Hi folks,

I got a Mark Levinson amp on my bench that supposedly wasn't working. I specialize in first gen LS'es and this one is new for me. The amp was in for repairs at a specialized shop, but never finished. When I opened it, I noticed two missing components, so it makes sense it doesn't work. However, I have no idea what the part numbers and specs of these two components (transistors) are, does anyone here have an idea? They are labeled Q491 and Q492, located on the heatsink under the large transformer. Any help is greatly appreciated!

 

These guys might be able to share info with you: https://www.unitedradio.com/

 

Without schematics you're groping in the dark. Last person to work on that should stay away from electronics.

 

That is obviously the switching power supply that converts the voltage from 12V to a higher value needed by the amplifier for high power.

So the missing transistors (Q's), must be a couple of N-Channel MOSFETs in the TO-220 package style, and the "D's" nearby are the high-speed switching rectifier diodes.

You can select a MOSFET that is relatively low-voltage, high current, and low on-resistance. For automotive applications like this, I often see the "IRFZ44N".

If you search the internet for "TO-220 power MOSFETs for automotive amplifiers", it will give you the following results:

IRFZ44N (55V, 49A, commonly used for high efficiency in power supplies)
IRFB7545 (60V, 95A)
STP100N8F6 (80V, 100A, a high-current option, often in automotive applications)
FDP038AN06A0 (60V, 80A-100A range, onsemi N-channel, suitable for automotive use)

From the above, any of the last two is preferred. All those are available in marketplaces like eBay.

NOTE:
My answer is based on what I can see in the pictures and my experience on electronics, but without a diagram, this cannot be 100% accurate, but highly probable.

EDIT:

Taking a good look at the traces, my answer will be 99.99% correct. You can see the primary coils of the switching transformer attached to the "Drain" of the MOSFETs, and the secondary coils attached directly to the rectifiers, so the MOSFETs listed above will do the trick, as long as there is nothing else damaged in the circuit, like the output stage of the amplifier.

 

Only go by a schematic or picture showing part # of parts with no guessing. Those missing parts could even be regulators.

 

Due to the configuration, not regulators. If you see the pictures, you will notice that they are configured as a push-pull circuit in the primary power winding. The power goes to a common point (center tap) of the two coils of the primary winding, where the 12V is fed. Then the other two ends of the primary coils go to the drains of each MOSFET, each one for a different phase of the wave. A pretty common configuration for a switching step-up power supply for an amplifier, due to the size of the transformer.

 

Thanks so much for your input RickRod, I'll order the components you suggested. Will let you know how it ended!

 

Even blowing this up I don't see all the traces but meter could verify where they go. It should not be hard to find out exact parts to go in but from looking at the picture blown up it looks like more problems anyway like caps leaking that most likely shorted out the parts. If I were still in business and this came into my shop I would turn it down. Of course if cannot locate correct part you can do as Rick suggested. Good luck.

 

Right or wrong this is what AI says.......Replacement Part Numbers
According to documented repairs of similar Mark Levinson automotive amplifiers, these circuit positions often utilize the following specific semiconductor parts:

• Q491 (NPN Transistor): Commonly identified as 2SD1913 (or a similar high-power NPN equivalent like 2SD2390 depending on the specific board revision).
• Q492 (PNP Transistor): Commonly identified as 2SB1273

 

What you mentioned that AI suggested is a complementary pair. Mostly used in the output stage of the amplifier, not in the power supply, and the missing transistors are clearly in the power supply.

But that's easy, if it is a complementary pair (using the bipolar 2SDxxx and 2SBxxx in your reply), you know that in that configuration, the Emitter is connected to ground in the NPN transistor (2SDxxx), but the Collector is the one connected to ground in the PNP transistor (2SBxxx). If, instead, MOSFETs are used, as I suggested, the third lead of both transistors (Source) will be connected to ground.

So, with the multimeter, he should check continuity to see which leads are connected to ground.

If the circuit is as I suggested, using MOSFETs, and Bipolar transistors are installed instead, as AI suggested, that is a new short circuit coming.

 

Agree and I would do more diagnosis before putting anything in there. Also even if correct part is put in it most likely will blow as majority of time something causes it to blow. Hard to tell but looks to me like caps are leaking. Customer should have originally sent this to a repair center for this amp.

 

I have ordered the parts, it took quite some time to find them. Hopefully, some STP100N8F6 mosfets will arrive anytime soon. I wanted to reply to Alextv' s post: The car was sent to a Lexus specialist who said they could fix the issue. When they couldn't, they sent it to a specialized repair shop at their own cost. The car belongs to a friend of mine who always has it serviced by this specialist. After six weeks, he got the car back, but without the amp installed. The Lexus specialist said that the amp was unfixable because the repair shop couldn't find the components.
It ended up in my workshop because I work a lot on the electronics of the first gen LS400 - which my friend also has. He asked me to have a look, and the first thing I noticed were the missing components and the scrathed circuit board. They scraped some conformal coating away to probe it, but they damaged the traces in some places.
I don't have my hope's up too high, but with missing components in the power supply, it will never work. So it is worth a try for me. I'll keep you posted, as promised.