PL400 Restoration

J

jtatknox

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Hi all,

Thanks for your help getting my two series 2 PL300's running again. I'm starting a new thread to keep things on topic.

I acquired a PL400 about 6 months ago and this weekend I finally found the time to replace the filter caps and install the Watts Abundant relay. Bias is normal and It came up fine on a DBT, has normal DC offset, and is dead quiet when connected to speakers but no source. I have tried connecting two sources, a laptop and a miniDSP 2x4HD with optical input. One is battery powered and the other is powered by a 2-prong wall wart. I am getting loud 60 hz hum with both sources but here is the interesting thing: if I touch the case or the power cable of the miniDSP with my hand, the hum goes away until I remove my hand. If I UNPLUG the miniDSP power, the amp still hums. The PL300s didn't give me any problems with noise using the same source. Tried three different known good RCA cables.

The RCA jack grounding scheme is exactly as specified in the Watts Abundant instructions: cut of all but 1/2" of the thick, bare, ground wire from the RCA jacks, clean off oxidation w/ a pocket knife and then solder a white wire to be run from the RCA ground along the bottom of the chassis to the filter capacitor ground bar using a solder-on lug provided in the kit. The relay board is grounded to a terminal strip much closer to the board as specified in step 24:

https://wattsabundant.com/sitebuildercontent/sitebuilderfiles/400_Installation_Procedure.pdf

I did some searching on this forum and found one related thread:

https://forums.phxaudiotape.com/threads/pl400-ground-loops-and-hum.8292/

Sounds like a grounding problem to me, but I'm not really sure where to start other than opening up the chassis and poking.
 
J

jtatknox

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#2
Also, forgot to mention about 20% of the time I plug the amp in the relay doesn't activate and instead the VU meters peg for a moment. Not sure what to make of that.
 
Gepetto

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jtatknox said:
I read your description of phaser linear latching here:

https://forums.phxaudiotape.com/thr...-up-tendency-in-phase-linear-amplifiers.2307/

Could what I am seeing have something to do w/ power cycling a lot as I mess with connections? Maybe I need to be more patient about that. I definitely want to replace the driver board eventually w/ a WOPL.
Click to expand...
Sure it is more likely if you are cycling a lot and not letting the caps fully discharge. Put in the Gepetto fix and it will cease doing that.
 
J

jtatknox

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I'll study your post and see if I can understand what needs to be done. I haven't started studying the schematic of the driver board yet.

Odd observation: I cooked dinner and came back and the amp isn't picking up any hum anymore. Dead clean. Would you expect the amp to behave better after warm up? I noticed it is most noisy right after power up. Just wondering if your fix might also fix the noise.
 
laatsch55

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Hard to say, could be a cold solder jt. Do you have any 1N4148 diodes? If not get 4...Twist the leads together on a pair with the polarity reversed to each other, C6 is the 10 volt, 100uf cap on the top of the board.. Solder the twisted pair of 1N418's accross the leads of C6 on the backside of the board. The 1N4148's will resemble a "C" when you are done...
 
J

jtatknox

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Surprisingly I do have the right diodes... I will take a look inside the board and post here my plan of attack but it may take me a few days.

Mean time I did another startup test on the amp. This morning I plugged the amp in and it again made tons of noise that I could silence by touching the case or wires of the source. I let it sit for an hour, now it's dead quiet. It's very confusing!
 
J!m

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Maybe something with the main caps charging? (Just an idea since I know nothing)
 
J!m

J!m

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You're going to need someone smarter than me to know if that is a valid thought train...
 
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Mohawk

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laatsch55 said:
Hard to say, could be a cold solder jt. Do you have any 1N4148 diodes? If not get 4...Twist the leads together on a pair with the polarity reversed to each other, C6 is the 10 volt, 100uf cap on the top of the board.. Solder the twisted pair of 1N418's accross the leads of C6 on the backside of the board. The 1N4148's will resemble a "C" when you are done...
Click to expand...
Sorry to just jump in here but what's the deal with the diodes across C6?

M
 
J

jtatknox

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The thing I described about pegging VU meters was consistent with a latching phenomenon when power cycling the amp. Gepetto describes the phenomenon and a fix he came up with in one of the links posted above.

Probably unrelated to the problem of noise that mysteriously vanishes after an hour of operation.
 
Gepetto

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We would all just be guessing without something to go on. Some detailed, high quality photos of what is happening inside might be a good basic start for us to do more than speculate.
 
wattsabundant

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First of all, hats off to JTAT for doing some research. Apparently the decision to install the relay board saved a speaker or two from lots of DC.

I haven't seen a PL14/20 board in the last 5 years that did not have one or more front end transistors (Q1-4) that have failed. Quite common is intermittent operation as seen here.

Traditional troubleshooting with a meter on the diode scale to read the junctions is elusive. Although I really like the $20 transistor tester on EBAY it doesn't put out a high enough voltage to see the failure. A curve tracer will catch it. Few people have one.

We all know the long term solution is a new driver board. The short term is Q1-4 with Q1-2 being first. Matched pairs are nice but not necessary. That will address the latch up.
 
J

jtatknox

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wattsabundant said:
First of all, hats off to JTAT for doing some research. Apparently the decision to install the relay board saved a speaker or two from lots of DC.

I haven't seen a PL14/20 board in the last 5 years that did not have one or more front end transistors (Q1-4) that have failed. Quite common is intermittent operation as seen here.

Traditional troubleshooting with a meter on the diode scale to read the junctions is elusive. Although I really like the $20 transistor tester on EBAY it doesn't put out a high enough voltage to see the failure. A curve tracer will catch it. Few people have one.

We all know the long term solution is a new driver board. The short term is Q1-4 with Q1-2 being first. Matched pairs are nice but not necessary. That will address the latch up.
Click to expand...
You're costing SEAS a lot of business here, friend!

Thanks for the kudos, but all the kudos goes to you for such a nice and functional kit.
 
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