COUPLING CAPACITORS:

In the past several years I have noticed a significant increase of IEC brand electrolytic capacitor failures in the Series 1 4000. These were used for several years and are clearly marked (black plastic wrap). The main culprits are the 1 and 2.2 uF values. I'm beginning to see an increase in failures of other brands but at a lower percentage.

The 4000 utilized a single ended 30-32 VDC power supply which required that all circuits be capacitor coupled. Each plug in board had electrolytic output coupling caps. Most were 1 uF/35 V. Early Logic boards with 5 uF coupling caps should be updated to 1 uF to reduce turn on/off transients.

Symptoms of bad caps are "pops" and "clicks" when switching the various circuits in and out. A good cap will measure less than 3 mv on the output (-) side. Voltages higher than this will create the aforementioned pops and clicks.

Tracing the bad caps is relatively easy with the help of the interconnect schematic in the service manual (see NOTES below). For example; if the Peak Unlimiter switch pops, it is either the caps on the output of the Peak Unlimiter or the Tone Amp PCB. Measuring the voltage on the output side of the coupling caps will reveal which ones have failed.

POWER SUPPLY CAPACITORS:

4000's up to approximately serial number 5000 had the early power supply design with a 500 uF/50 V capacitor for C2. These have low failures, regardless of brand.

Later power supply revisions used a 220 uF/63 V C2 and included a heatsink on Q1 right next to it. Heat from the heat sink may shorten the life of C2.

Typical voltages on C2 are 40-45 VDC. When C2 fails, this voltage may fall to as low as 20 VDC. Other symptoms may include failure to power up, failure to engage the switched outlet relay, and excessive hum.

Simply replace C2. Be sure to use a cap rated at 63 VDC. Do not turn the heat sink or it may short to the leads of Q1.

NOTES:

1. The 4000 service manual contains too many schematics to post on the web site. See the Owner's and Service Manual section for ordering information.

2. The Correlator PCBs contains electrolytic capacitors in the gate circuits (C13/14, C 22/27, C24 and C 25). Troubleshooting these are more difficult and is beyond the scope of this service bulletin. Because they affect gate timing, they should always be replaced in matched pairs.

3. The Peak Unlimiter also contains electrolytic capacitors in the signal path (C3/4, C15/16, C5/6, C17/18, C10/11, C22/23, C12 and C24). These should also be replaced in matched pairs.