MTX Road Thunder 2600red protect led

[vincegironda]

I have this early MTX road thunder 2600,

Symptoms -

powered it up and the led lights up in red

High pitched whining noise coming from around the power supply

Fan does not engage

Sound on right ch

low distorted sound on left channel and speaker pulsates if turned up.

No fuses blowing, no burnt traces or other visible damage


I have tried disconnecting the fan, made no difference.

The readings for the fets on the L channel vary from the right on outputs and power supply

Any thoughts gents?

images below

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[Perry Babin]

Remove and check the outputs in the dead channel.

 

[vincegironda]

Hi and thanks again Perry

I'll remove them this eveing and post the results.

I don't recognize the output codes, what would be suitable replacement?

Also the power supply fets are ones I haven't seen before

An externally hosted image should be here but it was not working when we last tested it.

 

[Perry Babin]

I generally use the IRF3415 in the 2300s I repair. If this amp has less than about ±70v of rail voltage, they should work in this amp as well.

The power supply have an HUF prefix. HUF75339P

 

[vincegironda]

Hi Perry

I pulled all the left side outputs and they all measured different, a couple were way out but no dead shorts.

I clamped the board back down and momentarily powered the amp back up, the led remained red and the fan failed to spin up.

 

[Perry Babin]

As far as I know, the LED is red. There is no protect indication.

The fan may spin up for a second on startup (not sure if it always does). It's controlled by the output level and possibly thermally.

Do you read infinite ohms from leg 1 to the other 2 legs on all of the FETs?

 

[vincegironda]

I just clamped it all down and powered back up, there is clean audio on the right channel now.

No overheating or high pitched whining.

I'll order the IRF's on your advice Perry along with the HUF's for good measure and with any luck the ol girl will be firing on all cylinders again.

Thanks Perry I'll update when Iv'e replaced the defective devices.

 

[Perry Babin]

You should try to find enough good FETs from thew dead channel to get it working. There's a good chance that there is more damage. Generally, there are no more than 2 shorted FETs on low to moderate power amps like this one.

 

[vincegironda]

Are you saying I should re install the good fets?

I thought it was best to replace the whole bank?

what more damage should I look out for?

 

[Perry Babin]

This is simply to see how extensive the damage is. If the amp plays perfectly with 2 missing FETs, order a full set of outputs for this channel. If it doesn't, troubleshoot until you find the rest of the defective parts. Sometimes there is extensive damage to the smaller transistors. Other times, none. Finding all of the defective components now will prevent having to make multiple orders.

 

[vincegironda]

I got the amp back on the bench as I didn't have much time for an extended test.

You were right Perry, there appears to be a problem with one of the small transistors - Q307 was getting very hot to the point I could smell it heating up.

I turned the bias pot down on the channel left channel which appeared to cool the transistor down, also turning the gain down appeared to stabalize the temperature further.

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