CurtPalme.com Home Theater Forum

[Nashou66]

There is this op Amp quad form Burr Brown that might work.

http://www.ti.com/lit/ds/symlink/opa4227.pdf

Athanasios

[CxTurbo]

I have been reading that the OPA4227 and OPA4228 are good replacements for the TLO84/TLO74. The 7 is a unity gain chip and the 8 is for a gain >5 what ever the frick that means.

Here is a ss of the TLE2074 and the OPA4228 (Mouser only stocks that one)

What am I looking for in Slew Rate? Higer or lower numbers?

[Nashou66]

James a gain of 5 is too much, it has to be unity gain as the original. That is why i suggested the 4227.

The only thing I am not sure of is the technology, one is Bipolar and the other is Bifet.

I found the 4227 on Ebay.

Athanasios

[CxTurbo]

Did some digging and it appears that switching bipolar opamps for bi-fet opamps is concidered a no no.

Think I'll just stick with the TLE's and EL's and be done with it. Those are proven to work and do well.

I am not informed enough or equipped well enough to start playing with that type work i.e feed back resistors etc. I am a parts replacer when it comes to electronics not a diagnoser. If you catch the car humor there...... LOL!


James

[Nashou66]

[CxTurbo]

Looks like I have the +2.5V and -2.5V @R64 and R77.

I checked all of my current sense resistors and I have continuity on them all so they are not open and ohm out fine.

So before I toss more money at this board in chips should I salvage my new parts from this board and replace them on one of my other boards?

VMD's seem to be cheap and maybe not worth the trouble?

I do not mind testing but I have no way of seeing wave forms as I do not have a scope......

I'll move on to the Focus board and HDM today while I *in my best James Earl Jones voice* Contemplate this on the tree of sorrow!

On a side note when I clean the chips on the CLM etc. with Deoxit do they need to be removed or do you just spray it on,let it dry and then you are good to go? I have only used it on audio connections before. Never on IC's

James

[barclay66]

[CxTurbo]

Thanks Barclay. That is exactly what I was thinking. I will move on.

I tested all the resistors. Checked for a bad/backwards diode. Checked for continuity between all points where I replaced caps for a lifted trace etc. Also checked for polarity.

The HDM that came from the same machine seems to also have problems as does the convergence module it appears to be on the Blue only. I have no left and right control on the blue or ability to control convergence on blue. Maybe something was just waiting to happen. That machine was in pieces when I bought it so I never did test it.

James

[CxTurbo]

Thanks Barclay. That is exactly what I was thinking. I will move on.

I tested all the resistors. Checked for a bad/backwards diode. Checked for continuity between all points where I replaced caps for a lifted trace etc. Also checked for polarity.

The HDM that came from the same machine seems to also have problems as does the convergence module it appears to be on the Blue only. I have no left and right control on the blue or ability to control convergence on blue. Maybe something was just waiting to happen. That machine was in pieces when I bought it so I never did test it.

James

[CxTurbo]

Wait a minute........

R920,921 are paralleled to make a half ohm on the -15v.
R922 is listed a a single 1 ohm on the +15v

I put half ohm in both positions. Did I eff up there or what?


James

[Nashou66]

From the VMD post.

For R720,721,820,821,920,921:
.47ohm 1/2 watt fusible resistor from Vishay 594-NFR25H0004707JA5

They should be 1/2 ohm, I have to look on my one board to see if I changed them out too now. I know that they are listed in the revision notes and added in Aug 19 1995 from the schematics. I think they should match the +15 line. So make them all 1/2 ohm

Athanasios

[Nashou66]

922,822 and 722 Are on the -15 james. Looks like on the schematic they had the positive 1 ohm and made the negative 1/2 ohm. But I think MP changed them all to 1/2 ohms.

Athanasios

[CxTurbo]

Ok, Nash. Thanks!

I Went with what was on the schematic. I just finished one of my other VDM's and it works Flawless so far.

