Transistor replacement options - PM 655vxi

[iaRIVR]

Hi All-

I have a HK PM655vxi that I may have (absolutely) toasted an output transistor on. The tech I work with also recommended replacing the driver transistors.

Output- 2SC3281, 2SA1302 (O or R)

Mouser has MJL1302 and MJL3281 which were recommended as replacements. I haven't been able to find matched pairs of original or the MJLs, best option as of now seems to be to replace all 4 outputs with the MJLs.

Driver - 2SA1111/2SC2591 (Q or R)

These are also no longer available, any advice on a suitable replacement?

They look legit as far as I can tell: Ebay: https://www.ebay.com/itm/1pairs-OR-...220-2SA1111-2SC2591-A1111-C2591-/162639799869

I am really out of my league trying to figure this out an mostly relying on what I've read in other forums, so any advice is greatly appreciated.

Plan so far is to replace the outputs with the MJLs from Mouser, and the drivers with the ones from Ebay if I can find a modern replacement.

Thanks, Mike

 

[FootFungus]

For the outputs the MJLs work, or the 2SA2121/2SC5949 are pretty much the exact same transistor from Toshiba.

Those drivers you linked to are remarks of some kind - the aren't original. They have the frosted ROHS look.

 

[ivandezande]

Don't buy electronic parts from eBay, no shot those are real. Just use MJE15032/3 for the drivers from Digikey/Mouser.
Don't worry about matching the outputs, normally ones from the same batch are close enough. Be sure to replace the offset and bias trimmers. I'm in the cities as well, feel free to shoot me a DM if things go south.

 

[iaRIVR]

Glad I checked, feels dodgy buying from Ebay.

ivandezande- thanks so much for the offer. I'll order the MJEs tonight and see if I can figure out which trimmers I need. Think I can do that.

 

[FootFungus]

This looks like a wide bandwidth amplifier (180khz), so the MJE150XX might be too slow. The 2SA1111/2SC2591 are probably the fastest TO-220 drivers made.
Bdent has these you might look at:

2SC4793/2SA1837
2SC2344/2SA1011
2SC4883/2SA1859
2SC5171/2SA1930 (these are the fastest, but have slightly lower power handling/soa at the knee)

EDIT: Yikes, compound darlington driver stage. Make sure you check the 2SC2235/2SA965 right before the 2SC2591/2SA1111. Make sure to double check for oscillations after any repair.

 

[iaRIVR]

This looks like a wide bandwidth amplifier (180khz), so the MJE150XX might be too slow. The 2SA1111/2SC2591 are probably the fastest TO-220 drivers made.
Bdent has these you might look at:

2SC4793/2SA1837
2SC2344/2SA1011
2SC4883/2SA1859
2SC5171/2SA1930 (these are the fastest, but have slightly lower power handling/soa at the knee)

EDIT: Yikes, compound darlington driver stage. Make sure you check the 2SC2235/2SA965 right before the 2SC2591/2SA1111. Make sure to double check for oscillations after any repair.

Yikes indeed! Looks like Digi-key has the 2SC4883/2SA1859 instock, but that's the only one I can find of the four you mentioned.

If I remember right I think one of the drivers was questionable, but not for sure bad and recommended replacing just to be safe. Should I just try replacing the output transistors first, or is there a risk of damaging them if one of the drivers is actually bad?

EDIT: see now bdent is bdent.com, so thanks! I'm really at the mercy of the forum here, of those four which would you go with?

 

[rickl]

Should I just try replacing the output transistors first, or is there a risk of damaging them if one of the drivers is actually bad?

I'd suggest replacing the drivers too. At least test them with a transistor tester before you install the new outputs. You might want to test the other transistors too (pre drivers, ...). I didn't look at the schematic but if other parts are bad, it would be cheaper (and quicker) if it was on the same order.

I recently fixed a marantz with outputs, drivers and pre drivers where blown (and a few other transistors in the power amp section). kind of a pain to test all transistors but made getting the amp working easier.

 

[FootFungus]

Assuming you don't need the metal tab I'd probably go with the 2SA1930/2SC5171 given they are the fastest. If the 2SA965/2SC2235 turn out to be bad, then the KSA916/KSC2316 are the Korean versions which are readily available.

 

[iaRIVR]

Assuming you don't need the metal tab I'd probably go with the 2SA1930/2SC5171 given they are the fastest. If the 2SA965/2SC2235 turn out to be bad, then the KSA916/KSC2316 are the Korean versions which are readily available.

