High Plate Current Indication in Eico 720 Transmitter

[KC7TEN]

I posted another topic about a year ago trying to understand how the meter circuit of my Eico 720 works, and got some pretty helpful replies. I have a bit more information now, but can't figure out what's happening with my transmitter.  The circuits look simple enough, but I'm struggling and would sure appreciate some education.

I've replaced the tubes in the transmitter and made many voltage and resistance / continuity checks without finding anything wrong.

When I go through the tuning process I can properly tune the grid circuit on every band (except 10 meters, but leave that aside for now.) Grid current peaks as expected, and the drive control works to limit grid current to about 3 ma. But when I switch the meter to read plate current and put the key down, the meter pegs.

But here's the interesting and puzzling part. If I leave the meter switched off, or in the grid position, I can tune the plate drive control to yield about 70 watts output to a 50 ohm dummy load as measured with a watt meter.  The output varies a bit on the different bands, but power output seems pretty healthy.

So... is that pegged meter actually an indication of excessive plate current?  Would the transmitter output 60 or 70 watts with excessive plate current?

With the key down, the bias voltage of pin 3 of V3 is about 180 vdc. B+ is a little higher than spec, but not excessive. (660v or so.)  The bias resistor, R9, is within spec. There's good signal coming out of the plate of V2 and it's coupled to the grids of V3 and V4 as expected. There's some distortion, as viewed with an o-scope, but nothing spectacular. Freq seems to hold steady after several minutes of warm up time. The signal sounds fine as received by another rig when I transmit into the dummy load.  R11 and R13 both test good.

So.... what am I missing?

Thanks!

Darrell
KC7ten

[AC5UP]

But when I switch the meter to read plate current and put the key down, the meter pegs.

Remember your basic DC theory when it comes to metering... A voltmeter is across the power source ( + & - ) while an Ammeter is connected in series with one leg of the circuit under test. Typically on the B+ line between the power supply and load. Look at the meter switch circuit of the EICO 720, between S4 and M1. You'll see R10, a .512 Ohm resistor.

That's the meter shunt for the ammeter function. With a low value resistor like that across a meter movement almost all current will flow through the shunt with only a trickle through the meter, and that's exactly what you want since a trickle is all the meter needs. If the shunt resistor was open, or S4 wasn't reliably switching it across the meter movement, or there was a bad solder joint preventing the shunt from carrying almost all the current while in TX and S4 was set to plate current...

You'd peg the meter.  Be glad you didn't smoke it.

I suspect the most likely fault is with S4, and could have been caused by flipping the meter switch HOT (while in TX) once too often which arced at least one contact finger. If absolutely nothing in the circuit looks arced or otherwise compromised, I'd re-flow the lugs of S4 and R10 with fresh solder. If any of the original solder joints are a dull grey or blobtastic they could be electrolyzed or were cold joints since Day One.

And that's just a wild-ass guess........................... 

[AC5UP]

Would the transmitter output 60 or 70 watts with excessive plate current?

One other thot....... The book sez the EICO is a 90 Watt transmitter. Don't forget that back then 90 Watts meant 90 Watts of DC input power to the final(s). 60 Watts out is a little high for 90 Watts in, but there's always the chance you have an overachiever power transformer or your AC line voltage is higher than the classic Reddy Kilowatt standard.

I should also mention it's good form to never assume any electronic equipment was 100% perfect when it left the factory... Sure, most of the gear we see is within 95%, but if it was a kit?

Assume Nothing.

[KC7TEN]

Thanks for the input. I'd already changed the shunt resistor, which had changed value over the years, but I didn't actually test the switch for high resistance. (It's switching, else there would be no plate current indication at all.) I'll check that out, though.

[KE3WD]

I'd be checking the actual Plate Voltage with a known good VTVM or DMM that could handle the range when the rig is in the state where the internal voltmeter pegs. 

That would give me a "place to stand" in determining what is really going on first.  Highly unlikely that the power supply could fault such that it would suddenly be able to deliver a Plate Voltage very much higher than its design limits.  But some older rigs might be capable of that.  Eico would not likely be one of them, consider that Eico was among the offerings for the middle class market, they were economically minded as to parts selection and count.

This is almost certainly a metering circuit problem.  Trace the entire circuit through from the schematic, then check every single connection and component involved with the Plate Voltage meter circuitry, to include any switches that may be involved.  A dirty or corroded switch could be all that is causing the problem.  Or a bad solder joint to Common...


73

[KC7TEN]

OK, I hereby confur upon KE3WD the official title of  "boat anchor genius".  Thanks for your help!

Turns out that there was about an ohm of resistance across the contacts of the meter switch. I took it out, cleaned it up, put it back in and as my old friend Gomer used to say, Shazam!

Seriously, thanks for your advice. You were right on the money.

73

Darrell
KC7TEN

[KE3WD]

Thanks for the flowers, but its certainly not genius. 

But, from the eham and qrz posts of problems with gear where the cleaning and lubing of controls is often bypassed or not believed to be the possible culprit, I can back up the notion and tell you how many customer pieces cross my testbench where the repair is just that are sometimes unbelievable in number.  Especially in the high fidelity audio world.

73