The output of my GEMINI GVX-SUB18P is weak and distorted. Also I measured the Voltages Ubase and Uemitter on the output transistors and both are the same: 0.40 Volt DC.

Blue color output is less or not is the problem to your set. Check the voltages at the blue color output transistor at the CRT base card. Check the voltage at the base of this transistor. It should be almost same as the voltages present on the other two color output transistor bases. If there is no voltage; means the chroma IC should be checked. Anyway; replace this transistor; even though it measures good with multi-meter.
If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/

you have some wires touching causing it to ground.check wiring

It seems to me that you have a bad transistor, T14 or T13 to be precise. T14 most certainly is in short circuit or its companion T13 (T14 is a small signal NPN transistor whereas T13 is a small signal PNP transistor). I just fixed one with the same symptoms as yours. You should be able to measure T14 collector voltage against ground (metal chassis) and the reading must be very close to +15VDC. Also, measure T14 emitter voltage to ground and the reading must be very close to -15VDC. If any of those voltages is compromised, you will not be powering the amplifier's operational amplifiers (U4, U5, U6) right. Check T14 and T13 with your multimeter and replace them as needed. Also, keep in mind that after replacing any of those transistors you will be required to readjust the BIAS current for your amplifier's drivers, which by the way, is a very easy task to accomplish. You know your BIAS current is misadjusted when your output transistors get really hot even under no signal conditions or when your amplifier distorts at very low volume levels or on high frequencies amplifying. To adjust the BIAS current for channel 1's driver transistors, wait until the amplifier has cooled down, locate and adjust VR3 until you read 80 mV across R2. To adjust BIAS current for channel 2's driver transistors, locate VR6 and adjust it until you read 80mV across R106. You need to wait until the amplifier cools down to set BIAS current right, If the output transistors' heat sinks get hot before reaching proper BIAS current adjustments, wait until they cool down to average room temperature and retake BIAS current adjustment. Once both channels BIAS currents have been readjusted, apply a thin coat of thick nail polish to the sides of VR3 and VR6 to keep vibration from misadjusting BIAS currents again... T14 goes down under severe BIAS currents shifting and it just makes me wonder as to why BEHRINGER does not apply a setting substance to VR3 and VR6 so they do not shift under sever vibration conditions, after all, almost any major amplifier manufacturer in the world does it. Good luck!

Transistor Problem


To replace the power transistors ( MN2488 and MP1620 )

• you need to have a technique in soldering.

( much faster is cut the transistors pins , but be careful only for

professional serviceman )


click on the link below:


Soldering Guide

After taking out the burnt transistors:

• get a multimeter and measure the DC voltages present on

each amplifier output stage ( center of the two resistors in series ).

• • it should be no more than 0.7 V, if it s, remove those pair of power

transistors and test again for output voltages.


the negative probe

of the multimeter to the chassis of the unit.


• Do not connect new power transistors yet.

you may need to power it without it.


Until you sure that the unit powers up and have some/other channel

working to prevent further transistors and components failure.





But if there are still more than 0.7 v on some of the outputs


• Check for VAS circuits and Differential Input Stages



Let me know how you make out with the inspection.

It is a good chance you will find something.


hope this helps, thanks for using Fixya!

Nnormally about a fifteen amp

It may NOT have a fuse. If too much power was sent or an amplifier drove DC to the speaker, the voice coil may have fried. The specs do NOT brag aboput protection, so it is likely either the speaker is blown or the internal wiring is broken. Open the unit and use an ophmmeter to tesst the speakers.

Hello

Blue video output section of your Soney tv has failed. It may be due to a open video output transistor or a resistor. Video output section can be located at the picture tube base card itself. in some models, Sony used transistors, and on others they uses IC for this section. Just open your tV, and measure the voltages at the Red, Green and Blue cathode voltages. It should be almost the same to each other. In your tVs case, the voltage at the Blue cathode will be high. Normally it should be due to an open resistor or loose or bad solderings. I advise you to resolder most of the suspected solder points with applying a little more solder, without making any solder bridge shorts in between. Check it out. If this does not help, check Blue output transistor, or if it is an Ic; whatever may be used, for open. If found faulty, replace it with same type and number. OK.

Hello
Have you measured the voltages at the collectors of the three video output transistors?. There must be a voltage over 124 to140VDC at least. All voltages at the base of that transistors should be between 2.5 to 3.75VDC according to the level of chroma signal from the chroma Ic.
You must check these voltages with a good quality digital or analogue multimeters, otherwise you will get only false values instead of the real. You just check the main voltage that goes to the video output stage, that is the high voltage comes to the crt base, from the main board. It should be more than 260VDC. Normally in all tvs this voltage is derived from the LOT. After rectifying it and filtered with a 10Mfd 350V capacitor, and given to the baseboard as the main supply to video output stage. If the smoothing capacitor [10Mfd] gets value changed, that is if it gets a low value due to age, otherwise whether it has an internal short, you won't get the above said 260VDC at the CRT base boad. At first locate this capacitor and replace it with a new one with same capacity and voltage ratings. and check all video output [ 3 of them] transitors too. Check the transistors for collector to emitter leak, not on circuit, but after take it out desoldered. Then only you will get the proper measurement of leak. Set the multimeter in MegOhms rage, and don't held the transistor by hand too. If you do so you will see some deflection in the meter due to your body resistance.
Check the capacitor first. Replace it. It is best. Check the rectifying diode that rectifies this voltage from the LOT winding. If it has a leak, you will get a low voltage at this rail.
Remember, LOT works at high frequecy. So the diode used there as retifier is a special type, fast recovery diode, and should be repaced with the same number. Never use ordinary diodes that we uses for line rectification purpose.
Remember, colector voltages at the video output transistors must be above 140VDC. Again recomends to use a good quality multimeter, as cheaper one cannot measure this voltage accurately.
Ok. Wish you all sucess.

At this point, I would be looking at the main power suply for the final output stage. You may be attempting to drive the output with the driver section rather than the final amp stage. The center pin of the output transistors should have B+/B- on them. Measure that to ground and report the value here.

Dan

If the output transistor is good you need to measure the voltage at the collector to see if it has the needed voltage to operate,is also important to check the conections in the horizontal driver transformer and also measure the volts at the collector of the horizontal driver transistor,before you check any voltages check for loose conections and overheated areas,after everything is nice and solid then you can check for volts,be careful while measuring voltages and try not to touch anything with your bare hands while the set is on.Let us know how it goes or if you need more information.