Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th December 2012, 12:09 PM   #31
wahab is offline wahab  Algeria
diyAudio Member
 
Join Date: Nov 2009
Location: algeria/france
I wonder if the schematic components values are genuine..

The input stage current is 10mA while the collectors loading resistors
are such that almost all the 10mA will be flowing in Q2 , with a massive
DC output offset as a result..
  Reply With Quote
Old 8th December 2012, 12:20 AM   #32
gilwe is offline gilwe  United States
diyAudio Member
 
Join Date: Nov 2007
Ian, Thanks !

I actually took out all transistors in the channel and tested them out of the circuit for both hfe and lack of shorts, all of them tested 250-450 (the BC239C in particular read about 450hfe), is that reasonable ?...

Anyway, I now got some fresh MPSA06, MPSA56 and BD911 I can install. I also have some BC549B which afaik can replace the BC239C (?) I just prefer to use all-new transistors in that channel...

So - with no transistors in the output stage (The two J13009-2, ZTX752, ZTX652, MPSA06 and MPSA56 out of the circuit), I could read 1.95v between E and C of BC550C with the trimpot at minimum position... I first placed two fresh BC549B instead of the original BC239C, but with them in the circuit I was only able to read minimum of 2.05v across the BC550C so I'm not sure I can use them instead, is that correct ?

Quote:
check the voltage to ground too - it should be within 2V of 0.

If that is the case, then adjust the 1k trimpot to give 0V or the lowest voltage possible.
I'm not sure I got it right, but I tested the voltage between the Collector of the BC550C to ground and I read 27v. When testing the Emitter to ground I get 25v....

Also, just wondering - is it ok to use a 2SB892 (60v transistor) instead of the (100v) ZTX752 as driver ? I have a few here.

Thanks a lot !

Last edited by gilwe; 8th December 2012 at 12:27 AM.
  Reply With Quote
Old 9th December 2012, 03:47 AM   #33
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour, on the east coast
OK - nice to see some voltages. "C" range transistor beta varies with the type. You can download datasheets by
Googling the part number and choosing Datasheetarchive, Alldatasheet or Datasheet 4U sites. These are free,
some are just sales gotchas. Lots of info on such details out there!
Here, 200 beta will fine everywhere but the input pair, which would best, but not essentially, be at 350+.
The issue is mainly whether they have any beta at all so you have some assurance that they do work.

Re: The VAS stage, which is what you are left at, without the output stage. It seems there is full rail voltage at
the bias generator which should be close to zero. That means one of the VAS transistors, apparently the positive,
ZTX752 rail one, is toast. Think of it as a wire between E and C and look at the circuit again - It must be close
to rail potential at the BC550 as it is only 2V between its E and C. Test the voltages at the ZTX 752 terminals -
any the same means shorts, right? I hope this all follows pretty easy. Compare your checks with the other channel
and report any discrepancies more than say, 5%.
You need a quantity of ZTX 753/653. to do proper replacements in Naim amps.
The 2SB892 is only a sluggish 50V part and your rails probably exceed 55V at times. If you must keep substituting,
use the BC640 and it complement BC639. Watch the pinouts. These are 80V parts, but not as suited to Naim amps
as the Zetex originals which they still use after 30 odd years.

Let's get the VAS straight first, then think about reassembling the output stage. When the VAS transistors
are functional, retest the E-C voltage of the BC550. The 2V across it is OK for the moment but strange it can't
be zero'd . Maybe a quirk of the design but lets proceed. as it may be I'm missing something
__________________
regards

Last edited by Ian Finch; 9th December 2012 at 04:04 AM.
  Reply With Quote
Old 9th December 2012, 04:39 AM   #34
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour, on the east coast
Hi Wahab. I copied this image link of the NAP 140 clone schematic for comparison. I understand it is not identical to any particular NAIM model but is often posted as a NAP250 schematic too. Call it "generic" but it does work and is similar to my NAP140.
http://www.diyaudio.com/forums/attac...m-nap.jpgAP140 You can see the common design elements easily enough.
__________________
regards
  Reply With Quote
Old 9th December 2012, 05:14 AM   #35
wahab is offline wahab  Algeria
diyAudio Member
 
Join Date: Nov 2009
Location: algeria/france
Thanks Ian ,
The LTP current source is set at 1mA so i guess that its 56R resistor
in the other schematic is actualy 560R.

