I am in the novice to intermediate range when it comes to making modifications, so you can rest assured that modifications listed here are relatively simple.  Some modifications are DX-centric, and others are audiophile-centric.  Sometimes these goals conflict, and such cases are noted below.

The procedures involved in re-aligning a tuner are not covered here, as that is NOT a simple procedure.  Contact a professional if you suspect your tuner needs re-alignment.

Where can I learn more?
As I said above, I am not an expert, but have picked up a few mods over these last few months and years.  If you're looking for detailed information and forums to exchange ideas and questions, try these:

Audio Asylum - audiophile-centric discussion groups for vintage gear and other hi-fi audio equipment, including "DIY - Do It Yourself" modifications.
FMTuners - Yahoo group with lots of high level discussion; many tech gurus and audiophiles
Tunerinfo DIY Mods - The premier tuner review website has a section describing many mods - written by experts.

What will I need?
Most of these modifications will require:

- phillips head screwdrivers of varying sizes
- canned air (Endust for Electronics; deoxIT recommended for deoxidizing (see caig link below) - for cleaning the interior, blowing out dust, etc)
- pencil soldering iron
- solder (rosin core)
- desoldering braid or desoldering bulb/vacuum
- good lighting
- magnifying glass
- chain saw (only used as a last resort)

And of course the electronic components as specified.

Precautions
- If you aren't comfortable with soldering in tight circuit boards, do not attempt these modifications
- always unplug the tuner before removing the top and bottom covers.
- Capacitors can store a large amount of electricity!  Be careful when working with the circuit board.  It is not too difficult to shock yourself, or zap some components.
- The compiler of these modifications assumes no responsibility for the results of these modifications.  Performing them means that you understand the risk to your tuner and yourself.
- Study and read each modification multiple times to become familiar with it BEFORE starting to perform it
- Take notes and digital pictures of the way things were before the modification
- Take notes to help quantify signal strengths, selectivity, and other measurements BEFORE and AFTER the modification, to insure success
- Ideally, use a signal generator for the before/after measurements.  If you do not have one, find a stable semi-local station to measure.  Remember that temporary atmospheric conditions, airplanes, and other things can also alter signal strengths.
 

THE MODIFICATIONS

IF FILTER MOD- primarily for DX'ers - improves selectivity
Summary: Most FM radios use ceramic 2 stage IF (Intermediate Frequency) filters to shape the "bandwidth" of a broadcast.  They are important to allow for reception of stations on frequencies alternate (.4 MHz away) and adjacent (.2 MHz away) to stronger local stations.  Manufacturers use "wide" filters to insure that all of the signal (and the audio "highs") get through, and to guard against variances in the tolerances of the filters.  These filters can be quickly replaced with inexpensive, narrower ones, allowing for improved selectivity.

Potential Benefits: Greatly improved selectivity.  In some cases, you may also improve sensitivity a little (some older "wide" filters actually had more insertion loss).  Improved image rejection.

Potential Detriments: Audio fidelity, especially on weak stations.  If the modification is done well, this might be negligible.  And a happy medium can usually be found where you improve selectivity quite a bit without encroaching on fidelity.  These mods could potentially also decrease your SCA and RDS capabilities.  Properly "matched" filters ideally centered exactly on the 10.7MHz IF rarely result in the audio, RDS, and SCA problems.  Finally, you could lose some sensitivity (insertion loss of narrow filters is higher than wide ones, unless the wide ones are low quality or very old).

Potential Roadblocks: These filters are pretty small, and sometimes hard to reach.  Some tuners (especially pre mid-1970's) used old fashioned components to crease IF filters, rather than these slick, relatively new technology, inexpensive ceramic filters.  Even some newer tuners didn't use these filters (see the tuner overview page).  Finding a source of small quantities (less than 100) of filters can be difficult.

The Details: This is a well documented, well used modification.  It is very straight forward, and often times, the most difficult part is finding a supplier willing to sell small quantities of the filters.

Pictures and description of this mod is located on the IF Filter page.
 

