OK so from the PT you have bias voltage( around 50VAC going into the board)
you have heater VAC ( around 3.2VAC on each leg).
Are you getting the HV in at the bias/rectifier board - around 345VAC on each wire? Measure this with your DMM AC setting.After the rectifier, change back to DC.
Have you trace measured from the PT outputs along the path to the rectifier and switches, remembering to keep one hand in the pants back pocket while doing this (hope I'm not teaching Egg Sucking 101 here, but TAG plays safe )
1- What is your primary voltages to the power transformer AC in.
2- What is you secondary voltage HV out of the power transformer in AC ( 345-0-345 ) 690vac no load.
3- What is your B+1 DC voltage ??? to the standby switch. Then B+2, B+3, B+4, B+5.
Martin,
Thank you. I do indeed have a blown fuse - certainly at least part of the problem. Step #1 - replace - underway. Hopefully that will resolve the problem
I replaced the .5A fast blo fuse and re-ran the start up checks with the bulb limiter in place. HV DC is present and, with the bulb limiter, the following readings occur:
V1:
1: 150.7 v
2: 0
3: .93 v
4: 3.59 v (AC)
5: 3.59 v (AC)
6: 151 v
7: 0
8: .9 v
9: 3.58 v (AC)
V2:
1: 151 v
2: 0
3: 1.25 v
4: 3.58 v (AC)
5: 3.58 v (AC)
6: 156 v
7: 0
8: 1.16 v
9: 3.58 v (AC)
V3:
1: 190.1 v
2: 17 v
3: 26.8 v
4: 3.58 v (AC)
5: 3.58 v (AC)
6: 182 v
7: 17 v
8: 26.8 v
9: 3.58 v (AC)
V4 & V5 (identical readings):
1: 0
2: 3.58 v (AC)
3: 322 v
4: 321 v
5: -37 v
6: 321 v
7: 3.58 v (AC)
8: 0
I realize that these readings will be slightly lowered by the bulb limiter. Unfortunately, as soon as I remove the limiter and connect regular AC line, the fuse immediately blows. Speaker load is connected whenever the tubes are in BTW.
Because of the fuse blowing situation, I can only perform measurements with the bulb limiter in place. With it set up in that manner, the PT secondary B+ reading at the board is 272 v (AC). I don't know if the bulb limiter lowers that reading, but I assume it does. The PT, OT and Choke were made by Mercury Magnetics specifically for this amp. Spec calls for 345-0-345
Obviously something drawing too much current from the HT for the fuse. The tube voltages look proportionally ok and high enough that it is not a short. I wonder if a slow-blow fuse would hold it? I also wonder why with the limiter in your filament voltage is high (over 7VAC) while the other voltages are low. Pins 1 and 8 on the power tubes should not read 0V. Are you sure you have the 1 ohm current sense resistors and bias test jacks wired correctly?
I do have a slow blow w/the proper rating. However, the layout calls for fast blow here and I don't have the expertise to comment on safety of slow blow. Can anyone advise if that swap is safe or not?
Martin,
I rechecked the filaments, and made a mistake when posting by looking at the wrong list for those. With the limiter in place, the filament pins are running 2.67 v each (total 5.34 vac).
I also rechecked pins 1 & 8 on the power tubes. With the limiter in place, these are reading 5.2 mv.
They are wired per the layout. Pins 1&8 share a leg of the 1R2W resistor which runs to the com point for the bias check, but are otherwise w/o any wiring.
My apologies for the error in my earlier post, and my thanks to you for catching it.
So the HT fuse blows immediately when you switch power on, with the standby off? If so then there must be some wiring error between the PT HT and the standby.
)
