Help on Marshall AVT150H gone bad..please

[bryanarzaga]

Ok we were trying out the AMP without no cabinet, we plugged a headphone on the headphone jack and its sounded amazing but we didnt have a cabinet so we tried plugging in to a crate combo amp, so we did AVT150's speaker out -> input of the crate combo amp 65watts, it sounded great so after 30mins or so i decided to power it off, but when i hit the combo amps power off switch the light of the the marshall AVT150 went flickering an i think i heard a pop sound after which the amphead gave off a burnt smell, so we opened it up and saw the fuse was burned out, my question is, is because of what we did is there any chance that we fried the amphead? or we just need to change the fuse? also where will we get a replacement tube?

Please advise..thanks

[deluxe reverb92]

oi. you're never supposed to plug the "SPEAKER OUT" on an amp into anything but a speaker cabinet.

[bryanarzaga]

oi. you're never supposed to plug the "SPEAKER OUT" on an amp into anything but a speaker cabinet.

yeah lessons learned but is there a way to fix it?

[quazcha]

Yep....your TDA7293's are smoked

The power amp IC's you are looking for are TDA7293's order at least 4 pcs from mouser or digitech- you will need 2 pcs- thats what the smoke smell was, and may go through another set (or two) if there are other problems, first disconnect all plugs to the 2 IC power amp pcbs, replace the fuse and listen to the headphone out-hopefully you will have sound. the power amp IC's need to be replaced as a set. do a search for AVT150 and MG100HDFX the both use the same IC. email me at: quacha at yahoo dot com if you need the schematic.

[music.man]

Quazcha: I am having problems with my AVT 150 blowing fuses even when I've replaced the (2) TDA7293 chips...I read in this last thread that regarding the 7293's, you mentioned "and may go through another set (or two) if there are other problems"...what other problems could there be? I wanted to check everything out before I lose another set of 7293's, and was hoping you could elaborate on what other problems there could be. thanks for all your time and help! Bob

[Mars Amp Repair]

Hello bob,
Have you replaced both of the TDA7293's at the same time? IF one is bad it will take the other one out.
Also I have seen if you have removed the jumper wires between the main board & the power ic boards where the internal connections to the connector ends get intermittent.

Any slight disconnection there will cause the IC's to blow instantaneously. glen

[music.man]

Hi Glen: Thanks so much for the quick reply!!! Yes...I've replaced both TDA7293's...my +15, - 15 and +5 Volt power supplies are good, fans run. Doesn't blow fuse until I connect either CN102 or CN103 (which are the 3 pin power amp connectors with +/-PSU VE and Tip) to power amp boards A or B. Checked thermal pads with each power amp = OK (no punch-through)...also checked interconnect wires from CN102 and 103 AND C189 and C230 to poer amp boards = OK. Full Bridge Diodes L100 and L101 ohm-out OK also. One curiousity was C141 and C171 (electrolytic caps right off the PSU+VE and PSU-VE rails) momentarily short to GND before they charge up (...I guess all the electrolytics on the board did this?) With the "Back PCB Circuit" (Main Board) completely out of the chasis and disconnected, I connected a 9v battery with a 1K resistor in series across each of the caps to see whether the caps would take a charge, and they did...but only to about 4 Volts and then bled off immediately when I pulled the battery out of circuit. With this scenario, (no "real" GND connection since the board is disconnected and sitting alone on my workbench) should those caps have held the charge? Also, can we exclude the 12ax7 tube and circuitry as being the culprit since all that seems to work off the 15 Volt rail? Thanks for all your support!...very very very much appreciated!!!

[Enzo]

Until you connected the power pins? Please tell me you are not connecting charged power supplies to your TDA boards. MAke sure those power supplies are COMPLETELY discharged before connecting them to your little boards. This is a place for the variac and current meter or at least a light bulb limiter.

[music.man]

Marshall AVT150 Valvestate 2000 Combo blowing main fuse

Hi Enzo: I'm not connecting the TDA boards with any charge on the caps...I just made the observation without anything connected...just the main Back PCB sitting on my workbench. I'm almost afraid to connect everything up again and apply power...I was just questioning how those caps should react when I do apply a charge voltage from a 9v battery. If I were to use a light bulb limiter, how would I hook it up? My assumption is to connect an 8 ohm load in parrallel with a light bulb to the speaker output jack...is that correct? and if so, what would I expect to see? Don't have a variac, so this would be my only resource. Thanks much for your time!

[Enzo]

Do a search in this forum for light bulb limiter. NO, not on the speaker, it goes in the mains conection - often connected in series with the mains fuse.

[Mars Amp Repair]

part of the problem here as Enzo mentioned is without at the very least a bulb limiter, those TDA7293's can fry easily without much provocation.
So...you may be dealing now with the new TDA's blown again. That may be your short you're tracing.

I beleive you'll need to replace them again & find a way to limit the current to the amp ie:bulb limiter before you fire it up again.

Note that the bulb limiter may not limit the current enough to keep the TDA's from blowing again.

All the business of the electrolytics acting initially like shorts is totally normal for a high value capacitor. Before the cap charges, it behaves much like a very low resistance. (ELI the ICE-man & all for those who remember ;-] ).

Ideally you would have a variac & bulb limiter to determine at a low voltage low current into the amp whether or not anything is drawing too much current before the TDA's blow from too much current.

glen

[SY12]

avt150H TRANSFORMER PROBLEM

Does any body know where i can get a transformer for the avt150h amp?
if not does anybody know what the outputs are from the transformer. voltage from the 6 male spade connector?i need a replacement as my transformer has broken
thanx

[Enzo]

Any Marshall dealer can order the part. or a MArshall parts seller like www.marshallparts.com