Tweet
In this instance, not the ivory one. This is instead driving the Porter-Davies Drum Stool or 'Throne' with their small potent BC-2 Kick Drum Throne Amp package. We have several in our Drum Dept inventory, while this one that came in yesterday was from Auntie M, who likewise has several. They periodically misbehave, so they're an interesting box.
This one that came in had no output. Not an uncommon problem. I have limited documentation on the BC2 amplifier package. Mostly that of the Power Amp, power supply and the Lamp section that's located between the Neutrik NL4 Output connector and the Power Amp. What I still don't have is the Preamp / Control Circuitry ahead of the Power Amp.
Over the years, I've used both a Roland TD-10 V-Drum Controller for a Kick Drum source, as well as 50Hz 1/3 Octave Pink Noise from my B & K 1027/1617 Sine-Random Generator in Pink Noise mode, with the 1/3 Oct Filter engaged, which produces random-amplitude 50Hz Sine Wave. Has a good 20dB or greater random amplitude output. In today's episode, I didn't have the drum throne handy. The light bank assembly, which sits between the Power Amp module (at the bottom of the inside view and the Input/Control section sitting at the XLR/knobs section lights up under drive.
Using ONLY my Ampeg 15" bass speaker as a load, I never see the lights light up...unless I plug in an 8 ohm resistive load across the speaker.
After pulling the chassis from the heavy half of the road case cube, is tethered by multiple cables attached to the Toroidal Power Xfmr mounted to the side of the case, so getting it positioned on the bench to troubleshoot it is somewhat of a challenge, unless you take the time to remove the transformer and grounding wires from the case. I usually struggle with the case attached. I wasn't getting any drive signal from the 'preamp' section over to the Power Amp assembly. So, using my scope probe to poke around finding signal on one of the two 4558 Op Amps, and one of the four quadrants of the TL084 op amp, I saw I had input signal from both the Mic Preamp and the Line Input. Common Signal and CLIP LED's present to give you confidence. I found signal present at the top of the Master Volume pot, but nothing on the wiper, but nowhere else.
Reseating the TL084 gave me output, so I let it run a while. Ran for about 5 minutes, then phttt..........no more signal. I pushed down on the TL084, and got signal again, so powered down, took that upper pot board off the stack and then replaced the IC socket, while looking at all the rest of the connections. Re-soldered the two bridge rectifiers on the XLR board, which has the power supply IC's....15VAC input and 12VDC bipolar regulators 7812T, 7912T. It has poor regulation...needs higher voltage for the supplies, as it falls out of regulation at 112VAC, then you get odd clipping behavior. Low Drop-out regulator versions of the IC's lower that drop-out range to around 106VAC.
The new IC socket soldered in seemed to work favorable, and I had signal running again for a good while. Went to the water closet, came back, still running. I took my reading glasses off to put my loupes back on, and just setting my glasses on the top of the case.....the signal stopped! Signal lights on (not output lamps), but no more sound. Pounded on the case, as I had re-mounted the panel back into the case.
I resumed the chase this morning, now suspecting that Master Volume pot, as I had found when I got the unit running, the other three quadrants of that TL084 seemed to follow the output of that pot, and when it has stopped, I only had signal from the one quadrant. 100k Linear taper pot. I had continuity from top to wiper, but not from wiper to bottom, and the reading from top to bottom was way more than 100k. I had opened the cover of that yesterday when I seeped in contact cleaner, and did have continuity on the pot.
Dug thru my pot collections, found another 100k linear, this one with a 1/4" shaft, where the one installed was a 6mm spline shaft. Knob size was 6mm with two set screws. So, had to ream that out to 1/4". Swapped the pots out, re-mounted the PCB assy and fired it back up. NOW I had the system working again, all four quadrants of the TL084, and the action of the Master Valume pot now seemed wider than it had all along.
Figures....hoodwinked by a stupid pot all this time. I let it run for a good while longer, then plugged an 8 ohm resistive load in along with the 4 ohm bass cabinet. Running at 50Hz, the impedance curve of the cabinet shows that is around 8 ohms at that frequency. The Drum Throne is resistive until the inductive reaction steadily rises up past 64 ohms at around 20kHz. With the dummy load connected, I now had the lamps dancing, as one has using the Drum Throne and kick drum to drive it.
