I have a Nakamichi (stasis) PA5 that has went bad. I was hearing this humming noise after the unit would heat up several minutes. I usually wouldn't play it long after that but I forgot one day and came back and the amp was dead. I found a bad thermal fuse on the limiter board but it keeps going after replacement. I don't have the exact replacement but sub a 2 amp pico. It blows immediatly. Does anyone have any ideas or a schematic they could email me? I sure hope its not the transformer-looks like one of a kind!
Hi gto127,
I'll try to give you a hand. I used to do warranty service on Nakamichi. Nice amp. We only ever had one blow, due to a fat copper staple for boxes that fell in (PA-7).
These amps also have an inrush current limiter circuit (resistor shorted by a relay). Make sure you don't blow it open. You should be using a variac for servicing. Short out the resistor when you power it up this way.
Nelson would understand how these amps work, but he most likely does not have an exact schematic, but you never know! He just might.
-Chris
I'll try to give you a hand. I used to do warranty service on Nakamichi. Nice amp. We only ever had one blow, due to a fat copper staple for boxes that fell in (PA-7).
These amps also have an inrush current limiter circuit (resistor shorted by a relay). Make sure you don't blow it open. You should be using a variac for servicing. Short out the resistor when you power it up this way.
Nelson would understand how these amps work, but he most likely does not have an exact schematic, but you never know! He just might.
-Chris
Thanks for the schematic anatech-I used the variac and brought it up to about 30 volts input(AC) and it started buzzing like it did when it first went out. Ifound this to be caused by cold solder on a ground wire on one of the main caps. The buzzing at low volts is gone now but it still blows the thermal fuse when powered at full volts. Thought I'd try replacing the spark killer next as well as try to find a source for the original thermal fuse. Does anyone know where I can find one of these special fuses. I can't find the original to look at the value and I feel sure it would be different than the pa7.
Hi gto127,
Do you mean the current limiting resistor? The only thermal fuse I know of is inside the transformer. It would be open then.
Should be around 2 ohms I think.
-Chris
Do you mean the current limiting resistor? The only thermal fuse I know of is inside the transformer. It would be open then.
Should be around 2 ohms I think.
-Chris
No, its labled TF on the limiter board-it's in series with a large 2.2 ohm resistor. I noticed some things on this(limiter) board were different than the PA-7 schematic. I remember this component being yellow and it was heatshrinked to the large ceramic resistor.
Hi gto127,
Sorry, you are quite correct. I forgot about those completely.
Look up at MCM or other jobbers. Coffee makers use similar things.
-Chris
Sorry, you are quite correct. I forgot about those completely.
Look up at MCM or other jobbers. Coffee makers use similar things.
-Chris
haven't had any luck finding this. I think it said 1 amp on the body of unit but I looked it up(months ago) and it had much higher momentary current. I put the fluke across the thermal fuse circuit and its drawing 5-6 amps momentarily then going down to .2-.3 amps after the relays pull. Is this too much inrush current ? If not do you know any device that would meet these specs?
Hi gto127,
-Chris
The thermal fuse seems to be perfectly fine. The relay contacts that short out the resistor / thermal fuse may have arced contacts (rare).
-Chris
I had already put in a generic 12v relay. I guess it may be too far off spec. Will it hurt the circuit to continue with this much start up current?
Probably nothing but I think I threw it away. It seemed like it was arching but it was probably caused by the poor ground on the cap. I cleaned out alot of parts a couple of months ago.-I had become a pack rat. It seems to be working ok now I just hear a slight hum breifly when I cut it on. It only does this mabye 1 of 3 times.
Hi gto127,
Try and get something closer. There should be 0 VAC across that resistor (0 mA through it) with the relay closed.
-Chris
Try and get something closer. There should be 0 VAC across that resistor (0 mA through it) with the relay closed.
Could simply be where on the AC wave you turn it on.
-Chris
I used the NTE equivilant. I doubt I could find an original, would you know another brand that might have tighter specs?
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