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So you want to learn more about radio repair

Discussion in 'General CB Services Discussion' started by Radio Tech, Oct 26, 2009.

  1. Radio Tech

    Radio Tech Radio Operator

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    Please ask questions by replying to the thread not with PMs. That way everyone gets questions answered and every one else gets to learn more.

    Scope
    • General
    • Shop equipment
    • homebrew test equipment
    • Basic trouble shooting
    • Receiver
    • Transmiter
    • Phase lock loop






    Years ago I started off in electronics with nothing more that a volt meter. Analog at that. I now have a well stock repair shop going with some decent equipment. But took years to accumulate. Back then there was no internet to look stuff up on. No great forums like this one to go on and ask how something works, or where to find a part. Well, at least there were local places to find parts! Everything was trial and error. At least for me.

    What I am hoping to do in this thread is share with you some of the tricks of radio repair. And maybe some of the other techs will jump in and add a few things. Reason I am doing this is that I get pm's and emails from cb'rs and new hams everyday asking different things about their radios. Just today I received one from the Netherlands asking about an article I wrote a few years ago on another forum.
    Also by reading the forums there are many folks that are trying to repair their own radios. These are the different folks than just the ones that want plug and play. So don't worry shop owners. You loose no business with this. The plug and play operators will continue to support you :)

    The art of radio repair is dying. Around my area there is only one other person that works on older tube stuff. And he is semi retired. There are three other shops that only repair solid state units. If you want to start off repairing your own rigs there is nothing better to start with than a CB Radio. Why?? Well it does not matter if you are a cb or ham radio operator. You have to start somewhere. CB's are cheap. You can pick them up at yard sales for a couple of bucks. So if you destroy it you are not out much and maybe a lesson learned.

    Standard Shop Equipment
    Here is some of the tools you will need to start repairing radios.
    A good set of hand tools.
    Regulated power supply.
    Dummy load. (A home-brew 100 watt is fine)
    Analog and digital volt meter.
    25 to 45 watt soldering irons.
    Solder wick
    Solder sucker (Radio shack has a nice solder sucker iron. Get them while they last)
    Assortment of test clips
    Magnifying lamp (Well I need one)
    Frequency counter

    I want but can not afford shop equipment
    Service Monitor
    Dual trace oscilloscope
    Transistor tester
    Capacitor tester
    Variable bench power supply
    Signal tracer


    The bottom list gets very expensive but are a must for professional repairs. But if you are affected by today's economy, you probably will struggle to get the stuff. And no need to take food out of the families mouth just for you to learn. So lets see if we can compromise a bit.

    Service Monitor.
    This is the next best thing since sliced bread. If you look around the net you can pick one up for a few hundred dollars. But the good ones are more like a few thousand bucks. I use the IRF 1200. And see them all the time for 3 to 4 grand. Take a look. You will see. But since we are starting off with cb, I will show you away you can test a radio.

    The Poor Mans Service Monitor / Generator (P.M.M for short)

    The poor mans monitor is nothing more than a converted cb radio. The first service monitor I ever owned was a modified cobra 23 channel radio. The radio was removed from its shell and mounted in a larger metal case. This way I could add the needed components to complete the unit.

    I added a internal power supply. This does not have to be a large amp supply but needs to be stable. The driver and final of the radio will be removed. The pre-driver will deliver all the power you need to test receivers with. You will need to build an attenuator so you can vary the output in generate mode. In the one I had I used a 12 position switch with resistors added of various resistance to achieve less db output.

    With today's export rigs, you can build one that will cover more frequencies and AM, CW, SSB, FM.

    List of things to add to monitor
    • Internal power supply
    • Dummy Load
    • RF attenuator
    • Tone generator
    • Frequency off % meter
    • Frequency counter
    • On Air pick up loop

    Some radio's of choice are the Cobra 25 GTL or Cobra 29 GTL (Classic). The older model. These were built tough and hold up good. So if you have an old beat up one lying around use it. One important thing to consider is you need to add some shielding to certain area’s. If you can find some very thin sheet metal you can solder it together. Double sided pc board works great also. Most of the shielded compartments will be small so it will not take much. There are several sources for this including RF Parts and Tesco.

    We will start looking at the radio antenna lead. We will need to isolate the area were the TX and RX comes together at. You will need locate D8 and R39, L10, C44, and C142.

    More to come
     
    #1
    Last edited: Nov 27, 2009
  2. GLR

    GLR Keeping It Good Natured

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    good post, I look forward to the next installment
     
    #2
  3. Radio Tech

    Radio Tech Radio Operator

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    Thanks GLR. I think this will be fun for everyone. And I may learn something while we are at it :LOL:

    I will keep editing the top post untill the P.M.M. is all layed out. Then with every new adventure I will try to keep it all in one posting. This way it is not all scattered through out the thread.

