HELP! Rescuing Dad's old PL400 S2 (pics)

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lightfall

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Yea if I have this preamp and amp hooked up as it should be to play an mp3 the drivers extrude a half inch and hum and it plays no music. It doesnt work with any combination of settings using the various knobs and switches. It will also show short on the DBT with any combo as long as the speakers are hooked up, notably whether the preamp is turned on or off. It won't short the DBT if the amp and preamp are set up and on without speakers attached.

I swapped interconnects and speakers wires and outlets to be sure that was not the issue.

I get no DC voltage out of the amp outputs nor any preamp inputs/outputs. I still need to perform this test with speaker wires hooked up.

Something about having the amp outputs hooked up is causing a short and driving the speakers with noise but I cannot figure out what that would be.

Lee I also need to perform the additional test you mentioned. I spent most of tonight cleaning the switches and pots and actuating in rounds of 50 with deoxit.
 
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I also asked about the resistors and caps on the speaker posts. That is known as the zoebel network and can affect a lot of stuff....
 
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lightfall

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laatsch55 said:
I also asked about the resistors and caps on the speaker posts. That is known as the zoebel network and can affect a lot of stuff....
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Here is the speaker posts, the resistors look OK to me, not tired I would say. The behavior occurred with two different sets of speakers so I am fairly certain I eliminated that. IMG_2365.jpg IMG_2363.jpg IMG_2361.jpg IMG_2359.jpg IMG_2355.jpg
 
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lightfall

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I just borrowed a different preamp, set of speaker cables, and rca cords and with everything connected if I turn it on with the DBT I get a bright glow indicating short. All volume knobs at their lowest settings.
 
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I just said with speaker cables connected YOU HAVE A LOAD, the bulb WILL burn bright....
 
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Measure those resistors to see if they are close. Unless I'm having a dummy moment, you should see 6 ohms across the pair of them.
 
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lightfall

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gadget73 said:
Measure those resistors to see if they are close. Unless I'm having a dummy moment, you should see 6 ohms across the pair of them.
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Resistance is even across both, and I do believe they look good. I will just post an update in case you are curious where I am at with this madness.

I went through the unit again after I tested the amp with the other working preamp and found it to work well together. At that point I was fairly confident the amp was actually OK for the most part. I replaced all of the interconnects and speaker cables.

Then I spent a very long time actuating each controller at least 50x then deoxit and so forth for about 5 sets. When I hooked the 2000 back to the 400 I found it to be working well, so at first several of the switches and perhaps some of the pots were malfunctioning in tandem. After trialing each adjustment and set of inputs I eventually found that when the tone controls are not activated the right channel and left channel are fine, when tone control is activated the left channel is ok and the right channel will still play music, but will intermittently pop and a bass hum is present. The audio instantly returned to normal with the tone controls deactivated.

I am a little nervous that the popping is DC output from the right channel somehow when the tone controls are active, so I am reluctant to try and fiddle with the right channel treble and bass to see if a pot is to blame, or if the tone control switch is to blame (not sure how a switch could cause humming and intermittent popping) or if something in that circuit is toast. So I am going to try and backtrack from the service manual schematic anything that looks to be part of the tone control circuit and look at it with the MM. I guess the tone switch could be bad, or the tone switch could be only partially cleaned at this point but I will report back when I find a culprit.
 
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In the troubleshooting section of teh 2000 service manual excessive distortion and channel dropping is due to a malfunctioning op-amp. Check some old threads on the 2000's to get and idea on what you can do about it...
 
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lightfall

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Lee where did you get your Burr-Brown OPA 2134 OP-AMPs and the Brown Dog RC 4136 adapters? Was that difficult to assemble and solder and install? I presume if you chose to upgrade a 2000 with that combination that it should be a good choice for this repair.
 
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Ah.....you found the thread!! From Brown Dog Adapters. You can buy them from BD already mounted. That will work for the 4136, there is no substitute for the 4739. If the 4739 is bad I've got some. And order some sockets if they are not socket mounted already...
 
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lightfall

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oh ok sorry I see you can get them presoldered. What socket part # do I need? order direct from bdog looks like what I will do and hopefully the unobtanium one is ok.

I was using my search function wrong and didn't go deep enough. That one immaculate 2000 you built was amazing!
 
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No , I bought the op-amps and adapter from Brown Dog Inc. The op amps were mounted (that's an option) ....
 
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lightfall said:
oh ok sorry I see you can get them presoldered. What socket part # do I need? order direct from bdog looks like what I will do and hopefully the unobtanium one is ok.

I was using my search function wrong and didn't go deep enough. That one immaculate 2000 you built was amazing!
Click to expand...
Wasn't as good as my Spec 1, but it's worthy...
 
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