Hi folks,

I've been using an MH1-B8 microphone for a while (few years), substituting for the rig's original MH-36D which had the disintegrating rubber cord problem.? Following up on recent suggestion on the Amsat forum, I ordered and received a replacement cord meant for the MH-36 with the RJ plug.? Cut off the RJ and connected the 8-pin round connector, and it seems to work, including the DTMF keys which is why I wanted to do the repair.

But, I'm getting reports that my audio has a sort of hum on it, and the volume is rather hot, even with the microphone gain turned all the way down.? I double checked the wiring, and I think it's right. The new cable has 6 wires instead of 7, so left the missing "Fast" line unconnected on the 8-pin.? The MH-36D has no such button, so no loss there.? The 2 grounds (mike and other) are connected together at the 8-pin connector; there's no "shield" on the cable, nor was there one on the original.

Is this behavior - the hum and loudness - "expected", or is there something else going on?? I see what appears to be a small potentiometer inside the mike, just above the connector.? Does that need to be adjusted?? What is it for?? I haven't touched it as yet, but I'm not the radio's original owner.

Thanks,

Greg KO6TH

I think you have the mic wired wrong. Pin-8 is the mic input not ground, which would explain the hum. Also, do not tie the mic ground (Pin-7) and the "other" ground (Pin-5) together. They are separated for a reason, so keep them that way. The POT controls the DTMF signal level and I would not suggest messing with it.

Right, pin 8 is mike, and pins 5 & 7 are the grounds.

So, leaving pin 5 unconnected seems to have helped a bit, but there's still a hum.? After some troubleshooting on the local repeater, we noticed that placing my hand on the transceiver itself while transmitting (holding the mike) made the hum go away, indicating a capacitive problem somewhere.? Bridging an extra decoupling cap across the power internal to the mike didn't change anything, however.

The new theory is that the problem might be a shorted cap in the microphone line (pin 8).? Guessing C104, according to That would explain the high voice volume, as well as perhaps the hum.

I'm measuring about 28k ohms across it (in-circuit), with the probes in either polarity.? So, it's not shorted, but perhaps really leaky?

Before digging in (it's a more invasive fix for me), does that sound plausible?

Thanks for the help!

Greg? KO6TH


Omni wrote:

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Hi Andrew,

Looking at the schematic (if this is correct:? ), I don't see one.? Where would it be?

Greg? KO6TH

A quick update...

I'm now thinking the problem has nothing to do with C104.? I tried inserting a capacitor in series with the mike line (pin 8 on the round connector), and the problem remains unchanged.? Further, I can make the problem dramatically worse by simply holding the top portion of the microphone (around where the mike holes are) against my face.? AC hum pick-up.? Holding the mike by the cord at its base, and using the MoX button on the radio, there is no hum.? So, it's like there's a lifted ground somewhere, but I can't find it.? Poking around inside the mike with a straight pin or needle held in my fingers, I can touch any of the ground traces and the hum disappears.? The schematic I found turns out to not match the MH-36D exactly - parts layout slightly different, some different parts - so this level of troubleshooting will be challenging.? It's for the MH-36A6J.? Does anyone know of a proper MH-36D schematic?

Perhaps unrelated, I notice that the mike volume control on the radio does not seem to have any effect on either the hum or the volume of my voice when in FM mode.? It does work in both SSB and AM mode, and functions the same with either microphone model.? Is that expected?? Just trying to rule out a rig problem as perhaps compounding the troubleshooting effort.

I opened up the MH-1B8 microphone that I have been using (with no hum), and see that it uses a shielded cable between rig and mike, with the shield connected to the "Mike GND" pin 7 on the round connector, and that goes to the ground side of the microphone element itself.? The other ground pin 5 on the connector is used for the up / down / fast buttons, with the two grounds only connected inside the 847.? Since the MH-36 microphone only has a single ground pin internal to the mike, and uses an unshielded cable, I'm presuming it should be connected to the Mike GND, not the other one.? Connecting it to both 5 & 7 didn't help; I haven't tried 5-only, as that doesn't make sense.? Yes?

I've got one of the cheap knock-off MH-48A6J mikes on order, since it uses the same replacement cable as the MH-36.? Hoping it will function with the 847.? Plan is to swap mikes on my cable that has the round connector spliced in, and see if the hum is fixed.? If it works, I'm done.? If not, it was a cheap troubleshooting tool.

Greg? KO6TH


Greg D wrote:

Hi Greg,

Not sure if you have tried this but it has worked for me in the past.

Take the mic connector apart and solder a bare 2" or so length of wire to pin 7. Route the other end of the wire back through the mic plug and wrap it around one of the screws that secure the cable.

See if this helps.

Max

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Hi Max,

So, you're connecting the mike ground pin to the chassis through the connector shell??

Chassis ground should be the same as pin 5, no?? They're separated now, but I had originally connected the two ground pins (5 & 7) together, and that didn't help.? Grounding is kind of magic, however, so I suppose it's worth the try.

Thanks,

Greg? KO6TH