Optonica SM-4000 amplifier NUDIES, plus problems

[leesonic]

Here is a rare bird, an Optonica SM-4000 amplifier that I'm looking at for a fellow AK'er. Solid construction, the thing weighs a ton, with a big transformer, 2 x 15000uF of power supply regulation, and two big heatsinks.

Only thing is, it won't come out of protection. I measured the offsets, the right amp board has 405mV :worried:, but the left channel has a whopping 41.5V (V, not mV) . I noticed some previous "repairs" in here, including this weird double resistor arrangement to replace an driver emitter resistor (circled in yellow) that had blown a crater in the board. This amp features push-on connectors for all the boards, and pulling the connection off for the left channel speaker output to the protection circuit gives the green LED and the relay clicks on.

The output transistors on the right channel are Hitachi 2SA753 and 2SC1343 pairs, which is what the service manual says they should be, but on the left channel are Toshiba 2SB556 and 2SC1079. Pulling and testing these, the 556 is open circuit from the Base to the Emitter. When I searched through the archives, I couldn't find much replacement info looking for the 2SA753, but searching for the 2SC1343 came up with OnSemi MJ15024G and MJ15025G used in a Marantz amp, that thread is here. Would these be suitable for this Optonica amp as well?

Lee.

 

[leesonic]

I was searching through the archives yesterday, trying to see if there are any suitable replacements for the other transistors used on this amp board. Just in case I have to replace more than the outputs. Pickings were slim, I can tell you. If this was a Pioneer amp, there would probably be complete lists of transistor substitutes. I did some searching around on datasheetarchive dot com as well.

Here's what I came up with, I would welcome comments on them :

2SA673AB = BC327
2SA836D = BC560
2SC1514B = MJE3439
2SA775B = BD240C or TIP30C
2SC1213AB = BC639 or KSD1616 (remembered this from my Kenwood amp)
2SC1410B = BD239C or TIP29C

There are also some diodes on the board. NV-46 is one of those that goes on the heatsink, like the STV-2H used in Pioneer amps. But there are also IS2076, IS2348 and KB365M diodes on there I could use help with suitable equivalents.

I posted a picture of the power amp schematic below in case anyone is interested.

Lee.

 

[HVHiFI]

Hey Lee, I like the previous "repairs". The On-Semi subs for the outputs look good to me. You may want to check out B+D Enterprises they may either have the original Hitachi outputs or can confirm the substitution for you.

Good luck with the project!

 

[ryuuoh]

You can look up transistor subs here:
http://alltransistors.com/crsearch....=175°C&ft=10MHz&cc=-&hfe=30/200&mnf=&caps=TO3

The HV diode is probably thermal tracking for the bias circuit.
The other diodes can be crossed by looking up their type, like
1S2348H Hitachi Semiconductor General Purpose Fast Rectifier
and finding one that matches the parameters of the old one.

 

[leesonic]

Desoldered all the transistors on the board and tested them. They all seem to be OK.



Lee.

 

[ryuuoh]

Looks like progress~
Make sure you use a current limiter when you fire it up,

 

[HVHiFI]

Have you narrowed it down to which section has the issue? I would bet on a diode or fried series resistor.

 

[leesonic]

Have you narrowed it down to which section has the issue? I would bet on a diode or fried series resistor.

It says up in the first post, a bad output transistor. I'm still not convinced how suitable replacements these were...

Lee.

 

[FootFungus]

I'd go with the standard MJ21193/MJ21194 or MJ21195/MJ21196 sets. They are beefier but a lot slower than the originals. It probably won't make much of any difference. You aren't going to do any better in a TO-3 package nowadays, anyways.

I'd go MJE15032/MJE15033 or MJE15034/MJE15035 for the drivers.

 

[leesonic]

Spent some time yesterday working on this amp. I re-installed the transistors on the board, and put in the new caps that I got from Mouser, plus the Bournes multi-turn trimmers. The "previous repair" resistor, I changed out to the correct one, modern 1/4 resistors look so tiny compared to 30-40 year old ones don't they? I mounted it using some ceramic stand-offs that came from my NAD amp. You can see the burn marks and a crater in the board from when the old one let go, I figured if it ever went again it wouldn't do any more damage to the board.



I also installed the new output transistors. No, it wasn't me that scratched the old transistor number into the heatsink.



One of the nice features about this amp is the boards are all plug-in, you don't have to undersolder any connections or remove any wire wraps. I powered it up with just this rebuilt board in, and adjusted the offset and bias. You remember the previous offset was 41.5V? Well, I got it adjusted down to 1.8mV, and set the idle at 33mV as per the factory manual.

Now I just have to re-cap the other amp board and the power supply.

Lee.

 

[HVHiFI]

Awesome work Lee! Looks great. You'll be happy with the ON semi outputs, I have heard them in many Marantz and Pioneers and they sound fine.

 

[leesonic]

Here are a couple of before and after shots of the power supply/protection board. The three capacitors with the darker blue sleeves just left of center are for the phono stage, and are rated at 200v because according to the service manual :

"The transistor at the final stage is given a high voltage of 120v to assure the amplifier of satisfactory dynamic range".

That's SOME headroom. These had been replaced previously with slightly higher values, but I decided to put them back to the factory specs.





Here is the second amp board from the working channel, with replacement caps. I also replaced the resistor that went bad on the other board, not because it was bad on this one, but because I wanted to space it away from the board and the leads on the original one weren't long enough. Keen-eyed viewers might notice the new emitter resistors, I had to steal one from this board to test the other board, so I replaced them both with new ones.

 

[leesonic]

I should have added, I replaced the outputs in the working channel as well, just to make everything the same.

Lee.

 

[Repairman]

Here are a couple of before and after shots of the power supply/protection board. The three capacitors with the darker blue sleeves just left of center are for the phono stage, and are rated at 200v because according to the service manual :

"The transistor at the final stage is given a high voltage of 120v to assure the amplifier of satisfactory dynamic range".

That's SOME headroom. These had been replaced previously with slightly higher values, but I decided to put them back to the factory specs.

Here is the second amp board from the working channel, with replacement caps. I also replaced the resistor that went bad on the other board, not because it was bad on this one, but because I wanted to space it away from the board and the leads on the original one weren't long enough. Keen-eyed viewers might notice the new emitter resistors, I had to steal one from this board to test the other board, so I replaced them both with new ones.

I just had one of these show up for repair today, sounds like a similar problem with mine. I do not see any pictures with the links, guess they may only stay up a short time. On the one I had come in today, the cap C518 was totally blown out and serverly burnt along with R556, a 2 Watt power resistor and of course the circuit board was seriously damaged under that resistor.

 

[Cesari0]

Interesting, I'm working on my unit, and looking at the diagrams and schematics, I can't find C518 nor R556.

My unit is giving me a headache, I had sound, then went dead...absolutely no sound.

 

[leesonic]

Fixed dead picture links.

 

[Cesari0]

Fixed dead picture links.

Thanks for fixing the pictures.

 

[Ranchman]

Fixed dead picture links.

Yes. Thank you for fixing the pictures

 

[ozzi013]

Tell me please .... how are there two chips .. It seems like a cutting amplifier And how they are called Model and characteristic

 

[Melvin Mora]

leesonic, did you have the service manual, I try to find out but only got the SM 4000H