Phase Linear 400

[BigE]

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]

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

G²

[BigE]

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]

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]

Imo, that is insignificant DC offset...

_-_-bear

[Mark Allen]

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]

Originally Posted by Mark Allen

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]

Wow, the mat12 is brutal to locate.

[wg_ski]

Originally Posted by BigE

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]

Originally Posted by wg_ski

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.