Last
update 10/8/07
|
|
|
|
HVTP2: 2 Tech Support / Updates / FAQs UPDATED 3/26/04 WITH AUDIO CHANNEL TEST DATA FAQs ____________________________________________________________ 7/8/04: Q I've built the tube kit, and one channel is working great, the other is low in gain. I check the tube orientation, swapped tubes, and checked DC voltages, and all that seems ok. Any thoughts. A: If the DC voltages are correct, then the amplifier stages are more than likely working, so the problem is likely in the point-point wiring, either in the DI jack, the "A+" jumper (the 4" coax that goes from behind the input XLR right to the tube), or going to the pot. The mogami coax included in the kit will melt if over heated, we have seen this happen in a couple kits we helped fix for people. In 3 separate cases, it was the coax that was compromised from over heating. Below I made a list of things I would try, in order: 1. Clean all the flux off the back of the boards with isopropyl alcohol and q-tips, they should be perfectly clean. Focus attention on the area where the p-p wiring connects to the DI jack, and the input XLR jack. Flux in the wrong place can cause problems. If needed, use a poker and GENTLY scrape and residue between and around the pads, especially where they are close together. 2. Replace the "A+" p-p section with any coax you have available, the run is so short, the quality of the cable is not super critical. You can even use a twisted pair. 3. Double check the DI jack p-p. One person who emailed had a gain problem, and found he had reversed two wires. If you plug a 1/4" jack into the DI input and push the pad switch in, you should read 250k ohms or close to it. It you see something low, like 5-10k (what I found in a couple units), and you have double checked all the resistor are correct, you probably have a problem in the "A+" jumper or DI p-p. 4. If the solder tabs on the pot where strained at all, they can cause the pot to malfunction. Before you remove it however, you can test the pot and its p-p in the following way: A. With the power off for at least a few minutes, remove the tubes. B. With a function generator, put a test tone into pin 1 of the 6072. C. Measure the return amplitude on pin 7 of 6072 at various volume settings, and compare this to the good channel. If something looks wrong, then replace the pot, and the wiring. If this doesn't fix the problem, please email back and I'll give you more things to try. SH ____________________________________________________________ 3/23/04: Q Which tube goes in which socket? A: The 6072 goes next to the input transformer (the smaller round one), the 12AU7A next to the output transformer. Its labeled on the back of the board. One of each tube in each channel! If you look at the schematic, it should be pretty clear which is which. Q: One of my channels is about 6-8dB lower in gain, any ideas? A: Check for proper tube orientation, that gain difference is about what you'll get if the tubes are mixed up.. ____________________________________________________________ 3/11/04: The schematic for the HVTP2 audio channel shows the input transformer drawn backwards. The dual primaries are on the input, the way they are on the board, and the instructions. Pin 1on the transformer (pin with the dot) goes to the pad labeled "1" on the board. Its a documentation error, not a board or electrical problem. All kits shipped after 3/8/04 have this corrected in the instructions. ____________________________________________________________ 3/11/04: The schematic for the HVTP2 psu does not show R58. It is in series with C51. It is on the board, and on the parts list so you should have the part. The board is correct, as is the parts list for that part. All kits shipped after 3/8/04 have this corrected in the instructions. ____________________________________________________________ 3/8/04: The PSU assembly instructions do not mention installing the LM329, it is shown on all the parts list and drawings. The easiest time to add it is before the large electrolytic caps, step 3, though any point in the assembly is fine. All kits shipped after 3/8/04 have this added in the instructions. ____________________________________________________________ HVTP2 Channel Card Assembly Photos
HVTP2 Power Supply Photos
HVTP2 Chassis Assembly Photos
HVTP2 DC testing. The red probe shows the point in the circuit where the measurement is made, the meter shows the DC voltage, all referenced to ground. In general, if these voltages are correct, the amplifiers and power supply should be working right. The pictures basically start at the beginning of the gain stages, and work through the end. Tubes vary, so the voltages you see will more than likely differ a bit, look for large differences. Another good trouble shooting method if you have a channel that is bad, is to put the negative lead of your meter (make certain it is isolated from ground, all handheld meters are) on a point in the circuit on the good channel, and the red lead to the same circuit point in the right channel. If everything is working right, the voltages should be roughly the same, so the DC volts should read zero. If a significant difference is measured, then look in that part of the circuit for the problem. This is referred to as taking relative measurements. 1. Voltage on cathode of first gain stage, pin3 of the 6072, where pin 3 of the 6072 joins with R17.
2. Plate voltage of first gain stage, where pin 1 of the 6072 and R16 join.
3. Cathode of the second gain stage, where pin 8 of the 6072 and R11 join.
4. Plate of second gain stage. Note that the voltage on this gain stage is running at roughly 50% of the plate supply = +200vdc. This biases the amp in its most linear region. The first gain stage on the other hand is under biased a bit at about 60%, why? If I tell you, I'll of course have to kill you.
5. Phantom power on the +48 switch
6. B+ = +200vdc coming into the pcb.
7. Voltage on the plate, pin 1 of the 12AU7A.
8. Voltage on the cathode, pin 8 of the 12AU7A. Pins 3 and 6 are connected together
|
