Sunn Alpha 112 missing two fuses.

Snoopdon

Member
Messages
5
I am repairing (tryin to any way) a Sunn Alpha 112 amp for a friend. The amp is completely dead. The only action is when you turn the power switch on, the light (neon bulb I think) in the power switch lights up. I tried connecting a patch cable from the accessories to and from jacks and also tried connecting headphones to the headphone jack and still no sound. There is no humming and no thump when you turn the power switch on. I've tried setting the output knob to various settings and the drive knob to various settings. I have disassembled the amp enough to unseat the control panel and stand it on end on top of the amp with the speaker still connected. There are three fuse holders on the circuit board. F1 is in series with the 110V AC input. F2 and F3 are in series with the + and (-) outputs of the bridge rectifier circuit, respectively. However, the F2 and F3 fuse holders have NO FUSES INSTALLED!! Seems odd but I thought I'd ask here if this may be some oddball normal configuration. At first I thought F1 (1.5A 250V Slo-Blo) was blown as the glass was cloudy but my ohm meter says zero ohms. But there is a black wire that connects to the circuit board next to this fuse holder and the end of it looks like it is melted from getting too hot. While I was reinstalling F1, this wire came loose from the circuit board, leaving a miniature cone of what looks like milky white epoxy. Any ideas?
 

Attachments

Steppin' Wolfe

Silver Supporting Member
Messages
5,057
F2 & F3 fuse the rectified secondary voltage feeds...+15v on one leg and -15v on the other leg.....to the circuit. Without those two fuses in place the circuit will not function. F1 fuses the primary side of the PT, and the pilot lamp is across that primary winding.
Note: solid state circuits are not my specialty, but a quick look at the schematic provides the little bit of info given above.
i would think that the amp is of an age that all of the electrolytics are suspect. That would be a start. Perhaps those with solid state circuitry will come on board and give you assistance. One would want to check the output transistors and all connected components...from what little I understand of SS amplification.
 

teemuk

Member
Messages
3,178
..and after you have put in new fuses DON'T turn it on before applying some kind of current limiting (i.e. old-fashioned light bulb in series with the primary side). May save you from further extensive damage, which happens in an eyeblink without current limiting. Solid-state devices can die fast.

If the bulb burns bright you know to look for more stuff you need to repair.
 

Snoopdon

Member
Messages
5
Hi teemuk,
Your light bulb idea sounds good. What wattage would you suggest? I would think a low wattage would be better as a 100W lamp will not limit current as much as a 4W night lite bulb and the 4W bulb would be more likely to glow, I would think a solid state amp like this should not draw much current, like maybe less than 1 amp since the input fuse is 1.5 amp. In that case the 4W lamp should not glow at all.
 

Snoopdon

Member
Messages
5
Turns out the speaker was fried, open circuit. Next time I'll check that first for an amp with no sound ;-) I installed a speaker I removed from my Peavey Studio Pro because it was partially blown (rattling sound) and it worked. My friend is ok with that speaker so we called it good.
 




Trending Topics

Top