Marshall mute

[Valvehead]

Can someone show me on this scem where the "mute circuit" is ?

Im told when these Marshall Mode 4 / MF350 models burn the tda7293 chips (this amp did for sure) they can damage the mute circuit .

marshall_mf350_sch.pdf

[Enzo]

Page 3, lower left. right below TUNER OUT.

Look on the page of only the TDA7293 module. page 5. See pin 4 of the long connector is called STANDBY.. That is the mute. Now back to page 3. See the four 15-pin connectors? One for each module.. All the pins 4 are wired together, and they then connect to the left to the mute circuit. WHich is: The two zeners ZD1, ZD2, TR1, and C5. If any of these goes to ground (shorts) that will mute the amp.

Easiest: with all four modules removed, Measure resistance to ground from the top of the two zeners. Either shorted? If so, replace it. Note, ZD1 on the left, has a cap and a transistor in parallel, so any one of them (or both) can be shorted and make the zener look shorted.

When a TDA7293 shorts out and burns up, SOMETIMES it can damage the mute circuit, but certainly not always.


I absolutely hate these amps, and I refused to take them in at my shop. I could not honestly tell a customer that his amp would be reliable, and I was not going to put my name on a repair with that outcome.

Never connect or disconnect a power module while the power supplies are charged.

If one module blows up, replace all four. The four all work together and this amp has a bridged output.

DO NOT LET ANY SIDE OF THE SPEAKER WIRING TOUCH GROUND.

DO not connect or disconnect a load while the amp is running.

The jacks are wired for different impedances. OBEY THE DIRECTIONS.

[Jon Snell]

Mute becomes Standby on pin 4 of the output modules.
Confuses a lot of people.
The Mute or Standby is in Mute or Standby when the voltage on pin 4 is pulled down. Pin 4 becomes pin 9 on the output ICs.
The Standby function is not used in these modules.

Always use commercially supplied ICs. NEVER fit TDA7293s from fleabay!

Observe the warning on the service manual

[Mick Bailey]

I had a batch of modules directly from Marshall where the electrolytics were installed reverse-polarity. In fairness, they sent me a whole box of them as a replacement when I reported the issue.

It's really important after replacing the modules (or ICs) to monitor the DC offset as you gradually power up the amp (no speaker load), ideally using a variac. If there's any DC offset at all beginning to show, power off and investigate. The ICs will fail at very low voltage, especially if the supply rails are imbalanced for any reason such as a bad connection, or even plugging in the modules with any residual charge in the PSU. The spec sheet for the ICs changed a while back and it's claimed that there's a board rewire that may be a fix for the issues, though I haven't tried this myself.

[Valvehead]

Thanks guys - i have 4 complete modules coming from England. I will check all of the mute circuit

I do know first hand about the bogus parts on Ebay

[Randall]

Where to buy these TDA7293's now? I need one for a Marshall 100 watt, but I can't find one, except from China on eBay.

Never mind. Newark has a bunch of them.

[Jon Snell]

Farnell or Element 14 have plenty; https://uk.farnell.com/search?st=tda7293
https://uk.rs-online.com/web/p/audio...er-ics/0218478 but Mouser are temporarily out of stock
No need to replace the boards, check the components on the board when you replace the chip and replace the mica insulator!

[Enzo]

I only replaced the chip most times. Only reason to buy a new little board was if the copper burnt off it when the old chip failed.

[Randall]

"No need to replace the boards, check the components on the board when you replace the chip and replace the mica insulator!"

??? This one has no mica insulator, nor plastic screw insulator. First thing i checked when I pulled it off the heat sink. What's up with that?

[Enzo]

The Marshall modules screw to a heat sink. On some models, the heat sink is grounded and mica must be used. On some other models, the heat sink is screwed to a plastic fan and so is not grounded. In those models we can screw the IC directly to the metal heat sink for better heat transfer. You are working on some 100 watt model while this thread is about the MF350. That is likely the difference.

[DrGonz78]

If the back of the IC is plastic then it is already insulated. The only transistors and IC’s that require insulator are when there is a metal backing on the chip. The metal backings are connected to a leg of the component. Then there are the TO3 types where the case is the actual 3rd “leg”.

[The Dude]

......and some of the Marshalls have screws that go through the heatsink, fan, and chassis, but the screws go through little plastic bushings where they attach to the chassis so that the heat sink is not grounded to the chassis.

[DrGonz78]

That tda7230 would normally require insulators. If the mounting doesn’t connect directly to chassis ground then that is exception. Pulling it off and seeing no insulation plus a failed part definitely makes you think that someone installed it incorrectly. At that point I bust out my ohms meter and investigate if the heat sink connects to the chassis. I always remember those old solid state Silvertone amps from the sixties with the isolated heat sinks. They had rubber grommets where they mounted to the chassis.

edit: On those Silvertone amps the heat sinks were mounted on the circuit board.

[Jon Snell]

"No need to replace the boards, check the components on the board when you replace the chip and replace the mica insulator!"

??? This one has no mica insulator, nor plastic screw insulator. First thing i checked when I pulled it off the heat sink. What's up with that?

Without knowlege of the suffix code on the TDA7293 , most TDAs have the case connected to the -ve rail unless it is the isolated type.

[Mick Bailey]

I always replaced the modules on these, as my trade account buy-in price from Marshall was cheaper than buying ICs + the labour time to replace them.