Thanks, I will also take your earlier advice and test all the transistors before I start ordering things. I was getting excited and impatient. Will post the results tonight or tomorrow.

 

[iaRIVR]

Out of curiosity, would the 2SA1930/2SC5171 or MJE15032/3 be lower performance than the original 2SC2591/2SA1111?

Edit- maybe starting to make sense. Are you referencing the transition frequency? Original is 200 MHz, which is the same as the 2SA1930, whereas the MJE15032 is 30 MHz.

 

[dlucy]

One of the big selling points or specs of that era of HK TOTL consumer gear is the wide bandwidth (and high slew rate).

The fT of a transistor, I believe, is the frequency the transistor stops amplifying. So, a transistor that has a fT of 30 MHz would amplify from 1 Hz to 30 MHz. Am I getting that right?

If so, then I'm failing to understand how that would affect the audio frequency performance of any amp. I know HK and others tout their ability to go far beyond 20 kHz, but I don't understand why that makes their gear better than another brand/design that only covers the audio range.

To the contrary, I know amps can overheat is they can amplify above 20 KHz and they actually get sent signal above 20 KHz. Don't many designs use lowpass filters (e.g. caps) to eliminate higher-than-audio-frwuency before it can get to the driver and output transistors?

Sorry to ask deep on your thread, but I'd really like to know why a great output transistor like MJE1503x isn't as good as the same with a higher fT.

 

[rcs16]

Sorry to ask deep on your thread, but I'd really like to know why a great output transistor like MJE1503x isn't as good as the same with a higher fT.

You can read up on bjt operation.
In some cases it is better, look at SOA curves, Pd and other DC specs as examples. people through these transistor part numbers around without much thought as to how they would work in the actual circuit. many of these parts are obsolete and make people resort to eBay for these which I frown on unless you trust the source.
In designing an feedback amplifier you are concerned about oscillation, the phase margin determines stability, faster transistors cause less phase margins, but also can oscillate more readily because they are so fast and support higher freq of oscillations. circuits require analysis, ltspice is free

 

[iaRIVR]

I'm really curious too. Hz was the only thing I saw on the spec sheet that looked like speed. Based on @dlucy that may not have been what @FootFungus was referring to?

This looks like a wide bandwidth amplifier (180khz), so the MJE150XX might be too slow. The 2SA1111/2SC2591 are probably the fastest TO-220 drivers made.

Why can't they just include extra transistors with the amp like IKEA does with screws???

 

[UncleBingo]

One of the big selling points or specs of that era of HK TOTL consumer gear is the wide bandwidth (and high slew rate).

The fT of a transistor, I believe, is the frequency the transistor stops amplifying. So, a transistor that has a fT of 30 MHz would amplify from 1 Hz to 30 MHz. Am I getting that right?

If so, then I'm failing to understand how that would affect the audio frequency performance of any amp. I know HK and others tout their ability to go far beyond 20 kHz, but I don't understand why that makes their gear better than another brand/design that only covers the audio range.

To the contrary, I know amps can overheat is they can amplify above 20 KHz and they actually get sent signal above 20 KHz. Don't many designs use lowpass filters (e.g. caps) to eliminate higher-than-audio-frwuency before it can get to the driver and output transistors?

Sorry to ask deep on your thread, but I'd really like to know why a great output transistor like MJE1503x isn't as good as the same with a higher fT.

I think of it as a transistors ability to do other work while it's doing that 20k; rarely do we see music that is a pure sine wave. Say there is a 20khz tone happening at the same time as a 20 hz tone, a 500hz, 5000hz tone and harmonics from complex instruments all happening in that same 1 second snapshot. The transistor has to be able to do all of that stuff in that time-window at a slew-rate that may demand large current surges. A faster transistor can navigate those transitions more cleanly and generate less heat doing so. The trade-off is that circuit design is more critical; parasitic oscillation at radio-frequencies can rob power.

 

[FootFungus]

Out of curiosity, would the 2SA1930/2SC5171 or MJE15032/3 be lower performance than the original 2SC2591/2SA1111?

Edit- maybe starting to make sense. Are you referencing the transition frequency? Original is 200 MHz, which is the same as the 2SA1930, whereas the MJE15032 is 30 MHz.