Since the current through the 1K collector load wont exceed 1mA it
would be convenient to give the VAS emitter a 10R or so resistor as
in such case the voltage across this resistor couldnt exceed about 0.5V ,
automaticaly limiting the VAS max current to 50mA in a very simple way ,
wich is surely the only advantage of anLTP loaded by resistors rather
than a current mirror.....
  Reply With Quote
Old 9th December 2012, 08:47 AM   #36
gilwe is offline gilwe  United States
diyAudio Member
 
Join Date: Nov 2007
Thanks Ian!

So basically I can use BC640/BC639 pairs just for the debugging,
and once having everything work - replace them with ZTX753/653 ?
  Reply With Quote
Old 9th December 2012, 08:05 PM   #37
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour, on the east coast
Well, they should be OK at the low rail voltages here. The main difference from a physical standpoint is dissipation. The tiny ZTX type can actually dissipate more heat but both types are rated 1 amp and pretty fast, depending on who made them. Philips were best, Fairchild OK, but On-semi not so good. They are actually BD139/40 in a tiny case.

The plan will be fine as long as the parts are genuine, you take account of the strange pinouts and you don't pay much, or there is little point.

Click the image to open in full size.Click the image to open in full size.
__________________
regards
  Reply With Quote
Old 10th December 2012, 08:38 PM   #38
gilwe is offline gilwe  United States
diyAudio Member
 
Join Date: Nov 2007
Ian,

I compared the voltages of the bad and good channels, here are the readings:

(VAS) ZTX752 E to C: Bad Ch: 2v , Bad Ch: 27v
(VAS) ZTX652 E to C: Bad Ch: 51.5v , Good Ch: 26.5v
BC550 E to C: Bad Ch: 2v , Good Ch: 2.2v
BC550 E to Ground: Bad Ch: 26v , Good Ch: 1v
BC550 C to Ground: Bad Ch: 24.5v , Good Ch: 1v

What do these readings mean ?

Quote:
Refit the upper (first or VAS) ZTX 752 and power up with the bulb and measure the voltage across Emitter
and Collector of the BC550. It should read sufficient voltage to bias the output transistors with about 30mA.*
That will be about 3 diode potentials or 1.95V. check the voltage to ground too - it should be within 2V of 0.*
Not sure I understand - what should I test to ground (and see 0v-2v) ? BC550 Emitter to ground and Collector to ground ? (and get 0v-2v as on the good channel ?) Should I then set the trimpot so I see as close to 0v in each of these two connections ? (meaning that the "good" channel should be also adjusted? as I see 1v currently...)

Thanks !

Last edited by gilwe; 10th December 2012 at 09:05 PM.
  Reply With Quote
Old 12th December 2012, 09:25 PM   #39
gilwe is offline gilwe  United States
diyAudio Member
 
Join Date: Nov 2007
Update:

I replaced ALL input stage transistors - I get the same voltages !

Used BC549B for BC293C
BC639 for ZTX652
BC640 for ZTX752
MSPA06 for MPSA06

What can cause it ? could this voltages actually be normal, while the different from the "good" channel is because I have no output stage currently there ?
  Reply With Quote
Old 12th December 2012, 10:03 PM   #40
gilwe is offline gilwe  United States
diyAudio Member
 
Join Date: Nov 2007
The voltage between E of ZTX752 (which is now a BC640) and ground,
is equal to the voltage between its C of and ground, which is 29.3v
(about half of the voltage between the positive and negative rails = 58.8v)
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
New Naim Output Transistors Charles Hansen Solid State 67 3rd June 2012 04:57 PM
Naim NAP90 amp schematics... DarkHorse Solid State 34 29th August 2009 07:20 PM


New To Site? Need Help?

All times are GMT. The time now is 08:33 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2