2270 Differential Pair Transistors

[highhosilver]

Hey everybody, I recently recapped my 2270 and replaced a few transistors (the two on the phono board H705 H706, and H760 on the main amp boards).

I also attempted to replaced the differential pairs H751 and H752, but after doing so, the bias jumped to 55mV and the bias trim pot had no affect on bringing this down at all. Furthermore, when listening through the headphones (and through speakers) there was a very high pitched static noise, comparable to adjusting a AM radio.
Not the best description I know, but it was obviously something that was being produced by the H751 752 transistors, because when I lightly tapped them with a Q tip it changed the frequency of the static noise.
Also, music would play at this point, but the noise was still there when music level was low.

After removing the new transistors and putting the originals back in, the bias returned to 5mv, the trim pots adjusted properly, and the static noise was gone.

So... now I need your help to tell me what I did wrong. the transistors I used were these:
http://www.mouser.com/Search/ProductDetail.aspx?R=ZTX795Avirtualkey52230000virtualkey522-ZTX795A

Were these the wrong transistors? I chose these from reviewing previous threads on replacing these transistors?

If these are in fact the right transistors, the only other thing that I could think of was that I installed them incorrectly, mixing up emitter collector.

I honestly checked and rechecked ten times, the transistor data sheets and the Marantz schematic to make sure I had everything in correctly.

Is the data sheet or the schematic wrong?
Here is the data sheet I used:
http://www.diodes.com/datasheets/ZTX795A.pdf



If anyone knows what happened here please chime in. Any input is appreciated.


I should mention that the 2270 is sounding great after putting the old transistors back in. By starting this thread I am just hoping to get the correct info so I can replace those transistors in the near future.
Thanks!

-Glenn

 

[rswojo]

It sounds like the originals are working good. I'd just leave well enough alone.

 

[highhosilver]

Right you are rswojo.
I would just like to know what the appropriate transistors would be, or if I did something wrong, so in the future if they need replacing I will have the knowledge to do so.

 

[avionic]

What serial number is your 2270.. What is the part number printed on the original H751 & H752's?

 

[highhosilver]

What serial number is your 2270.. What is the part number printed on the original H751 & H752's?

The serial number is 19981, and the original part number is 2SA640.
I am aware that there are some differences between early models and later models. And I am quite sure I made the appropriate part selection according to this fact.

But please let me know if there something important that I could have missed, especially concerning these transistors.

Thanks

 

[ManyMoonsAudio]

The Zetex parts should have been fine. You must have done something wrong. The 795A is a center base part, so you have to be careful installing them where center collector parts used to be.

 

[highhosilver]

The Zetex parts should have been fine. You must have done something wrong. The 795A is a center base part, so you have to be careful installing them where center collector parts used to be.

Thanks ManyMoonsAudio.

Thats all I can up with too, is that I did something wrong. I just don't know what that something is. Like I said, I checked and rechecked about ten times that I had the the collector emitter and base all oriented correctly, according to the schematic and data sheet. In this case bending the collector around the base (isolating it to make sure they don't touch).

All of the solder joints were good, traces all good.

 

[michaelj12]

Did you match the hfe on the diff pair before you installed the replacement?

 

[highhosilver]

Did you match the hfe on the diff pair before you installed the replacement?

I am not sure what the hfe is. Could you enlighten me?

 

[avionic]

Hfe = forward current gain of a bipolar transistor.Differential pairs need to be Hfe matched as closely as possible.
I usually purchase 20 or so transistors . Then measure each ones Hfe.Select the two with the highest matching Hfe and use them as diff pairs.

ZTX694B--example

These PEAK meters are awesome. But a little spendy--Buy direct from UK its cheaper.
http://www.peakelec.co.uk/acatalog/jz_dca55.html


But you can get close enough matching using something like this.
http://www.harborfreight.com/7-function-digital-multimeter-92020.html

 

[michaelj12]

I learned this from avionic himself just a few weeks ago, working on my 240 amp. Get a bunch of the differential transistors you'll be using (10-20 or more), and test all of them for Hfe. Pick the two that have the closest, highest readings and use them for your diff pair. Believe me, it works like magic! Brought my 240 back to life!

Thanks again, avionic, for your expertise, experience, and most of all, patience!

 

[avionic]

I just replaced the diff pair in a Soundcraftsmen MA5002. Reduced the DC offset from 50/65 mv down to < 10mv.:thmbsp:

 

[michaelj12]

Yeah man! Best part is, those transistors are cheap, so it's not like you're wasting money or anything.

 

[airtime]

Both channels the same static?

Are you sure you got them in the right positions?
Why are you changing out good transistors?

Check them to make sure they are good with a DMM or transistor testor
charles

 

[highhosilver]

Hfe = forward current gain of a bipolar transistor.Differential pairs need to be Hfe matched as closely as possible.
I usually purchase 20 or so transistors . Then measure each ones Hfe.Select the two with the highest matching Hfe and use them as diff pairs.

ZTX694B--example

These PEAK meters are awesome. But a little spendy--Buy direct from UK its cheaper.
http://www.peakelec.co.uk/acatalog/jz_dca55.html


But you can get close enough matching using something like this.
http://www.harborfreight.com/7-function-digital-multimeter-92020.html

Thank you very much Avionic. I was not aware of this, and to me it sounds like this is most likely the reason for my troubles.

Thank you very much

 

[highhosilver]

I learned this from avionic himself just a few weeks ago, working on my 240 amp. Get a bunch of the differential transistors you'll be using (10-20 or more), and test all of them for Hfe. Pick the two that have the closest, highest readings and use them for your diff pair. Believe me, it works like magic! Brought my 240 back to life!

Thanks again, avionic, for your expertise, experience, and most of all, patience!

and thank you michael for bring this to my attention.

 

[highhosilver]

Both channels the same static?

Are you sure you got them in the right positions?
Why are you changing out good transistors?

Check them to make sure they are good with a DMM or transistor testor
charles

Yes same sound in both channels. I know I used the word "static" but it was more of a high pitched low level squeal that was changing frequencies as I lightly touched the transistors with a Q tip.

I am positive I got all the legs in the correct holes, according to the schematic and data sheets.

I decided to change the transistors because I was recapping and replacing one transistor on each board, because of the service bulletin. After reviewing threads here on AK, I gathered that the differential transistors were also failure prone so I wanted to replace them while I had the unit appart.

Thanks to avionic and michael, it seems my issue was most likely poorly matched hfe.

 

[quadtech]

Perhaps you know this already, but just in case - it is desirable for the
differential pair of transistors to be in good thermal contact.

Dab a little heat sink compound on the face of the transistors
and use heat shrink tubing to hold them together. In a pinch, you could
also use a cable/plastic tie.

 

[michaelj12]

I'm not sure about the receivers, but the 240/250 amps have a handy little C-clip that holds each differential transistor close to each other. Neat-o!

 

[highhosilver]

Perhaps you know this already, but just in case - it is desirable for the
differential pair of transistors to be in good thermal contact.

Dab a little heat sink compound on the face of the transistors
and use heat shrink tubing to hold them together. In a pinch, you could
also use a cable/plastic tie.

Right, I was aware of this, but thank you for confirming it for me.