TWO TRANSISTOR IN THE RF OUTPUT STAGES ARE MISSING.....TYPE? CIRCUIT DIAGRAM? PRO 501 XL.THANK YOU

The big question is what caused the output transistors to fail...

Most modern audio amplifier circuits are dc coupled for good performance on the cheap and that means amplification stages can't be considered independently and several stages must be considered as a whole.
Some circuits are more sensitive than others and more highly stressed. If it has a complementary pair output stage the transistors should be a matched pair.

The reason for this in a dc coupled circuit the bias and the quiescent current for each stage is controlled by earlier stages - when an output transistor pops, the fault is likely to be several driver stages back.

When a transistor is replaced the gain is likely to be different to the original and so the bias and quiescent current must be checked and adjusted in order for the finished job to have long life and good performance and this usually means some component values have to be changed slightly...

The frame amplifier, like the line amplifier, will not work if there is no output from the oscillators for them to amplify.

Early 405 line sets didn't have a frame oscillator as they used the sine wave from the AC mains but I believe the later sets needed a frame oscillator and a sync signal from the rf demodulator to prevent instability.

Transistors rarely fail by themselves and it is often external factors that cause them to overheat and fail and an essential part of the repair process is discovering what factors caused the initial problem. Modern circuits use a lot of multi-transistor dc coupled stages and it is often the bias of an earlier stage that is the culprit.

The frame output transformer is usually very robust and much less likely to fail than a line output transformer. I suggest you ensure the correct operation of the oscillators before going any further.

In the form of a question???? I am assuming you are hearing it on another radio or someone has told you it is quieter. A shorted coax is more likely wattage and range reduction. Proportionally reducing audio volume.......modulation. If you have only changed antenna it could be broken or could you have put the Teflon washer on the bottom of the mount rather than insulating the antenna from the mount during installation. That would result in a direct ground. Thank for reading my blither.....take care.

Hi,If your blowing fuses then the problem is most likely to be faulty bridge rectifier or darlington output transistors.All readily available and not too expensive.Circuit diagram on Fender site or net easily found.Thanks Fender good customer service.

Check the video output stage circuit. Check whether there is any loose solder terminals at the video output transistors. If you wish to get more details about this fault, check this link. Video output section board can be located at the back of the picture tube, will be directly plugged on to it. http://electronicshelponline.blogspot.com/
Pull up previous posts by its left sidebar, or type in the brand name and model number to of your set, to its search box, and press 'Enter' or view it in Mosaic / Timeslide option.

Hello
If your TV screen looks more red its red video output section is in trouble. Some philips circuts uses transistors while others uses ICs in that section. Philips magnavox model have both types. We here in India, have transistorised video output stages in Philips Magnavox sets. Whatever the ciruit type may be, the fault and its remedy is same.
Check whether your set has transistors or ICs used. If transistors are used, check the collector voltage at red video output transistor. If ICs are used, some sets uses a single IC for Red, Green and Blue output stages. Some of them have three ICs on board as Red,Green and blue output stages.
The video output stages are integrated on the base card of your TV. That is, a small square or rectangular board, pluged into the neck of the picture tube. You just find out the red,green and blue output transistors, or ics whichever may be used. It is ver easy to understand the video output transistors. They may be a little larger than other transistors.
An easy way is to locate the Red,Green and blue cathodes of the picture tube base board points. Check the voltage of the three of them They must be almost equal to each other, provided a little, that is, 2 or 3VDC difference is allowable and will be. In your case, the voltage at the red cathode may be , not may be, must be low. As cathode voltage lowers, emmision of that cathode will rise, and poise that colour is in excess. If the Red Cathode voltage in your TV is found low, say difference betwwen other two cathodes is 50v or so, repace the red output transistor or the Ic if used at that stage. If a single Ic is used in your TV as R_G_B outputs, replce the same. If three Ics are there, replace the red output control Ic. ok
If you have any more problems, inform fixya. They will send me alerts.
Thank you

Wharfedale pro bx1360sd

I have had problems with this stereo mixer amplifier which had the symptoms of no sound output from either channel .

I opened up the unit and discovered that the amplifier was correctly amplifying my input signal up to a relay which switches the output through to the speakers .

This relay did not activate at all.

When the power switch is turned ON you should hear a click approximately 2 seconds later , if no click then there is a problem with the circuitry associated with this relay .

I traced out all the connections associated with this relay and made a circuit diagram .

The relay is a 24v relay and I will explain the way that I can see that this section of the amplifier works.

It has been designed to ;

1. Create a slow start delay ( 2 seconds ) when the power is turned ON.

This it needed because otherwise there is a current surge and sparks from the relay turning on immediately.

1. Thermal cutout mounted on heatsink . If the ventilation fan fails the heatsink and transistors may get too hot and the cutout will operate and cut power to the relay by going open circuit.
2. Speaker protection from blown output transistors. There are 2 transistors q802 and q803 both 2sc1815. They are connected together emitter to emitter and collector to collector to form a differential Op amplifier I.C. If there is a blown output transistor then DC voltage will be detected by q802 and q803 and the relay will be switched OFF to protect the speakers .

Q804 turns On Q805 which turns On Q806 to activate the relay .

Voltages on transistors

Q802 and Q803 2sc1815

Base 0V Collector 12.9V Emitter 0V

Q804 2sa1015

Base 12.9V Collector 0V Emitter 15.1V

Q805 2sc1815 (driver for Q806)

Base 0V Collector 7.2V Emitter 0V

Q806 2sc1815

Base 0V at Turn ON then after 2 secs increases to 0.7V

Collector 2.8V ( transistor is activated as a switch Turned Fully ON ( saturated) to turn on the Load which is the relay .

Emitter 0V

I have replaced the following transistors to fix my amplifier q805 2sc1815 and q806 2sc1815 .If you are going to use a different type of transistor it may not work and also the lead connections of a 2sc1815 for the collector and emitter are different to those such as on a 2n2222( transposed).

Faichild/Toshiba make the output transistors for the ep2500, part numbers 2SA1943 and the 2SC5200. Beware though there are a lot of dodgy fakes around on ebay. Profusion are excellent. Heres the links for them.
http://www.profusionplc.com/pro/gex/pcatdtl0?ipartno=2SA1943-O
http://www.profusionplc.com/pro/gex/pcatdtl0?ipartno=2SC5200-O

If the speaker is not blown, there must be an output capacitor. If no output capacitor, and you have a blown transistor the speaker will blow from DC current. You can further check the transistors with a multimeter. A transistor looks like 2 diodes and you should be abobe to test with a mulimenter like a diode test.