Flip69
Active Member
I have finished restoring an HF-81. When I fired it up for the first time nothing blew up, which is always a good sign. No hum or other unwanted noise, also good. However, the sound is not right. First, the volume level is very low. Second, the sound is coming through both channels, but the balance knob is not working, i.e., is not moving sound from one side to the other. Also, the tone controls only appear to work for one channel. Lots of fun still ahead before this baby is finished.
I note that I reused all but the original 250k volume pot that came with the unit. I replaced the volume pot with a 250k ALPS Blue Velvet. As I understand it, the pin configuration on the ALPS is: 1-ground, 2-wiper, 3-input. Can anyone confirm? Also, the balance pot that came with the unit is marked 500k. The parts list specs a 750k pot. Could the difference in pot values be causing any of my problems? It seems more like a wiring issue, but I need to ask. Do I need to use a 750k pot?
My next move is to go through and check the voltages using the Sam's schematic, as well as voltage and resistance using the table in the user's manual showing values for each pin of each valve. Regarding the pin values, I plan to check resistance for each pin with the unit disconnected from power, and then check DC voltage with the unit powered up with no signal/source connected. What exactly is the procedure for making these measurements? Do I take the readings from the bottom with all of the tubes inserted? Do I remove the tubes one at a time and check from the top? Do I take the readings with all of the tubes removed?
I did some initial spot checking of resistance from the bottom, with the unit disconnected from power, with all the tubes inserted. Many of the readings were dead on. Others were way off. I will ultimately make a table that shows spec'd values and actual readings. I expect I can use this, along with the schematics, to trace the source of my problems. Am I on the right track? What other troubleshooting steps should I take? I have posted a similar message on the Yahoo Eico Group page, so don't be surprised if you see it there.
I note that I reused all but the original 250k volume pot that came with the unit. I replaced the volume pot with a 250k ALPS Blue Velvet. As I understand it, the pin configuration on the ALPS is: 1-ground, 2-wiper, 3-input. Can anyone confirm? Also, the balance pot that came with the unit is marked 500k. The parts list specs a 750k pot. Could the difference in pot values be causing any of my problems? It seems more like a wiring issue, but I need to ask. Do I need to use a 750k pot?
My next move is to go through and check the voltages using the Sam's schematic, as well as voltage and resistance using the table in the user's manual showing values for each pin of each valve. Regarding the pin values, I plan to check resistance for each pin with the unit disconnected from power, and then check DC voltage with the unit powered up with no signal/source connected. What exactly is the procedure for making these measurements? Do I take the readings from the bottom with all of the tubes inserted? Do I remove the tubes one at a time and check from the top? Do I take the readings with all of the tubes removed?
I did some initial spot checking of resistance from the bottom, with the unit disconnected from power, with all the tubes inserted. Many of the readings were dead on. Others were way off. I will ultimately make a table that shows spec'd values and actual readings. I expect I can use this, along with the schematics, to trace the source of my problems. Am I on the right track? What other troubleshooting steps should I take? I have posted a similar message on the Yahoo Eico Group page, so don't be surprised if you see it there.