I need to contact Porter Davies again and plead for their Signal Processing section schematic(s). Always easier trouble-shooting when you have all the doc's
But, a fun project. More fun when you have that drum throne to sit on while it kicks you in the rear.
This one that came in had no output. Not an uncommon problem. I have limited documentation on the BC2 amplifier package. Mostly that of the Power Amp, power supply and the Lamp section that's located between the Neutrik NL4 Output connector and the Power Amp. What I still don't have is the Preamp / Control Circuitry ahead of the Power Amp.
Over the years, I've used both a Roland TD-10 V-Drum Controller for a Kick Drum source, as well as 50Hz 1/3 Octave Pink Noise from my B & K 1027/1617 Sine-Random Generator in Pink Noise mode, with the 1/3 Oct Filter engaged, which produces random-amplitude 50Hz Sine Wave. Has a good 20dB or greater random amplitude output. In today's episode, I didn't have the drum throne handy. The light bank assembly, which sits between the Power Amp module (at the bottom of the inside view and the Input/Control section sitting at the XLR/knobs section lights up under drive.
Using ONLY my Ampeg 15" bass speaker as a load, I never see the lights light up...unless I plug in an 8 ohm resistive load across the speaker.
After pulling the chassis from the heavy half of the road case cube, is tethered by multiple cables attached to the Toroidal Power Xfmr mounted to the side of the case, so getting it positioned on the bench to troubleshoot it is somewhat of a challenge, unless you take the time to remove the transformer and grounding wires from the case. I usually struggle with the case attached. I wasn't getting any drive signal from the 'preamp' section over to the Power Amp assembly. So, using my scope probe to poke around finding signal on one of the two 4558 Op Amps, and one of the four quadrants of the TL084 op amp, I saw I had input signal from both the Mic Preamp and the Line Input. Common Signal and CLIP LED's present to give you confidence. I found signal present at the top of the Master Volume pot, but nothing on the wiper, but nowhere else.
Reseating the TL084 gave me output, so I let it run a while. Ran for about 5 minutes, then phttt..........no more signal. I pushed down on the TL084, and got signal again, so powered down, took that upper pot board off the stack and then replaced the IC socket, while looking at all the rest of the connections. Re-soldered the two bridge rectifiers on the XLR board, which has the power supply IC's....15VAC input and 12VDC bipolar regulators 7812T, 7912T. It has poor regulation...needs higher voltage for the supplies, as it falls out of regulation at 112VAC, then you get odd clipping behavior. Low Drop-out regulator versions of the IC's lower that drop-out range to around 106VAC.
The new IC socket soldered in seemed to work favorable, and I had signal running again for a good while. Went to the water closet, came back, still running. I took my reading glasses off to put my loupes back on, and just setting my glasses on the top of the case.....the signal stopped! Signal lights on (not output lamps), but no more sound. Pounded on the case, as I had re-mounted the panel back into the case.
I resumed the chase this morning, now suspecting that Master Volume pot, as I had found when I got the unit running, the other three quadrants of that TL084 seemed to follow the output of that pot, and when it has stopped, I only had signal from the one quadrant. 100k Linear taper pot. I had continuity from top to wiper, but not from wiper to bottom, and the reading from top to bottom was way more than 100k. I had opened the cover of that yesterday when I seeped in contact cleaner, and did have continuity on the pot.
Dug thru my pot collections, found another 100k linear, this one with a 1/4" shaft, where the one installed was a 6mm spline shaft. Knob size was 6mm with two set screws. So, had to ream that out to 1/4". Swapped the pots out, re-mounted the PCB assy and fired it back up. NOW I had the system working again, all four quadrants of the TL084, and the action of the Master Valume pot now seemed wider than it had all along.
Figures....hoodwinked by a stupid pot all this time. I let it run for a good while longer, then plugged an 8 ohm resistive load in along with the 4 ohm bass cabinet. Running at 50Hz, the impedance curve of the cabinet shows that is around 8 ohms at that frequency. The Drum Throne is resistive until the inductive reaction steadily rises up past 64 ohms at around 20kHz. With the dummy load connected, I now had the lamps dancing, as one has using the Drum Throne and kick drum to drive it.
I need to contact Porter Davies again and plead for their Signal Processing section schematic(s). Always easier trouble-shooting when you have all the doc's
But, a fun project. More fun when you have that drum throne to sit on while it kicks you in the rear.
Attached Files