At Remote speaker-microphones and DMR portables I discussed the propensity for RF ingress to speaker microphones on DMR radios in digital mode.
This article looks at another speaker mic problem, mechanical compatibility of the plug / jack arrangement.
The MD-390 is IP67 rated, which means that it has a water-resistant gasket around the speaker mic jacks.
Above, the soft rubber gasket surrounds the speaker mic jacks, and if you look carefully, you will note that the metal part of the jack is recessed in the gasket. This is not an unsual arrangement.
Compatible speaker mics MUST project sufficient plug shaft into the jack to make good connections, ideally they should have a boss that seals to the recess in the gasket.
Accessories designed for the MD-380 might not work properly with the MD-390.
Above is a speaker mic plus set that works properly with the MD-390. Note the boss at the housing end of the plug shafts, this fits into the gasket recess and there is the standard length of plug shaft projecting into the jacks. This makes good electrical contact and may well be IP67 compliant. The moulding partly covers the boss on some parts, and the gasket can have extrusions that cover the mating face preventing proper seating… this is Chinese quality, bit of an oxymoron really.
Many after market speaker mics and indeed the Retevis speaker mic that claims to be original equipment does not have the boss, and lacking adequate projection of the plug shafts, they do not properly engage the jacks and connection is non-existent or poor and intermittent.
Another problem with lots of speaker mics is that the cable plug strain relief does not have sufficient relief to allow the plug set to be fully inserted in the jacks.
So… the challenge increases… finding a speaker mic with a plug set compatible with the MD-390’s IP67 gasket, and resistant to RF ingress and ‘motorboat’ audio interference on transmit.