D104 Crystal Damage Prevention?

[ka4koe]

I've had experience many years ago with damaging D104 crystals. I remember when I was 14 and playing around with CBs, I had an old S120 Halliscratchers sitting next to my CB radio. I believe the S120 was one of those versions of the all American 5. Anyway, if you recall that if you reverse the line plug you will get AC on the chassis, since the radios do not have an isolation transformer.

The D104 must have bumped into the side of the S120, as they were sitting adjacent to each other. Right after this, the microphone sounded awful as the crystal was apparently damaged.

My question is this: I've already installed a proper line cord on my transmitter. I measured a small amount of voltage to ground at the mic jack, perhaps 1 VAC. I assume this small amount was floating due to the old power cord. Should I insert a 0.01 uF cap in line with the microphone audio line as an extra added measure. My D104 is a straight model with the unamplified base.

Thanks.

Philip

[kb3rdt]

Your High's in your response might be gone! Try a 500pf cap but I just  a use home brew audio amp using mpf102.

[wa3dsp]

Not sure what you are saying about 1VAC at the mic jack. Do you mean from the shell (chassis) to ground or from the actual microphone audio input pin on the connector?

If the rig is grounded properly there should be no or very low potential difference between the chassis and ground mainly because they are (or should be) bonded. If you are depending on the AC outlet ground there could be a small potential difference to another ground reference like you station ground to a rod outside your shack due to the resistance of the ground run. This is why it is important to tie all grounds together. Sometimes there could be a large enough to feel potential difference between the AC ground and a ground coming from another source if the two sources are widely spaced, say two ground rods 50 feet apart. This could happen in a densely populated area where a neighbor (or you) has lost a neutral connection to the power pole and depends on the earth ground for a neutral return. Some of there current could seek a ground return through your ground. Thus the importance of bonding all grounds together.

Anyhow that is getting off the point. If you are measuring 1VAC from the mic audio input pin to the shell then there is something wrong and you undoubtedly would have a big AC hum on your signal. Not sure where that would be coming from unless there is some kind of strange coupling in your rig.

The D104 mic element is very high impedance. Loading its output with a low impedance greatly effects its response and quality. Most tube rigs have rather high input impedance but still often lower than optimum for the D104. The input grid leak in many cases is 1 Meg but a more suitable value would be 5 megs or more. I have often used 10 meg grid leaks.

Connecting a D104 directly to any low impedance input such as most all of the modern transceivers will most certainly bring mediocre results. Most of these rigs have 600 ohm inputs. A widely used fix ws to put about a 33K resistor in series with the element and the mic output. This helps but still loads the mic far more than optimum. I have used transformers like an old carbon mic to grid input in reverse. Something like 250K to 50 or 100 ohms. That works but unless shielded is highly susceptible to AC hum from outside sources.

A good answer, and one that has worked well for me, is to use a FET source follower at the base of the microphone. The element goes directly to the very high impedance gate of the FET and the source output is low impedance. This is analogous to a tube cathode follower. This has the added benefit of isolating the microphone element from the outside world. The characteristics of the microphone does not change with differing load impedance. The low impedance output can be used with any input, low or high. I use it very successfully on my low impedance solid-state rigs as well as high input impedance tube rigs.

Power for the FET can come from a 9V battery with an unused microphone PTT contact to switch power or come from the rig. All modern rigs have 8VDC available on the microphone connector and some on the microphone lead itself.

[W7TFO]

Ambient over 110 deg will kill them, too.

73GD

[ka4koe]

D104 doesn't want to work with the rig, but EV638 does (albeit with poor audio). Using the EV I get full scale swings with modulation but it sounds, shall we say, sub par. Its a HiZ model EV.

Will need to check wiring of D104. I hope the switch leg wasn't wired incorrectly by the seller via swapping with the audio line. Rig keys normally with the PTT. So, I'm wondering if the D104 element is faulty.

The cord wiring is assumed (ass-u-and-me, Walter Matthau, "Bad News Bears") as follows:

Braid/shield-ground
Red - PTT line
Translucent/clear - audio.

I get some indication of modulation on the output waveform displayed on the Flex if I yell into the D104, but you really have to look closely.

