Help With Nakamichi Sr-2A

[Freddymac2]

As Mannegizen said if you turn it on and in 5 seconds you don't have a click of the protect relay, you have DC at the output, or another problem. On the SR 4a I have a led that blinks if it is in protection mode. The 2 is only a 30 wpc unit, but it is still a Stassis, and it has a very clean output, that you should hear before you dump it. I think it is worth fixing. If you take it to a shop it will probably be $150 to fix it. If you turn it on and the Heat sink get warm both front and back, it is probably working but has a simple connection issue. See if a local AKer would look at it. I will bet that it is something simple.

 

[Mannegizen]

I've looked on schematics now, F401 is one of the rail fuses for +/-37V. Does F402 look OK?

As one rail fuse is gone the protection relay will not engage.

The cause of that can be many things, shorted rectifier (D401), smoothing caps (C401/C402), output/driver trannies (Q710-713 or Q810-813) or as I had in one case, bad solders on the to220 transistors in the +/-18V regulation (Q405 and Q406).

When I blew the rail fuses on my unit I noticed that I got loud burst of noise when moved the receiver, I suspected there might be a bad solder some where and gave it a good thump. This caused both rail fuses to blow with a bright blue flash and a bang from the speakers that almost scared the living daylight out of me.
After fixing the bad solders on Q405 and Q406 and replacing the fuses it still works today, gave it to a nephew along with a pair of Dali 104 and a CD-player some time ago.

If a output transistor is shorted, the most likely cause is that someone tried to measure the bias and shorted the pins.

There is good guides on how to measure rectifiers and transistors with a multimeter.

 

[casiokorg]

hi - been a while but I still have this unit - I figured id ask - does anyone want it by any chance?

 

[dogwan]

I'm in Portland, OR. I would give it a home if nearby.

 

[Ulysse]

Got one SR-2A today with no sound. I found this thread and it is very helpful.
Looks like TO220 transistors are toasted. No click in relay either.
Should I just replaced these transistors? What's the replacement part?

Thanks!

 

[Freddymac2]

For starters just clean connections, remove old solder and re-flow with quality solder, no need to replace transistors.

 

[northpaw]

+1 on Freddymac's suggestion.

I know it looks bad where the transistor pins meet the board, but they all look like that. I've carefully scraped away that brown gunk that has accumulated on the board surface. At first I thought it was just blistered board, but I am not so sure anymore. It is not unlike the brown glue found around the base of some capacitors (and maybe it is the same, and was used to hold that transistor in place for wave soldering). But it does scrape away when probed with a small flat-bladed screwdriver. Then clean that board area up with some alcohol, and then do as Freddymac suggests, and remove the solder on the bottom side and resolder. If you are new to soldering, you might want to practice on a scrap piece of electronics or get some help, as this isn't the easiest thing to learn on.

And clearly, these two transistors (Q405, Q406) run hot. I ended up installing bigger heat sinks that I had laying around. No need for an insulator as the bases are free-floating; just make sure the heat sink doesn't touch any of the components on the board.

Mannegizen has made a suggestion for possible replacements (http://audiokarma.org/forums/index....2a-channel-problem.71484/page-2#post-10300861), but I am with Freddymac here too: no need to replace unless it becomes clear they have failed, and you are not there yet.

 

[Ulysse]

Freddymac, northpaw, thanks a lot. I'll do the cleaning and test the transistors as suggested.

Haven't studied the schematics, what's the purpose of these transistors? Rectifier?

 

[northpaw]

As Mannegizen indicates above, they serve as power regulators on the +/-18V circuit.

 

[Mannegizen]

Got one SR-2A today with no sound. I found this thread and it is very helpful.
Looks like TO220 transistors are toasted. No click in relay either.
Should I just replaced these transistors? What's the replacement part?

Thanks!

If speaker relay doesn't click, it's most likely blown fuses (F401 and F402), caused by blown outputs or bad solders on Q405 and Q406, or/and a high DC-offset in one or both channels.

My bet is bad solders on Q405 and Q406 that has blown the fuses.

 

[Ulysse]

Took out the transistors and tested good. Put them back, still no relay click no sound

 

[dogwan]

I had a similar issue with a TA-3A once. Took me a long time to track it down to the small auxiliary transformer in the power supply. Turned out the unit had been dropped. The small transformer ran a couple of sub-circuits including powering the relays. Well, that small transformer was soldered to the power supply board instead of being bolted down separately. When it was dropped it was enough mass to break a couple of the solder pads loose. The only way I found it was by starting from the power input all the way through the unit. Had to pull the power supply board and inspect the bottom to finally locate the problem. Once I secured to the transformer and repaired the broken traces with jumpers the Nak fired right up.

Anyway, just an idea.

 

[Mannegizen]

Took out the transistors and tested good. Put them back, still no relay click no sound

That's good. Checked the fuses F401 and F402?
Without those working, the speaker relay, the (+/- )18V regulator nor power amplifier section will have power.

 

[Mannegizen]

I had a similar issue with a TA-3A once. Took me a long time to track it down to the small auxiliary transformer in the power supply. Turned out the unit had been dropped. The small transformer ran a couple of sub-circuits including powering the relays. Well, that small transformer was soldered to the power supply board instead of being bolted down separately. When it was dropped it was enough mass to break a couple of the solder pads loose. The only way I found it was by starting from the power input all the way through the unit. Had to pull the power supply board and inspect the bottom to finally locate the problem. Once I secured to the transformer and repaired the broken traces with jumpers the Nak fired right up.

Anyway, just an idea.

A good one, beside the fact that SR-2 SR-3 doesn't have a transformer on the power supply board.
But the advice is applicable on the rest of the models.

 

[Ulysse]

Yes, the fuses F401 and F402 are blown. How do I remove them? Looks like they are soldered in. Don't have 4A 125V fuse on hand, can I use 3A 250V instead?

 

[Mannegizen]

Yes, the fuses F401 and F402 are blown. How do I remove them? Looks like they are soldered in. Don't have 4A 125V fuse on hand, can I use 3A 250V instead?

Never seen fuse soldered on these units but a previous owner might have made modifications. Usually they are glass fuses with metallic caps in holders that are easy to pry out (turn it off first, been there done that, got my fair share of electric shocks. ).
Could you take a picture?

The 3A 250V fuse might work, but I'm not sure. Some one else might be able to tell if it's OK.

 

[Ulysse]

Yeah, this one looks weird.

 

[Ulysse]

OK, soldering in two 3A 250V fuses and it works!!! Hope it lasts.
Thank you guys for the help!

 

[Mannegizen]

Looks like it was delivered like that tho. Never seen it on a Nakamichi before tho.

Gratz on a working unit, I bet it will need some DeOxit tho.

 

[Ulysse]

DeOxit is the first thing I did. Now I need to check DC offsets tonight before I put it all back.