harman/kardon hk725 preamp rebuild

[z-adamson]

I now have everything that I was waiting on. All parts needed are in my hands and I am wondering about transistor installation.

How do I determine which way to install transistors? How do I identify each leg of the new transistors and then identify each leg of the old transistors and thus confirm that the legs of the new transistors are being soldered to the correct solder pads? Example...a three legged JFET transistor?

Some of the transistors that I am installing get bolted to a heatsink and only go into position one way so I dont think I can mess that up, but others have more room for error so leg identification would be more of a necessity.

 

[jheu02]

Look at least at the first post and attached pic here:

http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

You can figure out how to determine each leg from that post and using a DMM. However, it's easier if you have the datasheets and know what the symbols mean. Don't assume that since it's bolted to a heat sink, the new transistor has the same pinout as the old.

Is the board screened with the transistor symbols so you can tell which is the emitter, base, and collector? Have you downloaded the datasheets of your new transistors and the ones they're replacing?

 

[z-adamson]

Look at least at the first post and attached pic here:

http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

You can figure out how to determine each leg from that post and using a DMM. However, it's easier if you have the datasheets and know what the symbols mean. Don't assume that since it's bolted to a heat sink, the new transistor has the same pinout as the old.

Is the board screened with the transistor symbols so you can tell which is the emitter, base, and collector? Have you downloaded the datasheets of your new transistors and the ones they're replacing?

No screening or labeling on the new ones...the old ones arnt near me to check, but I don't recall seeing anything.

With the heatsinked transistors, once it is bolted to the heat sink everything only goes one way and there is only one way to bolt it to the heat sink, if the pinout is not the same, then they cant be used.

I do have the datasheets for the new transistors, although one of them is not 100% clear. The NTE152 and NTE153 pic does show which leg is which in a drawing, but the drawing does not make it clear wether the drawing is viewed from the front or back.....front I would guess. NTE133 datasheet is clear as to the terminal identification.

But then what about the transistors that I am replacing? No datasheets. It's DMM there unless they are labled.

 

[jheu02]

The screening I meant was the circuit boards. Sometimes the board will be screened with the transistor symbol for an NPN or PNP, other times the holes will be marked EBC, BCE, ECB... Then, you just confirm that the screening matches up with the old data sheet pinout (to make sure the screening was correct, this can also be done by checking the trace of each hole to see what component it attaches to and cross referencing the schematic) and then make the legs of the new ones go into the correct hole.

If you google the transistor number you can find many of them to at least find out the pinout. You can also type that same number in the AK search bar and find threads with recommended replacement transistors. NTE, while they may work, aren't necessarily your best choice as far as replacements go. There are many excellent, better spec'd replacements from Fairchild, On-semi, etc.

Yes, you can bend legs for heatsink mounted ones if the pinout is different. You just need to make sure you insulate the leads so they don't short each other.

Do you inow for sure the one's you're intending to replace are bad? A (very) quick look at the SM doesn't reveal and typical "problem child" transistors that I know of. There were some from the early 70s like the 2SC458 that have known issues with noise or shorts, etc, but the 775 is from the mid 80s correct?

Not trying to be a killjoy, but it seems you're moving perhaps a bit too fast. Stop and smell the proverbial roses. Enjoy the process. Research, research, research. There's loads of great info right at your fingertips.

 

[z-adamson]

The screening I meant was the circuit boards. Sometimes the board will be screened with the transistor symbol for an NPN or PNP, other times the holes will be marked EBC, BCE, ECB... Then, you just confirm that the screening matches up with the old data sheet pinout (to make sure the screening was correct, this can also be done by checking the trace of each hole to see what component it attaches to and cross referencing the schematic) and then make the legs of the new ones go into the correct hole.

If you google the transistor number you can find many of them to at least find out the pinout. You can also type that same number in the AK search bar and find threads with recommended replacement transistors. NTE, while they may work, aren't necessarily your best choice as far as replacements go. There are many excellent, better spec'd replacements from Fairchild, On-semi, etc.