    So every one keep looking up :D
     
    #3
  4. loosecannon

    loosecannon break on through

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    bravo radio tech!!!

    i think having a dedicated test set is one of the best things any cb hobbyist can do for themselves.

    now, if we can figure out how to make it transmit a signal modulated at 30% with a 1000Hz tone, and have it put out a 100uV signal; we will REALLY have something.

    i look forward to reading more of this!
    LC
     
    #4
  5. Radio Tech

    Radio Tech Radio Operator

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    Thanks LC,
    I agree 100%.
    As far as the 100Hz tone we should be able to work this in. So I have to look around for a tone generator schematic. So if anyone wants to take that on please do not mind.
    Well business is back to normal this week in the plant. The past two weeks have been a nightmare here at work. Also behind like crazy in the shop due to work. So this project should be getting updates real soon.

    Keep watching the first post for updates.
     
    #5
  6. unit_399

    unit_399 EL CAPO

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    RadioTech -

    Most receiver alignments require the injection of an RF carrier modulated to 30% by a 1000 hz tone,. If you remove the driver and final from the rig you're using as the basis of the PMM it will not produce any modulation, and as an alignment aid it will be pretty much useless.

    - 399
     
    #6
    Last edited: Nov 2, 2009
  7. Radio Tech

    Radio Tech Radio Operator

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    I have to agree with that. You are 100 percent correct since the modulation is tied to the final and the driver. But if we leave those components in place the output power will be to great. This will damage the receiver we are trying to align. Most likely smoke our attenuator and or pass through with out any attenuation. The goal is to produce 500 milliwatts to 1 watt of power. We will do this in later steps of construction.
    Now you could leave them in place, build a dummy load inside, then pick of the rf. Then you would have to use a lot of shielding or you will end up with a unit you can not attenuate and saturate the receiver under test.

    Good eye catch though.
    But if you have a different way then please share. Ideas are always welcomed.
     
    #7
    Last edited: Nov 3, 2009
  8. unit_399

    unit_399 EL CAPO

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    There are lots of ways of reducing the output of a CB: Detune the final tank circuit; Reduce the predriver output to the driver and final; ect.. Probably the easiest way is to use a fixed resistance to reduce the collector voltage to the driver and final. You can use a rotary switch to switch in various resistors to get diffferent power levels. Easy and Cheap.

    - 399
     
    #8
  9. Radio Tech

    Radio Tech Radio Operator

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    Now there is an idea!
    See, that is why a thread such as this needs others to join in.
     
    #9
  10. BigJeff

    BigJeff Member

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    This is just meant to go over the basics of building a lab power supply out of an old atx power supply. The first thing i will do is go over the basic pinouts & wire colours you will need to worry about.

    Green - Power On to power supply
    Gray - Power OK (for your status light)
    Black - Common (ground)
    Purple - +5v DC Standby (not used)
    Orange - +3v DC
    Red - +5v DC
    White - -5v DC (removed in the 2004 atx power spec)
    Yellow - +12v DC
    Blue - -12v DC

    Please note, that the -5v may not be there. It was eliminated in newer atx power supplies. Also that the negative voltages do not have a high amperage (they are limited to approx 5 amps or less). A full guide to the 20 pin molex connector pinouts can be found here: ATX power supply connector pinout and wiring @ pinouts.ru

    The first thing that needs to be done before you start hacking into the unit is to test it. I prefer to use an old hard disk. plug it into one of the molex connectors, and jumper the green wire to a black wire. This should give you a base load and allow it to start up so you can verify your voltages.

    Once you have the supply tested, you can go about to hacking up the wiring. the first thing to look out for a smaller orange and red wires, these are for a signal reference, and must be connected somewhere to their respective wires, or the power supply will not operate, or could fry the voltage regulator.

    So, as for building your case, and installing your binding posts, that is up to user preference, but i recommend attaching all the corresponding colored wires to their respective binding posts. it allows for a larger draw through the binding posts. How i did mine, was i twisted the green power on wire to a black common wire, and put a led & resistor inline to the black wire from the gray wire. Now depending on how lazy you want to be, you can either use a old hard disk for your base load, or use a 10w 10amp resistor across the 5v bus (between red & common). Without a load on the 5v bus, most of these power supplies will not give full voltage. Also make sure you have the smaller signal reference wires attached to your binding posts so that the power supply will regulate the voltage correctly, and that should be it

    I hope this helps somewhat :)

    -Jeff
     
    #10
  11. Radio Tech

    Radio Tech Radio Operator

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    Thanks Jeff for the contribution to the thread, and welcome to the forums.

