TK5710-K speaker bias

[N5APC]

Our FD runs several TK5710-k in trunking mode. Within the last two months, we have experienced speaker failure on two units. The speaker lines have 6 volts across them causing the speaker to become hot and fail.
The first failure was in a unit that was due to be replaced soon so we did a work around with an electrolytic cap in the line. The second unit to fail is in the station on an AC PS. These units have been in service for a couple of years with no incidents and then 2 identical failures within a few weeks.

Just curious if anyone else has experienced similar failures.

73's
Mike

 

[cmdrwill]

Someone wired the speakers WRONG.

 

[RFI-EMI-GUY]

Our FD runs several TK5710-k in trunking mode. Within the last two months, we have experienced speaker failure on two units. The speaker lines have 6 volts across them causing the speaker to become hot and fail.
The first failure was in a unit that was due to be replaced soon so we did a work around with an electrolytic cap in the line. The second unit to fail is in the station on an AC PS. These units have been in service for a couple of years with no incidents and then 2 identical failures within a few weeks.

Just curious if anyone else has experienced similar failures.

73's
Mike

See below, this radio, like most these days has a BTL audio amplifier. If one side of the speaker line is inadvertently grounded it can damage the amplifier and/or speaker. Check the installation to make sure the proper speakers were installed and that no other equipment has been installed on the speaker lines. It is common in fire trucks to have an intercom system wired to the radio, and in fire stations to have a PA system wired in. For these custom installations, a transformer should be used to isolate the radios amplifier from the load.


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8. External Speaker (KES-5/6:Option)
The speaker output from the transceiver is as follows:
1. The KCH-14 has a built-in speaker (3W/8 ohms).
2. The KCH-15 does not have a built-in speaker.
3.The external speaker output from the accessory
connector (9-pin) on the rear of the transceiver is 13W/4
ohms. Use the KES-5 or KES-6.
4.The speaker output from the accessory connector (12-
pin) on the rear of the control head is 2W/4 ohms. If the
remote kit (KRK-5, KRK-6DH) is used, use the KES-5 or
KES-6.
Note :
Since the transceiver uses a BTL audio amplifier, do not
ground the speaker output

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BTL is an acronym for "bridge-tied load." A bridge-tied load configuration consists of one amplifier driving one side of a load and another amplifier (with an inverted signal from the first amplifier) driving the other side of the load. This results in 2x more voltage swing across the load than you would have in a single-ended configuration where one side of the load is tied to an amplifier and the other side to ground. Twice the voltage swing across the load equates to 4x the power to the load (P = V^2/R). So, a BTL load configuration offers 4x more power to the load than a single ended configuration from the same supply voltage. Be aware, that BTL amplifiers dissipate 4x the heat of a Single-Ended amplfier under the same supply voltage and load conditions. Refer to the TPA7x1 series parts in the applications section for more on BTL. Also note that the output DC blocking capacitors are NOT required in the BTL configuration. Since the load is tied across two amplifiers with the same DC bias and the signal across the load is the difference between the amplifier outputs, the DC bias is removed.

 

[ramal121]

So the question is what is your speaker configuration? A single speaker connected to the correct output should cause zero problems for the life of the radio.

Are you running a tap to an unbalanced pa amp input? Do you have multiple speakers running off the radio? Something is not right here because speakers should not "become hot and fail".

 

[jim202]

To put it in a little different light, there has been many threads about Motorola radios not grounding the speaker wires on either side of the speaker. Doing so would destroy the radio speaker output amplifier.

Not that much has been mentioned that the same thing can happen with the Kenwood mobiles. The best way to insure that you don't do damage, is to pull out your trusty volt meter and measure both sides of the speaker while the radio is on. This will show if there is a voltage to ground on the speaker wires. If so, you need to find another way to connect anything external that will ground one side of the speaker.

Most radios of the commercial class have other connections available to use as an external connection for RX audio output. But there is a good chance that even these connection points have a DC bias voltage on them. The use of a simple series coupling capacitor of around 1 uf. or so will block the DC bias voltage. You can then connect this connection into an external audio amp to drive a speaker system for building use as needed.