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Author Topic: Service manual or info for an Azden PCS-7800h 10 M FM radio.  (Read 1824 times)
N1AUP
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Posts: 254




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« on: January 23, 2018, 01:53:08 PM »

I know they're dinosaurs, but does anyone have a service manual or service info for an Azden PCS-7800H 10 M FM radio?  I'm not getting any output power.

Thanks
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KD0QL
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Posts: 1




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« Reply #1 on: June 19, 2019, 10:39:49 AM »

Check for faulty pin diodes around the power module. They're troublesome.
Check the driver transistor.
Check the connection to the PL-259 (and on the circuit board)

Bryan, KDØQL
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WB8VLC
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Posts: 667




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« Reply #2 on: June 19, 2019, 01:02:41 PM »

The PCS7500 6 meter and the similar PCS7800 10 meter radios don't use a Pin diode t/r switch nor do they use a power module.  

They use a low cost at the time bipolar lineup consisting of a 2SC1972 or similar driver transistor along with a 2SC2097 or similar final transistor and a classic old 12 volt T/R relay to perform TX/RX switching.

A common failure in these radios are both the Hi and low power control pots becoming noisy.

These pots are easily identified as they are of a blue or gray color depending on the radio year of production and they have a cross adjustment access for a phillips or a flat head screwdriver on their tops, all of these pots are on the main RF board.

There should be adequate silk screen markings next to these pots to see Hi and low power marking on the PCB next to them.

The radio has a simple VSWR protection circuit which is also or'd with the main power control POTs control voltage, either Hi or low power pots, and from this or'd output the pre-driver transistors Collector voltage is increased/lowered to control the output power and protect the final amplifier transistor.

In one case for a PCS7500 that I worked on 4 months ago it was as simple as rotating the hi and low power adjustment pots around their ~270 degree rotation range to make it come back to life on tx.

In another I had to move several of the pots on the radio as Rx audio, squelch and tx power were all lost.

If you do move any of the control pots first using a sharpie pen, mark the initial settings on the sides of each pots with the marking going down to the PCB so the pot can be placed back in its initial setting.

Next clean all of the pots with good old contact cleaner which in my case resulted in bringing the radio back to life.

 I haven't had to replace a relay yet in several radios that I worked on but it is possible to have bad relays as we all know.

Another major failure point on these radios is one of the LCD back lights, they are all small Grain of wheat incandescent DC light bulbs that burn out and during replacement a Backside to frontside circuit board trace connection which is made by the bulbs leads can result in a major troubleshooting headache.

If someone un-solders the burned  out bulb it can disturb a particular circuit board VIA/trace in the process which results in a loss of a front to rear circuit board path that this bulbs leads makes with the result being the loss of Voltage to the entire radio making it appear to be dead.

So if you ever need to replace a burned out LCD/display bulb or bulbs make sure that the replacement bulbs leads are soldered adequately on both sides of the circuit board.

« Last Edit: June 19, 2019, 01:05:50 PM by WB8VLC » Logged
WA2CWA
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« Reply #3 on: June 19, 2019, 10:44:28 PM »

I know they're dinosaurs, but does anyone have a service manual or service info for an Azden PCS-7800H 10 M FM radio?  I'm not getting any output power.

Thanks

I remember talking  to the U.S. distributor and "Azden go to person" years ago about the Azden manuals and I believe he said there was no service manual available for distribution for the 7800 and a number of other models. Before he passed away, he sent me what paperwork he still had on the various Azden  models but the only 7800 and 7500 related stuff he sent were the schematic diagrams, block diagrams, related circuitry, etc. Unfortunately, no voltages were marked on any of the components shown in the schematics.

Pete, wa2cwa
www.manualman.com
« Last Edit: June 19, 2019, 10:51:18 PM by WA2CWA » Logged
WB8VLC
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Posts: 667




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« Reply #4 on: June 22, 2019, 10:58:15 AM »

You may be correct about the service manual not not being available.

I used the fold out schematic from the operators manual which included 2, 1 1/4, 6 and 10 meter schematics.

The LED backlight issue of course was not evident in the schematic and this issue took a few hours of chasing/swearing at Azden.
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