Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: Bill McKenna on February 07, 2019, 11:27:20 pm

Title: Help with 1964 Standel S-50 I Blew up a Cap today?
Post by: Bill McKenna on February 07, 2019, 11:27:20 pm

Hello Solid state experts.
I tend to stay on the tube side of the forum and have been happily building and fixing tube amps for a number of years without needing much help until today :) A friend brought by a 1964 Standel S-50. I think I found a later schematic below. The amp was working  but seemed not loud enough and breaking up early.  Did a little checking and found that the power section Caps were pretty far out of spec so decided to do a cap job. As they were over 50 years old. After shotgunning all the filter Caps. I brought it up on the variac and it seemed ok for a couple mins, was pulling less than .5 amp at 120vac when one of the 1000uf@25v caps  off the negative leg of the rectifier went off like a gun shot . :laugh: . scared the crap out of me and spread cap guts all over the place. Powered down pulled the fuse to the output transistor and powered up again on the one leg that was working and took some measurements From the Rectifier at 120V AC I see 25 vdc on the positive leg of the rectifer and only 16vdc on neg, the side that blew?  Here are some pics from similar 67 era Studio Standel.
Any thoughts on why I'd see -16 one side and +25v on the other? Need to do more checking but a bit lost on these solid state amps.
ThanksBill

Title: Re: Help with 1964 Standel S-50 I Blew up a Cap today?
Post by: sluckey on February 08, 2019, 05:18:00 am

Replace the blown cap. Did the voltage come up to -25V?

Title: Re: Help with 1964 Standel S-50 I Blew up a Cap today?
Post by: shooter on February 08, 2019, 05:33:03 am

Yup, like he said.  transistors are just like tubes, they both start with T  :icon_biggrin:
If it stays low, the split rail feeds the PA, Q3/4, there ain't much to the circuit.

be real careful on polarity, our brains are wired to NOT want to put + to ground. 

Title: Re: Help with 1964 Standel S-50 I Blew up a Cap today?
Post by: Bill McKenna on February 08, 2019, 08:45:08 am

I slept on it last night,  and believe I am a moron  Think the Neg leg should be Positive lead of the Cap to ground on the neg side of the rectifier. Getting a bit slow I guess after years of doing tube amps.
Will check it today and see how it works :)ThanksBill

Title: Re: Help with 1964 Standel S-50 I Blew up a Cap today?
Post by: Bill McKenna on February 08, 2019, 12:19:13 pm

Yep that was it, all good now. Standel seems much happier with new caps.ThanksBill