Autotek MM 4000.1

[S Merlington]

there are a total of 16 output fets in this amp..

I get 31.2 volts of rail on 4 of them per side and the voltages drops down to 2 volts.

On the remaining 8 outputs 4 per side i get 2.1 volts of rail voltage and it stays steady

 

[Perry Babin]

If you're getting rail voltage, pin 1 on the 7500 has to be dropping below 2.5v. It's not important now but you may want to look at it again in case you need to see this type of pulse in the future.

Do you get positive and negative rail voltage (re-check both sides)?

Did you confirm that you had positive and negative rail voltage when the driver boards were out of the amp?

 

[S Merlington]

When i first try to power the amp up i get both positive and negative rail voltages but after 5 secs the rail voltages all switch to positive on both sides..

Without the driverboards out of the amp i get both + - 115 volts of rail on the outputs..

With the driverboards out i measured the voltages on the pads for the driverboards..

On all 21 pins for 1 board voltages look good..

On the pads for the other driverboard PINS 6 and 16 have pulsing voltages and thoose pins connect to where the tace has been cut..

 

[Perry Babin]

Do you read anything near 0 ohms between ground and the legs/pads for any of the output transistors?

 

[S Merlington]

No

 

[S Merlington]

I read nothing near 0 ohms on any of the output legs . I checked other things but can't seem to find the fault..

Any ideas where to go from here?

 

[Perry Babin]

In the first seconds with the driver boards installed, do you see ±115v then it goes to +115 and 0v on the negative rail?

 

[S Merlington]

With driver boards installed I only get. + - 30 volts of rail and the negative rail voltage goes to positive voltage 2.5 volts

 

[S Merlington]

Ok i found 3 problems in this amp finally..

i wanna deal with one problem at a time..

1st problem is what ever is making the amp shut down is on both audio driverboards..

2nd problem is on pad 6 and 16 for 1 of the driverboards i get pulsing voltages..

3: not sure if this is a problem or not but the + - 15 volt regulators are only putting out 11 volts..

Pads 6 and 16 for the driverboards go thru some 47mh chokes then go to a d1303 muting transistor the muting transistor tests fine but the middle leg is pulsing the same voltages..


What should i check to see why i have pulsing voltages there?

 

[Perry Babin]

Do you see DC pulsing on any of the output filter inductors?

 

[S Merlington]

No when i first power the amp up 1 output filter inductor has about 0.13 volts on it and the voltage is steady..

Another output filter inductor has -0.13 volts and remains steady..

The other to remain at 0.00 volts

 

[Perry Babin]

Did you perform the same repairs on both driver boards?

If so, which components were replaced on both boards?

 

[S Merlington]

I replaced the same components on both boards..

Replced both IR21844S's

Replaced U6 LM293
Replaced U3 TL072
Replaced U4 LM211

the folllowing transistors were replaced also

Q1:1A
Q2:2D
Q3:2A
Q4:2D
Q5:1D
Q7:2D

 

[Perry Babin]

Check the two halves of all 4 driver ICs as shown in the attached image. Do you read anything below 1000 ohms in any of the probe/pin combinations?

 

[S Merlington]

1 driverboard all are well above 1000 ohms


Other driverboard 1 IC i get

Pins 11-12:181.5 ohms
Pins 12-13:71.7 ohms
Pins 11-13:56.6 ohms
Pins 5-7 meter remains at OL

Other IC on same board

Pins 6-7:7.8 ohms
Pins 11-12:181.4 ohms
Pins 12-13:71.4 ohms
Pins 11-13:156.5 ohms

 

[Perry Babin]

Both of those ICs appear to be defective. It could also be that the capacitor and/or diode near pins 11, 12, 13 could be defective.