Bulletin Jbl2000-04 - JBL BP300.1 Service Manual

Service Bulletin JBL2000-04 - November 2000
To: All JBL Service Centers
Models: BP300.1, BP600.1, BP1200.1
Subject: Muting during intense bass passages
The JBL BP series amplifier is designed with the following protection circuits:
Thermal (excess heat)
DC voltage on the output terminals (overdriving or "clipping")
Short Circuit (excessive current)
Under certain large signal conditions during intense bass passages, the unit will go into protection. These
amplifiers are behaving as designed. As the unit is driven into hard clipping, the protection circuit found in BP
Series amps was engineered to protect your loudspeakers from being destroyed by a severely clipped output
signal. This can be remedied easily by turning the volume down somewhat on your music source.
In the event you receive a BP series amplifier with the complaint: "Under a large signal input, with
music that is composed of high-level bass, the amplifier mutes for 2-3 seconds, and then returns to
normal operation", the following modification will effectively increase the amount of clipping that is allowed
to occur before the protection circuit engages by increasing the threshold at which the amplifier mutes - for
the benefit of customers who use the amps in SPL competition, or for customers who insist on driving the
amplifiers into this potentially hazardous zone.
1) Turn the amplifier upside-down and remove the Phillips screws holding the bottom cover on.
2) Locate and change R510 from 47K to 4.7K ohms 1/5W (JBL part# RE212047200).
3) Locate and change R511 from 22K to 2.2K ohms 1/4W (JBL part# QAF0450-222).
4) If it is not already present, add new part - 4.7uf 50v electrolytic capacitor (JLB part# EC070114760) in
parallel with R511. Polarity: The negative side of the capacitor must be connected to the junction of
R510/R511, which can be discovered with an ohmmeter.
NOTE: Using the correct soldering techniques, you should be able to make these changes from the top of the
PCB without its removal.
5) Replace bottom cover; test amplifier
Model
BP300.1
BP600.1 All Serial numbers affected
BP1200.1
Serial number
Service Bulletin
Status
Modify if Needed
Add 4.7uf 50v electrolytic cap in parallel with R511
10
Warranty labor rate: MINOR
Action
Change R510 from 47K to 4.7K ohms
Change R511 from 22K to 2.2K ohms