Re: Wanted Tohtsu CX600NC relay

Richard GD8EXI

After reading the comment re different versions of CX600N relays I thought I better retest mine, which is about 30 years old and silver-plated inside and out.
53dB at 30MHz
49dB at 50MHz
39dB at 144MHz
30dB at 432MHz
21dB at 1296MHz
Measured as per the Tohtsu spec sheet with a 50ohm load on the com port.

These figures are ~ 9dB worse than the current manufactures specs adding weight to the idea there are two versions. The nearly flat contacts on mine are only 1.4mm apart and 5.7mm in diameter so a calculated capacitance of 0.65pf so the poor isolation is no surprise.
The danger level for some preamps is about 0dBm, 1mW so there is not adequate isolation on VHF, even for a 10 watt Tx without a second isolation relay!
No surprise I destroyed a few GASFETs on 432 MHz!
I also tested the isolation of two CZX3500, one dated, 2016, the other brought in about 2009, date now rubbed off.
Both >85dB at 432MHz and >80dB at 1296MHz .
So plenty of isolation even for a QRO station on 1296MHz
The trouble with a second isolation relay is that it will increase your path loss on RX, which could be critical on 432MHz and above for EME and even extreme terrestrial. Certainly not needed with a CZX3500, which is a double relay anyway.
As to power handling of the CZX3500s, no I have not destruction tested them yet! However I have used them on both 432MHz and 1296MHz for 400 watts SSB and they survived for years. However with 500 watts of CW on 432MHz the CZX3500 runs hot, so as Alwyn stated not suitable for EME with a 100% duty cycle.

On 09/09/2020, 14:47, "Ian White" <gm3sek@...> wrote:


By having the common port pointing the opposite way from the other two, the CX520D saves a lot of space inside a preamp box. I wish there was a wider choice of relays with that connector layout, and at a similar price point (which rules out the big, expensive EME relays) but I have never found even one alternative.

However, the problems with poor isolation and poor power handling are surprising. I used the same CX520D in the 70cm EME preamp for many years, handling 1kW CW and connected directly to the GaAsFET preamp, all with no damage. So does the performance really fall over a cliff between 432MHz and 1296MHz?

A possible explanation for poor isolation is that there have been at least two versions of that relay, sold under the same model number. In the modern ones, the isolated contacts are grounded onto gold-plated contacts that are screwed into the body. The older ones relied on grounding onto the body itself, which is a poorer contact and also less reliable... bearing in mind that the relay will be operated tens or hundreds of thousands of times, but preamp only has to be unlucky once.

Yet another thought is about the settings of the moving arms and the actuator plungers. These settings are quite critical to achieve the correct contact pressures, and the TohTsu relays aren't exactly built like Swiss watches.

As I said, I do wish there were other relays with that highly practical port layout and at a similar price point.


73 from Ian GM3SEK






On 09/09/2020 13:29, Colin Ranson wrote:


Andy,    I think the CX3500 is a copy of the venerable Amphenol 316-10744 series, of which I was fortunate enough to rescue several from a defunct H.O. RF switching station many years ago.   One was 48v, the rest were 26v except one which was 12v !    These are just as good, if not better than the Tohtsu, in build quality at least.


What is rare, and I would love one for an application, is the same build but the common (centre) N connector coming out at 180 degrees so that the common can poke out of a plastic (mast head pre-amp) box for connection to the antenna.   Unfortunately the Tohtsu CX520 just does not cut it as far as isolation or power handling is concerned. I burnt one out with 23cm at 240w even when properly sequenced.


Regards Colin de G8LBS.


Sent from  Mail <>  for Windows 10



From: Richard GD8EXI <mailto:perwick@...>
 Sent: 08 September 2020 23:05
 Subject: Re: [UKMicrowaves] Wanted Tohtsu CX600NC relay


     I have one which was retired from active service many years ago as the isolation was not good enough on 432MHz and above, destroying the preamp device. I might sell it but, what band is it for and how much isolation do you need. I can re measure it but from memory it was less than 40dB of isolation on 432 MHz and above, not enough if you are running 100watts plus.
 A CX3500 is much better, if you need good isolation, as it has a double switch and the unused bar is grounded both ends. The isolation is so good it is difficult to measure, being in my experience greater than they 65dB they claim, more like 80dB even on 1296MHz.
 On 08/09/2020, 21:20, "Andy GD1MIP" <gd1mip@...> wrote:

Hi, I need a Tohtsu CX600NC 12v DC relay.
 Before I buy new from a store does anyone have a spare they'd like to sell?
 Thanks Andy GD1MIP




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