    This is the type of responce I am looking for here. And for your first post, that was a great idea.
    When you are on a budget this type of supply comes in real handy. Old pc's can be picked up at yard sales for less than 10 bucks. And you can also use some of the low voltage outputs. Make them varible and then you have a source to inject missing voltage on a radio for testing puposes.

    Thanks Jeff.
     
    #11
  12. BigJeff

    BigJeff Member

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    anytime. The power supplys are capable of producing voltages of 3.3 to 24vdc, just through normal pinouts, and i have seen some information on re-wiring them into a variable voltage power supply. They are handy little devices.

    Something else i would like to add, is for those of us who do wiring diagrams, a good PC based program (that is free), is called TinyCAD, available freely from sourceforge (big freeware repository) at TinyCAD - The open-source schematic capture program for Windows
     
    #12
  13. hypo

    hypo Member

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    FREE SPICE and Schematic software.

    Here is the link to some other Schematic and design simulation software that has a really good Yahoo support group with Lots of additional parts libraries. The Yahoo support group has links to instructional videos as well.

    Linear Technology - Design Simulation and Device Models

    LTspice : LTspice/SwitcherCAD III

    Linear Technology primarily makes chips for Cell phone power supplies and switching power supplies but also has complete transceivers on a chip in the cell bands. Amazing technology but not really applicable to CB.

    I have some old dead Uniden 510's I might have to resurrect. It would be neat if we could get a schematic of the old Sencor CB test sets and try to duplicate them. Over on CB tricks there was a suggestion by Mr.Ward that starting with a dead Galaxy 99 or any SSB radio with a freq counter would be a starting point. Not quite enough resolution on the counter though. Older lower than 100 MHz HP counters go for less than $100 on eBay pretty often. 7 digit resolution there.

    I was thinking of just getting the 510's to output a carrier at a low enough level to feed into a SA602 or 612 which is a common active mixer in SSB radios. Would have to unbalance it to reinsert the carrier. I will use the schematic of my 2510 as a starting point since it has an adjustable carrier level. I guess any low level modulation circuit would be a good start. Maybe even a diode ring mixer since the loss through one is usually around 6db and would help attenuate the signal. Look at some of the older Ten Tec schematics from the Omni A or Titan radios from the late 70's and early 80's for reproducible circuits. Schematics are in the manuals. They made amps as well.

    Ten-Tec: Downloads

    Here is a really good read as well, all 100% homebrew, and very clear and easy to understand.

    CRYSTAL SETS TO SIDEBAND

    A Guide to Building an Amateur Radio Station
    By Frank W. Harris, KØIYE

    download it here for free

    Book Copyright (c) 2006, Frank W. Harris / HTML and PDF assembly Copyright (c) 2006 AmSoft

    I'll have to make do with my basic items till the day comes and I can get a
    HP 8920 or 8924c. I did score a Tektronix 2230 scope for less than $80 a couple years ago. Still need a decent counter but until I get back full time it will have to wait. Either that or go back OTR, but then I won't have any time for learning radio. Making it driving a forklift at a LTL company now till they add drivers or some land I own sells and I go back to school. Ordered my transcripts to see where I stand in that regard.

    I love this topic. Make it a Sticky if it isn't already.
    Thanks,
    Hypo
     
    #13
  14. nat19

    nat19 Active Member

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    Hi I am somewhat new to radio repair but I love it and was wondering if you had any tips on becoming succesful.
     
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  15. Robb

    Robb Yup

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    Nat19; this is the list as previously mentioned in the first post. It is the list for tools most needed. Besides tools, one needs also to practice good soldering and de-soldering skills. Learning to repair will take patience and starting with the easier things to do. Such as wiring an aftermarket mic to any given radio, doing your own coax connectors, understanding basic circuits - like power supplies, etc. Unless one is an apprentice at a radio shop, going to school to learn would be another means to acquire the necessary skills and knowledge. I have only modest amounts of repair knowledge and have been successful in some repairs/modifications.
    I haven't yet ruined anything; but there is a first time for that - too...


    Standard Shop Equipment
    Here is some of the tools you will need to start repairing radios.
    A good set of hand tools.
    Regulated power supply.
    Dummy load. (A home-brew 100 watt is fine)
    Analog and digital volt meter.
    25 to 45 watt soldering irons.
    Solder wick
    Solder sucker (Radio shack has a nice solder sucker iron. Get them while they last)
    Assortment of test clips
    Magnifying lamp (Well I need one)
    Frequency counter
     
    #15

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