Yes, you can bend legs for heatsink mounted ones if the pinout is different. You just need to make sure you insulate the leads so they don't short each other.

Do you inow for sure the one's you're intending to replace are bad? A (very) quick look at the SM doesn't reveal and typical "problem child" transistors that I know of. There were some from the early 70s like the 2SC458 that have known issues with noise or shorts, etc, but the 775 is from the mid 80s correct?

Not trying to be a killjoy, but it seems you're moving perhaps a bit too fast. Stop and smell the proverbial roses. Enjoy the process. Research, research, research. There's loads of great info right at your fingertips.

I will look at the board for any screening to indicate orientation.

I arrived at NTE via a recommendation from the tech at Mouser. They didn't have what I was looking for and he suggested NTE product that Newark sells. I will make a note that they arnt preferable. Although the specs match up nicely with the ones I am replacing and build quality seems good just looking at them.

There is no way the legs are going to be bent on the heat sink mounted transistors. The terminals are bladed, each blade is short, thick, wide and they are all close together. The only way that I could rearrange the terminals is to remote mount the transistor and solder extensions to each bladed terminal.

I don't know that they are bad, I just know that there was a TON of heat coming from them so I am replacing them as well as anything nearby that got heat soaked from them (caps, resistors, diodes). And yes, HK725, 775 are from the early 80s.

I am aware that I bit off more than I can chew for the moment, but that is a problem that I am ok with having. If my biggest problem in life is that I have lots to learn about this stuff then I guess that means I am doing well.

 

[jheu02]

Great attitude. The NTE products should work and should be correct pinout as they were marketed as aftermarket parts. If the option was a functioning unit using NTE or having a door stop, I'd go with NTE also. Not seeing your NTEs, I can't really comment re. bending leads. However, I can say that Ihave successfully bent in-line leads to work due to different pinout. Remote mounting of transistors is a topic that has definitely been discussed and performed for at least the Pioneer SX-1980 due to heat issues.

 

[z-adamson]

Great attitude. The NTE products should work and should be correct pinout as they were marketed as aftermarket parts. If the option was a functioning unit using NTE or having a door stop, I'd go with NTE also. Not seeing your NTEs, I can't really comment re. bending leads. However, I can say that Ihave successfully bent in-line leads to work due to different pinout. Remote mounting of transistors is a topic that has definitely been discussed and performed for at least the Pioneer SX-1980 due to heat issues.

The transistors look like this...

http://bentronics.com/index.php?main_page=product_info&cPath=16_85_385&products_id=8054

....by the time the transistor is mounted to the heat sink, there will be about 1/16in of terminal hanging past the solder pads on the other side of the board. Bending to change the configuration wont happen without soldering extensions. At that point the terminals would have to be shortened to accommodate extensions or a remote mount would have to be set up.

Since these are cross references of the original part, using the part numbers from hk's service manual and the numbers on the old parts themselves, I would have to assume terminal pin outs are the same, but I would still like to be certain for peace of mind, plus the learning experience.

 

[jheu02]

No screening or labeling on the new ones...the old ones arnt near me to check, but I don't recall seeing anything.

I do have the datasheets for the new transistors, although one of them is not 100% clear. The NTE152 and NTE153 pic does show which leg is which in a drawing, but the drawing does not make it clear wether the drawing is viewed from the front or back.....front I would guess. NTE133 datasheet is clear as to the terminal identification.

But then what about the transistors that I am replacing? No datasheets. It's DMM there unless they are labled.

No screening or labeling on the new ones...the old ones arnt near me to check, but I don't recall seeing anything.

With the heatsinked transistors, once it is bolted to the heat sink everything only goes one way and there is only one way to bolt it to the heat sink, if the pinout is not the same, then they cant be used.

I do have the datasheets for the new transistors, although one of them is not 100% clear. The NTE152 and NTE153 pic does show which leg is which in a drawing, but the drawing does not make it clear wether the drawing is viewed from the front or back.....front I would guess. NTE133 datasheet is clear as to the terminal identification.

But then what about the transistors that I am replacing? No datasheets. It's DMM there unless they are labled.

