...isn't after all. It hit the big desk yesterday for a nice, slow power-up so I could start working on it. Previously was sidelined upon purchace (or theft, considering price!) due to bad hum, i mustve had selector in phono or tape head position then. Hooked up antenna & speakers, switch on fm, started twisting up the juice, 150w bulb in the power source glowing orange- damn, I hear something besides hiss as I slowly advance the big knob on the throttle. It's alive! tuner working !
Getting to work, started poking about with the dvm,voltages as follows:
Siemens bridge- +4.2vdc, -21.3vdc.
375 volt bus to pin 8 on 7591's- 397.5vdc
Plate voltage (pin 3) 435vdc on v8 & 9, 434vdc on v10 & 11.
Bias volts on pin 6- -16vdc on all 4.
Voltages at c91a-212v, b-312v, c-362v, d-397v.
Voltages at c92a-164vdc, b&c- 188vdc, d- 214vdc.
Tuner working fine, except for too-high stereo capture threshold, hum when in stereo mode which is directly proportional to volume level. Not present in mono mode.
Schematic not clear where the diode bridge feeding to relay gets its power, but it's the same source that feeds to selector switch section 3.
Other major issue is LOUD hum in both phono inputs & HELLISH hum in tape head position. Being that this preamp, v16 & 17 ,is powered from 215v source at c91a/r133, shall I suspect that 20uF can cap section may be at fault? Yeah, I know-replace all can caps,right? Prior to that, I want to find out & fix what ails it before I shotgun ALL the caps.
This unit, s/n 78778am, is all original, right down to the tubes & light bulbs, which all still work. I'd like to keep it as original as reasonably possible, but will start out with upgrading bias resistors & caps, cathode ground resistors, mpx electrolytic, don't need the inrush limiter just yet, busted & bypassed power switch will be difficult to replace. Metal bone maybe still has a couple, but I don't deal in bitcoin, which is what it'll take to pry one out of him according to a recent visit to his site. (Dammit!) & maybe a 10k pot for basic bias adjust.
I don't think this unit ever got much use, due to condition of the tubes & bulbs. The 7591's all show in excess of specced mho on the 533a tester, they're electrically like new. Only cosmetic issue, not counting knob brights (I have them all in a safe place) is darkening of the signal strength meter & stereo beacon dummy faces, likely UV damage from sunlight exposure. Nothing I can do for that but eventually replace. Would be nice to have an em84a in place of that stupid bulb/relay set-up, it would just fit in that faceplate slot, too.
So, Larry, Dave & company, your hallowed opinions & advice greatly appreciated! As I have two of these things plus the 400, they'll eventually all get the treatment, but one at a time.
Last question : are there any known discrepancies in the factory service manual for these things? And believe it or not, this one came mounted in an end table with a lift-off ceramic top. No pics of that yet, I'd need to dig out of storage.
Getting to work, started poking about with the dvm,voltages as follows:
Siemens bridge- +4.2vdc, -21.3vdc.
375 volt bus to pin 8 on 7591's- 397.5vdc
Plate voltage (pin 3) 435vdc on v8 & 9, 434vdc on v10 & 11.
Bias volts on pin 6- -16vdc on all 4.
Voltages at c91a-212v, b-312v, c-362v, d-397v.
Voltages at c92a-164vdc, b&c- 188vdc, d- 214vdc.
Tuner working fine, except for too-high stereo capture threshold, hum when in stereo mode which is directly proportional to volume level. Not present in mono mode.
Schematic not clear where the diode bridge feeding to relay gets its power, but it's the same source that feeds to selector switch section 3.
Other major issue is LOUD hum in both phono inputs & HELLISH hum in tape head position. Being that this preamp, v16 & 17 ,is powered from 215v source at c91a/r133, shall I suspect that 20uF can cap section may be at fault? Yeah, I know-replace all can caps,right? Prior to that, I want to find out & fix what ails it before I shotgun ALL the caps.
This unit, s/n 78778am, is all original, right down to the tubes & light bulbs, which all still work. I'd like to keep it as original as reasonably possible, but will start out with upgrading bias resistors & caps, cathode ground resistors, mpx electrolytic, don't need the inrush limiter just yet, busted & bypassed power switch will be difficult to replace. Metal bone maybe still has a couple, but I don't deal in bitcoin, which is what it'll take to pry one out of him according to a recent visit to his site. (Dammit!) & maybe a 10k pot for basic bias adjust.
I don't think this unit ever got much use, due to condition of the tubes & bulbs. The 7591's all show in excess of specced mho on the 533a tester, they're electrically like new. Only cosmetic issue, not counting knob brights (I have them all in a safe place) is darkening of the signal strength meter & stereo beacon dummy faces, likely UV damage from sunlight exposure. Nothing I can do for that but eventually replace. Would be nice to have an em84a in place of that stupid bulb/relay set-up, it would just fit in that faceplate slot, too.
So, Larry, Dave & company, your hallowed opinions & advice greatly appreciated! As I have two of these things plus the 400, they'll eventually all get the treatment, but one at a time.
Last question : are there any known discrepancies in the factory service manual for these things? And believe it or not, this one came mounted in an end table with a lift-off ceramic top. No pics of that yet, I'd need to dig out of storage.