Need help w/ old school HiFonics Pluto

[THUMP LUMP]

I have an old Series 7 HiFonics Pluto amp that has went dead on me in storage. When I put power to it, nothing lights up and there is a bit more spark when I touch the ground to my power supply than what would be normal. I have never messed with repairing an amp before but I have replaced components on other things. I know that caps will go bad over time and thought that maybe a cap on the power side of the board may have gone bad just sitting. Here are pics of the board top and bottom.

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

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

 

[Perry Babin]

Clamp the transistors to the heatsink and insert a 10 amp fuse in the B+ line feeding the amp. Power up the amp (incl. remote) and post the DC voltage on all 16 pins of the TL594. Place the black meter probe on the amplifier's ground terminal. Place the red meter probe on the point where you need to measure the voltage.The attached image shows the numbering scheme. Copy and paste the following list into your reply.

TL594
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:

 

[THUMP LUMP]

First, thanks for the help Perry. I connected it up to my power supply and tried to get readings. I couldn't get anything but I did find that once I powered up the amp, the power supply quit supplying power (zero volts). I disconnected the amp and the supply went back to 13.68v. The 10amp fuse is also intact. I rechecked the supply afterward, to be sure it was working, and it is fine. I thought I had killed my 20 amp power supply. Sorry but I can't get any readings on the IC. Though, the fact that the power supply went to zero, I hope, will tell you something.

Thanks for the help!! I really don't want to toss this fine old amp. It fits in really well with my Cupid and Cyclops HiFonic amps.

 

[Perry Babin]

On the IRFZ44s on the corner of the amp, do you read anything near 0 ohms when you measure the resistance between the legs of each individual transistor. Measure from leg 1 to leg 2, 1-3 and 2-3.

 

[THUMP LUMP]

I measured the IRFZ44s and found, on the one closest to the corner to measure

Pin 1-2) - 13.9ohm
Pin 1-3) - 13.9ohm
Pin 2-3) - 0.3 ohm

And on the other one next to the one in the corner, with the insulator still on it in the above photo

Pin 1-2) - 0.9ohm
Pin 1-3) - 0.9ohm
Pin 2-3) - 0.3ohm

From a quick look and the data sheet, it mentioned "low resistance" so I am gathering the first one is bad. But only relative to the second as I have no other point of reference. I'm trying to learn!

 

[Perry Babin]

The second one appears to be defective. Both need to be replaced. Remove them and then power it up. Does it still drag the power supply voltage down?

 

[THUMP LUMP]

I removed both of the IRFZ44s and reconnected the amp. The power supply maintained the 13.68v output.

I have also read where it's a good thing to replace the caps on older amps as well. Which ones would need replacing?

 

[Perry Babin]

The larger ones are the ones that generally leak (3 from what I can see in the photo).

Are the two resistors connected to the first leg of each of the Z44s within tolerance?

What is their value?

 

[THUMP LUMP]

Thanks for the info about the caps. I have read that an amp this old should have them replaced.

The bands on the resistors are white, brown, black, gold. 91 ohms @5% tolerance, if I read the chart right. They both measure within tolerance. 90.2ohms and 90.4ohms.

 

[THUMP LUMP]

Is there anything else I need to check? Will this work for replacements for the IRFZ44s?

 

[Perry Babin]

Those should work. You could power it up and confirm that you have approximately 5v on the first leg of each of the power supply transistors.

 

[THUMP LUMP]

OK. I did that and they both are just over 5v. One at 5.12v and the other at 5.11v. I'll shoot and order out tomorrow for the IRFZ44s and the caps. Thanks for all the help!!! And if there is anything else you think I should do or need for this amp before I send in the order, please let me know.

Thanks again!!!

 

[Perry Babin]

Check the output transistors to see if any are shorted.

 

[THUMP LUMP]

Shorted to ground or from one leg to another?

 

[Perry Babin]

Check between the legs of each individual transistor.

 

[THUMP LUMP]

Checked all 4 output transistors. None showed any continuity from one leg to another.

 

[Perry Babin]

Do you mean that the meter never read anything near 0 ohms or did the meter never change when you touched the probes to the transistors terminals?

 

[THUMP LUMP]

I set the meter to continuity and it didn't show any connection from any one leg to either of the other two on any transistor. I wasn't really looking at the meter for any ohm reading, just listening for the tone to confirm continuity. I never got a continuity tone. Should I look for resistance?

 

[Perry Babin]

It's probably better if you set the meter to ohms or diode check in the future. If you're going to use the continuity mode, at the very least, touch the probes together before and after testing the transistors to confirm that the meter is working properly.

 

[THUMP LUMP]

I normally do touch them before and during just to check that I am getting tone. I have come across broken leads in the past and found then out by touching. I went back and used the 3k ohm scale and got flash readings right at first and then the reading went over, but this was only some of the time at random leg tests. Touching the leads using the 3k ohm scale, it read .001.