Another AVT-150/TDA 7293 Thread

[J M Fahey]

I accept fault for assigning so much reverence for some of you who regularly contribute to issues of others.

Just stop wasting time here and go straight to the Source:
https://marshallamps.com/support/contact-marshall/

[AmpFix]

Assuming that the heatsink mounting topology is correct, do the new ICs fail with no load (i.e. Speakers)?

[Jazz P Bass]

I presume you saw my thread ?

http://music-electronics-forum.com/t31030/#post324354

Well , no need to repeat myself although I didn't get any comments on the accuracy of my "re-vamped schematic".

Summary- Replaced both pairs of 47uF caps on board and... the 2 x 22uF caps (bootstrap) on main board near where
other end of output IC cable plugs in.

It appears that you redid the Slave Sig Ground to -Vdc.
According to the ST schematic, -Input + Input should also be tied to -Vdc.

ST TDA7293.pdf

Just saying....

[oc disorder]

52$Bill

I'll add here, that it seems that Marshall has redesigned the circuit for these modules. They seem to have added static diodes to the input of the chip. I haven't been able to find a schematic for the change, but the new modules come with the added circuitry.

http://www.fullcompass.com/prod/5056...n-M-KITS-00003

JP Bass ..

It appears that you redid the Slave Sig Ground to -Vdc.
According to the ST schematic, -Input + Input should also be tied to -Vdc.

Thanks for that , it's a bit disconcerting to do a search for a diagram via Google and see my incorrect diagram appear in the results.
It's now in my "to do" list along with everything else !

I am placing importance on the bootstrap caps even though I may be misguided. Well voodoo still exists doesn't it ?
Otherwise I have no explanation.

[Jazz P Bass]

I had one of these beasts that kept eating output chips.

I do remember replacing all the caps around the IC (that would include the 'bootstrap' caps).
That seemed to fix the problem.

[Mick Bailey]

Man, I've just read through thinking I'd contribute, but won't bother. How can someone new to the forum get so disrespectful so quickly?

[The Dude]

Man, I've just read through thinking I'd contribute, but won't bother. How can someone new to the forum get so disrespectful so quickly?

+1
I apologize for forgetting to take my hat off and not bowing.

[oc disorder]

Steady on chaps .. One must know one's station in life mustn't one ?

[Allen.Gordon]

Enzo, I'm aware of and respect your knowledge on electronics. Many of your posts are the reason I decided to add this forum to the long list of other forums of which I've been a member.

I understand the issue with noobs; hence the precise information I articulated in my first post of this forum, and coincidentally, this thread.

My frustration is that you and other members didn't read the information provided in my post before posting your typical response to these threads.

For instance, I was quite clear on the Arctic Thermal Compounds, and you're first reply was to repeat the very information provided in the original post.

The fact that you point out my post count and your generic response leads me to believe that you don't read posts, but look at post counts and reply accordingly.

Seeking resources elsewhere isn't a threat. I'm simply disappointed by progress of this tread after reading other threads where information was read, assessed, and addressed.

Allen, you have three posts. Many people come here and ask questions. Some of them know nothing at all, and some know quite a bit. We have no idea after your first post what you do and do not know. We are also cognizant of the many other people reading or who will read the thread. Many if not most of them do not go and read through all the other 7293 threads. So we cannot assume they have that information ahead of time.

We didn't know how aware you might be about various arctic silver compounds. We also didn't know which heat sink scheme this model amp used, as Marshall used both. So we had to ask. We had to ask before giving further advice, because if the sink were grounded and the tab not insulated, then sparks fly.

And at the risk of sounding snarky, this is a forum, a place to discuss things, not an answer service. So if we give you extra information not directly aimed at your specific question, that is just part of life. If you walk into a sports bar and ask "what was the final score in the Spartan game and how many points did Valentine score?" Chances are you don't get two numbers and then silence, you get a discussion of the game. Even if you watched most of it already.


