Author Topic: Is this feasible ? And I apologize for the vagueness.  (Read 10796 times)

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Offline HextejasTopic starter

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Is this feasible ? And I apologize for the vagueness.
« on: July 16, 2018, 12:01:39 am »
I am a raw beginner to the electronics world and am studying anything that looks to be worthwhile.
In my closet is a Nad 402 am-fm tuner. Well it dont work and I want to make it work. I thought that it would be a good beginners project.
I checked the minimal stuff that I know how to do. Checked for an open circuit by putting my vom leads on the end of the power cord. Yep, shows as open. The tuner is fused so I tested the 2 fuses and one was burned. If memory serves me, they were 415ma, 250v. They were odd in that they had a small resistor in the fuse. A ham buddy of mine said that they were slow blow. That was the replacement that bought. One capacitor looked odd, kinda swollen so I replaced it. Another site, somewhat kiddingly said that it would be a diode. So I replaced 2 of them. 1N4001 with 1N4007 cause that's all I had.
The tuner was acting very strange. After I replaced the fuse the 1st time, the control panel lit up and I tried to select a station. Poof, lights out and another burned fuse. Replaced the fuse and now it won't even light up.
Its beyond my skills to repair this thing so I want to hire a knowledgeable person to repair it as part of a teaching session with me as the student.

How do I go about doing this ? I am willing to pay $nn per hour to do this, whatever the market deems fair. I have the Nad service manual and it has a schematic so thats a help.

Is this anyway possible.
I live in Houston TX if anybody wants to try it. :=)
 

Offline james_s

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #1 on: July 16, 2018, 01:35:27 am »
Well I'm nowhere near Houston or I'd offer to help, I've been known to do this sort of thing in exchange for pizza & beer. Good call knowing when to stop though, one of the most common noob mistakes is forging ahead without realizing until it is too late that they're in over their head and have turned a small problem into a big problem.

In the meantime you might look around on trash day or post on craigslist looking for broken electronics to practice on, and then take a few days to study electronics fundamentals, just basic stuff like linear power supplies. Start on stuff that's beat up or otherwise not worth much because you'll probably break a few things beyond repair before you get the hang of it. Also if nobody replies here you might look around to see if there's a hackerspace or other geeky gathering nearby.
 

Offline Relayer

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #2 on: July 16, 2018, 01:53:10 am »
Hello Hextejas,
It would be ideal if you can scan the power supply section of the radio and post it
up in here, as it will make it easier to diagnose your problem.
In the mean time though, you can check with your VOM set to low ohms, the main
filter capacitor. It is usually the biggest cap in the power supply section and should
be rated at least 200VW (VW = Voltage Working or just Working Voltage).
If you find that its reading very low ohms and stays in that state for several seconds,
then you have a short circuit. But it would be extremely doubtful that the capacitor is
the cause of the short. It will most definitely be  semi-conductor component.
If you find that the VOM reads low ohms to start off with, then it starts climbing up to
infinity, then the main primary voltage rail is OK. You need to look elsewhere.
Either way, try checking any diodes, transistors etc, to see if any of them have shorted.
Please let us know how you get on.
Regards,
Relayer
 

Offline coromonadalix

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #3 on: July 16, 2018, 02:58:10 am »
here's the service manual:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjRiPzVz6LcAhUJ3YMKHeAaCqQQFggoMAA&url=https%3A%2F%2Fwww.vintageshifi.com%2Frepertoire-pdf%2Fpdf%2Ftelecharge.php%3Fpdf%3DNad-402-Service-Manual.pdf&usg=AOvVaw3XkxulnRb7VW2I7vjnuh0d


Normally  you check for shorts on all the power supply (ies) in it, when its possible you isolate all the circuits, ex: preamp, power amp, tuner section, and work your way thru

Pictures would help  or tell us witch part number you have changed, you wrote having changed an swollen capacitor, did you put the same capacitance value and or put it in the right polarity ??? 

The 32v power supply section act as an voltage doubler of somekind, strange way to save an transformer secondary output.

The fuse is the last thing to blow, damage has been made, changing fuses wont help.

You have to find the cause ... this is a very basic tuner, you can measure a few things if you have an basic electronic knowledge

Is the transformer is set to the right main ac voltage selection ???

You have 4 regulator sections,  do they have an short at their outputs ? dead capacitors may cause or can cause a short, capacitor in reverse polarity can do the same.  Blown rectifiers diodes can do harm ...

Do you have an variable current limited power supply ? you can substitute some voltages in it, check if your power supply ex: limited to 50 milli amps goes into overload ???