Plan to swap out R25 to 22Mohm and R3 to 25Kohm/10w.

Philip

[W3RSW]

Take the 104 element out and solder directly to input line of connector in any audio amp that is good for low levels to test. Impedance not super important to see if it even works.
At least half of these xtal mikes sold at festers, etc. are defective.  Eliminate all "seller" scenarios with a direct test. You probably already thought of or have done this.

As 3DSP and others have said, I'd really be tracing out lines to see about that stray voltage if it is truly on the mike signal input pin. Also if your using a sensitive, high impedance VTVM or DVM and your PTT line is at 120 vac you can easily see stray on adjacent, even shielded, audio cable if you have defective or partial ground, leaky old shielding, etc.

[wa3dsp]

W2ENY is selling D104 element "upgrade" kits at -

http://members.bellatlantic.net/~vze3rys4/D104Replacement/

for $20.

There is also discussion on eham that this is just a Korbitone element available from Mouser for $5. Mouser has a slew of Korbitone elements so I am not sure which one but someone commented that there was one about the size of the original D104 element that he A/B tested and no one could tell the difference.

If anyone has the Korbitone part number I would be interested. I would like to try it out.

[W2VW]

Kobitone replacements are on epay. Tried one and no way do they work anywhere near as well as a good D-104. Not even close. They are an improvement over a broken D-104 or one terminated in a low impedance such as that presented by a bipolar sand state 3 legged fuse.

A condensor electret mic is the best cheap mic on the planet but has tone issues for typical AM. Too much mid bass and a lot of proximity effect. Requires monitoring one's audio locally which can lead to severe brain damage. Used one for a long time which may explain a few things.

[ka4koe]

Here is what I found:

http://www.rbmicro.com/index.php

Will check the element tonight with the oscope. Already planning on swapping out the 1M first audio stage resistor to at least a 10M. The original has a pin hole in it (I presume to improve response), so it has been tampered with in its life.

We're getting there. At least the Valiant didn't vaporize during re-energization after all the electrolytics were replaced.

Anyone used RB Micro's product, or is it a Kobitone clone?

The bottom line on this happy discussion is that here is further, undeniable PROOF that I am cursed as an electrical engineer because I have insulted the Spirits of Volta, Ampere, and Ohm.

Philip

[Pete, WA2CWA]

The bottom line on this happy discussion is that here is further, undeniable PROOF that I am cursed as an electrical engineer because I have insulted the Spirits of Volta, Ampere, and Ohm.

Philip

No; it's just normal amateur radio.

[ka4koe]

Pete,

For once, ONCE, I'd like something to work first time out.....

Probably too much to ask.

 

PAN

[Pete, WA2CWA]

Pete,

For once, ONCE, I'd like something to work first time out.....

Probably too much to ask.

 

PAN

If it did, I would really wonder.

[ka4koe]

Getting about 25-30 mV P-P on conversational voice 1" from mic element, so it is perhaps good.

[ka4koe]

No ac or dc voltage between audio line and ground. Keyline measures out to 190V. Think the line cord, 3 terminal, may have gotten rid of what little ac/dc was riding there. Measured with a 1 Mohm input Tek DM43 and 465.

Rig acts like no audio is getting in whatsoever. Thought perhaps connections to pins 3/5 are good? if I'm reading schematic properly, then audio is not going where it is supposed to be going if these do not have continuity.

Getting about 25-30 mV P-P on conversational voice 1" from D104 mic element, so it is perhaps good.

[Pete, WA2CWA]

Have you checked the output of each stage in the modulator while feeding an audio signal into the mike input?

[ka4koe]

Willing to bet something is intermittent here. It works a little bit with an EV368. Haven't gotten to that point yet but its next.

[wa3dsp]

Having 190V on the key line is just crazy. I know it is normal for many rigs but still crazy. It would not be unusual to have some millivolts  to a volt of AC there. There are many way to correct that from using a secondary low voltage relay, changing the existing relay, using a high voltage bipolar or FET to key the existing relay, etc.

Of course this has nothing to do with why you are not getting the correct audio.

[ka4koe]

Used a digital meter and an oscope....not a nary trace of voltage or ripple on the audio line, so the 3 wire line cord took care of what little I had.

Philip