Basic Troubleshooting of Electronics
and how to cobble together these
modules into a working radio set!!
and how to cobble together these
modules into a working radio set!!
Basic Troubleshooting and repair of older
electronics can be fairly straightforward.
Here is a simple circuit Im working on right
now. It is a rather rare RF converter that I
bought at a ham radio flea market in
Southern Ontario this past year.
An Amtron 144 to 146 mhz vhf converter
that converts this frequency down to 26 to
28 mhz.
It is kinda rare. Most Vhf 2 meter
converters use the 28 to 30 mhz frequency.
By the way..the nerdy cheap glasses are
actually magnifying glasses I bought from
the dollar store. They work ok for working
on reading schematics and reading part
numbers on parts ect. I use them while
working on the bench.
It took me only a couple of minutes to come
up with the fault.
I started measuring the DC voltages around
the crystal and I used my oscilloscope to
see if the crystal was oscillating. It was not.
The collector voltages of the transistors
was only about 3.3 volts. Rather low indeed.
Should be more like 12 or 9 volts.
I took a look at the manual and it should be
about 9 volts. There is a 9 volt zener diode
on the line. The 9 volt zener has been
replaced and the DC voltages are now
pretty normal. The oscillator transistor
(near the crystal is going to be replaced
because the oscillator is not working
properly at all and I suspect the transistor
is bad. I am replacing it with a 2N5109 which
has better high frequency response than
the original transistor. You can see I
removed some 2N5109s from a surplus
circuit board in my junkbox (as per the
photos on this page.
This converter uses all discrete
components such as resistors, capacitors,
diodes and transistors (including 2 dual
gate mosfets). I have all the parts in my
stores to fix this item.
You can see a tantalum capacitor (with a
yellow top) near the top middle of the
chasis. Sometimes these old tantalums can
be problem. Im going to replace this one
with a 4.7 uf electrolytic.
If you have an item such as this that you
would like to repair..I just might be able to
supply you the parts you need at a very low
price.
I also buy items such as this one. I can also
do some repairs of items such as this at a
very low price.
Some of the repair shops charge at least
$40 per hour and their replacement parts
are not cheap. My bench rate is only $10
per hour and the parts I have are cheap
(sometimes used tested pulls)..but the unit
will work and work well when Im finished
with it.
electronics can be fairly straightforward.
Here is a simple circuit Im working on right
now. It is a rather rare RF converter that I
bought at a ham radio flea market in
Southern Ontario this past year.
An Amtron 144 to 146 mhz vhf converter
that converts this frequency down to 26 to
28 mhz.
It is kinda rare. Most Vhf 2 meter
converters use the 28 to 30 mhz frequency.
By the way..the nerdy cheap glasses are
actually magnifying glasses I bought from
the dollar store. They work ok for working
on reading schematics and reading part
numbers on parts ect. I use them while
working on the bench.
It took me only a couple of minutes to come
up with the fault.
I started measuring the DC voltages around
the crystal and I used my oscilloscope to
see if the crystal was oscillating. It was not.
The collector voltages of the transistors
was only about 3.3 volts. Rather low indeed.
Should be more like 12 or 9 volts.
I took a look at the manual and it should be
about 9 volts. There is a 9 volt zener diode
on the line. The 9 volt zener has been
replaced and the DC voltages are now
pretty normal. The oscillator transistor
(near the crystal is going to be replaced
because the oscillator is not working
properly at all and I suspect the transistor
is bad. I am replacing it with a 2N5109 which
has better high frequency response than
the original transistor. You can see I
removed some 2N5109s from a surplus
circuit board in my junkbox (as per the
photos on this page.
This converter uses all discrete
components such as resistors, capacitors,
diodes and transistors (including 2 dual
gate mosfets). I have all the parts in my
stores to fix this item.
You can see a tantalum capacitor (with a
yellow top) near the top middle of the
chasis. Sometimes these old tantalums can
be problem. Im going to replace this one
with a 4.7 uf electrolytic.
If you have an item such as this that you
would like to repair..I just might be able to
supply you the parts you need at a very low
price.
I also buy items such as this one. I can also
do some repairs of items such as this at a
very low price.
Some of the repair shops charge at least
$40 per hour and their replacement parts
are not cheap. My bench rate is only $10
per hour and the parts I have are cheap
(sometimes used tested pulls)..but the unit
will work and work well when Im finished
with it.
As an alternative to completely building electronics equipment from
scratch or even kits, you can buy or scrounge broken or dysfunctional
equipment and fix it up then cobble together modules and assemblies to
build yourself a creation you can call your own..and unique. Here I start
out with a converter that I may well turn into a transmitting converter
incorporate into a complete transceiver. I bought this unit for $2.00.
To build it from scratch would cost maybe $50 and many hours and I
seriously doubt the circuit board and box would look as good as this one
does. I can modify the circuitry as well to permit different frequency use.
scratch or even kits, you can buy or scrounge broken or dysfunctional
equipment and fix it up then cobble together modules and assemblies to
build yourself a creation you can call your own..and unique. Here I start
out with a converter that I may well turn into a transmitting converter
incorporate into a complete transceiver. I bought this unit for $2.00.
To build it from scratch would cost maybe $50 and many hours and I
seriously doubt the circuit board and box would look as good as this one
does. I can modify the circuitry as well to permit different frequency use.
The 2n5109 is a great little transistor good up to over
1200 mhz.
For an oscillator such as in my AMTRON
converter..this transistor should work well as a
crystal oscillator.
I use this radio shack desoldering iron. I bought a
number of them when they were on sale. It works fine.
