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-   -   Phase Linear 400 (http://www.diyaudio.com/forums/solid-state/180441-phase-linear-400-a.html)

BigE 4th January 2011 09:02 PM

Phase Linear 400
 
Since there is considerable DC offset on my PL400 (125 and 75 mv per channel), I need to replace the input differential pair Q1,Q2 which are currently TIS97's.

Is it safe to use an MAT12 ?

This will guarantee a good match.

Thank you!

stratus46 4th January 2011 11:48 PM

Looks to me like your only problem will be getting it connected due to packaging differences.

G

BigE 5th January 2011 01:13 AM

Stratus46: Thanks for checking it out!

There should be no trouble, since the TIS97's are close together on the pcb. The board is the later PL14B model and it looks like the leads on the MAT12 are 19mm long.

Hopefully, the unit won't need to be re-biased. The service manual seems to treat the bias and offset as independent quantities.

I'll post back the results after changing them in both L/R channels. I'll order a few so that the hfe can be matched between the pairs with a DMM.

Thanks again!

Mark Allen 5th January 2011 01:21 AM

You might just try replacing the small electrolytic caps on the main board first and see if that takes care of it. If one of them is leaky it could cause a dc offset... especially on something this old. It's hard for me believe but I repaired my first Flame Linear 400 in 1973!! you may also find that you can't get the offset as low as you'd like. The model 700 had a DC offset pot... the 400 only had a bias pot if my mind is still cookin.

Mark

bear 5th January 2011 01:31 AM

Imo, that is insignificant DC offset...

_-_-bear

Mark Allen 5th January 2011 01:51 AM

Agreed! However...always re-cap an amp this old first... then go from there. It does you no good to have 30 year old electrolytics in there that were originally el=cheapo 10 cent caps anyway. I remember typically <25mv when all is said and done on a properly working 400. The model 700 could do much better but had to be tweeked occasionally.

Mark

BigE 5th January 2011 02:53 PM

Quote:

Originally Posted by Mark Allen (Post 2421705)
You might just try replacing the small electrolytic caps on the main board first and see if that takes care of it. If one of them is leaky it could cause a dc offset... especially on something this old. It's hard for me believe but I repaired my first Flame Linear 400 in 1973!! you may also find that you can't get the offset as low as you'd like. The model 700 had a DC offset pot... the 400 only had a bias pot if my mind is still cookin.

Mark

If the caps are replaced, won't rebiasing also be required?

http://www.hifiengine.com/manuals.shtml has the service manual, so that's not really a big deal.

Any capacitor should be better than the originals correct? Like non-polarized metal film.

BigE 5th January 2011 06:05 PM

Wow, the mat12 is brutal to locate.

wg_ski 5th January 2011 08:50 PM

Quote:

Originally Posted by BigE (Post 2422272)
Any capacitor should be better than the originals correct? Like non-polarized metal film.


Let's see you try to replace the boot strap cap with one of those. Won't fit.

And the bootstrap is a little weird on this one - it connects to the driver transistor and not the output. And in that position, just a little leaky will give you a DC offset. A lot leaky, and the amp can latch up.

BigE 5th January 2011 08:53 PM

Quote:

Originally Posted by wg_ski (Post 2422700)
Let's see you try to replace the boot strap cap with one of those. Won't fit.

And the bootstrap is a little weird on this one - it connects to the driver transistor and not the output. And in that position, just a little leaky will give you a DC offset. A lot leaky, and the amp can latch up.

I assume that's C11?

So, I will change C11 first, then test DC offset.


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