Marshall AVT150HX Schematic

[Enzo]

Yes, they redid the power amp for the H version. Let me see if this file will take,

Nope, OK have to resize it.

[901lonestar555]

Thanks, Enzo.

This is definitely what's going on in the AVT-150HX.
It will definitely come in handy.

If you will, please repost the marshall avt150x-61-02-2 ps and tube (original reply #14 on 02/05/2010) and I should have
something close enough to the complete set for the AVT-150HX. The current pdf (posted 02/05/2010) is still opening to a
blank page.

Your assistance is greatly appreciated.

[Enzo]

I'll see when I can get to it. Isn;t the rest of the amp a stock 150?

[901lonestar555]

Yes, it does appear to be the stock 150 as far as the other boards are concerned. It's just that the other pdf you posted indicated 150x-61-02-2 and the 150 schematic pages are numbered differently. Maybe there's a difference and maybe not. I haven't checked all the boards out yet but I think they may be numbered the same as the 150. I'll be taking it apart in the next day or so (depending on whether or not my next amp arrives) to get the buttons from sticking (faceplate was applied crooked so the oval holes don't line up correctly with the chassis front which causes all the push buttons to bind). I received my box of small half-round files so this will be an easy fix.

[diydidi]

old thread new question

Hi
Ok so I have an AVT150X on the bench for repair. The output stage uses the SAP16P/N transistors.
I can't get the darn power amp section to work, Replaced saps, checked all the other components, no luck.
My next thought of action, is to replace this board with a TDA7293 board.
My question however, did these versions of theses amp use one or two TDA7293's. One or two boards?
I would seem they used one for the internal speaker and the other for the external. but I'm not sure.
Can anybody verify this for me. ALSO, pictures of the innards of this version using the TDA's would be great.
I'm gonna make the boards myself from scratch, so just wanna be sure.

[J M Fahey]

It will be easier to repair the SAP type power amp than to put TDA7293 ones there, which are a very poor design to begin with and guaranteed to fail.

What is your exact problem?

Besides it's "modern mask" it's just a very classic Lin type power amp, design technology from the early 70's , go figure, just fitted with power Darlingtons which also include compensation diodes in the chip (good) and emitter ballast resistors (horrible, who wants an electric heater on the same power transistor die?) .

Post a couple PCB pictures, both sides.

If you are ready to make a new PCB, you might clone the SAP one (there is a reason for Marshall designing a new amp to replace TDAS7293) .

Only problem is that since SAP are obsolete, you might have gotten FAKE ones.

Do your homework, search for some power Darlington with similar specs but conventional, 3 legs only, post link here, I may suggest some mods to add external discrete biasing and ballast resistors.

Show also how it's attached to heatsink, etc.

Guess space must be quite tight.

[oc disorder]

I had one with the o/p chips which had me stumped. The power supply must be discharged before connecting the new chips.
Here is my last post with a redrawn version of the o/p stage(s) (haven't had anyone check it btw) and what I found with the fault I had.

Valvestate AVT 150 o/p chips failure

The original schematic (with 2 x TDA 7293) AVT150 / AVT150H / AVT275

Marshall AVT150 complete.zip

[diydidi]

Hi JM
The fault I'm having is excessive current draw. Have bulb limiter connected. Medium brightness. TR9 heating up and SAP's following suit.
I don't exactly understand the circuit. I see FB1 &FB2; where does that go? Also is the feedback resistor on another board?
Maybe I should give the STD03's a go.
Anyways I have plenty SAP's here...

It will be easier to repair the SAP type power amp than to put TDA7293 ones there, which are a very poor design to begin with and guaranteed to fail.

What is your exact problem?

Besides it's "modern mask" it's just a very classic Lin type power amp, design technology from the early 70's , go figure, just fitted with power Darlingtons which also include compensation diodes in the chip (good) and emitter ballast resistors (horrible, who wants an electric heater on the same power transistor die?) .

Post a couple PCB pictures, both sides.

If you are ready to make a new PCB, you might clone the SAP one (there is a reason for Marshall designing a new amp to replace TDAS7293) .

Only problem is that since SAP are obsolete, you might have gotten FAKE ones.

Do your homework, search for some power Darlington with similar specs but conventional, 3 legs only, post link here, I may suggest some mods to add external discrete biasing and ballast resistors.

Show also how it's attached to heatsink, etc.

Guess space must be quite tight.