Hello e-music,
I deeply apologize for the long absence.
My wife's PC decided to pack it in, and she had important work to complete, so she had to
sponge my computer until she was done.
I have to admit that I require some time to answer any posts, as I like to think of as many
alternatives as possible before uttering a word. :smile:
OK, first up, I need to know exactly what component/s have been replaced in this set. We'll
get to the other telly later.
When ever you do any static measurements, to be 100% on a component, you need to either
lift one leg out of circuit or in the case of capacitors, remove them entirely. This is to ensure
that no other components in circuit will affect any of your readings.
SMD is the same, you need to remove the part entirely, as unfortunately you can't lift a leg.
E-design is correct.
How can you measure 320 VDC on C604, but get no voltage on pin 1 of the STR?
It definitely makes no sense.
Due to the PSU not oscillating you should be getting an identical voltage on Pin 1 of IC601, as its
only passing through a virtual short circuit via the chopper transformer. But when it is oscillating,
the 81.7 volts shown in the schematic would be correct, and not be at the same potential as C604.
I need you to measure R615. But make sure you lift one leg out of circuit, then get your reading.
This component should be the start-up resistor for IC601.
If you have a look at the picture below, it will show you the drive needed for Q3 of the internal
diagram:
If you find that R615 is OK, then we need to check the 3 diodes in the PSU for leakage.
Once again, lift one leg on each of the diodes (D602, D603 and D607).
Set you multi-meter to diode test and measure them both ways. One way (Positive probe to anode
and negative probe to cathode [Forward-Biasing]) you should get a reading. Something like .607
or there-about's.
You should get an open circuit reading the other way around [Reverse-biasing].
If you find the readings correct, we need to do what they call, "a punch-through" test on them.
Set your multi-meter to the highest ohms setting it has, then measure each diode reverse-biased.
If you get any reading at all, then there's a good chance the diode/s are leaking.
No point doing a forward-bias test, as the readings won't mean anything on high ohms.
If you need to replace D603 and/or D607, you can replace either with an 1N4148. They are as common
as muck.
You can replace D602 with an FR504, BYT52G or BYV13. Not as common, but source-able.
If all is well, we now need to start measuring resistors, that have either gone high in value or open
circuit.
Do an in-circuit static ohms measurement on each one first, and if you're getting obscure readings on
your meter, you'll have to lift one leg of the component out of circuit. And in the case of SMD resistors,
you'll have to remove them entirely, sorry to say that. :sad:
Concentrate on the easiest to remove or lift a leg/s up first.
If all good, then you're going to need to remove Q601 and Q603 and do a forward and reverse bias test
on both of them.
I have to say at this point, it isn't easy to remove a three legged surface-mounted device using just a
soldering iron, but here's a fairly easy way of doing it:
Place a blob of melted solder onto each pin of the transistor.
Now alternating between the three elements with your iron as quick as you can, allow each ones solder to
melt to the point that the transistor starts to move, then just slide it to one side away from other components
and/or joints and let it cool.
Lastly on this point, try not to heat the transistor for too long, as this may damage it.
If you're finding it difficult at first, then allow it to cool, then try again. Keep repeating this, so you don't
damage the bugger.
You may encounter one problem, and that's if the transistor itself has been glued down at the factory. If so,
it won't be easy to remove.
The only way I know of how to do it with just a soldering iron, is to place a blob of solder onto the Collector of
the device, leaving the iron in place, get a small thin sturdy piece of metal and try levering it up until one side
lifts from the board. It may take a bit of pressure, as that glue is super strong. :sad:
What ever you do, make sure your iron is constantly on the Collector. I Doubt you'll be able to melt or soften the
glue though.
OK, follow the next procedure when replacing or refitting the transistor:
Using de-soldering braid, remove all the solder off each pad.
Next, place a small blob of solder onto say the Base of the pad, then, with a set of tweezers, place the transistor
onto the pads and heat the blob so the base pin moves into its proper place, at the same time, ensuring the
Collector and Emitter pins are lined up as well. Make sure its laying flat as as possible.
Once you're happy with its placement, solder the other pins, making sure all of the pads are wetted, but don't
over-do it.
OK, once the transistor/s are out here's a hint on measuring them:
See the picture below for the pin configurations:
Lets say you're going to measure Q601 (2SA1162), which is a PNP device.
