LC584AL arrived today but channel 4 is not working :(

[tautech]

is this the type of material that is on the board covering those diodes and surrounding area?
http://www.ebay.ca/itm/Kafuter-100g-K-5902T-transparent-silicone-industrial-adhesives-waterproof-glue-/292277339286?hash=item440d164496:g:IicAAOSwaB5XvAD3

If what's on the board is pliable and not hard, yes. That ^ seems ideal. 
If it is hard it may be hot melt glue.

WRT BAV99, have you looked up any spec to indicate that is indeed temp sensitive ? Maybe there's a temp graph in the datasheet.
I'm struggling too to get my head around why it's where it is and covered in silicone ? 
Yes, point C should be 0V, sorry for the previous confusion.

[analogRF]

yes, I did take a look at BAV99 specs. They are labeled as ultra-fast silicon switching diodes
which I don't know why it should be the choice in this particular circuit. They dont seem
particularly more temp sensitive than other diodes. There are a couple of graphs showing the
VF or IF vs T but they look like any other diodes to me, not particularly temp sensitive.
For example in one graph from Fairchild, Vf drops from 0.75V at 25C to 0.70V at 50C
and then to 0.60V at 110C (all at 10mA forward current which is close to the 11.3mA used in this circuit)

yes, the silicone on the board is kinda soft and I can poke into it by multimeter probes
but not that soft which can be removed by hand, for example.

[analogRF]

...but the thing is if the two diodes are matched, even if their Vf changes with temperature still their sum will be zero!

besides, is this really a good idea for protection against overload? I mean, if overload happens
it takes a while for the diodes to get warm and do their thing and by that time the damage is done!
there must be another path for overload detection, too, I guess but I cannot see it from the schematics yet...

LC684 is using the same concept as this one except it uses LM60 temp sensor instead of BAV99 diodes.

I am not even sure if juts purely resistance measurements + multimeter diode measurements on board is a reliable way
to tell me this diode pair are at fault here except that the values I get are different that the other three channels.
besides, when I did the in-circuit diode measurement, the forward bias was OK and exactly as other channels
the reverse was off but in-circuit diode testing on reverse bias is kinda meaningless and totally depends
on the surrounding elements...so concluding that the diodes are at fault here may not be correct at all...

[nctnico]

If you see different readings between working and non-working channels you are very likely in the part of the circuit where things are wrong. There may be multiple components which are defective. If you have good soldering skills you could replace all the semiconductors in the front-end (presumably some diodes and transistors/fets).

[analogRF]

I thought I'd give a final update on the repair of LC584AL:

After I found out the overload protection circuit is at fault and extracted the schematics (see post #19) I could not explain how that circuit could protect anything!
If the double diodes are perfectly matched, any change in temperature would not trip the opamp (their sum voltage remains zero). Anyways, I went ahead and simulated this circuit in NI Multisim (I have access to at work) and still could not figure how this thing can protect again overheating of the 300 ohm resistor. I think this thing does not protect against anything unless there is some mismatch, but not too much of it, between the diodes.

If they are mismatched by a lot the opamp trips at room temp which is what has happened to my scope I guess and if they are perfectly matched then nothing will happen with temperature. In LC574 there was a potentiometer to adjust this circuit but it is eliminated in LC584. Besides, even if it were to work as intended, it would take a while for the over-voltage at input to warm up the 300ohm resistor enough so the diodes voltage difference can trip the opamp. By that time the damage is probably done. So the ability of this circuit to protect the input is very low (none). And it only works at 50Ohm coupling, however, if it trips, the input becomes disconnected and does not work on 1M either until the overload is cleared.

So the solution for me was to just DISABLE this subcircuit instead of going through the painful and expensive job of removing the silicone putting and replacing the diode and re-applying the silicone. The easiest and cleanest way to do this was to just remove the 4.7K resistor (R4081 at the output of AD705 on the back of the mainboard). This way the output of LM339 comparator remains high and the input relay will not disconnect the port anymore. At the same time I have not changed the biasing conditions of any of opamp/comparator pins. So this repair costs nothing!

I did this and the scope works like a charm. All 4 channels work perfectly and I wish I had done the same on other channels when it was open.
I borrowed a GPIB cable from work and updated the firmware to 9.3 (last version) and some of the bugs I had noticed in operation are now gone.
The scope has so many software options and looks pretty much brand new :-)
The floppy also works!

thanks for your comments and help.