I remember reading your post, but don’t remember where. It’s also very difficult to remove parts from this board. I think I may have to sell this one as is/for parts…
Those readings are not unexpected. There are feedback resistors in parallel with the capacitor,
and those determine the DC voltage. It's unlikely that the capacitor is bad. If you look at the schematic,
the voltages noted on it are in that range, depending on the faults present in the circuit.
Don't try to diagnose the circuit, that takes experience. Just measure the node voltages as discussed before.
The important DC voltage to measure is from the 12BH7A grid to ground.
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Surprise! I changed the tubes in PS, and it started to work, AGAIN. Hopefully it will stay this way. Through 5.6uf output caps I'm gettin on output around 7 mv dc, HV stays steady at 390v (AC outlet at 119v), consumes 464 mA at idle. I didn't connect any music to it yet though... Did I get lucky?
Originally I bought it to keep. I have Sprague Atoms and BC Vishay caps. I know about damaging the foil, I screwed up while soldering in FETs in the phono. Previous “modifier” clipped them off but I had to remove remnants…. I also found a mistake he made: 1000uf / 40v caps were soldered parallel to each other, instead of going to different sides of that 10 ohm 8 watt resistor
You could rewire the socket to take a 7044 tube, it has a watt more of dissipation
Than the 12BH7, the heater arrangement is the same. ARC used this tube in their MCP-33 the power supply is basically same setup.
640 is awfully high, I would find a way to drop the input voltage, 500V should be sufficient.
So the thing is working. Need to find out the following: 1. Is it normal for it to take it’s sweet time (3 min) to calm down to acceptable dc on output, and 4 min 15 sec to really low (1 - 6 mv)? Or is there something wrong with the regulation? The process happens like so: first 30 to 44 sec dc is around 400 mv, then s regulated voltage starts to rise, it goes haywire and dc starts to bounce around, reaching as high as 30v, then after about a minute and a half it begins to calm down…. If it was connected to a power amp, it’d destroy the speakers and possibly the amp for sure… Auto mute works only for the very first 10 - 15 seconds, but all the spares begin to fly about a minute into this severely complicated turn-on process. I’m teaching retirement age, and I’ve had many, many preamps, but if this is a normal behavior, then I’ve never seen it before.
It sounds like the PS is really unstable, I’ve breadboarded several different ARC power supplies, and their hybrid setups aren’t very reliable, mainly oscillation problems. I decided then that either all tube or discrete solid state components is the way to go, the op amp setup in a High Voltage environment only works well, if it floats on the high voltage B+.
Thx. I can’t imagine it was doing this when it was new. I mean who’d buy it, if they had to wait for 5 minutes for it to come down? Besides the point of poor design, if this is the case, I think there’s still something wrong with it… I will try to replace the oppo-amps, I will install the bases, so they can be easily replaced. Then there are a few transistors that may be the culprits. Mystery continues ))). I want to listen to it and decide for myself if it sounds as good as people say, and if it’s worth pursuing this till the end. If it sounds s great a they say, perhaps sending it to ARC and paying them exorbitant amounts of money is worth it )))
The OPA134 is a great op amp, JFET input, and very low noise if You want to experiment with different op amps.