TECHNIKA LCD26-207 Sound but no (black) picture... advice please.

[Markie76]

Re: TECHNIKA LCD26-207 Sound but no (black) picture... advice please.

SD1

Hmm, ok - I tested this with two different multimeters (they were there so it was out of curiousity) and getting different results;

UNI-T UT10A(auto-ranging only)
DIODE TEST
(-) on cathode:0.254
(+) on cathode:0.741
RESISTANCE TEST
(-) on cathode:6.54k
(+) on cathode:21.43k

Precision Gold Academy PG017(manual-ranging)
DIODE TEST
(-) on cathode:0.268
(+) on cathode:1.082
RESISTANCE TEST
(-) on cathode:9.70k
(+) on cathode:113.1k

I've tested a few random resistors with both meters and they seem consistent on those.

Is it just quirkyness with auto-ranging?
What would you suggest?
Which one should I trust/continue with?

Thx

[momaka]

Re: TECHNIKA LCD26-207 Sound but no (black) picture... advice please.

Is it just quirkyness with auto-ranging?

You know, there was another person here who noticed the exact same thing when he used a manual vs. an auto-ranging meter. In his case, the differences were even more severe.
Unfortunately, I don't have an explanation for this. My best guess is that since we are doing in-circuit measurements, diodes and other components with non-linear resistances are causing the differences.

What would you suggest?
Which one should I trust/continue with?

Well, I've been using a cheap manual-ranging multimeter (a 830B variant) for the many years and I've never had a problem with it.
If you're conducting a diode test or just measuring voltage, I would say use whichever one you like. However, when measuring those capacitors I asked about, I'd recommend you use the manual-ranging one, starting with the 2000 Ohm scale.

What's so special about the 2000 Ohm scale? Well, at least from my observation, it seems that this is the only scale that will measure diodes properly on most manual-ranging multimeters. (with that said, did you use 2000 Ohm scale on your manual multimeter when you measured SD1? And if so, did you not get a reading?).

As for diode SD1, your diode test measurements (from both multimeters) indicate that it's good.
Out of circuit, a good diode will show a reading in the forward direction ( (-) probe on cathode), usually around 200 mV to 1000 mV and open circuit in the reverse direction.

I would say go ahead and test those ceramic capacitors.

[Markie76]

Re: TECHNIKA LCD26-207 Sound but no (black) picture... advice please.

(with that said, did you use 2000 Ohm scale on your manual multimeter when you measured SD1? And if so, did you not get a reading?).

Yes, started on 2k range and turned up until it gave a reading instead of OL.

Capacitor resistance tests;
CP2; 54.7k
CC17; 4.81k
CG18; 0.934
CD28; This one gives OL on all ranges except 20m - when you rub it counts down from 19m to 5.34m stays there for a second or two then starts increasing....
CD29; 32.2k
C403; OL on manual range or on auto-range scope it rises to 40m then goes OL
C385; OL on manual range or on auto-range scope it rises to 40m then goes OL
CD24; 4.72k
CD30; OL on manual range or on auto-range scope it rises to 40m then goes OL
CD31; OL on manual range or on auto-range scope it rises to 40m then goes OL
CD33; OL on manual range or on auto-range scope it rises to 40m then goes OL
CT30; 55.3k
CD23; OL on manual range or on auto-range scope it rises to 40m then goes OL
CD27; 254k
CD26; OL on manual range or on auto-range scope it rises to 40m then goes OL

Any good?

[momaka]

Re: TECHNIKA LCD26-207 Sound but no (black) picture... advice please.

Any good?

Yes, excellent. Thanks.
Well from your measurements, it appears that none of the capacitors are shorted. That's good, because at least we know they are working right. Not good because we still have to find the problem.

The next step is to check the diode packs and the transistors in that circuit, so brace yourself for more resistance/diode measurements...

First check diodes D7 and D8. According to the EL7585A datasheet and based on the way the diodes are wired in the circuit, the configuration for those diode packs is like this:

Measure them just like you measured diode SD1 - i.e. put negative probe on cathode and positive on anode, note readings on multimeter, then reverse probes and see if the reading is higher (or just post the results here). Cathode is again the side marked with the straight line and anode is the side with the triangle.

After the diodes, check transistors Q3, Q4, and Q5. Measure resistance or perform a diode check between pins 2 and 3 (using the same pin numbering as for the diode above) and post the results here.