I am now working on the Focus board am will call it a day.

James

[Nashou66]

James I was looking at the schematics for the VDM and your problem. I wonder if one of the Fets went out Q2 or Q3. can you test them?

In case you forgot( I did and looked it up)

Instructions
1
Consult the FET's data sheet or a semiconductor replacement book to identify the position of the gate, source and drain pins.

2
Set the multimeter to the diode range setting.



3
Connect the multimeter's negative probe to the FET's source pin.

4
Touch the meter's positive probe to the gate pin while keeping the negative probe in contact with the source. This discharges any internal capacitance in the FET.

5
Remove the positive probe from the gate and connect it to the drain pin. There should be a low reading indicating conductivity. The device is now "turned on" due to the FET's capacitance being charged by the meter.

6
Touch a finger to both the source and Gate pins while continuing to hold the negative probe in contact with the source and the positive probe in contact with the drain. It is fine if your finger also makes contact with the drain pin. The gate will be discharged by your finger connecting it with the source and the FET will stop conducting. This causes the multimeter to read a high resistance. This means that the FET is functioning normally.

Here is another test source

http://howtoblogpost.blogspot.com/2011/09/how-to-test-mosfet.html

Athanasios

[barclay66]

Hi,

And of course:

0. Remove the FET from the circuit...

Regards,
barclay66

[Nashou66]

Yes! lol

Nashou

[Nashou66]

Might as well test the transistor Q6 as well. If one of the fet's shorted to Drain to gate then it will send the voltage back into the circuit which can blow another transistors or fet's on the circuit.

Athanasios

[barclay66]

Hi,

If You're really (I mean REALLY!) willing to play around and take the (SIGNIFICANT!) risk of damaging Your tubes You could try out the following.

Introduction:
This procedure is intended to diagnose a faulty VDM. The VDM incorporates several circuits which monitor the VDM's outputs (e.g. the amplifier outputs for each channel). This is necessary because without vertical deflection the entire electron beam energy (i.e. the entire brighness of a picture) will be concentrated on a single horizontal line at the tubeface. This -most likely- will result in phosphor burn and You will end up with a useless tube. In order to provide protection against this event the monitoring circuits control the VFAIL signal which will tell the CLM's processor to shut down the PJ if something is wrong. What we're about to do is to disable this signal in order to allow the PJ to start up. As we don't want to damage the tubes it is necessary to lower the beam intensity to the absolute minimum possible so that no burn will occur.

I cannot guarantee that this procedure won't do any harm to Your tubes or other parts. Therefore I can't assume responsibility if something goes wrong. You're doing this at Your own risk!!!

This is the procedure:

- Switch on the Marquee with a working VDM.
- Reduce Brightness to zero
- Reduce Contrast to zero
- Save the settings (UTIL -> 1. Source Setup -> 3. Current Setup -> Set to "Locked")
- Shut down the Marquee
- Take the defective VDM and shorten Drain and Source pins of Q1 (see picture below) -> Inhibits VFAIL signal
- Mount the modified VDM
- Switch on the Marquee with the modified VDM
- It might be necessary to carefully increase the contrast until You can see anything on one of the tubefaces

If -for example- one of the VDM's outputs is dead You will see the horizontal line on the corresponding channel. The working ones will most likely show an extremely dark picure or maybe no picture at all (too dark). The channel with the line on it may now be further diagnosed.
It might be possible too that the waveform of one or more channels is completely off. This should be visible due to an abnormal picture too.

I've done this once and it has helped me well in diagnosing and repairing a faulty VDM. In my case a diode (D801) at one of the Amplifiers was bad...

Regards,
barclay66

[Nashou66]

Good procedure Jorge. That is why I have a set if crappy tubes I use in my test bed marquee. If I spot burn them I dont care.

[CxTurbo]

Good info for the brave Barclay.

I am not that man.

James