[MRCA] Troubleshooting an RT-524 - K1TEW

[Ray Fantini]

Nice pictures! A6300 and A6400 are the key to understanding the transmitter in the VRC-12 On top of A6400 the isolation amplifier is J6001 that feeds the power amplifiers and if you can adapt the signal from that point to a receiver or spectrum analyzer that will tell you what’s going on being that’s the first place you are seeing the real output of the FMO that’s on carrier. This is all the stuff right in the center of the radio after you remove the top. J6003 right next to it provides a small sample that’s feed back to the receiver to provide side tone that’s sampled prior to Q6402  the small RF amp in A6400
I have the benefit of using a spectrum analyzer that makes much of this work easy, without an analyzer think maybe you can do something with a service monitor or a receiver but keep in mind that the level at J6001 will be around -20 to -10 DBm and a SDR may not like it.
Not certain but have idea that if you’re not getting side tone or hearing your own voice when transmitting that’s a good way of knowing the FMO A6300 or A6400 is dead being they are before all the amplification junk in the driver and PA where the side tone channel monitors the transmitter FMO directly via J6003
In my limited experience with the set I have discovered that the slide switches inside the sub-assemblies did not age well and become intermittent and will result in poor or no operation on some bands and result in just what you are experiencing. Remove the assemblies and clean the surfaces on the contacts and it will work like new. My biggest problem has been not the switches in the transmitter section but the switches in the front end of the receiver the VHF Tuner Assembly A1000 that will result in reduced receiver sensitivity.
It’s anecdotal but it’s a policy I try to live by, it’s never alignment its always bad contacts!

All this brings up the point of it being a real plus to have a communications service monitor. A lot of the old Cushman CE monitors can be bought for a couple hundred dollars.
I have a Motorola 2001 that I bought at a ham fest for $250 being almost all commercial radio has now migrated to narrow band digital and all the old FM service monitors are just about useless to the two way shops. TM 11-5820-401-34 spends most of its time showing how to do diagnostics with the IFR service monitor but being the IFR monitors are a bit smaller than the Cushman’s they tend to sell for more money.
I have an old Motorola R1200 service monitor that I would gladly part with for $250 that has the manual, cover and best of all comes with a spare unit for parts. Shipping is the biggest issue with it being it’s huge and heavy! So would prefer to sell local.

Hope all that helps and sorry about the promotion of trying to sell my old Motorola.

Ray F/KA3EKH


From: mrca-bounces at mailman.qth.net [mailto:mrca-bounces at mailman.qth.net] On Behalf Of Todd Tew
Sent: Monday, October 02, 2017 4:12 PM
To: mrca at mailman.qth.net
Subject: [MRCA] Troubleshooting an RT-524 - K1TEW

I am working on a transmit issue on one of my RT-524.

Currently the RT-524 appears to receive in both frequency bands and I have tested this on multiple frequencies.

However when I attempted to transmit on 51.0 there was no outbound transmission = nothing.  The radio does spin up with that whine like she's trying but I heard nothing on my other radios.  I had an additional 524, a 1523 and my Flex radio all monitoring the frequency but nothing.

I was about to assume that perhaps someone had really demilled the radio but instead flipped over the radio into the other frequency band at at 74.000 the radio transmitted just fine into a dummy load of-course since that's outside of my HAM frequencies.

Having two RT-524's with one being a fully functional I pulled both covers and made sure all modules were in place, no internal cables broken or disconnected and no modules missing.  I could find no internal differences that could account for the lack of transmission in the lower bands so I am left with that I believe is a bad module or internal problem with the radio.

Attached are some pictures of my setup.

The RT-524 is connected up to an antenna with coax but I did not have the antenna control cable in place - I manually placed the antenna base in the proper bands for the frequency in use.

Would really appreciate some suggestions on where to continue the troubleshooting.

Todd
K1TEW
ttew22 at mail.com<mailto:ttew22 at mail.com>


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