18Watt.com

I know this is a site for tube amps, and I have built a couple myself, but I just thought someone here might be able to offer some advice or point to somewhere I can get help.


I have an old VOX Royal Guardsman that I am trying to repair. I cannot find a schematic for it. The vox amp site does not list this model, and I was told that it is an american made model by a company that is no longer in business. Yes, it is a solid state amp, but hey - it was free. The previous owner has told me that it just immediately blows the fuse when power is applied. I have taken it apart, and started looking from the power transformer outward. It did not take long to find a problem, as 2 of the four rectifier diodes are bad (and yes, I checked them after removing them). The numbers on the diodes are 66-7862 and they have a standard schematic symbol to indicate installation direction. A local luthier shop would not even look at the amp, saying it was probably the output transistors, and that they are no longer available. I have checked the transistors, and they appear to me to be good. I used the hfe test on my meter, and they both tested at 1462 for a gain reading on the PNP setting. The pre-amp section connects to the power amp/power supply section with a connector, so I should be able to test the power section by itself. I am reluctant to just replace the diodes and turn it back on. After all, sonething had to blow these things. Anyone have any advice? (other that toss it and get/build a new one?)



Thanks
Sybex23

not much to tell you,these amps are a mess to work on,and alot of people won't bother. As you said,its most likely something else took out the retifiers. As for parts and crossing the transistors.i would suggest going to Ampage,and talking to Enzo,he has posts there. He is the guru when it comes to the SS stuff..he should be able to tell you where to find the replacement parts,or what to sub for them.
Richie

The best approach is to replace the diodes. Then connect the power cord of the amp to a variac (variable voltage transformer) and set up an AC ammeter to measure the current drawn by the amp (you can usually put it in series with/inplace of the amplifiers AC fuse). Then you can very slowly raise the voltage applied to the amp and watch the current. Everytime you raise the voltage from the variac the current will rise and then drop again as the power supply capacitors charge up. If the current rises and does not drop then there may be still something wrong with the amp. If there is a power supply fault or the outputs are blown then the current will rise very quickly when you turn the variac. The variac allows you to stop before something else gets destroyed and look for further faults.
I think some of these amps used a driver transformer as a phase splitter for the output devices (2n3055s?). Although this type of design has long since fallen out of fashion it can be easier to fix than direct coupled SS amps. 2N3055s are still available.

As a safety caution remember that a variac is an auto transformer and so the amps line cord, ammeter etc should be treated as if there is 120V AC on it even when the variac is turned down.