC is too close to nominal line voltage to be ‘trivial’.
So, more than just replacing the front panel gain pots to get this unit solid again.
Attached Files
Comment