crispy relay and half wave smoke

[Opcom]

Today I finished wiring up the relays in the Tucker transmitter and started firing it up. Unfortunately one of the rectifiers decided that today was a good day to die. I am not sure how much current passed through the center set of normally open relay contacts or if it was even sinusoidal. kind of a moot point but I guess I get a few points for burning up something today.

The high voltage was about 500V instead of 3000V because one MV rectifier was apparently arcing. It was beautiful blue glow all over the outside of the plate. That's what I learned later. The good one would conduct, and the bad one would short out gaseously.

What I saw first was a bright gree-blue flash apparently coming from inside the PA grid bias meter. Thankfully it was coming from the relay behind he meter! this lasted only as long as it took me to release the test key, maybe 1/2 second. That is how long it took to do this to the relay. I have never burned up one like this before.

If one mercury rectifier is flashing and the other is not, what goes on? I suppose that makes the HV power supply looks like a short.

Anyway, enjoy the nicely burnt relay with metal parts melted and a contact missing somewhere. The burnt pole is rated 13A, I wonder how much current passed to do that to it.

[WQ9E]

Nice zorch! 

MV rectifiers are like high school cheerleaders; very pretty to look at but very high maintenance.

Did you run the MV rectifiers long enough cathode voltage only to vaporize any residual balls of mercury?  As I recall 10 minutes is the recommended minimum if there is any chance the tubes were disturbed (mechanically).

I vaguely recall reading somewhere about a problem with choke resonance (or something along those lines) causing the immediate destruction of one of a pair of MV rectifiers.  OK, I am editing this to add the information from the Orr Handbook.  To paraphrase Orr, filter resonance is sometimes a problem with HV power supplies.  If the product (value of choke in henrys times value of capacitance in ufd) = 1.77 then the filter is resonant at 120 Hz., if the product is 7.1 then the filter is resonant at 60 hz.  With a swinging input choke these product values may occur at different current draws.  Symptoms with MV rectifiers are one will dim or extinguish and the other will become very bright. Did you stay with the same value filter capacitance as original?


The only thing I currently have that still has MV rectifiers in it (other than my B&K tube tester) is a Hallicrafters HT-19.  This transmitter was a basic death trap when I got it since the plate (around 1800 volts) and screen supplies were brought to bare binding posts on the back so it could be used with an external modulator for AM in addition to its built in NBFM and CW capability.  I don't think these ever went into full production and every manual I have seen for them is stamped prototype.  It doesn't get much use but I did build a plexiglas cover over the binding posts and it gets used with the matching SX-42 for a CW qso now and then.

Hopefully the next test firing of the Tucker will go better!

Rodger WQ9E

[Opcom]

I let the rectifiers warm up for 30 minutes since the rig had not been on in a couple months.

The filter is a pi-network with a cap input of 8uF, a fixed choke of 4H, then an output C of 24uF. I should be well away from any resonance there.

For now I have disconnected the offending rectifer. Controls seemed to go OK then, minus the function of that relay. All 3 sections are in parallel so I might cut off the bad part and try again later before risking a new one. I will replace the rectifier first.

The function of that relay and a corresponding one in the other HV supply is to bypass a ceramic heating element. The element is one of those old cone-shaped ones that the old buzzards used to cut in for tuning up. All I can figure is that with the HV transformer partly shorted by the rectifier, the relay must have arced upon opening or closing and the transformer may have acted as an inductive ballast keping the arc alive.

This rig has a very peculiar circuit for hi/lo power, except the 120V for the hi/lo relay comes off the variac and if it is not turned up high enough the relay will buzz - see the schematic. I think the best way to deal with it is to set up those two relays to get their coil power from the 120VAC line (hi/lo power switch in series with the coils not shown) rather than after the variacs. I do not know why COL. Tucker designed the relay circuit as it is. It resembles an inrush circuit, yet I am not seeing the effects of inrush. It was certainly intended to reduce the power level by putting the 660W heating elements in series with the AC input.

This relay is in the same electrical position as the one that was very burnt before. I am thinking the tube has gone sour in the past and the OM disregarded it. I don't seem to find my spare 673's since my apprentice cleaned up the lab and have no spare relay right now so I called it a night. The relays are very expensive - $6.00 each!

[W3SLK]

Wooops! If it were me, (and its not), I would shitcan all the MV rectifiers and install 3B28's. I ran into an issue where the MV's were arcing through the pin base to the screw holding the ceramic socket. It took out a chunk of the socket. So I put the 3B28's in and never had a problem again.

[Opcom]

As much as I like mercury, the thought occurred to me to change rectifiers.

Even the 872 or xenon equivalent is going to be short on inverse voltage. The problem is the ratings. I'm afraid SS rectifiers would also be very costly in case of a fault.

Ignoring the secondary transformer connections because the diagram is old and wrong, The PIV in the circuit with the HV turned up is 8.5KV and peak current each cycle is 3A with 3500VDC output @ 600mA DC. I prefer a better safety margin due to surges and the like, so I would rather stay with a 15KV rectifier. I realize also these power levels are excessive, but this is what the beast was designed for and I don't want to castrate it. I could possibly go from the 4-1000 to a 3-1000 in the PA and then use a lower B+ like 3000V and come up with a 7300V PIV, but I am just trying to get it working like it is supposed to.

575A/673:
15KV PIV
6A peak repetitive
1.5A continuous
100A fault

4B32 / 872 / 8008
10KV PIV
5A peak repetitive
1.25A continuous
50A fault

866A / 3B28:
10KV PIV
1A peak repetitive
0.25A continuous
20A fault

====
I will try to cook in that rectifier for a few days just in case there's a glob of mercury in the cathode shield or somewhere, and see if I can recover it. I am open to more suggestions.

[N3DRB The Derb]

If it were me, (and its not), I would shitcan all the MV rectifiers and install 3B28's

do it now. 3B28's are much superior. I ran 4 of them in the HB rig - never a moments trouble.

[Opcom]

I got a whole case of 3B28's, they just won't fill the bill for voltage and current. Are you saying they can take more than 10KV? I am also looking at 500-600mA, and they are rated a bit less I think.

[Opcom]

I bought 8 new surplus relays at $5 each.. being frustrated that I didn't think to buy spares. So with the bad 673 rectifier tube's anode disconnected, I can make high voltage. I might have fused one side of the power reduction switch. Can't reduce high voltage on the RF amplifier, only the modulator.

The stange thing is that the relay actuates, but no change takes place. But when I pull the relay out, the power reduction comes ON as expected because the series resistance is no longer bypassed. This is very weird. I can see it is going to devolve into one of those afternoons spent with head and shoulders inside the rig, with a work light and an ohm meter. Must be a shortage or crispiness somewhere else.