C Crane FM transmitter Mono Mod Observation

Hi Everybody:

My C Crane FM transmitter has been
a wonderful work horse for 6 or 7 years.
It has gotten a little beaten up, though, so
I am going to be getting a brand new one
from C Crane.

Before I go on, I have to mention that my
C Crane transmitter has always had an interesting
characteristic. I'm not sure how to describe this,
but if you listen to the transmitter unmodulated,
about 40 dB down you will hear an audio whine.
With my C Crane, the whine consists of a couple
of different audio frequencies. When the unit
is fully modulated, you never hear the whine at
all, although it is still there - again, about 40dB

I have wondered for a long time if turning off the
stereo generator in the transmitter would eliminate
that whine.

Just for the sake of experimentation and my own
education, I decided to try to do it.

If you look back at the Forums on this board, you
will see various discussions about how to do the
above. I believe the best way is to open a line
that is after the stereo chip. I am mentioning this
in a simple manner - I do not have sufficient eyesight
to accomplish that modification.

That other way that has been mentioned to achieve
mono operation is to disconnect the X1 crystal. I
do not recommend this, because all it does is disable
the stereo pilot signal (I think.) There might still
be other output in the above audio range (such as
left plus right and left minus right audio components.)
However, a stereo radio will receive the signal in the
mono mode, because the pilot is not there for the
receiver stereo circuitry to lock onto.

Anyway, I went with the latter option just for the
sake of experimentation, and because I can't see
well enough to do the correct mod, and also because -
I wondered if the negative 40 dB whine would go
away if the pilot tone was off.

Lacking vision to do it any better, I took a pair of
pliers, and pulled X1 off the circuit board. So for
me, this experiment is not reversible. This transmitter
will not ever deliver a stereo signal again. But that's
OK. I'm getting another C Crane transmitter anyway.

So, did I achieve my objective, which was to eliminate
the negative 40 dB audio whine??

Nope, it's still there.

So, I recommend that you don't try to do what I did.
The whine still exists in the signal and the mod
cannot be reversed.

Still, I'm not sorry I tried it. I really wanted to know
the answer.

Best Wishes,
Bruce, MICRO1690/1700

P.S. The transmitter still sounds great and still
goes a couple of hundred feet on the car radio.
The radio in the car just thinks it's hearing a
mono transmission now.

radio8z's picture


I like your no holds barred experiment but I regret the reason you had to do it this way.

Here's my guess as to the possible cause of the whine you mentioned. This device most probably uses a phase locked loop frequency synthesizer. These can cause the whine in two ways I can think of.

The crystal reference oscillator is divided down to a lower frequency which is then compared with a divided down frequency sample from the transmitter output. It varies but some synthesizers divide down to audio frequencies in the kHz to tens of kHz range. Maybe this is what you are hearing.

Phase locked loops "hunt" to find and maintain phase lock and if the loop filter is not optimized this hunting can occur at audio frequencies which could be another possible source of the whine.

Other than a redesign of the synthesizer I can't think of a fix.


kk7cw's picture

The audio whine you hear is very likely the audio feed in the stereo generator. For example, if you feed these cheap stereo generators with computer audio, there is present on both left and right channels a very low volume audio hash produced by the digital to analog converter on the sound card. When these two signals are combined they can produce a low level whine (beat frequency) not detectable under full modulation. The phase difference between the 2 noise sources hetrodyne to create the audio you hear. Most stereo generators with on-board chip modulators will pass the high frequency audio beat note(s) or whine. Some of the new commercial FM audio processors have a built-in band pass filter for the audio input to circumvent the whine. So, the whine is not in the stereo generator oscillator circuit; either the 19KHz or the 38KHz.

And BTW. mono and stereo receivers will receive and decode mono FM signals just fine. The only thing the "pilot" or stereo subcarrier produces is the left minus right audio signal for stereo. Left plus right audio is already on the mono baseband audio signal.

Marsh Johnson, Sr. - North Bend, Oregon, USA

MICRO1700's picture

Thanks Neil and Marshall for your detailed
You guys are the best.

Bruce, MICRO1690/1700


Carl Blare's picture


This thread got me wondering if I can detect a whine in the audio down around -40dB, and I do not hear anything but a hum that sounds like ground loop, but I'll talk about that in a minute. This C.Crane FM is newer than yours, Bruce, about 1-year old. And I thought about another cause of squeals and whines that it is worth mentioning.

Consider the possibility that your c.crane is picking up RF from the computer, a modem, a router, maybe an AM transmitter or a wi-fi transmitter.

In my retired Amiga computer I had several times when add on cards interfered with each other and I spent hours trying to get everything quiet.

Now the hum. You know what a ground loop sounds like. A deep AC hum at 60-Hz. That's what I hear on my c.crane. Here's what the audio input is like: the computer audio card output is plugged to a splitter that divides into two stereo lines, and one of them runs to the c.crane, the other is open for later use. As I count it, there are possibly two ground loops. The first one might be where the computer grounds from left and right are combined to the single shielded line of the audio cable, then split again within the c.crane to left and right. It's sort of hard to guess without tracing circuit wiring.

What I used to do when I had a studio was install one ground post to every piece of equipment and used the "star" system where everything is grounded only once. To make this work I had to avoid common audio cables and make custom cables with shields grounded only at one end. But just one slip up and hum everywhere.