300 OHM BALUN REMOVAL - useful for audiophiles and DX'ers - improves sensitivity
Summary: Most FM Tuners allow for connection of two antenna feedline types - 300 ohm impedance twinlead, and 75 ohm coaxial feedlines.  Coaxial feedlines are the BEST to use, since they are shielded (if you're a serious DX'er or audiophile, you should be using 75 ohm coax as your feedline).  Twinlead is not shielded, and thus, also acts like an antenna unless it is very carefully run, potentially degrading the directional capabilities of your antenna, and potentially also picking up stray RF generated by your tuner and amplifier.  With this in mind, if you are NOT using your 300 ohm antenna input, you can remove the ferrite core balun that is on the interior circuit board.  These almost act like a splitter, so when it's removed, you gain back a dB or two of sensitivity!

Potential Benefits: An extra 1-2 dB sensitivity

Potential Detriments: You will no longer have a 300 ohm antenna input to use.

Potential Roadblocks: Some tuners do not even have 300 ohm antenna inputs.  So in those cases, this modification obviously doesn't apply.  I've yet to see any, but I'm told that some tuners actually use the balun on the 75 ohm antenna, so be sure you know which input the balun is associated with.

The Details: This is a VERY easy modification.  As always, document the "before" condition in case you need to revert, and for comparison sake.  Make sketches, take a digital photo, measure signal strengths, etc.

1) Locate the BALUN - it should be very close to the antenna inputs on the back of the tuner, located on the circuit board.  If it a donut-shaped black ferrite core, usually about an inch long, with wires wrapped around it (see pics)
2) Identify the points where it is soldered to the circuit board.
3) De-solder the balun from those points.
4) Save the balun somewhere safe, in case you ever want to revert.
 

SCRATCHY TUNING FIX- useful for DX'ers and audiophiles - helps tuners lock onto signals and removes that annoying scratchy sound
Summary: OK, in a lot of cases this isn't even a modification.  It's a maintenance procedure that might be needed on occasion.  I'm sure we've all had a TV or radio that makes an annoying scratchy static sound when you adjust the volume, level, or tuning knob.  This is often caused by dirt and dust leading to shorts in the tuning capacitor, switch, or potentiometer. Worse yet, a plate on your variable capacitor is bent and will require straightening (we'll ignore that possibility now). And to be complete, there is always the possibility of stray DC or other causes, but if you've replaced your electrolytics and visually checked the board, this likelihood is relatively minimal.

We will assume that you've visually inspected for bent plates, have new non-leaking electrolytic capacitors, and have also inspected for other obvious problems.  This means that the simple case of a dirty connection is the likely cause.

Potential Benefits: Smooth tuning with no static

Potential Detriments: None

Potential Roadblocks: It is sometimes difficult to reach the appropriate spots in the tuner - your "canned air" will need to have a small straw or needle to point into those tight spots.

The Details: There are a few methods to approach this one with.  Note that you might need to remove an interior metal cover over the tuning capacitor (the big thing with parallel metal plates that rotates as you tune).  The first approach is to use "canned air".  Radio Shack, Electronic Fry's, and many computer stores sell these products (Endust for Electronics; deoxIT recommended for tough cleaning, avoid silicone based products).  They are a small canister, often with a "straw" that can be attached to blow air into tight places.

Follow the directions of use on the can, and blow air on the capacitor from as many angles as you can.  Do it with the capacitor in many different positions (i.e. tune to 88 MHz and blow air on each plate, tune to 98 MHz and repeat, and tune to 108 MHz and repeat again).  This should help, and perhaps resolve the problem.

The second approach involves using thin swabs and actually physically cleaning between the plates.  Some electronics stores sell these - they look like tiny tongue depressors with a material wrapped around the end.  Be gentle, as you do not want to bend the capacitor's plates.  You do not want to use anything like cotton that might leave behind small fibers.

Finally, frequent use often clears up the problem over time.  Don't let the dial sit in one position for long.  Give that dial a spin frequently, and after a few days, the problem should subside.  A few spins from 88 to 108 and back again might help quite a bit.
 

AGING ELECTROLYTIC CAPACITORS- primarily an audiophile fix, but can be of use to DX'ers
Summary: Electrolytic capacitors (those small tin-can looking polar capacitors) can and frequently do degrade overtime.  In fact, it is not too uncommon to see them physically leak after years of use.  If in the AF path, they can slightly degrade the audio quality.  If in the power supply/filter section, many other problems can arise.  This modification simply replaces these electrolytics with new ones, and in some cases, other types of capacitors.

