Paramount + soft start not working [resolved]

[Neuronal]

Hi everyone! I have a quick and possibly weird question. I have a Paramount V1.1 with the soft start upgrade. I've upgraded a bunch of caps but it is otherwise stock. One of the two monoblocks suddenly stopped working - no sound - after 7 years of continuous service. The small tube glows bright but the 300b does not...despite the fact that it is heating up physically. If I swap tubes I see the same phenotype in whatever 300b I use. All the components look totally fine - nothing looks blown or funny. I took resistances, and other than what I expect is modest drift (everything is within 15% of my last measurement) everything is normal.

My questions: is it safe for me to take voltages? Which ones are most important to take (given how crowded that layout is)? I ask because in the manual it says to take voltages only if both tubes glow, and this is not my situation.

Thanks in advance for the advice and help!
Bob Datta

 

[Paul Birkeland]

Hello Bob,

What often happens is that the little 1N58118 filament rectifier diodes on the power supply board give up. You can carefully trim each one off by clipping the leads at the body, then solder in a 1N5820 in the same orientation in place of the 1N5818.

I suspect this will bring your amp back to life, and when it does, please consider performing the same work to your other amplifier as well.

-PB

 

[Neuronal]

Thanks, will try this ASAP and get back to you

 

[Neuronal]

quick ? - when I measure resistance it looks fine? (high in one direction like megaohms, and lower in the other). That is of course in circuit - still a good idea to replace?

 

[Paul Birkeland]

If the 300Bs aren't glowing, these parts are primarily responsible for this. If you want to dig in and do diagnostics, there's a red/black twisted pair entering the FC-1, and another red/black pair of wires exiting the FC-1. You'll want to measure the DC voltage between the red/black wires where they enter the FC-1, then another measurement where they exit. I suspect both voltages will be low.

 

[Neuronal]

Thanks for this! As I took voltages, I noticed that one of the ground wires to the FC1 was actually loose - there was a bad joint that failed. I wonder whether it was related to the incredible cold we've been having here in Boston - the unit is right next to a window, and so there are lots of cold/hot cycles with especially big swings recently. Fixing it brought the music back! Thanks so much for pointing me in the right direction, and I hope you are keeping warmer that I am