A few minutes work and I removed the components
shown below.
I used a 2n5179 in the crystal oscillator.
The 2n5179 is an industry standard vhf/uhf transistor
and I feel I could not go wrong by using it as a
replacement in this circuit. The original transistor was
rated at 800 mhz.
SUBSTITUTE vs ALtERNATE item.
An alternate item is a NATO term meaning it could be
the same but likely a bit better in
specification/performance or reliability. A substitute
item is usually not quite as good but will work in the
circuit. I'm not sure I am completely accurate in my
definitions..I have been away from NATO components
for a while now.
Divide and Conquer is a rule of trouble shooting. First DC and AC. Do a test of the DC
voltages and functions. Then if this is OK then trouble shoot the AC (RF in this case).
DC wise..this unit was defective. 9 volts was not present on the collectors of the
transistors and the obvious culprit seemed to be the 9 volt ZENER diode. It was
defective and replaced.
NOW TO THE AC ANALYSIS of the circuit. First go to something simple..such as the
crystal oscillator. It is simple. A crystal and a transistor basically.
Quite often..the semiconductors are to blame in electronics such as this. The
capacitors and resistors are fairly stable entities but semiconductors are somewhat
more likely to fail.
The oscillator is not oscillating. I will replace the transistor to see if that solves the
problem..I suspect it will. I checked the oscillator in the following way.
1st..I unplugged the crystal (it was in a socket) and tried it in my little nand gate crystal
testing oscillator. The crystal oscillated fine. So I suspect the transistor is defective.
voltages and functions. Then if this is OK then trouble shoot the AC (RF in this case).
DC wise..this unit was defective. 9 volts was not present on the collectors of the
transistors and the obvious culprit seemed to be the 9 volt ZENER diode. It was
defective and replaced.
NOW TO THE AC ANALYSIS of the circuit. First go to something simple..such as the
crystal oscillator. It is simple. A crystal and a transistor basically.
Quite often..the semiconductors are to blame in electronics such as this. The
capacitors and resistors are fairly stable entities but semiconductors are somewhat
more likely to fail.
The oscillator is not oscillating. I will replace the transistor to see if that solves the
problem..I suspect it will. I checked the oscillator in the following way.
1st..I unplugged the crystal (it was in a socket) and tried it in my little nand gate crystal
testing oscillator. The crystal oscillated fine. So I suspect the transistor is defective.
Navigation link back to my start page (earl andrews. com click here to go
| the transistor near the crystal was replaced with a 2N5109. |
| The tantalum cap 4.7 uf was replaced with a 10 uf electrolytic |
| This half is the oscillator section. The crystal is in the 39 mhz range and triples up to 118 mhz (which is the Local Oscillator frequency). This Local Ocillator is pased on to the two Dual Gate Mosfets (little metal can transistors). |
| The 144 to 146 mhz signal (ie from an external antenna) enters this connection. The two diodes are clamping diodes and they ensure the peak to peak signal going to the mixer dual gate mosfet does not exceed about .7 v p-p. |
| OUTPUT FREQUENCY OF BETWEEN 26 AND 28 MHZ. |
The converter now works. I tested it with a 2 meter ssb
transceiver and a transceiver that is capable of
receiving 26 to 27 mhz ssb.
Seems to be working good on the bench.
The problems were a bit more involved than I thought.
The two dual gate mosfets had to be replaced. Perhaps
they were damaged by a high voltage spike or reverse
voltage connection of a power supply or something. I
replaced them with two dg mosfets (type 3n201 plastic
case). I mounted them under the circuit board.
Not the same exact physical configuration but she works
now.
transceiver and a transceiver that is capable of
receiving 26 to 27 mhz ssb.
Seems to be working good on the bench.
The problems were a bit more involved than I thought.
The two dual gate mosfets had to be replaced. Perhaps
they were damaged by a high voltage spike or reverse
voltage connection of a power supply or something. I
replaced them with two dg mosfets (type 3n201 plastic
case). I mounted them under the circuit board.
Not the same exact physical configuration but she works
now.
I have added a second page with some scans of the schematics and some notes on
trouble shooting techniques. click here to go
trouble shooting techniques. click here to go
| Pictured LEFT is an old circuit board with vhf transistors, vhf coils ect. I pulled 5 transistors (4 2n5179s which are good vhf transistors) and I also pulled some capacitors. Took me a few minutes only. I tested the transistors for hfe to make sure they were good. |
| When I need parts, sometimes I pull them from old circuit boards. THIS IS OK for my own hobby circuits. When I sell parts..I sell them as new or if they are used and tested pulls..I will let the person know in advance. I can test the parts before using them as replacements. The only used parts I try and avoid using if possible are Electrolytic Capacitors and possibly Tantulums as well. The Electrolytics can dry out with age. NOTE: I now have an ESR tester for testing Electrolytics for leakage. |
I use this homebrew NAND gate
oscillator circuit to test crystals. It has
worked very well for me.
I check the frequency of oscillation
and the quality of waveform
(amplitude) on an oscilloscope to
confirm the crystal.
I have a crystal testing test circuit kit
of higher quality that I will eventually
build and use as well.
oscillator circuit to test crystals. It has
worked very well for me.
I check the frequency of oscillation
and the quality of waveform
(amplitude) on an oscilloscope to
confirm the crystal.
I have a crystal testing test circuit kit
of higher quality that I will eventually
build and use as well.
| All my work and my components and repairs ect are for HOBBY PEOPLE mainly ham radio people and the odd CBer or SWL or electronic tinkerer or even student. |