Potential Benefits: Slightly better audio characteristics (and I do mean slightly).  If power supply/filter electrolytics are going bad, other problems can result.

Potential Detriments: none

Potential Hazards: Capacitors store electricity, so be careful when removing.  If you aren't careful, you might get a small jolt, or you could fry a sensitive component.  See the Precautions section at the top of the page.

Potential Roadblocks: Some capacitors might be hard to reach.  Old circuit boards can be brittle - do not use excessive force when removing components.

The Details: This modification can be approached in a couple of different ways.  If you have no audiophile interests, then you will likely only want to replace the power supply related electrolytics.  If you do have audiophile interests, you will also want to replace the AF path electrolytics.  But you have a choice - many people only replace the "larger" electrolytics.  This is somewhat arbitrary, though it does have technical basis.  The most common cut-off points seem to be in the 4.7uF to 47 uF range (i.e.
replace everything larger than 4.7uF, or replace everything larger than 10uF).

Replacing the Power Supply Electrolytics
1) Locate the power supply - usually a big metal thing inside the chassis right next to where the power cord enters the chassis.
2) Close to the power supply should be several large electrolyitc capacitors.  They will be physically larger than others in the tuner, and should be labled.  You will usually have a few in the neighborhood of 2200 uF, 1000uF, 470uF and/or 220uF clustered together.
3) Make note of the quantity, uF ratings, and voltage ratings on the caps.  Many of these will likely be at least 16 or 25 V.  Make sure you buy caps with the same uF value (some people do get bigger caps) and at least the same V rating (you can use higher WVDC rated caps).  Also, insure you get polar electrolyitcs (they do make non-polar), and the right lead types (usually radial, more later).  Some folks replace the power supply capacitors with higher uF rated caps, too.  That is your call.
4) Make note of how they are situated on the board (i.e. which side is negative, and which side is positive).
5) Desolder 1 capacitor at a time, and replace it with a new one of the exact uF value and AT LEAST the same V rating.  Insure you are using the polar electrolytics (i.e. ones that have a positive and negative terminal), and insure you install with the polarity correct.
6) Note that most tuners will use radial lead capacitors - both leads on one side of the "can" (as opposed to axial leads, which have a lead on both sides, forming an axis).

Replacing the AF Path Electrolytics
1) Decide if you are going to replace all your caps, or just those above a certain value (like 4.7 or 10uF).  Make an accurate count of the number and uF and V ratings of the caps on the circuit board.
2) You have a choice - replace them with new, like-valued electrolytics, or "upgrade" to polypropylene or one-film capacitors.  Check some of the resources above for more info on why you might want to consider polypropylenes and one-films.  Usually you stay with electrolytic caps for larger sizes, I'm told, but 10 uF and under, consider the polypropylene and one-film caps.  I'm sure most of us non-audiophiles won't be able to tell the difference between these and electrolytics, so if you're only concerned about the age of the caps, just do a one for one replacement.
3) Make notes as to what caps are where, and what the alignment polarity-wise is.
4) Remove and replace one at a time.

OTHER MODS
There are a lot of other simple-looking mods that can be performed.  Many of these are documented in the sites above.  While they appear simple, I have not performed these on any tuner, so I won't pretend I know exactly how to do it by creating instructions.  These mods include:

- replacing older diodes with low noise schottky diodes
- hardwiring the de-emphasis to bypass several extra inches of wire that are frequently run to the switch on the back panel
- isolate the power supply from the chassis with small rubber grommets
 
- replace carbon film resistors in audio section with metal film resistors (to reduce thermal noise).
 

Resources and Further Reading
Defintely check out the resources at the top of the page.  Here is some additional information:

Picking Capacitors - a reprint from Audio Magazine, Feb and March 1980
Replacing Passive Components - a good overview of caps and resistors
Electrolytic Capacitor Life, Failure, and Usage - a .pdf file with concise bullet-point info on life of an electrolytic, failure modes, usage, and more
CAIG Laboratories - a source of deoxIT
Mouser Electronics - an excellent supplier of many electronic components.  This link takes you straight to the electrolytic page.
Murata IF Filter Guide - includes info on how they work, how to use them, and part numbers and specs (note: Murata has changed part numbers recently - go to their site for the new numbers)
Service Manuals - Get service manuals and schematics here