marquee 9500 neckboard repairs.

Yes I replaced with 100nF and I hope the replacement is 200V as I could not check that. This 100nF is parallel to the big electrolytic capacitors that are on  the board. Those capacitors where replaced with 160V, 33uF, 105 degrees. like suggested in one of the maintenance threads of the Marquee. In the picture theye are still the blue 85 degrees electrolytics.

I think it might become a bit confusing as now I handled two problems on two neckboard but the next problem was even more difficult to solve. This third problem was the gain on a high bandwidth neckboard. I agree that normally you would just toss the neckboard into the bin when you are facing a problem that the gain is just to low. But this neckboard had been modified and the bandwidth was very good but the gain was very low. In practise this ment that I could not use the board on green. I had to set the green drive to 100 and the red and blue drive to 0 and even than the greyscale could not be calibrated. There was to little green. Aside from that the light output was something like 5ftL and that is just to low.

So I had to hunt this board for gain issues.

I started by measuring the voltages on the power rails as I reasoned that low power means low gain. However these measured +-85V and +-5V so nothing wrong there. One of the 400 Ohm resistors measured 370 Ohm so to be sure I replaced with another one that measured 390 Ohm but than the gain was even lower.

In the next post I will show how I replaced two of the motorola transistors in the end and how I found out they where wrong. I still have questions trying to understand how this board could function at all with two dead main transistors. Perhaps someone can explain that. I will write what I understand of the schematics.

Thanks! Talking to someone else often gives the ability to solve things.


In the above picture you see the donor board that I used for parts. The lowest hole is where Q17 used to be. While I was checking each and every resistor that could cause a gain issue the very last one I checked was R51. R51 measured 70Ohm while it should be 100Ohm. Now I compared to other good working boards and there it measured 90 Ohm or so. So I was happy that resistor seems broken! I took it out and measured damn it measured 100 Ohm outside of the board.

So I put the multimeter to diode check again and measured between the legs of Q17. So I found Q17 was conducting in a direction it should not. On the other boards it did not. So now I knew this one was broken. How to fix that. I looked at my donor board and tried to get a comparable MRF549 from that board. The first one was not succesfull as one of the leggs detached, that seems to happen very easily. First you have to remove the screw on the bottom side very carefully not to turn the transistor and then you have to desolder the leggs without breaking them. After a long time I succeeded in replacing the transistor and all measured well. R51 measured also the same now as on the good boards. So I turned on the light and was very disappointed as nothing seemed to have changed

So back to the schematics. I get the impression that Q17 is in a common base situation. Reading literature is it possible that this stage does not amplify but only takes care of very low output impedance? Does someone know? If so that would explain why nothing changed as it was amplifying 1 time only.

Anyway back to the diode tester as now I tested all those Motorolla transistors. Arriving at Q13 I again measured a deviant situation. In the modified board (I was trying to repair) the diode D12 was removed and measuring there this revealed that Q13 was conduction in wrong directions just like Q17. When I compared to another board where D12 was not removed I also found conduction but that was the D12 diode! So replacing Q17 this time it went better and a little faster I turned the lights on and yes! There was the gain I was looking for.

Now the second question is why did the board operate without Q17? Is that transistor just there as a compensation for the Miller capacitance? Is this last stage a common emitter? I still am not sure about that it seems to the biggest part in the amplification.

When youre removing those transistors, are you using a solder sucker or a heat gun?

The factory solder is high temp. It helps to flow some new solder and mix it in with the old first and then wick or desolder it.

Q19 and Q21 (Q12/Q17) form a so-called cascode amplifier stage with R28 (R13). By the way in the service manual section of this page you can find the detailed circuit description of the neckboards (for all other boards too) straight from the Electrohome engineers
Q1 and Q22 (Q13/Q14) is a basic push-pull power amplifier stage, it has several advantages over the single transistor (single ended) solution.

High frequency electronics are very sensitive, one must see things beyond the signs of the schematics, there are a lot of other influencing factors not shown in the drawings, like grounding, circuit arrangement, etc. In case of the neckboards the stray capacitances have the most influence on the circuit behavior. All the capacitors and coils placed in the current flow have a good reason to be there, they are canceling out parasitic effects, though parasitic effects are not always the same, all parts have their tolerances, some happened to be tuned better than others.