Carl Blare

mighty1650's picture

I have that tiny whine too, but unless you crank up the audio in with no modulation occurring its not a problem.
I'd just do that to have it be mono so the signal gets out farther.

MICRO1700's picture

Thank you Carl and Mr. Crow!

You guys are the best, too.

I appreciate your comments, and
I may have some additional data to
add to this experiment.

Best Wishes,
Bruce, MICRO1690/1700


dlpoole's picture

A schematic of an early version of the unit can be found at the FCC's Office of Engineering Technology web site. Begin at https://fjallfoss.fcc.gov/oetcf/eas/reports/GenericSearch.cfm then search on the grantee (BYG) and product code (006) as shown on the product label. The original schematic will be found in within the table of exhibit details for the original application.

There are two different frequencies and two different causes for what you are hearing:

First, the phase detector (within processor IC101 and part of the phase locked loop) produces a small but audible 300 Hz ripple. This is not the 25Khz at which the phase detector operates, but rather an internal hardware or software refresh rate. The ripple can be reduced by bridging a larger capacitor across existing .047uF capacitor C110 in the loop lowpass filter (between Q102 and Q103.) On my unit, bridging 0.47uF worked well for the purpose while leaving some measured phase margin for stability. Your results may vary; if you hear instability (anything from a very low frequency "thumping" to a hum,) you must bridge a smaller value and tolerate more whine.

Second, DC-DC converter IC7, which develops the negative side of a split supply for audio processing and the output buffer, uses a charge pump switched at 5KHz. This puts a 5KHz load on the switched, 2 volt positive rail and introduces an audible ripple into the audio chain. If you have "adjusted" your unit for more transmit range as recommended elsewhere, you have increased the load on this converter and hence the amount of ripple it creates. This ripple can be reduced with a 1000uF capacitor in shunt with C35, effectively doubling its value.

Note: Your results may vary and of course, any modification, albeit a harmless one, voids the manufacturer's warranty and the FCC certification.

Carl Blare's picture

It was interesting, dlpoole, that you referred to an "earlier version" of the C.Crane FM transmitter. Can anyone describe why revisions occur in a circuit that was originally certified?

Each revision would need to be RE-certified, an expensive step. But it would also suggest that some deficiency in an earlier version was being remedied. What improvements have been made?

Ours is Model FT-007, with no version or revision number shown.

Carl Blare

dlpoole's picture

The loop lowpass filter of my FT-007, purchased in mid-2003, has a second R-C branch in its feedback network that is not shown on the schematic dated in 2001 from the FCC's Equipment Authorization pages. It appears that an additional "zero" was thus added to the filter's transfer function around 500Hz, most likely to improve loop stability or resolve a production yield problem related to loop stability.

An Equipment Authorization search shows that a "Class II Permissive Change" was requested in 2003 under conditions of confidentiality, as is the applicant's right. There is thus no newer public schematic nor explanation of what was changed or why. A "Class II" change implies that the certification test results were affected and new reports had to be filed before it was put into production, but it apparently didn't require changing the model number or the FCC ID.

I found the link to the 2001 schematic posted on another forum. I was surprised to find it since confidentiality is almost always requested on schematics for competitive reasons.

RichPowers's picture

Yes, I'm unclear on what exactly an X1 crystal is, but just speaking in a mechanical sense; Why would pulling an X1 crystal off the board be irreversible?
Why couldn't it just be put back in again?

Rich Powers Part15, Take 2..

Carl Blare's picture

Thank you dlpoole for the interesting account of how revisions can be made to certified products without triggering a complete re-certifying.

And to Rich Powers, since no one else has replied to your question, I'll give my theory. Because the circuitry of the CCrane FMT-007 is extremely small and tightly packed, I think MICRO1700 resorted to a brute force removal of the X1 (which is the standard symbol for "crystal") perhaps by twisting it free with pliers. As MICRO has revealed to us, he is visually handicapped and the task of doing surgery on tiny circuitry is not an option.

In reality, very few of us (opinion) would be able to un-solder and then re-solder the crystal from that circuit, without causing damage to surrounding parts.

Carl Blare

radioman310's picture

I just bought one of these on eBay used and it works fine... but it is transmitting in Mono not stereo. I have not had a look inside yet but after seeing this I may do, could it be the X1 crystal has been removed or is there something else I can or should look for?
Thanks in advance.

Carl Blare's picture

I know that this is an older thread, and what I have to say wasn't yet known when this thread started, but it might be of use to someone now.

The Wholehouse 2.0 FM Transmitter is FCC Certified and it performs exactly like a C.Crane, except that it has a button for turning off the stereo.

Having said that, I haven't check to see if it totally turns off everything that FM stereo involves, the 19kHz pilot tone, the L-R signal generator and anything else.

Carl Blare

RFB's picture

"I haven't check to see if it totally turns off everything that FM stereo involves, the 19kHz pilot tone, the L-R signal generator and anything else."

The pilot (19Khz) is derived from the 38Khz sub carrier generator, which relies on that tiny crystal. When taken out, it kills both the 38Khz and 19Khz.

There are TX's where these two frequencies are generated independently, but most low power types just divide down the 38Khz for the 19.