[Markie76]

Re: TECHNIKA LCD26-207 Sound but no (black) picture... advice please.

Ok,

Resistance
D7; 2-->1; 2.96m 1-->2; goes to OL
3-->2; 3.11m 2-->3;goes to OL
D8; 2-->1; 3.04m 1-->2;goes to OL
3-->2; 2.97m 2-->3;goes to OL

Q3; Resistance 2-->3; 7.18m 3-->2; goes to OL
Diode 2-->3;1.539 3-->2;OL

Q4; Resistance 2-->3;OL 3-->2;6.70m
Diode 2-->3;OL 3-->2;1.620

Q5; Resistance 2-->3;0.25m 3-->2;0.47m
Diode 2-->3;1.262 3-->2; 0.941


After the diodes, check transistors Q3, Q4, and Q5. Measure resistance or perform a diode check between pins 2 and 3 (using the same pin numbering as for the diode above) and post the results here.[/QUOTE]

[momaka]

Re: TECHNIKA LCD26-207 Sound but no (black) picture... advice please.

Resistance
D7; 2-->1; 2.96m 1-->2; goes to OL
3-->2; 3.11m 2-->3;goes to OL
D8; 2-->1; 3.04m 1-->2;goes to OL
3-->2; 2.97m 2-->3;goes to OL

Q3; Resistance 2-->3; 7.18m 3-->2; goes to OL
Diode 2-->3;1.539 3-->2;OL

Q4; Resistance 2-->3;OL 3-->2;6.70m
Diode 2-->3;OL 3-->2;1.620

Q5; Resistance 2-->3;0.25m 3-->2;0.47m
Diode 2-->3;1.262 3-->2; 0.941

Weird. Your measurements indicate that all of the above components are fine.
Well, the readings for Q5 seems a bit strange since you got a reading both ways. That's still plausible, though, because this is a in-circuit measurement. The resistances for D7 and D8 are also odd, but the fact that you get a reading in the forward direction of each diode and not in the reverse indicates that D7 and D8 are also fine.

I wonder if the LX output of IC320 is even trying to do anything before the fault protection kicks in...
See if the voltage on test point VOFF purses to a negative value when you turn on the monitor. Also see if it stays negative.
If not, check the voltage on VDD. Should be 1.3v to 3.6v.

Apart from those measurements, I guess the only thing that's left to be checked are the resistors. I highly doubt they would be the problem, but it's probably still worth checking, especially since we've gone this far. Hope I'm not wasting too much of your time, though.

To check the resistors, first note the 3-digit value that's written on them. This value follows standard resistor coding, where the first two digits indicate the numerical value of the resistor, and the third digit indicates the multiplier (or number of zeros to add after the value, if it's easier to remember this way). So for example, let's take RD59 (located to the left of IC320). The 3-digit value on it is 103. This means the actual value of the resistor is 10 x 10^3 Ohms, which is just 10000 Ohms, or 10 KOhms.

Since no component is perfect, a 5% tolerance is allowed for each resistor value. So taking RD59 as an example again, it's value is allowed to range anywhere between 9500 Ohms and 10500 Ohms.

Now, because you'll be measuring the resistors in-circuit with your multimeter (unless you want to do nothing else but remove and measure resistors for the next few days ), the value of some resistors may appear much lower than what is stated on them. This is OKAY. The reason why this is acceptable is because a resistor may be in parallel with another resistor (which you may or may not be able to see), and the total resistance across two parallel resistors is always lower than any of the individual resistances of each resistor.
Also, resistors almost always fail open-circuit or go high-resistance (much higher than the 5% allowed value). Therefore, when you measure a resistor in circuit, if it measures lower than its stated value or up to 5% higher, then it's more than likely good. However, if it measures much higher than 5% of its stated value, it's very likely bad.
So in summary:
lower or 5% higher resistance is good
much higher than 5% resistance is bad
----
Let me know what you get for VOFF and VDD. As for the resistors, you don't need to post any measurements here - just see if any of them are out of tolerance as I described above. Also, you don't need to be very exact for this (in other words, don't get out a calcualtor and try to figure out how much %5 fo each resistor is ). Just measure the resistance of each resistor and see if it's *generally* close to its stated value (or lower).