I was referring to transition frequency. The originals are spec-ed at 200mhz at 50ma. The 2SA1930/2SC5171 are 200mhz at 300ma. If you look at the graphs for the originals (check out the 2SA1535/2SC3944 plastic versions datasheets) they are near 300mhz at 300ma. Toshiba doesn't provide graphs for ft on the 2SA1930/2SC5171. If I measure a 2SC2592 vs a 2SC5171 on my B&K 530 (at around 10ma IIRC) the 2SC2592 is 90mhz, a 2SC5171 is 50mhz. A MJE15034 is 25mhz and a MJE15032 is 20mhz. The datasheets show the MJE15030 at 60mhz at 300ma, and the 15031 at 75mhz at 300ma. The MJE150XX parts are big transistors - the original MJE15031 (there's been at least 1 die shrink) had capacitance around 150pf. A MJ21196 is 180pf at 10v. The 2SA1111 is 30pf at 10v.

This is not only a fast amp, but they also used a somewhat uncommon compound darlington (CFP) driver setup. So there is a nested feedback loop within the output stage that can cause additional oscillations. In the schematic there is an inducter in parallel with the base resistor to the final outputs from the compound driver pair. I'd definitely recommend rigging up a dim bulb tester and use it when powering this up for the first time.

One of the big selling points or specs of that era of HK TOTL consumer gear is the wide bandwidth (and high slew rate).

The fT of a transistor, I believe, is the frequency the transistor stops amplifying. So, a transistor that has a fT of 30 MHz would amplify from 1 Hz to 30 MHz. Am I getting that right?

If so, then I'm failing to understand how that would affect the audio frequency performance of any amp. I know HK and others tout their ability to go far beyond 20 kHz, but I don't understand why that makes their gear better than another brand/design that only covers the audio range.

To the contrary, I know amps can overheat is they can amplify above 20 KHz and they actually get sent signal above 20 KHz. Don't many designs use lowpass filters (e.g. caps) to eliminate higher-than-audio-frwuency before it can get to the driver and output transistors?

Sorry to ask deep on your thread, but I'd really like to know why a great output transistor like MJE1503x isn't as good as the same with a higher fT.

ft is the gain bandwidth product. So if its spec-ed at 30mhz, the transistor would provide a gain of 30 at 1 mhz, or a gain of 3 at 10 mhz. If we need a gain of 100, then we get 300khz of bandwidth.

 

[iaRIVR]

@FootFungus, and all, thank you for taking the time with this. I pulled the drivers tonight, and tested with my cheap component tester and multimeter on diode. Initially the component tester recognized the ones from the bad side as diodes, but after checking back later and running through twice the all tested as PNP or NPN transistors, Hfe 120-140, and Uf around 615-630.

Multimeter results:

BE BC EC EB CB CE
1111 672 OL OL OL OL 673
2591 OL OL 685 681 OL OL
2591 OL OL 695 692 OL OL
1111 677 OL OL OL OL 680

So, it seems like they might be good, but my brain is a little dead and I didn't have time to read through transistor testing again but hopefully you can tell by this. This is where I'm at:

1) Consensus that there are good replacement options for the output transistors and pre-driver transistors. I'll replace both of those for sure.

2) Two leading candidates for replacing the driver transistors may be too slow, or fast but prone to oscillation.

What say y'all - put the stock drivers back in and try it, or replace? I don't have a dim bulb tester, but I could bring it back to my tech guy if you're really worried about powering it up for the first time.

 

[FootFungus]

Go ahead and try it with the originals. I still would suggest building a dim bulb tester. You can put together one for under $10 bucks at your local mega-hardware store. It will prevent further major damage if there are other problems or you make a mistake.

 

[rickl]

Go ahead and try it with the originals. I still would suggest building a dim bulb tester. You can put together one for under $10 bucks at your local mega-hardware store. It will prevent further major damage if there are other problems or you make a mistake.

I was going to suggest the same thing. If you lived closer, I'd let you use my DBT. Pretty easy to make. A DBT will protect the new transistors if the existing drivers are marginal.

 

[iaRIVR]

I think I have everything to build one lying around! I'll do that just in case and read up on how to use it.

I ended up ordering the new drivers, they were $4 so less than shipping. I'll do the dim bulb, and pull and test the 2SA965/2SC2235. Couldn't get those from bdent, so that would be another order anyway.

Seriously thank you again!!!

 

[rickl]

I think I have everything to build one lying around! I'll do that just in case and read up on how to use it.

we can go private if you want to see pictures or how I use mine. I have an outlet, switch and bulb socket. kind of simple. I used a 'remodeler' electrical box from HD (I think or might have used what I had laying around too). I use a lot of those blue boxes for test equipment. Cheap and easy to source.