PL-400, Q8 and Q9

AvSat44

New Around These Parts
Joined
Jan 11, 2022
Messages
2
#1
I've opened up a PL-400 and discovered the 2 main caps were both open circuit and the meter lamps were dead. I replaced the caps (at great expense) and fitted new lamps. With the supply disconnected from the amp, I found the supply was sitting at around 74v ( both plus and minus). So far so good. Re-connected the supply and have very distorted sound in both channels. Off again and discovered a lot of mismatched output devices so I decided to replace Q11 to Q18 in both channels and am now going to replace all of the electrolytic caps and transistors on the PC board. I thought I'd gone this far so why not. I've now discovered that a some point in the past, possibly when the mismatching took place, that Q8 and Q9 in both channels had been removed. Why would anyone do this? I understand that these are the current limiting transistors. Has anyone experienced this before.
 

mlucitt

Veteran and General Yakker
Joined
Jun 24, 2011
Messages
2,727
Location
Jacksonville, FL
#9
Is that the same as the Speaker protection circuit for the 400II and 700II? and disconnect it by lifting the 2-Diodes?
Be careful with the term "Speaker protection" because nothing will protect your speakers except for the wattsabundant (his Phoenix tag) DC Protect Delay/Relay, not even a speaker fuse because the melt time is too slow. The 2 diodes and 2 transistors of the "Protection circuit" on the control board only protect the amplifier during a short circuit at the output connection. If the speakers happen to be saved, that is a bonus.
The logic here is - if there is a short across the outputs, the speakers are downstream from this and will receive very little current flow. However, the amplifier 'sees' an impossibly low impedance and will try to send the maximum current possible to that load, overdriving the amplifiers output section. The "Protection circuit" senses the current overload and immediately unbiases the predriver transistors preventing further output.
This was the final test at the Phase Linear factory, to short the outputs and ensure the protection circuit worked. Nothing to do with speakers.

Let me know if I got this right.
 

MusicSteve

Journeyman
Joined
Mar 6, 2022
Messages
139
#10
Be careful with the term "Speaker protection" because nothing will protect your speakers except for the wattsabundant (his Phoenix tag) DC Protect Delay/Relay, not even a speaker fuse because the melt time is too slow. The 2 diodes and 2 transistors of the "Protection circuit" on the control board only protect the amplifier during a short circuit at the output connection. If the speakers happen to be saved, that is a bonus.
The logic here is - if there is a short across the outputs, the speakers are downstream from this and will receive very little current flow. However, the amplifier 'sees' an impossibly low impedance and will try to send the maximum current possible to that load, overdriving the amplifiers output section. The "Protection circuit" senses the current overload and immediately unbiases the predriver transistors preventing further output.
This was the final test at the Phase Linear factory, to short the outputs and ensure the protection circuit worked. Nothing to do with speakers.
Let me know if I got this right.
Yes, sorry, I was talking about the "Protection circuit" which I think they were talking about it in the previses posts they call it "current limiting" I also have a separate Speaker protection board.
Good to know what it does.
so if I disconnect the the PL the "Protection circuit" and accidently short the speaker outputs what does that do to my speakers? and to the PL amp? pro's and con's
 

wattsabundant

Chief Journeyman
Joined
Apr 26, 2011
Messages
601
Location
Central Ohio
#11
It doesn't hurt the speakers if the amp doesn't blow. If there is no music playing when the short occurs nothing happens. A "short" can mean many things. Temporarily touching the wires together at high output levels may just blow the fuses. However it's likely there is arcing when the wires touch and it could result in amplifier failure and very likely 80 VDC going into the speakers which may cause a lot of speaker damage. At that point the fuses prevent a fire. The damage is already done. Fuses are not fast enough to protect against transistor failure.

Every time I test an amp I do the short circuit test called out in the manual. I use a shorting banana jack on the output binding posts which is different than touching leads together at the end of speaker cables. When the amplifier protection circuits are functional the amp survives the test. Disable them and all bets are off.

Depending on the relay used in the protection board, the contacts weld. That is typical of most circuits. The relay board I designed simultaneously shorts across the speaker when it detects DC. During development I subjected a cheap speaker to multiple amplifier failures and the relay board did it's job every time, without causing speaker failure.
 
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