Repair your Cambridge Audio 840E volume contol issues
Many people with Cambridge Audio 840E (and I think 840A) preamps have reported strange volume behavior where the volume levels will suddenly change. Recall that these are the pre-amps with the relay controlled discrete resistor ladders and the Terrapin modules. There has been a lot of speculation posted on the web as to the root cause for this problem, including bad relays and bad volume knob encoders. Cambridge has since changed the design, removing the relays altogether and replacing them with solid state switches. Some, like myself, believe this is a step backward and would like to find a solution to the original faults with the discrete relays.
I own an early version of the 840E that was sent back to CA for re-work. Their service department added a separate relay board (containing the 28 relay array) with Omron G5V1 relays instead of the Massuse ME-3 relays that originally came from the factory. I was assured by Cambridge Service that the new relays cured the problem, but a few years later my volume control issues returned. I purchased a replacement set of Omron relays and replaced the Omrons supplied by service, and this did the trick for another 4 years or so. When looking into the specs on the G5V1, I found a most unusual disclaimer by the manufacturer:
"• Long-term Continuously ON Contacts
Using the Relay in a circuit where the Relay will be ON
continuously for long periods (without switching) can lead to
unstable contacts, because the heat generated by the coil itself
will affect the insulation, causing a film to develop on the contact
surfaces. Be sure to use a fail-safe circuit design that provides
protection against contact failure or coil burnout."
So what Omron is saying is that these were never designed for continuous use, as one might have in a volume control left at the same volume level for long periods of time. Further, it appears heat and high temperatures are at the root cause of the issue. If you look at the relay array for the volume control, the Cambridge design has these relays mounted as close together (side by side in two rows) as they can fit, and the center relays can get very hot. Due to the design of the volume ladder, not all relays are on at any one volume setting. There are also volume settings where the power generated by the relay array is maximum. From a document sent to me by Cambridge audio, it show that the current drawn by the relay array is a maximum of 271 mA at -32 dB, but drops to 131 mA at -30dB and 178 mA and -38 dB. The -32 dB area is where I see my volume instabilities, so it appears it may be related to power generation (and high temperatures) in the center mounted relays in the array. There are also high current levels at -16 dB and -8 dB (271mA) and -4, -2 dB. Of course, with the volume up above -8 dB, volume jumps to 0 dB are much less noticable when they occur.
So what can be done?
I was told by CA that the Omron replacement was the only one available. Turns out not to be true. I have found that Fujitsu makes a better relay that is plug and play replacement (SY-9W-K) for both the Omron and the Massuse relays. The Fujitsu relay has two contact options as well, one of which is much better for the ladder network. Let me explain. In the Omron spec sheet for the G5V1, there is a MINIMUM current rating of at least 1 mA. That means the circuit must flow a minimum of 1 mA to keep the contact resistance stable. Another condition violated by Cambridge Audio Engineering. The Massuse ME-3 is similar. The Fujitsu, however, has two contact configurations, one at 1 mA minimum and the second at 0.1 mA. The latter is more suitable of low current audio use. I believe the contact material (Pd/Ag under Au) is better as well. Finally, the Fujitsu relays are Japanese made whereas the Omron and Massuse are Chinese made. The Fujitsu relays have no continuous duty disclaimer in the spec sheet like Omron has.
Besides relay replacement, it is wise to do everything you can to keep internal temps down. I have my upper cover mounted on 1/4" stand offs to allow air to circulate inside the pre-amp. Mine is an early version without the vents. I have also mounted small aluminum heat sinks on the relay arrays to re-distribute the heat generated in the center units and lower their temperature.
Hope this post helps some of you who like the 840E and do not want to trash an otherwise fine sounding piece of gear due to the irritating volume control issues.