The amp ICs are functional blocks. If they have proper power connected, and a proper load, and no unwanted DC on the input pins, and not muted, then it should work, and if not, is likely bad. We have discussed not blowing them up in those many other threads, especially the MF350 threads.

[Enzo]

Allen, I only mentioned to post count to point out that we don't know you from Adam. I do read the posts. What I didn;t know was how well YOU understood the different heat greases. if you do, fine, but we couldn't assume that. We did read your post, we just didn't respond to it in the way you expected.

I once had a guy come to my counter and asked about checking out an amp he had not making sound. One thing I suggested was he check the plate voltages. Up until that point he had been discussing it in a seemingly knowledgeable fashion. He said to me, "Right, plate voltages. Now the plate is that metal part the transformers are bolted to?" I swear to god he said that. SO I learned not to take someone at face value instantly.

If your experience level is way higher than we may have thought, great, that will become evident as threads progress. I have a darn good idea the expertise level of guys here like Jazz and Fahey and Chuck and the guys in this thread, so if I don't understand something they write, I know it is either a typo on their part or a misread on mine. I don't yet have that luxury with you. I hope you become a valuable member of our forum.

[Allen.Gordon]

I must apologize to you all. Enzo was right when he said I "expected a different reply."

When someone takes the time to elaborate clearly on an issue and is asked questions that were already addressed in the original text, it can cause disappointment.

I allowed myself to react emotionally to that disappointment.

For that, I'm sorry.

Allen

[oc disorder]

After Enzo's reply I feel a bit of a heel for contributing to the developing negative tone.
Australians often cut through with comments but this is normally resolved with a few beers
and often some drunken scribble on the back of beer coasters.
I can remember my first post.
I felt a bit of an ignorant fool from some small town backwater when I made my first post seeking help.
Some of the technical expertise from the members can be overwhelming.
Luckily the backstabbing and borrowing as in some of the scientific community doesn't exist here and for an
electronics forum to me, it has in general a very humanistic supportive tone regardless of how much one knows.
I make mistakes and often get things wrong. I prefer to be pulled up for it rather than let it drift.

Should point out my forum name came from open circuit coupled with the fact that in this area of electronics
guitar amps , effects pedals etc ,it often appears obsessive and compulsive but I think the Golden Ears brigade
of Hi-Fi really win this prize overall !

Now that's out of the way "Welcome to the forum" Alan and lets get back to what we're all here for.

That's to mention Mr Trump as much as possible isn't it?
I thought it was time to elect another actor and he seems to fit the bill.?

That's another section of the forum.

Back to the bloody AVT's!!

O.C.

[g1]

If you tried fresh complete modules (not just power IC's), and had the same bad sound, then I think it's safe to assume the fault is not on the output modules, and may even be a second, separate issue.
I think it should be safe to run without the power modules, to see if the noise is on the FX send or preamp output, also to check your supplies. Then on to check for DC on any of the inputs to the power modules (at connectors with no modules installed).

[Valvehead]

I'll add here, that it seems that Marshall has redesigned the circuit for these modules. They seem to have added static diodes to the input of the chip.

I haven't been able to find a schematic for the change, but the new modules come with the added circuitry.

yes i just bought 2 of those , they go for about $66 per module

[Mick Bailey]

At the risk of being criticised for repeating what's already been posted, I follow the procedure outlined in post #23 here;

http://music-electronics-forum.com/t3873/

This works for me every time when reinstalling new modules and has saved me many times from destroying a new set.

My observation is that a lot of these ICs fail due to the voltage rails not coming up evenly. The sort of momentary DC offset that would create a thump in a regular SS amp will destroy them. The reason they fail if reinstalled with charged PSU caps isn't the actual voltage, but the fact that both power rails aren't connected at precisely the same time. When I've seen repeated failures at switch-on in normal operation and there's no evidence of any other fault, it's been due to rail imbalance.