Phase Linear Dual 500 Diagnosis Assistance Requested

zeppelinofled69

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#1
I have recently acquired a Phase Linear Dual 500 Series 2 amplifier, and am in need of some assistance in diagnosing a potentially dead channel.

Upon receiving the amp, it was missing the mains fuse, along with the fuses for channel A. The channel B fuses were present, but blown. I popped the top off the amp to see if there was anything obviously blown up or black, and after a really good look at the driver board. R105 and R205 (2.7k ohm) were blown up, so I have replaced them with new resistors from my stash.

I got the proper mains fuse (ABC 15) and decided to try to power the power supply up without the channel power fuses installed. With the amp on a bulb limiter, it came up fine, so I took the limiter out and let it start up, again without the channels power hooked up. It started without issue, and the main caps are holding at solid +/-122V DC.

I then checked for continuity on the channel power supply fuse holders (Transistor side of the fuse holders) and found that I was reading continuity at about 3 ohms between on channel A between positive and negative. Channel B does not read any continuity between positive and negative.

At this point, I think I may have a bad output transistor or two in the channel A, but I wanted to see if anyone who is experienced with these amps could provide some insight as to the best way of going about testing for and fixing the problem. The blown resistors on the driver board have me thinking that there is something else going on, so I certainly don't plan on trying to start it unless I can find a capable tech, or find the issue myself.

Thanks
 

zeppelinofled69

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#3
Thanks a lot for all of the information Ed. It was a pleasure talking to you as well.

I did a little more experimentation, and while the outputs on Channel B are working, something on the driver board is not right, and the audio comes in and out when passing signal. The meters never stop or moving with the input, which is why I suspect something on the driver board.

Once I have a little more time to spend with the amp on the bench, I will start testing everything on the board. Could even be that when I was moving the board around, I weakened some of the wire joints, so I'll be checking for bad solder joints as well.

Any other suggestions would be greatly appreciated.

Thanks,

- Joe
 

grapplesaw

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#4
Joe you may want to check your RCA plugs. They are old and may not be engaging the plug. Sometime a wingle of the plug can give you an idea if that is it. Jump your back channel signal to the inside of the plug and see if hat is it.
 

zeppelinofled69

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#6
I have noticed something else about the amp. It seems to be stuck in High Z mode, and the selector switch on the front panel does not make a difference when changed.

I have ordered a full service manual for the amp, as the stuff I have found online has been very sparse and incomplete. Hopefully that gets me what I need to get this thing going.
 

oldphaser

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#7
Joe,

It appears that you forgot that I mentioned the RCA jacks issue to you on the phone and how to test and reform them. Before you fire the amp up again, I would suggest performing as many resistance measurements as you can. This service manual states:

"Most transistors and diodes in the Dual 500 can be checked in circuit using an ohmmeter on the RX1, RX10, or 2K scale. With the Dual 500 power switch OFF, measure the forward-biased resistance of the particular diode or transistor junction and verify a reading of 400 to 700 ohms on a typical DVM for a good device. Actual readings on good devices will vary from meter to meter, and some meters do not have sufficient test voltage to turn on a semiconductor. Nevertheless the shorted or open device is usually revealed quickly using this method".

We event talked about how to use your Klein Tools MM700 digital multimeter and how to use it.

By the way, there are (2) versions of the Dual 500 service manual. The first version was #1078 (October 1978). It was revised 5/81. The revised version includes some typographical fixes as well as (3) service bulletins including a Hi-Z /Lo-Z modification.

Ed
 

zeppelinofled69

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#8
Thanks again Ed.

I am actually running through the amp now, testing at all of the transistors. I was hoping that all of the issues were focused to the driver board at the back of the amp, but it seems with the High Z/Low Z switch not working, the face plate needs to be removed and I need to test that board as well.

We actually did discuss adjusting the RCA plugs, but in this case, that does not appear to be the issue.

Thanks

- Joe
 

zeppelinofled69

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#9
I got a few more hours in with the amp on the bench this evening and I think I have found the cause of the amp not switching to Low Z mode. The LED indicator for the Low Z mode seems to be integral to that circuit functioning, and it currently doesn't read on my multi-meter in either direction like the other 3 LEDs on the board do.

Any Idea what a good replacement part is for this green LED?

I have also ordered a bunch of new electrolytic caps for the front board, as a lot of them are testing out of spec. Driver board has had maintenance done on it at some point, as all of the caps are relatively new.

Does anyone have a source for legitimate TP9054 output devices? I am sure we are into people's stashes of them by now... hoping there are enough around to get this thing going.

Thanks,

- Joe
 

zeppelinofled69

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#11
Based on what the manual states, shouldn't I replace all of them if I move to a new part number?

I had just planned on replacing the bad ones...
 

laatsch55

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#12
They are the same part, just rebadged under a PL part number. I would replace all of them....they're gettin pretty old...
 

zeppelinofled69

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#13
Thanks for the confirmation that the parts are the same, Laatcch55.

It looks like I can get the transistors from Mouser for about 7 bucks a piece, so about 200 bucks for both channels.
 

zeppelinofled69

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#16
I see they are about half of that on Ebay (even from US sellers) but I really don't trust ebay sellers, as I have heard there are a lot of fake parts out there. What are the reputable sources for these devices other than something like Mouser or Digikey?
 
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