Sometimes you remove some parts to get an good measurement, the measured values can be changed or interfered with the parts around ...

good luck
« Last Edit: July 16, 2018, 03:07:53 am by coromonadalix »
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #4 on: July 16, 2018, 11:56:02 am »
Wow ! Good stuff here and I will proceed as suggested.
Another thought I had while waiting in the dentists office would be to contact an engineering school and see if they have a smart student that wants to earn a few $$$.

Onward
 

Offline Wimberleytech

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #5 on: July 16, 2018, 02:13:23 pm »
Wow ! Good stuff here and I will proceed as suggested.
Another thought I had while waiting in the dentists office would be to contact an engineering school and see if they have a smart student that wants to earn a few $$$.

Onward

I grew up in Houston and learned electronics while taking Vocational Radio and TV class in high school.  We routinely repaired stuff for teachers, friends, etc.  We needed broken stuff to learn with.  We did it for free.  Alas, they dont have those anymore.

How important is this thing to you?  I am not in Houston, but I am relatively close.  Am tempted to help.
 
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Offline Old Printer

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #6 on: July 16, 2018, 06:26:48 pm »
Many good suggestions here, I'll add another. Is the main goal to get the radio working, or is learning a bit of electronics more the case. If you want to learn electronics hop on YouTube and poke around. You won't have to look hard, there is a ton. Two good channels with shortish videos aimed at basic knowledge are by GreatScott and Afrotechmods. There are hundreds and our host here, Dave has an excellent assortment but tend to be longer and more advanced. If you want more let me know, I have watched a lot. I download them onto my iPhone and listen commuting to and from work, don't look at the screen while driving :)
 

Offline Wimberleytech

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #7 on: July 16, 2018, 06:35:46 pm »
I see from the schematics, that the 5V supply is always powered once you plug it in.  The other power rails are controlled by the power switch.

If the unit does not blow a fuse with the power switch off but plugged in, then the 5V rail and load is probably OK.

You can verify this by plugging it in with the power switch off and measuring the 5V rail.
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #8 on: July 16, 2018, 08:14:12 pm »
Many good suggestions here, I'll add another. Is the main goal to get the radio working, or is learning a bit of electronics more the case. If you want to learn electronics hop on YouTube and poke around. You won't have to look hard, there is a ton. Two good channels with shortish videos aimed at basic knowledge are by GreatScott and Afrotechmods. There are hundreds and our host here, Dave has an excellent assortment but tend to be longer and more advanced. If you want more let me know, I have watched a lot. I download them onto my iPhone and listen commuting to and from work, don't look at the screen while driving :)

Primarily, I want to get the radio working and secondarily learn a bit of electronics along the way. Thank you for the youtube suggestions. I already subscribe to Dave's patreon service and I read here often.
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #9 on: July 16, 2018, 08:16:20 pm »
To any responder, how do I go about checking voltages at a capacitor when they are wired so tightly to the pcb ? Absolutely no room at all so as to get under it.
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #10 on: July 16, 2018, 08:23:05 pm »
here's the service manual:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjRiPzVz6LcAhUJ3YMKHeAaCqQQFggoMAA&url=https%3A%2F%2Fwww.vintageshifi.com%2Frepertoire-pdf%2Fpdf%2Ftelecharge.php%3Fpdf%3DNad-402-Service-Manual.pdf&usg=AOvVaw3XkxulnRb7VW2I7vjnuh0d


Normally  you check for shorts on all the power supply (ies) in it, when its possible you isolate all the circuits, ex: preamp, power amp, tuner section, and work your way thru

Pictures would help  or tell us witch part number you have changed, you wrote having changed an swollen capacitor, did you put the same capacitance value and or put it in the right polarity ??? 

The 32v power supply section act as an voltage doubler of somekind, strange way to save an transformer secondary output.

The fuse is the last thing to blow, damage has been made, changing fuses wont help.

You have to find the cause ... this is a very basic tuner, you can measure a few things if you have an basic electronic knowledge

Is the transformer is set to the right main ac voltage selection ???

You have 4 regulator sections,  do they have an short at their outputs ? dead capacitors may cause or can cause a short, capacitor in reverse polarity can do the same.  Blown rectifiers diodes can do harm ...

Do you have an variable current limited power supply ? you can substitute some voltages in it, check if your power supply ex: limited to 50 milli amps goes into overload ???

Sometimes you remove some parts to get an good measurement, the measured values can be changed or interfered with the parts around ...

good luck
Yes, on the capacitor, same value, i paid attention to polarity.
Xformer is set correctly, as came from factory.
4 voltage regulators ??? I dont know how you determined this. Remember my newbieness.
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #11 on: July 16, 2018, 08:40:36 pm »
I see from the schematics, that the 5V supply is always powered once you plug it in.  The other power rails are controlled by the power switch.

