PL-400 series ii issue
- Thread starter Michael F
- Start date
It fascinates me how the Phase Linear amps were built with junk parts, but many still work after 40 years and have had no maintenance. In this case, one channel works and the other appears to be heat sensitive and the result is the output goes to zero.
What's interesting is it appears that there is no significant DC offset. If something in the front end let go there would be high offset. If something in the output stage failed there would likely still be half of the waveform. The symptoms suggest the gain of the amp goes to zero.
I was on the road this week and couldn't test a theory. Today, storms in Houston and airline crew issues set me back 5 hours. I was going to investigate in the morning but it was bugging me. So I pulled a 400 off the shelf and lifted one end of the feedback electrolytic on one channel. I used a couple jumpers and a slide switch to reconnect the cap. I verified that the channel still worked and then opened the switch. As I expected the output dropped from 40 volts to a few millivolts with no significant offset.
The feedback network on the 400 and 400II is mostly the same. Both use a 100uf cap to limit low frequency response. In the 400II it's C11, 100 uf/6.3 volt. I suggest inspecting the solder joint on C11 first. Then try tacking in a cap in the range of 25uf to 100uf across the original C11 and test it. If it works for several hours then remove the temporary cap and confirm the problem returns. This is a somewhat laborious task, but confirms the failure and is far better than just throwing parts at it and hope for the best. If the amp is to remain stock, see the service bulletins. Also replace R101, (7.5K/2W) in each channel. I've seen several of these fail and the amp goes DC.
If I totally missed the boat, then it's the cable between the outputs and the binding post.
What's interesting is it appears that there is no significant DC offset. If something in the front end let go there would be high offset. If something in the output stage failed there would likely still be half of the waveform. The symptoms suggest the gain of the amp goes to zero.
I was on the road this week and couldn't test a theory. Today, storms in Houston and airline crew issues set me back 5 hours. I was going to investigate in the morning but it was bugging me. So I pulled a 400 off the shelf and lifted one end of the feedback electrolytic on one channel. I used a couple jumpers and a slide switch to reconnect the cap. I verified that the channel still worked and then opened the switch. As I expected the output dropped from 40 volts to a few millivolts with no significant offset.
The feedback network on the 400 and 400II is mostly the same. Both use a 100uf cap to limit low frequency response. In the 400II it's C11, 100 uf/6.3 volt. I suggest inspecting the solder joint on C11 first. Then try tacking in a cap in the range of 25uf to 100uf across the original C11 and test it. If it works for several hours then remove the temporary cap and confirm the problem returns. This is a somewhat laborious task, but confirms the failure and is far better than just throwing parts at it and hope for the best. If the amp is to remain stock, see the service bulletins. Also replace R101, (7.5K/2W) in each channel. I've seen several of these fail and the amp goes DC.
If I totally missed the boat, then it's the cable between the outputs and the binding post.
UPDATE
So I took some freeze spray to the driver board and it made no difference but applying the spray to the back plane produced some interesting results the further down the negative bank I went.
Have a look at a clip that I made at that time, I hope the link works.
https://streamable.com/f22fpt
When I applied cold to Q108 and its emitter resistor, the amp sprang back to life (with a load). It remained that way long after the freeze wore off and no amount of probing the TO-3 socket or resistor would replicate the issue.
I did decide to inspect the insulator on that driver and was surprised to find it had a silicone pad instead of a mica insulator.
Did those pads even exist in 1978? I replaced it with a new one and gave the emitter and base sockets a little squeeze just as a precaution.
I let the amp idle for a while and then applied signal to it and ran levels ranging from 10-300 watts and no amount of level changes had any effect on its integrity even when the dummy load started to protest due to prolonged high power use.
The amp spent the rest of the evening playing music without a hint of trouble.
I think it would be safe to call this a win. Im pleased that it wasnt a major issue and that it didnt cost anything to resolve, a BIG thanks to the good folks here that chimed in.
Now Im off to the 4000 sii.
So I took some freeze spray to the driver board and it made no difference but applying the spray to the back plane produced some interesting results the further down the negative bank I went.
Have a look at a clip that I made at that time, I hope the link works.
https://streamable.com/f22fpt
When I applied cold to Q108 and its emitter resistor, the amp sprang back to life (with a load). It remained that way long after the freeze wore off and no amount of probing the TO-3 socket or resistor would replicate the issue.
I did decide to inspect the insulator on that driver and was surprised to find it had a silicone pad instead of a mica insulator.
Did those pads even exist in 1978? I replaced it with a new one and gave the emitter and base sockets a little squeeze just as a precaution.
I let the amp idle for a while and then applied signal to it and ran levels ranging from 10-300 watts and no amount of level changes had any effect on its integrity even when the dummy load started to protest due to prolonged high power use.
The amp spent the rest of the evening playing music without a hint of trouble.
I think it would be safe to call this a win. Im pleased that it wasnt a major issue and that it didnt cost anything to resolve, a BIG thanks to the good folks here that chimed in.
Now Im off to the 4000 sii.
Good work, You got lucky it did not totally short out 
I'm still one of the few that use the Micro Insulators and Heat sink grease (never use computer silver grease)
I heard of the silicone Pads doing this, Even Gene had one short out when installing a new transistor & cut when installing)
I'm still one of the few that use the Micro Insulators and Heat sink grease (never use computer silver grease)
I heard of the silicone Pads doing this, Even Gene had one short out when installing a new transistor & cut when installing)
George and Gene
Here is a Very Long video of what you just said, Tony show how to use the Micro Insulators w heat sink grease (I still use Micro, hard to overtight them) but he also explains the different between the 2 , good and bad. it's a very good video and about haft way he talks about the silicone Pads. Good luck
Here is a Very Long video of what you just said, Tony show how to use the Micro Insulators w heat sink grease (I still use Micro, hard to overtight them) but he also explains the different between the 2 , good and bad. it's a very good video and about haft way he talks about the silicone Pads. Good luck
Fixing to watch the video. But the hard insulators that easy split into thin sheets are "mica" a naturally occurring mineral.
https://en.wikipedia.org/wiki/Mica
Joe says the silpads offer better heat transfer than mica. Certainly are easier to use. Going to watch the video now.
https://en.wikipedia.org/wiki/Mica
Joe says the silpads offer better heat transfer than mica. Certainly are easier to use. Going to watch the video now.