CurtPalme.com Home Theater Forum

[gjaky]

Let the pictures speak...
This one from a fairly old Barco Cine 9 or 909 neckboard:


Interesting that they changed the peaking on the newer hybrids (2x75Ohm vs 2x220 Ohm and 1.2k vs. 330 Ohm), hard core tinkerers might want to try if there is any visible difference...

[cmjohnson]

No, don't. Those are ceramic substrates and are not to be messed with unless you have the very specialized tools and knowledge
required simply to remove and replace the resistors. Using ordinary solders will DESTROY the traces.

If you are not FORMALLY TRAINED and QUALIFIED to do component repairs on thick film substrates, you do not know HOW to do it
and your attempt to simply replace two resistors WILL end in DESTRUCTION of the circuit.

I can not stress this strongly enough. DON'T DO IT. YOU WILL FAIL.

Even I don't mess with those. I have Mad Skillz with a soldering iron and a stereo zoom microscope and SMT handling tools, but
I am not thick film qualified and wouldn't touch that job with a 30 foot pole.

[Tim in Phoenix]

Yeah

Those look real Sturdy!!!!

[cmjohnson]

I just did some reading on this and for this to work, APPARENTLY, you need solders that contain Indium which are formulated specifically for thick film soldering jobs. And as I gather it, temperature control is critical and the solder alloy has to be the right one for the chosen thick film substrate and metalization.

In short, don't try this at home, kids.

[gjaky]

Of course I was not serious about modding the boards, but hoarders might look up their parts stash and find if they have different hybrids on the boards and then try those boards...
BTW I have seen some hybrid circuits already and I have the feeling this one is a quite low budget. No thick film resistors at all. And Ive never seen bond wires this long for connecting to transistors. long thin wires and traces mean stray inductances, those are unwelcome with high frequency circuits.

[Curt Palme]

I've had lots of failures of these custom BArco chips. Usually there's simply no video output, although I've had ones come on full brightness, and one with retrace lines. There's no repairing these AFAIK.

[Curt Palme]

I've had lots of failures of these custom BArco chips. Usually there's simply no video output, although I've had ones come on full brightness, and one with retrace lines. There's no repairing these AFAIK.

[gjaky]

More porn!

I drawed back the circuit diagram of this hybrid.


On the right there is the schematic of a Sanyo VPA hybrid which is essentially same as a Marquee neckboard design as well, it is recognized in engineering handbooks as cascode stage with output buffer, this is the pretty much standard circuit for CRT amplifier.
On the left side there is the Barco hybrid's schematic diagram. If you look at them from far they appear quite similar, but at second glance there is a differences.
-The power resistors are tied to Q1's collector instead of it's base (or close to it).

Here are some close ups from the transistors:
Q1:


Q2, please note the grey parts circled, those parts are probably burnt out already, high frequency power transistors are often constructed of several "smaller" transistors tied in parallel, with an internal fuse, a part of these small transistors might burn out in an event of failure, still the overal transistor might remain functional, and this may be the case here as well, since this transistor still measures good from the outside.



Q3, was somewhat damaged when I ripped-off the RF shielding, still the transistor is functional.



Q4:

[gjaky]

Also what is worth mentioning in the current barco design is that they paralleled Q3 and Q4 probably to fit in the transistors dissipation limit, but both transistor have to stand the whole voltage (and in this circuit Q3 and Q4 is under the most stress both in dissipation and in voltage) once they took the pain to use two transistors for that stage they would have been better to went with a so-called sliding cascode circuit, where two lower voltage transistor could be connected in series and adding the two base resistors would ensure equal voltage division between the two transistors. And as you know with lower voltage transistors it is easier to find faster.

[gjaky]

Some more shots, this time off a Sanyo VPA13.
Interesting is that Q1-Q3 transistors look structurally different from the ones used in the Barco, probably because this is the "FBET" process they mentioned in every datasheet.

Q1-Q3:


Q4, low voltage transistor looks similar to the barco transistors


The diodes are also in bare form in the VPA:

[gjaky]

This time I caught an MRF548 (with broken emitter lead), thought I have nothing to loose if I disassemble, and in the end I even managed to repair it...
It is interesting that it is actually two transistor dies in one package (tied parallel).