Set your meter to diode test.
(1) - Place your negative probe onto the Base of the transistor and the positive probe on the emitter.
You should get a reading of around .700 or something near that.
(2) - Leaving the negative probe on the base, place the positive probe onto the Collector. You should
again, get a reading similar to above.
(3) - Now place your positive probe onto the Base and the negative probe on the Emitter. You should
get an open circuit reading. Same reading when you place the negative probe onto the Collector.
(4) - Next, place one probe (it doesn't matter which one), on the Collector, and the other on the Emitter.
You should see an open circuit reading.
(5) - Swap the probes, and once again, it should be open circuit.
Now we need to do a punch-through test on the transistor. You pretty much repeat the above process,
but there is no need to do Steps (1) and (2). They will be meaningless.
Set you meter for its highest ohms range and proceed to do the punch-through.
The same exact method will apply to measuring Q603 (2SC2712) NPN, except that your readings in step
(1) and (2) will show that they will be open circuit.
Its not until you do Step (3), that it should show a reading. It should be similar to the PNP test, around
.700 or so.
If you need to replace either or both transistors, these are recommended:
2SA1162 ---> BC856A
2SC2712 ---> BC846A
The .700 readings, or the .607 reading on diodes just tells you how much voltage is required to bias the
device. Therefore a .700 reading will indicate a 700mV forward conduction potential required.
ALL readings I've indicated are just approximates and can vary quite a bit between devices.
i.e. Schottky diodes usually have lower conduction requirements than standard diodes. Zener diodes
should be higher than standard diodes.
Same goes for transistors, though they are pretty much on par with each other.
The above tests will allow you to determine if an unknown transistor is either NPN or PNP.
By having the positive probe on the Base and getting readings on two of the other elements will tell you
its an NPN. Positive probe to the P of the device.
The reverse is true for PNP transistors.
The above won't determine what pins are what, but at least it will determine its polarity, as well as identifying
the base pin.
So, when ever doing the above and you get two readings, then just look at what color probe is on the one
pin that stayed where it was. That will give you its type, as well as the Base pin.
As for FET's and MOSFET's, it is not possible to do static test as shown above. Here you would need a
component analyzer to get any accurate readings. The best you can do is to determine if the component
has gone short circuit.
But when doing a short circuit test with the part, you need to first make sure you have no static in your
body, then just place a wet finger across the three legs and then do your measurements.
The reason for doing the wet finger test is to discharge any internal capacitance charge that may be present.
Many a time I've measured brand new devices and in some cases they showed me that they were partially or
completely shorted. This is where a wet finger helps.
One last thing on transistors, they are NOT all made the same.
Some have internal resistors which make it difficult to conduct a fairly conclusive test.
Other transistors actually have two transistors in one, connected in parallel. These are high gain types and
are usually called Darlingtons.
There are many with internal diodes, mainly across the Collector/Emitter junctions, which are mainly seen
in Horizontal Output transistors. The internal diode is required to dampen the collapsing magnetic field of a
flyback transformer. Without it, the transistor would blow in micro-seconds.
In the early days, they used a separate dampening diode, but due to technology improvements, they were able
to integrate it within the device itself.
Lastly, there is NO guarantee that doing the above measurements is totally foolproof. I have been caught out
in the past, where I thought the device was good, yet when I replaced it, the set worked.
Once we had such a device and we put it through a bevy of tests such as, its gain (βeta), set it up in a biasing
circuit and it worked well and we also did curve tracing on it with its parameters being very close to what the
datasheet info gave us.
No reason for it to be faulty, but there you go. :bang:
The only conclusion we could come to was the fact it didn't like high voltages. This transistor we tested was the
blue cathode output transistor on the CRT board of a Sharp TV. Unfortunately when we bias tested it, we used low voltages
only. Those output transistors require 150 plus volts for proper operation.
So, my advise to anyone doing repairs, if in doubt, replace it. Provided you've done as much testing as possible.
Otherwise it will only confirm that that's one component you know works.
Please let me know how you get on, and sorry once again for the lengthy post.
Regards,
Relayer
P.S. @e-music I'll try to answer you queries about the other set tomorrow. Sorry, but I've spent too much time on this post and
I need to do stuff. :smile: