Earcraft
New Member
I dug out a 3020A (SN: A3243376) that I had packed away for about 15 years. An old boss of mine used to buy up defunct recording studios and this came out of one in NYC. Since it wasn't "Pro" gear and he had no idea what it was, he gave it to me. I decided to fire it up and it had the classic problems i.e Turn on thump (VRRRUUUMMPPP), and a major 60cycle hum in one of the channels. I have not turned it on since...
I set it aside till this winter so I could do a recap. The thing is in such good physical condition, i'm keeping the work to a simple restoration. I decided to replace all the electrolytics first to see where that got me. I used Elna SILMIC II in the audio circuits and a combination of Nichicon FG,FW,KA, and KL for power and other circuits. I replaced a few diodes and resistors that had corroded due to the brown glue. I finally replaced all the resistors in the “soft-clipping” circuits with metal film 1% equals to hopefully make it a little quieter.
After firing it up and adjusting bias and DC offset, I did a listening test. The audio is was completely distorted at low level and consistent across both channels. That's when I checked the fuses to find both 4A (F3,F4) blown. After checking cap polarity and a good inspection of the traces looking for shorts, I tried another pair and they went pop the way only slow-blows can.
After hijacking Leesonic’s post http://www.audiokarma.org/forums/showthread.php?t=460678&page=4 I got some advice to check out the output transistors and drivers which I did. While I cannot confirm they are working as spec’d, the tested out OK with my multi-tester using the diode setting. (I did not go as far as to pull the Bias transistors but plan to a little later today.) Also as suggested, I powered up with the drivers and output transistors out to check to make sure the rectifier was still good. It did not blow fuses, so I assume it is.
After a careful reassembly and making sure not to short the main output transistors to the heat sink, I again tested the unit. This time I built a quick dim-bulb-tester and used it. The 150W bulb glows dimly and pulsates at a slow rate (approx. 2 pulses a sec). I also noticed the the Output Transistors heat up fast and get really hot. I have a 7020e that I use and it is not nearly as hot after being on all day and driving music.
Since I am still suspicious about the Transistors, I started looking to buy replacements to have in case I want to pop them in to see what happens. The main 2N3055/MJ2955 are easy to find. The drivers are 2SD699A and Bias 2SD699A/2SB649. I have looked for a few days and cannot find direct replacements or a cross reference that I can understand. In earlier units I seen people using BD139 but to me, the data sheets don’t look all that similar to the 699A. Any AK’ers know of compatible replacements?
I’m a bit stumped, and I need some advice.
I set it aside till this winter so I could do a recap. The thing is in such good physical condition, i'm keeping the work to a simple restoration. I decided to replace all the electrolytics first to see where that got me. I used Elna SILMIC II in the audio circuits and a combination of Nichicon FG,FW,KA, and KL for power and other circuits. I replaced a few diodes and resistors that had corroded due to the brown glue. I finally replaced all the resistors in the “soft-clipping” circuits with metal film 1% equals to hopefully make it a little quieter.
After firing it up and adjusting bias and DC offset, I did a listening test. The audio is was completely distorted at low level and consistent across both channels. That's when I checked the fuses to find both 4A (F3,F4) blown. After checking cap polarity and a good inspection of the traces looking for shorts, I tried another pair and they went pop the way only slow-blows can.
After hijacking Leesonic’s post http://www.audiokarma.org/forums/showthread.php?t=460678&page=4 I got some advice to check out the output transistors and drivers which I did. While I cannot confirm they are working as spec’d, the tested out OK with my multi-tester using the diode setting. (I did not go as far as to pull the Bias transistors but plan to a little later today.) Also as suggested, I powered up with the drivers and output transistors out to check to make sure the rectifier was still good. It did not blow fuses, so I assume it is.
After a careful reassembly and making sure not to short the main output transistors to the heat sink, I again tested the unit. This time I built a quick dim-bulb-tester and used it. The 150W bulb glows dimly and pulsates at a slow rate (approx. 2 pulses a sec). I also noticed the the Output Transistors heat up fast and get really hot. I have a 7020e that I use and it is not nearly as hot after being on all day and driving music.
Since I am still suspicious about the Transistors, I started looking to buy replacements to have in case I want to pop them in to see what happens. The main 2N3055/MJ2955 are easy to find. The drivers are 2SD699A and Bias 2SD699A/2SB649. I have looked for a few days and cannot find direct replacements or a cross reference that I can understand. In earlier units I seen people using BD139 but to me, the data sheets don’t look all that similar to the 699A. Any AK’ers know of compatible replacements?
I’m a bit stumped, and I need some advice.