Author Topic: Amplifier Research 75A250A - Help With Repair  (Read 356 times)

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Offline Bruno123

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Amplifier Research 75A250A - Help With Repair
« on: October 22, 2019, 12:11:38 pm »
Hello,

I'm trying to repair an Amplifier Research model 75A250A, as I detected a short circuit in the input circuit. It is a 75 W 10kHz - 250 MHz amplifier.
The overvoltage/surge protecion device CR2 (no marking code), the device AT1 (marking code 06) and U2 (marking code H346 - voltage-variable attenuator from analog devices) seem to be all short-circuited.
I couldn't identify CR2 nor AT1 (seems to be an attenuator). Does anyone have the service manual/schematics for this amplifier or have an ideia about what these devices could be? The input is rated at 1mW.

« Last Edit: October 22, 2019, 12:34:43 pm by Bruno123 »
 

Online fzabkar

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Re: Amplifier Research 75A250A - Help With Repair
« Reply #1 on: October 23, 2019, 05:48:29 am »
The absolute maximum RF Input Power for U2 is +18 dBm, and its RF1 pin is DC coupled and matched to 50 Ohm. 

https://www.analog.com/media/en/technical-documentation/data-sheets/hmc346ms8g.pdf

Does this suggest a suitable candidate for CR2? I'm thinking that it must consist of two diodes connected in antiparallel in order to clip both positive and negative signal excursions.

As for AT1, perhaps you could get some idea of its internal makeup by measuring U3 ("03") at the output of U2.

Edit: Could ATI and U3 be pi-section filters? (1 x series inductor plus a grounded capacitor at each end)

Edit #2:  The amp's input rating would suggest that a BAT15-04W Series silicon RF Schottky diode pair might be suitable for CR2. ICBW, though.

www.testequipmenthq.com/datasheets/AMPLIFIER%20RESEARCH-75A250A-Datasheet.pdf

Quote
INPUT FOR RATED OUTPUT............................................................1.0 milliwatt maximum
INPUT IMPEDANCE...........................................................................50 ohms, VSWR 1.5:1 maximum

https://www.infineon.com/dgdl/Infineon-BAT15-04W-DS-v01_00-EN.pdf?fileId=5546d46265f064ff016638961e2b4e86
« Last Edit: October 23, 2019, 06:26:03 am by fzabkar »
 


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