If the unit does not blow a fuse with the power switch off but plugged in, then the 5V rail and load is probably OK.

You can verify this by plugging it in with the power switch off and measuring the 5V rail.

Wimberley, for a teachable moment, how did you determine that the 5v was always on ? The 2 diodes, D8 and D9 (I think) 1N4001 are the ones that I replaced with 1N4007. I hope that the substitution was ok.
Oh yeah, with the power on and my meter probes on each side of the diode, I get 0v readings.

Thanks again
 

Offline coromonadalix

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #12 on: July 16, 2018, 08:51:46 pm »
to check capacitors you can find any thru hole part who's been connected to each capacitor side ... without going under the board, you have to understand the electronic schematic

not to be rude, you seem to have very very low knowledge ...  to check diode  you do not power the tuner, you have on the meter a diode check:

here's some infos :

https://www.fluke.com/en-us/learn/best-practices/test-tools-basics/digital-multimeters/how-to-test-diodes-using-a-digital-multimeter

How is a diode marked?

The positive side is called the anode, and the negative one is called the cathode. The diode circuit symbol, with the anode and cathode marked. Current through a diode can only flow from the anode to the cathode, which would explain why it's important for a diode to be connected in the correct direction.

youtube: 

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&cad=rja&uact=8&ved=0ahUKEwj9tOXYvqTcAhWj5IMKHRxtBPgQwqsBCGUwDA&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DmMXDa5hVzXA&usg=AOvVaw3EHzaY8_FVGf5_uMVNk9RT

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=14&cad=rja&uact=8&ved=0ahUKEwj9tOXYvqTcAhWj5IMKHRxtBPgQwqsBCGgwDQ&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3Dgzh8ICLMXzQ&usg=AOvVaw2FRddKQf43-uwNQCvluBVW


Can you test a diode in a circuit?

The multimeter then displays the voltage drop when the test leads are connected across a diode when forward-biased. The Diode Test procedure is conducted as follows: Make certain a) all power to the circuit is OFF and b) no voltage exists at the diode. ... Set the multimeter to measure ac or dc voltage as required.

I know there is 4 voltages regulators since i have 30 years of experience in this.
 

Offline coromonadalix

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #13 on: July 16, 2018, 08:59:37 pm »
On page 10 of the service manual  you have 3 legged to-220 casing fixed regulators named :  lm7812, lm7912 and lm7806

lm7812 is a positive 12 volt regulator
lm7912 is a negative 12 volt regulator
lm7806 is a positive 6 volt regulator

and the fourth is a transistor regulator made with a few parts around it : q17 printed on the board, part number is bc64 xxx something ????

your 1n4001 diode can be replaced by any number higher than 1n4001, ex : in 4002, 1n4003, 1n4004, 1n4005, 1n4006 and 1n4007, the 4007 is an 1000v diode
 

Offline jmelson

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #14 on: July 16, 2018, 09:35:09 pm »
Wow ! Good stuff here and I will proceed as suggested.
Another thought I had while waiting in the dentists office would be to contact an engineering school and see if they have a smart student that wants to earn a few $$$.

Onward
You might check out TX-RX labs, a makerspace in Houston.  They have some REAL electronic geeks there.  They also have some classes in electronics, where you might fix this unit as a class project.

Jon
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #15 on: July 17, 2018, 06:12:08 pm »
Coromon, I take no offense at your statements, and I will admit my lack of knowledge re electronics "stuff". I have a lot of experience designing and writing computer programs and the appearance of the Raspberry PI, Arduino, Etc., has caused me to try and learn enough electronics so as to be able to design (HAH) and wire devices so as to be able to attach to the aforementioned processors. Believe me that from my perspective, electronics are a whole lot more difficult. And I really appreciate all the help. I will get involved in the TX-RX group here at their next meeting plus a lot of youtube viewing,

Now back to the tuner.
Wimberly with his noticing that the 5v rail was on without the switch being pressed was eye opening to me ( Thank you Wimberly ).

*****What I just discovered was that the fuses that seemed to be good were measuring 35M ohms. Sheesh, what the heck could have happened to them ? *******

Anyway, I put 2 more known good fuses in,did NOT turn it on. and it still blew the fuses. I think that I better have a look at the power supply board.

Another curious thing was that along with the TP (test points ?) that I saw scattered around the board there was also one named "earth" along with one named "gnd".
Gnd I understand but Earth ? Are they different ? I guess they could be in that earth refers to the middle wire coming out of the wall and gnd was the boards ground. I sure dont know.

As I was taking these voltage readings, for gnd, I was using the case. Is that a safe protocol or is there something else to look for in the event that the designer had a difference reference for gnd. My finding the "earth" post prompted this question.

