Hi all,
I'm fixing a pair of NAPA boards from a NAP250 and need to replace a BC239C due to it having a broken leg. I've searched around and have found advice about replacing the long tailed pair with BC546s, among other things, but nothing detailed about the original BC239Cs.
In regard to the BC546s, the advice I seen in multiple places is to ensure that the transistor used for TR1 has an hFE at least 5-10% greater than that used for TR2, with similar values across the pair of boards.
I have read also that BC239Cs used for the long tailed pair need to be 'matched', to keep dc offset in the preferred range, but can find no detail about that process. Does anyone know whether the objective of this matching is to get as close a match, in terms of hFE, as possible or, as with the BC546s, to aim for a target delta between TR1 and TR2 ?
I would like to stick to BC239Cs, to keep the board as stock as possible.
Thanks for reading; any help would be appreciated.
Cheers,
Bo
I'm fixing a pair of NAPA boards from a NAP250 and need to replace a BC239C due to it having a broken leg. I've searched around and have found advice about replacing the long tailed pair with BC546s, among other things, but nothing detailed about the original BC239Cs.
In regard to the BC546s, the advice I seen in multiple places is to ensure that the transistor used for TR1 has an hFE at least 5-10% greater than that used for TR2, with similar values across the pair of boards.
I have read also that BC239Cs used for the long tailed pair need to be 'matched', to keep dc offset in the preferred range, but can find no detail about that process. Does anyone know whether the objective of this matching is to get as close a match, in terms of hFE, as possible or, as with the BC546s, to aim for a target delta between TR1 and TR2 ?
I would like to stick to BC239Cs, to keep the board as stock as possible.
Thanks for reading; any help would be appreciated.
Cheers,
Bo
Naim matched them. They did *not* unbalance them by 5 - 10%. I've also seen that, but I have no idea where it came from.
Julian had a neat little trick of matching them in the amp. Leave the non-broken BC239 where it is. Substitute replacements into the other side of the LPT. When you find two that produce the same DC offset at the output you have a match. Remove the non-broken BC239 and insert your matching pair.
Julian had a neat little trick of matching them in the amp. Leave the non-broken BC239 where it is. Substitute replacements into the other side of the LPT. When you find two that produce the same DC offset at the output you have a match. Remove the non-broken BC239 and insert your matching pair.
Thanks very much for your response. I suspected that match really meant match in this case but I'm very grateful for your confirmation. Excellent plan for finding matches, too, I'll give it a go.
Cheers,
Bo
Cheers,
Bo
What does the print on the BC239 device say ?
Just BC239 or -A-B-C-BP-CP--there is a difference .
More modern version are NOT identical -which have 2 additional numbers after the BC239.
Just BC239 or -A-B-C-BP-CP--there is a difference .
More modern version are NOT identical -which have 2 additional numbers after the BC239.
Thanks for the warning. I found somewhere with old stock so have managed to get hold of some BC239Cs, as per the existing transistors.
As I am about to build your NAP250 clone, just to make sure if that is what I have to do in regard to the BC239s: build the hole board, power it up from the regulator board, leave input and output unconnected, put TR1 into the LTP and observe the DC offset with more transistors in place of TR2 as you described i.e. match them and finally solder a matched pair to the PCB, right?
Well, that's sort of the beauty of the Naim way -- you don't have to know which is more important, or if you're matching at the right current.
But if you do use a tester, I'd try to match both. Vbe should be easy, and then get hFE within a reasonable range.
But if you do use a tester, I'd try to match both. Vbe should be easy, and then get hFE within a reasonable range.
Thanks! In case I can't find a matched pair: is it OK to just put the higher hFE one into TR1? I have only a hand full of BC239Cs and don't even know the manufacturer of them. They have the printing on the top: I attached a photo, does anybody know the manufacturer of them...?
Attachments
Many ( but not all ) big manufacturer,s I know (granted its a long time ago ) had the identification on the flat side not the top for this type of device and those that did had it marked in a smaller way and had initials for the manufacturer
I would suspect they are far east copies or if not then they could be NOS of a cheaper manufacturer to put into old transistor radios.
I recognise the round pins and the centre kinked pin as typical of the period then for PCB,s but I doubt they are from a top manufacturer.
I would suspect they are far east copies or if not then they could be NOS of a cheaper manufacturer to put into old transistor radios.
I recognise the round pins and the centre kinked pin as typical of the period then for PCB,s but I doubt they are from a top manufacturer.