I agree that the NTE datasheet could be better. However, the drawing of the transistor is as if the metal tab were on the far side of the transistor, and the block below is closer to you. Thus, it's BCE pinout as shown once mounted to the heatsink.

The 2SC1419(C) datasheet is here ("C" references the CENTER collector). The NTEs have good specs except where current gain is involved (hFe). OEM is 35-320, NTE is 40-200. I'm sure they'll be fine here.

2SC1419 datasheet: http://www.svntc.com/TPDF/1947.pdf

I would be careful with the NTE133 jfet. It's only rated to 25V and the 2SK106 is rated to 50V.

http://www.semicon-data.com/transistor/tfet/2sk/tfet_2sk_17.html

 

[z-adamson]

Good catch on the voltage difference with the net133 and the 2sk106. Between digikey, mouser there is nothing for the 2sk106. At newark there is the net133 and that's it. I have no choice but this is reuse the old ones which are toasted from the heat, that's how they look anyways and that is what motivated me to replace them. They are RIGHT next to the big heat sinks. How would one determine if the 25 volt rating will be problematic?

 

[jheu02]

I don't see any voltage markings in the SM, so to be sure you'd have to put the old back in when you're finished, fire it up, take voltage readings in the area and see what you have. This one goes to 40V, but it's on order at mouser.

http://www.mouser.com/ds/2/149/PF5102-889951.pdf

This one is in stock and may also work. Incomplete info on the SK106 so not sure which parameters matter most.

http://www.mouser.com/ds/2/68/2n4391-4393-53044.pdf

 

[z-adamson]

There is what appears to be screening in the board for the 2sk106 / net133. Not sure how to interperate it though. For q604 there is a "G" there, for gate presumably.

 

[jheu02]

Yes, G is for the gate. Can't see in the pic, but I presume the other transistor locations also have screening on the board.
BTW, I'd attempt to remove the baked glue seen in the pic. Some formulations got corrosive over time and can harm components it touches. 91% isopropyl alcohol or acetone applied with a q-tip will soften it a little and then it can be scraped away with a flat xacto blade.

 

[z-adamson]

Everything soldered down. Came out very nice as near as I can tell. My best so far. I tried to position the caps near heat sources as far from the heat as I could, that's why some caps look like that are leaning.

 

[z-adamson]

The biggest pain in the you know what with this hk gear is putting it together and taking it apart. That and nothing unplugs....you are dealing with dangeling components and cord spaghetti full time. The way the chassis comes apart and goes together is not very user freindly.

 

[N8Nagel]

subscribing just because I have one of these beasties.

 

[jheu02]

Dim bulb it when put together to check for shorts and or components installed incorrectly and then let us know how it sounds.

 

[z-adamson]

Hooked it up without dim bulb (dang it) and no sound. Nada. Nothing.

Sooo, played around with my meter. 120v going in, 30v at each leg at the board from the transformer. Voltage bouncing around from .5v to 1v at the aux in (cdp). The manual has listings of all the transistors and how much voltage there should be at each leg. Example...q105, base...21.1v...emitter....21.7v....collector....19.7v. I don't get any of this. I get close to 0v everywhere, this transistor and all others. Hmmm, what to do.

 

[z-adamson]

I made a dbt....didn't think to hook it up because I thought it was for amplifiers, receivers, devices that draw a lot of current.

 

[jheu02]

You just put a lower wattage bulb in for tuners, preamps, etc. 25W - 60W is fine. Gives the visual cue as to whether there's a short/excessive current draw somewhere. Are you referencing chassis ground when you check voltages? Also, make sure that only the very tip of your meter probe is exposed, i.e. insulate the rest of the probe. Slips do happen and shorts can quickly follow.

 

[jheu02]

I just looked at the voltage chart in the SM, had forgotten about it. I'll need to look at it more closely but I'd say that the NTE replacements for the fets Q603,604 are a no go. Voltages are expected to be 32.5 and -32.5 depending on which one. A 25V replacement isn't going to cut it.

As to the zero voltage readings, recheck any ground leads connection points, wires to chassis, etc.