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Circuit schematic wanted: David Clark DC Comm 500 intercom

10 Posts

Hi, I have a David Clark DC Comm 500 intercom unit with an intermittent fault that is difficult to pin down. Would anyone have a circuit diagram for it. DC do not make schematics available unfortunately.
Thanks
Geoff

Australia

Australia

I have nothing from D-C in my large avionics manuals collection, and amazingly google turns up nothing on this too. I wonder if it was made by somebody else and they made a batch badged for D-C? Pilot Industries is one candidate which used to make intercoms like this.

What is the intermittent fault?

Normally, when trying to repair something for which there is no circuit diagram:

  • check PCB solder points and rework any looking iffy
  • check PCB tracks leading to any larger items which get stressed e.g. connectors
  • replace electrolytic capacitors; they tend to break off the PCB plus these are the main “lifed” item in electronics
  • replace switches and pots

If the fault is intermittent it is most likely mechanical.

Administrator
Shoreham EGKA, United Kingdom

Hi Peter, thanks for your post.

Yes it’s a DC box, now discontinued. The fault is with an Aux Audio i/p that works in association with a externally wired ATC/AUX switch. When switched to ATC, a signal then received by Com1 will cause the auxiliary audio i/p to mute. It is this function that is intermittent. As there are a number cmoss 4066 analogue switches it is possible one of these with a floating enable i/p. There is a FPGA type device that provides the logic control for the unit also in the mix of possible offenders.

I’ve search the boards with a binocular microscope for dry joints but nothing obvious.

Geoff

Australia

OK; sounds like you know what you are doing. This is a real bugger… and rigging up a test takes time. In years past, I spent days troubleshooting some simple thing for which there was no circuit diagram, and that was with an oscilloscope available.

Avionics circuits are normally closely guarded, to support the business of the authorised repair shops, or to block them completely (as with e.g. most Garmin stuff for which even the MMs are totally bland). Installation manuals are also guarded, to support the authorised installers. When someone set up a website with a load of manuals on it, Garmin lawyers would contact him asking it to be taken down (although this was some years ago; I doubt they bother now). I collect everything I can find

What is the P/N of the FPGA device? That is really surprising; quite an overkill. Can you post a photo of the PCB?

Administrator
Shoreham EGKA, United Kingdom

Hi Peter,

The logic device is a Altera EP610DC-30, a PLA. If the issue is with the PLA I might have problems in solving it. I haven’t fully studied the PLA data as yet as I’m still tracing. But if it is, it may well have had a software fuse blown to prevent reading to enable a write to a replacement. I"ll cross that ditch when and if!

There are 2 pcbs, one a logic and the other a relay board. This unit is not just an ICS but controls 2 Com units as well (my Com 2 is various Ham radios) . It has 3 o/p circuits being the pilot, copilot and pax. The Com 1,2,3 switch directs the Coms to the various o/p circuits whilst the Radio/Crew/All switch controls what is heard. A mini audio panel if you like. I don’t have the faulty unit with me on the bench as its still in my PA22. I can live with the fault as it only means the ADF audio has to be listen to through any Com 1 traffic with the ATC/AUX switch set to Aux. Annoying when the cricket is on.

Pictures attached is of an old unserviceable unit I’m using to buzz out a circuit, and my fabricated test box.

I have done some reverse engineering (more to be done) with a DSO and DMM. The PLA controls the 4066s’ switching in accordance with the front panel switching. The Aux muting issue I’m having is likely controlled via an op amp to the PLA and then to a 4066. That’s the area I will be concentrating on when I pull the unit out.

Having a circuit would greatly reduce the time factor.

Cheers
Geoff


Australia

Helitack, I am impressed with your electronics skills! It’s great to see… he says while slaving away in Protel PCB 2.8 (vintage 1995 – originally from your country ) doing an ARM 32F417 board which generates loads of DRC errors which move around randomly. Protel 99 imports the design but is no better.

I am almost as impressed with how many components they managed to pack in there, in the pre-SMT days. It’s a bit like this intercom from PS Engineering from over 20 years ago. One would not think there was enough functionality in there to justify all that…

It is pretty normal to set the “no read” fuse on any programmable device. It’s a no-brainer to do that, since you can usually still re-use it by erasing it, and it blocks direct chinese copies.

What aircraft is this for? You should be able to pick up a used PS Engineering box on US Ebay for not too much… I still have that intercom in the above link; free in the post to you

Administrator
Shoreham EGKA, United Kingdom

Thanks for your generous offer Peter. I am however needing the Com feature to allow selection to direct where the com units go to in the cabin. And I hate getting beaten by a box! I will keep tracing the circuit bit by bit as I compile a circuit for the unit. Time is the price. Its for my aircraft, a Piper PA22-150 Tripacer.

I use DesignSpark PCB. Got to love the DRC feature in these packages, sure takes care of the trolling through in an attempt to find floating tracks, clearance issues and the like. Just did a 4 layer board for an auto tune mobile HF antenna controller using an AVR. Can’t believe how the Chinese can do these types pf pcbs for the price.

I’m in the shack now so best get back to it. Thanks for your help.

Cheers
Geoff

PS, love to know how to get emails when a post is made on a thread. I did the “Receive Watch Notifications” in Preferences but it doesn’t seem to work.

Australia

Above each thread:

Administrator
Shoreham EGKA, United Kingdom

Ahhhhh

Australia
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