In any respect a new rectifier is on order so I cant do much more until it arrives.

Thanks
« Last Edit: July 17, 2018, 06:49:49 pm by Hextejas »
 

Offline james_s

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #16 on: July 17, 2018, 06:29:49 pm »
Yes TP usually means test point. Gnd or Ground often means the ground reference point of the circuit, while earth ground is usually the metal frame connected to the ground pin on the plug that literally connects to the earth. These may or may not be tied together, in some cases earth is the safety ground while the circuit ground is just the negative side of the power supply.
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #17 on: July 18, 2018, 04:23:28 pm »
I am thinking that this voltage regulator is bad. Its a LM7805, the 5 volt rail. I desoldered it and wired it up in a breadboard and it lets a lot more than 5V through. it seems like it passes the input voltage straight through.
Would that cause the fuse to blow ?
 

Offline james_s

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #18 on: July 18, 2018, 04:27:48 pm »
That could cause all sorts of things to blow, are you sure you wired it up correctly?
 

Offline Wimberleytech

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #19 on: July 20, 2018, 02:47:38 am »

Wimberley, for a teachable moment, how did you determine that the 5v was always on ? The 2 diodes, D8 and D9 (I think) 1N4001 are the ones that I replaced with 1N4007. I hope that the substitution was ok.
Oh yeah, with the power on and my meter probes on each side of the diode, I get 0v readings.

Thanks again

Just got back to this thread...

I see from the comments that follow, you are blowing fuses even without turning on power.  The problem is in the 5V rail--at least.

Yes could be the 7805, but if it is a short input to output, all the other chips downstream are probably fried as well...in which case...you have a nice boat anchor.

 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #20 on: July 20, 2018, 03:43:46 am »
Yikes, I hope not.
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #21 on: August 05, 2018, 07:22:38 pm »
Just back from a long road trip and back at this beast.
I replaced the 7805 and the rectifier and the fuses.
It powers on now and doesn't blow the fuses. The front panel lights up but nothing is displayed.
So where do I look next ? Measure where ? I was thinking of trying to input a 1khz test tone and measure somewhere.

Plus I would like someone to answer a question about my soldering of the 7805.
I have a picture with a toothpick pointing to the area in question.
When I took the old 7805 out I damaged the PCB.  When I put in the replacement I only soldered one of the pins as it seemed to be the only one that went anywhere on the back of the board.
See the attached picture and thanks.
 

Offline james_s

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #22 on: August 06, 2018, 01:37:19 am »
Um, all three of the pins go somewhere, how could it possibly work with only one pin soldered? You can see traces leading to other components right in that photo.
 

Offline coromonadalix

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #23 on: August 06, 2018, 02:07:13 am »
Seeing the damaged traces are giving me goosebumps,  sorry i have to say this, you're damaging the tuner more and more instead of repairing it ... its a messy job

To help you, i have to be frank, that's not intended to insult, nor offend you, that is a learning process.

You should practice on dead boards, practice and develop soldering skills IE: good iron temperature, working with copper wick, or an vacuum pump to de-solder parts, learn to read schematics and symbols, parts functions, read books, see some youtube videos ... learn to use tests equipment's, understanding read values  etc ...

Sure in life you have to start somewhere, but not on equipment you want to make work as a start.

Even with tons of experience (30 years in my case) you may damage some parts or equipment, i had this happened to me recently. We tend to be overconfident sometimes. Doing things too fast, overlook things or don't look well enough, we automate our judgment, do mechanical  / repetitive movements without thinking  etc etc... the list goes on and on.

Members here are trying to help you, it is not easy, we may use technical terms or see thing differently  EX: it would take maybe 10 minutes or less to see what is the problem in your tuner for experienced people. This is a very basic tuner, nothing fancy.

Even the only pin soldered may be bad, the pcb trace may be cut where the solder is ...

Do you have a friend who's knowledgeable in electronics, or some professors at school, an nearby shop who could help you understand ???
 

Offline HextejasTopic starter

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Re: Is this feasible ? And I apologize for the vagueness.
« Reply #24 on: August 06, 2018, 03:17:47 am »
Um, all three of the pins go somewhere, how could it possibly work with only one pin soldered? You can see traces leading to other components right in that photo.
Thank you James. My confusion with my soldering question is based upon this. Using the back side of the board and measuring the continuity from my unsoldered pin to a small distance along the trace, and it shows no continuity, ie no connection.
Measuring from the same pin to a pin at the end of the same trace shows continuity.
Hence my reasoning that the circuit is satisfied by the connection on the front of the board.
Where am I wrong.
I was trying to  avoid having to solder a jumper? onto the back.
 


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