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10-27-2021, 02:04 PM
The owner agreed to replace the transformer and today I installed a brand new Hammond 290DEX. We're back to healthy heater voltages (6,35V under load) now and with a GZ34 (which I had to substitute for the 5U4GB anyway) I'm getting 437V on the plates, which is the exact expected voltage according to the original schematic. There's 30 volts more on the driver node and further down the supply chain, but I'm going to leave it as it is. I installed 1K/4,7K dropper resistors as on the schematic, replacing the 2,2K/10K ones that I found in the amp so that's probably why I'm seeing more volts right now.
There's some more restoration work left to do as it's definitely been a long time since this amp was thoroghly serviced for the last time, but it already sounds OK. I installed all new electrolytics and I could use single caps after the choke as the voltages are now within their specs. Interesting that the amp has the ground switch installed, but it was only used as soldering lugs for the power cord. The slider lug has no solder on it, so the death cap wasn't installed. I'm going to use it as a NFB loop selector.
I think the original transformer could have been damaged by occasionally shorting indicator light connection in the 6,3V heater circuit. Someone has snipped off original artificial center tap resistors and soldered new ones leaving old legs sticking out very close to soldering lugs. There was also another weird problem as the amp wouldn't turn on every time the power switch was flipped on and I thought it will have to be replaced, but once I cleaned up the indicator wiring the amp will now turn on every time no problem.
I was told the amp was on someone's bench 4 years ago and the voltages were fine then.
Tomi
There's some more restoration work left to do as it's definitely been a long time since this amp was thoroghly serviced for the last time, but it already sounds OK. I installed all new electrolytics and I could use single caps after the choke as the voltages are now within their specs. Interesting that the amp has the ground switch installed, but it was only used as soldering lugs for the power cord. The slider lug has no solder on it, so the death cap wasn't installed. I'm going to use it as a NFB loop selector.
I think the original transformer could have been damaged by occasionally shorting indicator light connection in the 6,3V heater circuit. Someone has snipped off original artificial center tap resistors and soldered new ones leaving old legs sticking out very close to soldering lugs. There was also another weird problem as the amp wouldn't turn on every time the power switch was flipped on and I thought it will have to be replaced, but once I cleaned up the indicator wiring the amp will now turn on every time no problem.
I was told the amp was on someone's bench 4 years ago and the voltages were fine then.
Tomi
