Relay Problems and How to Solve

Problems with the 4 pole relay in the 830/530.  Why?

This is a troublesome part on the TS530/830S.  After years of use they give up the ghost and cause various problems.  Although the relay does last a long time, it really should last a lot longer.  Here is what is going on.

When Kenwood came out with the 530S and the 830S in 1980 it was a complete redesign from the earlier 520/820 series.  One of the things that was upgraded was this relay, which is used to do the switch from receive to transmit and back again.  The earlier models had a relay that looked like this:

It looks like it is sealed in plastic, but it really is not.  It’s just a plastic cover that does not seal where it joins the bottom of the relay.  Moisture/oxygen/ contaminants are free to circulate in and out of the relay.  The main enemy is moisture in the air.    These relays were reasonably reliable, but were subject to problems if the radio sat in storage for long periods of time in a humid environment.

 

The Relay used in the TS830S and 530S is a completely sealed unit.  No air can move in or out.  A sealed relay is perhaps one of the most reliable and long lasting electro-mechanical devices used in electronics, when used within its design specifications.  Even though they lasted a long time, they really should be almost trouble free with hardly any problems, ever.

On the 530/830 series, they do go bad, with surprising frequency.  They shouldn’t be failing.  Turns out that they are being used out of spec, overcurrent.  The problem is caused by the repeated discharging to ground of a charged capacitor through one of its contacts.  When a capacitor is shorted to ground, a large instantaneous current is produced that exceeds this relays current rating.  And of course it causes the gradual degradation of that contact.

One arm (the moving contact between two other contacts) is hooked to ground, and this is the one that causes the problem.  The solution is very simple…. merely insert some resistance in the circuit, to limit the current to a figure within design specifications.   These relays are typically rated for 5,000,000 operations or so, they should not be failing, no matter how many contests you have worked. 

A simple calculation with ohms law will illustrate. 

E/R = I

Let’s say the cap is charged to 10 volts.  When the relay is actuated, and the contacts short it to ground, the current will be equal to E/R, which, with no resistance in the circuit, is 10 divided by zero. 

10/0 = infinity

That would give us an infinity of amps.  In reality the ceramic cap has an equivalent series resistance of a couple hundred milohms and the printed circuit traces, plus a little stray inductance add up to something.  Let’s be generous and say all that stuff adds up to 2 ohms. 

10/2 = 5

This gives us 5 amps of instantaneous current.  Adding a 100 ohm resistor will reduce this to 0.1 amp, much more manageable.

10/100 = 0.1

Now, surely it seems like a little 0.01uf cap could not be causing this problem, or be capable of flowing this kind of current.  Let’s look at it a little further.  With 0.01uf and 2 ohms of resistance the time constant will be:

Tc = RC   (C is in farads)   so…

Tc = 2 X 0.00000001  

Tc = 0.00000002 second,   or o.02 microsecond,    or 1/2 of a cycle at 25 Mhz. 

During that time the current will fall to 37% of its initial value of 5 amps.  So, for a very,  very brief moment of time, about the time of 1/2 cycle at 25Mhz, the current across the relay contacts will go from about 5 amps to a little less than 2 amps.  This figure will also be affected by the actual relay contact resistance, which as the arm is moving and making contact, starts out as a very small point of contact and quickly grows as full contact pressure is applied.  It is during this brief instant that damage occurs to the contact surface.

Now this is probably more detail than most of us even care about.  But it gives you an idea of what happens at the micro contact area and micro second level with a relay that is  acting to short a charged cap to ground.  It’s almost impossible to measure this discharge current accurately.  But we can know that when the relay does not hold up as expected, something like this is going on.

The replacement relay we sell is rated at 1 million operations at 1 amp, and 10 million operations at 0.5 amps.  Getting the current down is going to give us a much longer useful life.

A 100 ohm resistor inserted in the circuit will reduce the current and should allow the relay to last a lot longer.  There are two ways to do this, either add the resistance between the relay arm and ground, or add it at the capacitor.  Turns out it is much easier to add it at the cap.  We have done it both ways.  First the hard way, at the relay arm…

 

 And the easy way.  Remove C51, a 0.01uf disc ceramic and replace with a 0.01uf cap in series with the 100 ohm resistor.   The resistor and cap simply get soldered together in series and mounted on the board in the C51 location.  The resistor goes on the end furthest away from the relay.

C51 replaced by a 0.01uf cap soldered to a 100 ohm resistor. It’s the green cap and resistor.

 

So, what about the 820/520 series of these radios?  The transmit/receive switching circuitry is almost identical.  In the 820 the offending capacitor is on the RF board, C17.  Here’s a picture of the same fix

C17 has been replaced by a new 0.01 cap and a 100 ohm resistor.

 

On the 520, the capacitor is also on the RF board, C43. 

On the 530S, the circuitry seems to be a little different.  One arm of the relay has a 220 ohm resistor already in circuit, so perhaps this was designed in at the factory.

Summing up…  It’s quite typical for any product, no matter how well designed, to have design flaws.  Another example on the 830S is the nylon bandswitch coupler.  In reality, a manufacture designs for a certain lifespan, and I would imagine Kenwood was looking at 15-20 years, which they more than achieved.  Now, however, we’re trying to get another 15-20 years, which brings us to  50-60 years, so we need to clean up little flaws like this.

So, if you’re replacing the relay, you might as well go the extra mile and do this fix.  It’ll be one less thing to go wrong in the future.  Another reason to do it, is….. these replacement relays are currently still available, but they are no longer in production.  When current stockpiles are gone, it’s going to be a really tough fix. Remember it’s the unavailability of electro-mechanical parts that eventually kills a vintage radio.

This fix comes from a document at N6WK.com, Gordon’s site.  It is entitled “TS830S Survival Guide”, by Olaf Rettkowski, DL9AI.  It’s well worth your time.

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