Another AVT-150/TDA 7293 Thread

[Allen.Gordon]

Hi guys and gals, new to the forum.

I have spent a few weeks searching as a guest and reading all the posts regarding the TDA723 IC's.

I have a decent background in tube amplifier circuits with some classroom training in the mix. I haven't spent much time applying that theory to SS amps, so the "extra" or unfamiliar components can often cause confusion.

I accepted an AVT-150H from a regular client as a favor. Initially the complaint was simply the IC's overheating and causing the amp to intermittently lose output.

I ordered the TDA7293V's from Mouser, and replaced them.

As you may guess, the replacements failed. VERY quickly, might I add!

I used Arctic Silver 5 which is highly conductive. I replaced the IC's again, this time using Arctic MX-4 electrically non-conductive/non-capacitive thermal paste.

This time, no explosion, but a loud noise from the speakers so I immediately ceased that operation.

I ordered a fresh pair of complete modules and of course, the same noise.

At this point, I have 4 more TDA7293's and the 2 unmolested modules with the original IC's considered dead.

My main question right now is how to test the IC's to see if they are, in deed, dead.

I have the ST Datasheet from September 2010.

Best guestimate, I aligned the IC as depicted in the pinout diagram and tested Pins 7 and 8. Both have continuity to ground, so I assume this can't be good as Pin 7 is +Vs. Of course, Pin 8 would be grounded to tab as denoted by the datasheet.

IS this the proper way to determine if the IC's are functional?

Should the IC's be dead, Can someone explain the safest process to reinstall the modules to prevent eating through more IC's?

I ALWAYS drain caps before repairing and amp, but apparently this may not be sufficient.

Thank you for your time and attention!

Allen Gordon

[elipsey]

Hi,

I fixed one of these a while ago (maybe you saw my thread). I'm not sure how much I can help here, but maybe someone more knowledgable will chime in.

What does the "loud noise" sound like?

Do the other parts on the 7293 modules test OK?

Did you check for power supply ripple on the modules?

My approach to the repair was to look for easy stuff first, and hope I got lucky.

[Enzo]

Don't use conductive heat grease. There are two kinds of heat sinks in these Marshall AVT series, some are mounted ON the plastic fan housing, and are thus insulated from chassis. The other kind are bolted to chassis and so are grounded. The tab of the 7293 is connected to V- internally, so it MUST NOT be grounded. Make sure you did not omit any insulating mica washer between sink and IC. If you have a fan mounted sink, then make sure it is not somehow shorted to ground somehow.

[elipsey]

Don't use conductive heat grease. There are two kinds of heat sinks in these Marshall AVT series, some are mounted ON the plastic fan housing, and are thus insulated from chassis. The other kind are bolted to chassis and so are grounded. The tab of the 7293 is connected to V- internally, so it MUST NOT be grounded. Make sure you did not omit any insulating mica washer between sink and IC. If you have a fan mounted sink, then make sure it is not somehow shorted to ground somehow.

Good catch, but I think he means thermally conductive. The manufacturer claims AS5 is dielectric:

Not Electrically Conductive:
Arctic Silver 5 was formulated to conduct heat, not electricity.
(While much safer than electrically conductive silver and copper greases, Arctic Silver 5 should be kept away from electrical traces, pins, and leads. While it is not electrically conductive, the compound is very slightly capacitive and could potentially cause problems if it bridges two close-proximity electrical paths.)

Arctic Silver Incorporated - Arctic Silver 5

EDIT: Re-read OP, I guess he meant it was electrically conductive, but I guess it's not supposed to be? My bad.

[Enzo]

I've never used either, I just was under the impression that at least some of their line was conductive electrically. The OP made the distinction so I assumed...

I used Arctic Silver 5 which is highly conductive...(then)... this time using Arctic MX-4 electrically non-conductive/non-capacitive thermal paste.

(added bolding is mine)

[elipsey]

I've never used either, I just was under the impression that at least some of their line was conductive electrically. The OP made the distinction so I assumed...

(added bolding is mine)

I thought it was too, until just now. Anyway, your point stands about isolating the tabs.

[drewl]

I wouldn't trust grease as an insulator.

We have rolls of various sizes of insulating material and even insulating tape.

[g1]

I wouldn't trust grease as an insulator.

No, but I'm not sure anyone meant that. More so that even with insulators you should not use conductive grease.

[Allen.Gordon]

Seeing how neither question I asked was answered, but arguments abound over rather specific details, I'll seek help elsewhere.

The purpose of this thread was to seek specific information not available in EVERY OTHER TDA7293 thread. Once these answers were provided, we could cluminate them into a sticky with specific how-to's.

Alas, some thrive in confusion and repeating the same information as though it's somehow new information.

THe first question I had was answwered by my own trial and error.

Let's see if we can get this one covered.

The last question in my original post....

Can you provide a step-by-step safe method of reconnecting the repaired modules?

I ALWAYS drain caps before I work inside an amp, but this doesn't seem to be enough for these apparently sensitive IC's.

Thanks to those who read the information and answered accordingly!

[elipsey]

We don't know. You have to diagnose the problem.

Is the heatsink isolated, or not?
What happened when you checked the other stuff I suggested?

[Jazz P Bass]

Step by step instructions for hooking up the modules:

Drain the supply caps.

Hook up both modules.

Turn on the power.

[oc disorder]

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.

[Allen.Gordon]

Thank you, Jazz, for the step by step. Now the specific questions I asked have been answered. Unfortunately, something is killing my IC's. The caps, resistors, and diodes on the modules are fully functional. I will have to probe several different caps through the amp to see if substantial charge is hanging around despite being grounded during service.

Some of you didn't read the original post comletely. Arctic Silver 5 IS electrically conductive as it's a synthesis of Silver, one of the most electrically conductive metals.

The other paste, as stated, isn't electrically conductive.

Paste indeed shouldn't be used to provide insulation, that's what the Mylar spacers and the isolation washers on the screws are for.

Paste is applied to provide a UNIFORM surface for superior heat transfer.

I accept fault for assigning so much reverence for some of you who regularly contribute to issues of others. I felt like I "knew" some of you through all the research I performed before even joining the forum.

Thanks for all the genuine help.

[Enzo]

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.

[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.