Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Why does the 2IF amp act like this?  (Read 4360 times)
KE4JOY
Member

Posts: 1335




Ignore
« on: October 28, 2012, 08:35:11 AM »

This is a heathkit SB 303. It has a 3 stage IF amp each stage has a can/coil for tuning.

The first stage uses a dual gate mosfet amp/mixer. The second and third stage use a NPN amplifier.

The first and final stages show a nice sharp peak, the second stage however doesent rise till the slug is run all the way down to the stops and rises very slowly and broad and cannot be run past the 'peak'.

I wouldent really worry about it but it does seem a little numb.

Any thoughts or is this kind of normal?
Logged
G3RZP
Member

Posts: 4391




Ignore
« Reply #1 on: October 28, 2012, 08:38:27 AM »

Have you checked the tuning capacitor?
Logged
AC5UP
Member

Posts: 3825




Ignore
« Reply #2 on: October 28, 2012, 09:56:51 AM »

The first and final stages show a nice sharp peak, the second stage however doesent rise till the slug is run all the way down to the stops and rises very slowly and broad and cannot be run past the 'peak'.

The primary purpose of an extra tuned circuit in the IF is to sharpen the bandpass of the amplifier. Also adds a little gain without too much noise as the active elements aren't being pushed as hard. You get better selectivity, improved dynamic range and a lower noise floor. What's not to like?

It's possible that Heathshkit didn't feel the need to make the center coil as sharp as the others, but I can't imagine why. It's rare (in my experience) for a tuned circuit to have a Q as low as you described. You don't need a tuned L/C combo to build a barn door, a simple feedthrough condenser can do that.

Either the center IF can has gone bad (most likely with a failed internal condenser as G3RZP has suggested) - or - there's something across the coil which is severely de-tuning it. Look for a leaky DC blocking condenser or a high-value resistor across one of the coils gone south. If you absolutely can't find anything that could be a problem, pull the IF can and open it up to check the internal parts. They can go bad, and it's also possible you have a broken core slug with either a piece missing or stuck in the wrong place.

At least you know it really is a Heathshkit.............    Tongue
Logged

Never change a password on a Friday                
KE4JOY
Member

Posts: 1335




Ignore
« Reply #3 on: October 28, 2012, 01:04:13 PM »

Ugh, I popped the coil out of the circuit and found a lack of continuity in one of the windings  Tongue

Now what?
Logged
AC5UP
Member

Posts: 3825




Ignore
« Reply #4 on: October 28, 2012, 01:24:22 PM »

What would you do if you found a 1/2 watt resistor that was wide open?
Logged

Never change a password on a Friday                
KE4JOY
Member

Posts: 1335




Ignore
« Reply #5 on: October 28, 2012, 01:28:52 PM »

Swap it for a 1 watt  Grin

Not quite the same thing. Resistors are easy to come by, IF cans not so much.

If anyone has a source please let me know.

BTW I found the disconnected winding, stripped a little insulation off it and checked it for continuity, its somewhere up inside the coil... geesh.
Logged
AC5UP
Member

Posts: 3825




Ignore
« Reply #6 on: October 28, 2012, 01:48:36 PM »

The problem with finding a suitable replacement involves several issues:   It's old.   The physical size and mounting dimensions.   The frequency.

If you're lucky and it's a 455 kc transformer there's always the cheap clock radio from a thrift shop route. Might need to make up an adapter PC board or similar. An alternate is laying hands on a 455 kc ceramic filter. They're usually 9 kc wide but a little creative soldering could make it fit, and cheap clock radios have been known to use them. Yet another alternative is to forget the whole bandwidth thing and jumper the transformer with a 200 mmf ceramic cap.

If you want to preserve the original characteristics of the radio you will need to search out a parts source for something Heathshkit or the Miller / Meissner equivalent. Then you try to find one......... Or, you could explore the possibility of rewinding the bad coil. All you need to know is the size of the Litz wire and how many turns were used.

There's also the possibility of selling the radio to N4NYY, but since he's about to become an urban sailor you may have to wait until low tide..........
Logged

Never change a password on a Friday                
KE4JOY
Member

Posts: 1335




Ignore
« Reply #7 on: October 28, 2012, 02:02:14 PM »

Its not litzwire it is single conductor, very fine. Insulated and about as think as a human hair.

Any idea on where to get such wire?
Logged
AC5UP
Member

Posts: 3825




Ignore
« Reply #8 on: October 28, 2012, 02:11:41 PM »


Yo........... Vinnie............. Do I recall that you did some garage cleaning last month?

Logged

Never change a password on a Friday                
W8JI
Member

Posts: 9304


WWW

Ignore
« Reply #9 on: October 28, 2012, 05:21:28 PM »

Its not litzwire it is single conductor, very fine. Insulated and about as think as a human hair.

Any idea on where to get such wire?

What frequency is the IF?

It shouldn't be that difficult to repair it, or to find something that will work....   



Logged
N4NYY
Member

Posts: 4742




Ignore
« Reply #10 on: October 28, 2012, 07:01:59 PM »


Yo........... Vinnie............. Do I recall that you did some garage cleaning last month?



I have NOS old stock 455 KHz IF cans, but those are the big mommas that are used in AA5 (chassis mount). He is welcome to them if he can retrofit it. How big is his can?
« Last Edit: October 28, 2012, 07:08:40 PM by N4NYY » Logged
AC5UP
Member

Posts: 3825




Ignore
« Reply #11 on: October 28, 2012, 08:47:10 PM »

Isn't that a personal question?
Logged

Never change a password on a Friday                
G3RZP
Member

Posts: 4391




Ignore
« Reply #12 on: October 29, 2012, 01:54:27 AM »

If it's the usual Heath 3.395 MHz, rewinding should be easy.
Logged
W8JI
Member

Posts: 9304


WWW

Ignore
« Reply #13 on: October 29, 2012, 06:53:57 AM »

If it's the usual Heath 3.395 MHz, rewinding should be easy.

That's what I was thinking.

I seem to recall the IF was up around 3 MHz. If that is the case, it will be very easy to rewind the transformer or substitute a TV set 4.5 MHz sound IF transformer with a little extra padding capacitance.
Logged
KE4JOY
Member

Posts: 1335




Ignore
« Reply #14 on: October 29, 2012, 12:53:15 PM »

Yes the IF is indeed 3.395 Mhz.

Yes it should be fairly easy to re-wind altough one winding is center tapped. However I do not have any wire that fine. I do have some magnet wire around 30awg I think (Radio Shack Red). Is there a source for the ultra fine wire?

Where can I find info on how to figure out how many turns? The tube and slug are in good shape. The original windings only take up about half the tube so I might be able to wind with the heavier wire.

The can is a little thing basically 3/4" cube anything much taller wont let the board slide in and out.

In the meantime I have some old TVs around for junk parts and see what I can dig out of those.
« Last Edit: October 29, 2012, 01:03:50 PM by KE4JOY » Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!