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Thread: Tait TB8100 VHF 100W Tx problem - current imbalance

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    Default Tait TB8100 VHF 100W Tx problem - current imbalance

    I have a Tait TB8100 VHF 100W that I picked up recently from a UK Ministry of Defence surplus sale. I have never even seen one of these before let alone played with one and it seems rather complicated. When I got it, the thing was programmed in simplex mode on 142.325MHz on what the channel name suggests is a paging channel.

    I am testing it out and the receive is OK, but when transmitting, the base gives a CURRENT IMBALANCE alarm. It does however transmit at almost 42 Watts, both with an external microphone and when using the diagnostic screen. That is a the only alarm showing and given that the base is able to kick out 42 Watts the problem doesn't seem like it should be terminal. I'm wondering if it is an issue with the programming or configuration of the modules making up the unit. The modules are

    RECITER: TBA40B2-0L00 136-156MHz TaitNet RS232

    POWER AMPLIFIER: TBA90B10000 136-174MHz 100W

    POWER MANAGEMENT UNIT: TBA30A0-0100

    Any suggestions would be gratefully received. I'm from a sales background, not an engineering one and while I can blunder my way through most Motorola problems, Tait kit is a bit of a mystery to me, so I'm rather out of my depth here.

    Thanks in advance
    Andy

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    Sounds like 1 of 2 parallel finals has blown in the PA. This is evidenced by the approx. half power output. FWIW paging systems are rough on the TX/PA as they are transmitting 80-100 % of the time.

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    I agree with Alpha. The diags and the power out are telling you half the PA section died. You can open it up to see if there is something obvious that you could fix. Otherwise, it's depot time.
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    The current imbalance alarm would definitely point to 1 of 2 blown finals. I also noticed there was no indicated reverse power. That would make sense if you have a really good DL absorbing the power. I'd think you'd see some small amount of reflected power.

    I mention such because the monitoring capability <appears> fairly robust with watching for PA issues (current/temp/vswr) etc. A robust system should be able to self-protect. There could be another secondary issue lurking in the PA module beside just a blown transistor.


    Bottom line: I think the PA module is bad and needs to be repaired/replaced. It's not a programming problem.

    M

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    Thanks for the responses

    I was checking it into a Motorola R2600 service monitor, so I would assume that wouldn't give much reflected power, but I've never had cause to check.

    I have taken the lid off the PA now and there is no obvious burnt components or anything else that looks untoward. It's way out of my league to do anything with it. I'll give Tait a ring in the next day or two and see if they have a fixed price repair service in the UK, however given the price they charge for spares I'm guessing it will be prohibitively expensive. If that's the case it will be going on the for sale board as a repair project. If anyone is tempted by it drop me a PM, shipping across the Atlantic is no problem, I have a FedEx account.

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    It was $500USD a few weeks ago to fix a Coast Guard RFPA.
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