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.
dhl93449
dhl: What a great post !
... I too purchased a new/demo (NOS?) 840E 2-months back. IT IS, a fine piece of kit (as you point out) !

Yet, I am discovering the volume issues ...arrghh.

Before it goes back for service (I will ask the service centre --C.A.-authorized) what type of relay they'll use (thanks to your excellent post). I'm told the volume "encoder" will be replaced. Does this sound right ?

Many thanks,

 peter
Peter:

They will use probably the Omron or perhaps the Massuse (original OEM) relays. Both types will fail again in two or three years in my experience. The encoder is not the issue, but if they replace it no harm will be done.

I have repaired mine firstly with Omron relays which worked OK for three or four years, then recently failed. The Fujitsu relays are working flawlessly, but to be truthful, it may be too early to tell re their reliability. But they are clearly better suited for duty in this application, unlike the Omron relays which the manufacturer states (on the data sheet) are NOT suitable for continuous duty. When I pointed this out to CA tech support, they never responded back.

I would check to see if CA will replace with the Fujitsu relays, or if you can purchase the Fujitsu relays and have them install. Or, you can replace the Omron's with the Fujitsu relays yourself after they complete the repair. They are mounted on a replaceable pc board that can easily be removed from the main board.

Another tip which may extend the life of these relays is not to use the volume ramp on turn on or turn off.
How many relays are required?

Hi
This is my first post on this site.

I've had an 840A V2 for about 4 years now. It is a very impressive amp.
It has been displaying volume issues for the past 6 months or so.
The volume has increased without input from me. When using the volume control on the amp it would be very random even increasing the volume when trying to turn it down. This was more noticeable when rotating the volume knob rapidly after the volume had increased by itself. Using the remote the volume operated as it should.

I've read the previous posts with interest and do not discount their validity. Though I have achieved a satisfactory result to this issue in a very simple way with the use of Deoxit 5 spray.

I removed the front panel and separated the circuit board from the front panel to gain access to the volume control mechanism.
Discovering the volume mechanism was a sealed unit I placed a tissue to catch overspray then sprayed Deoxit on the rotating spindle of the volume control.
Then rotated the spindle back and forth numerous times. I repeated this process 4 or 5 times. 

The result was a much smoother rotation of the volume control.
After putting the amp back together I now no longer have any random volume issues. The volume control moves freely and corresponds correctly to up and down movements.

In hindsight as the mechanism is sealed it is not nessasary to dismantle the amp front panel. Just remove the volume control knob and spray directly onto the spindle where it enters the mechanism.

This makes it a very simple procedure and well worth trying first.

Dear Friend,

I purchased the Fujitsu relays to replace the Massuse in a 840E unit.
I will report the results here.
Could you send a picture of the relays that must be replaced, please?

Thank you very much,

Edumar
Edumar:

PM me. I cannot post photos in the forum.

If you open your pre-amp you will see a cluster of relays right in the center of the large pc board. They should be mounted on a separate pc board. There are two clusters of 14 relays (7 in one row, 7 in another); 14 relays per channel. These are the volume ladder relays. Remove the connectors, and the two nuts holding the pc board in place on the main board. Then you can access the back of the board for desoldering. I would put socket strip connectors on the board, then plug you new relays into the socket strips. 
BTW, as an update, the Fujitsu relays are working great. Have not had a volume issue since I put my set in last September.
Dear Friend,

Thank you very much for your reply.
I understood what you said.
Is it a good idea to use socket strips? Couldn´t it cause bad contacts after some time?
What type of socket strips did you used?
It is true that it is very interesting to make future replacements easier.

Thank you for sharing this solution with us. 
Socket strips are optional, of course. If you are careful with the soldering iron, and have a good vacuum de-soldering tool, soldering in the relays is OK. With socket strips, you only solder once, and any replacement afterward is easy. Do not overheat these pc boards or the traces will lift and then the board is useless. If you solder, you will probably not get more than two relays change outs before the board is destroyed unless you have a nice temp controlled industrial vacuum de-soldering tool like a Hakko. This is because the boards use plated through holes and it is very difficult to get all the solder out of the holes on the topside of the board where the relays sit. If the solder is not completely removed and you pull the relay off, you will lift the traces with the relay as it comes off.

I use high quality gold plated MACHINED socket pins/strips. You can get these at Mouser or Digikey electronics distributors. They are a bit tricky due to the odd spacing of the relay pins, and Cambridge did not space the relays exactly on 0.1" spacing. I cut the unused pins on positions that are not used but left them in the strip. I believe I ran the strips horizontally across the rows of relays. This requires the fewest socket pins, but the running length must be broken up into two or three sections, as the spacing tolerance is not exactly 0.1".