Vintage Heathkit IG-1271 function generator troubleshoot and repair

[Radio Tech]

While home today I decided to put a recent flea mall find on the bench and have a go at it.
This is an old heathkit IG-11271 function gen. After a few moments I was able to determine what the fault was, but finding a shorted component can be a choir sometimes. To aid in finding the short I used a 1.5 volt battery as a constant load to trace out the circuit until the problem area was found. Once this was done it was simple to find the bad component.

BTW I am no professional video blogger and this head cold does not help.


Links to pics:
http://gokarters.com/smf/index.php?topic=2486.0

Video:

 

[Robb]

That was xlnt, RT.
Do some more videos like this when ya can!
Great stuff to learn by.

BTW; you are going to put another tantalum back in that spot - right?
They are used in freq depenent circuits for accuracy and stability - right?
Nice find.

 

[Radio Tech]

That was xlnt, RT.
Do some more videos like this when ya can!
Great stuff to learn by.

BTW; you are going to put another tantalum back in that spot - right?
They are used in freq depenent circuits for accuracy and stability - right?
Nice find.

Thanks Robb,
Even though I was sick at the time of the video I did enjoy running through it.
You are correct on the tantalum capacitor. Most circuits that are frequency dependent a tantalum is use due to its more stable design.
As in the circuit where C15 was shorted causing the zener to short is a good example of where a tantalum is needed. This circuit feeds voltage to Q12 which is an amplifier for the square wave generator. And needs to be stable. I will be replacing all caps and doing an alignment on this unit soon.

I will be doing a lot more videos in the future. And with all my videos I will be doing this the “old fashion” way. Not everyone has a shop full of the latest test gear. Why I used a simple 1.5 volt battery as a constant current load in this one.
Thanks for taking the time to view the material.

 

[Radio Tech]

After going back and reviewing this circuit I have to ask myself, why the heck did they use tantalum capacitors in this circuit. The whole unit only has 2 electrolytic caps in it. And those are the power supply filter caps. For some reason the designer decided to use only tants in this circuit.

Now given that tantalums are a more stable solution over electrolytic caps, we have to also look at the failure rate of tantalums. Tantalums need to be protected from voltage spikes while electrolytic are more forgiven. When a electrolytic shorts from what ever reason, modern day caps pop the vent and leak and older ones pop the top off leaving a nasty mess to clean up.
But when tantalums short the mess to clean up can be a lot worse. From sending small fragment projectiles everywhere to actually bursting into flames! This can be very dangerous, specially if you are not there to stop a fire....

Just a little information I wanted to throw out.
I will be doing more videos soon. Will see how the response goes. The more response to fuel the fire in the need to learn basic the more desire I have to produce them.

 

[unit_399]

Back

After going back and reviewing this circuit I have to ask myself, why the heck did they use tantalum capacitors in this circuit.

I think you have to remember that this box is close to 50 years old. Back then, tantalum caps were the best you could get for tolerance and stability. Those old electrolytics might have claimed a tolerance of +/- 10%, but were probably closer to +/- 20. The designer wanted the oscillator to be stable, and using tantalum was the best way to guarantee this at the time.
- 399

 

[Radio Tech]

Back


I think you have to remember that this box is close to 50 years old. Back then, tantalum caps were the best you could get for tolerance and stability. Those old electrolytics might have claimed a tolerance of +/- 10%, but were probably closer to +/- 20. The designer wanted the oscillator to be stable, and using tantalum was the best way to guarantee this at the time.
- 399

And that is most likely an accurate answer my friend
Tantalums were widely used back then. Probably cheaper than electrolytic, not sure. Plus you have to think that when this was designed, they had no idea folks would be using them 50 years later....

 

[unit_399]

you have to think that when this was designed, they had no idea folks would be using them 50 years later....

But it goes to show that a well designed piece of quality test equipment is ageless.

- 399

 

[Robb]

Well, many of the new and older radios still do use tantalums. Either surface mount or thru hole - too. Had to replace a couple of tantalums in the pll circuit of a Galaxy 959 last year after accidentally shorting one. They don't take kindly to a short or reverse voltage in them, as doing this will vastly shorten their lifespan.

 

[Radio Tech]

That is true 399.

Robb,
Wait till you get ahold of a smd tantalum that has exploded on a multi layer board. Most of the time there is little left to repair.....