Codar AT5 low output

[John Mann]

Hello all, I have recently acquired a Codar AT5 but cannot get it to load into a 50 ohm dummy load. Best output, though low, is at minimum capacitance. I can actually get slightly more RF (although still very little) into my aerial although it is not yet properly rigged or tuned, just draped over the roof.

Before I start chasing faults, it occurs to me that the unit (with its TV co-ax connectors) may not have been designed to work into 50 ohms? Opinions from anyone familiar with the AT5 gratefully received!

[Cobaltblue]

Coax TV connector ISTR are 75 Ohm and indeed the AT5 handbook mentions 75 Ohms.

It also says that when working into low impedance antennas you may need to add additional capacitance.

The Handbook can be found on the VMARS web site here

Good luck!

Mike T

[Station X]

I've had no problems operating my own AT5 into both 50R and 75R dummy loads.

[John Mann]

I'm going by the forward deflection on the SWR meter. I've tried two, one a "CB type" the other home made, however I've never tried them at such a low frequency so just perhaps they are less sensitive at 3.5 MHz. The dummy load is getting warm which is a good sign. Only getting 20mA on the meter - the manual suggests 45 or more. Incidentally the manual shows the meter connected to the cathode whereas mine is connected to the anode, hard to tell if this is a factory mod as the unit has clearly been hacked about a bit. I guess the current should be the same though.

[Sean Williams]

The metering being in the anode will not account for the screen current of the PA, however, this will be only a small proportion of the anode current.

Cathode metering is certainly a better way to go, if the set has been hacked around, then you may well suffer with previous repair issues.

CB type SWR metres are a variable quantity - normally though, if there is sufficient coupling at 27mhz, there should be ample at 3,5 mhz.

Might be worth checking the supply voltage to the PA valve (DC side of the anode RFC) just to see if there are enough volts.....

HTH
Sean

[g4aaw pete]

Just to clarify John, are the readings you quote on 80m or 160m?
This is quite important, as the the AT5 obtains an output on 80m by the method of 'doubling' in the PA. This means the efficiency is somewhat lower on that band compared to 160m.

When observing the resonant 'dip' in PA current, its often possible the obtain more than one. The deepest one should be on 160m, followed by a smaller one on 80m, and an even smaller one on 40m (tripling in the PA).

Regards
Pete

[Herald1360]

Ever so slightly O/T

Are you the John Mann that worked at OMT Eynsham some years ago? The name is familiar as is a recollection of a Riley 1.5 or Wolseley 1500.

[G4XWDJim]

I always find it useful when I'm tuning up low power transmitters to use a 15 watt mains bulb as a dummy load whilst listening on a nearby receiver set to the required frequency. That way you get an immediate reaction, both visually and audibly to each movement of the controls.

It won't necessarily tell you that it's going to match 50 ohms but will give you a very good idea whether it works fairly well or not and what the next step to take should be.

Jim

[MichaelR]

Originally Posted by John Mann

Incidentally the manual shows the meter connected to the cathode whereas mine is connected to the anode, hard to tell if this is a factory mod as the unit has clearly been hacked about a bit. I guess the current should be the same though.

That is a factory later modification. Are you tuning in cw mode ?.

Surely it would make more sense that you check the kit you have is not modified from the original circuit first,otherwise the instructions are useless to you.

Mike

[Keith]

A useful dummy load for the AT5 comprises 4off 12V 2.2W MES bulbs connected in series/parallel. This produces a 65ohm (close enough), 8.8W load which provides a visual indication of power output and modulation. As you can see from the state of it, mine has been in use for many years!

[G6Tanuki]

For tuning-up my current project AT5-alike I've been using a 5-watt 24-volt lorry sidelamp-bulb. At full brightness this has a reaistance of something just over 50 ohms.

With all these small transmitters with a pi-tank output the trick is to tune for maximum-smoke and not fret too much about things like SWR. Small "CB" type SWR-meters are rather too insensitive for use with low-power 1.8 or 3.5MHz transmitters: in typical strip-line directional couplers the degree of coupling is related to the square of the frequency being measured, so a meter designed for 5 or do watts on 28MHz or VHF will be pretty hopeless (the diodes operating on the non-linear bit of the curve) with 5 or so watts on 3.5MHz.

[John Mann]

Thanks for all replies.
Sean - Yes all HT voltages seem OK
Pete - I've been testing mostly on 80m as I've done the "short out half the PA coil" mod, but it's similar on 160m
Mike - there are two obvious user mods (PA valve 22K grid resistor has had a bypassed 100 ohm resistor added in series to ground, and the VFO temperature trimmer has an ancient wax 270pF added in parallel) otherwise it is as shown in the manual, apart from the meter. There is a lot of ham-fisted soldering (melted insulation etc) around the VFO.
Keith - Thanks for the tip, I will see if I can find some bulbs to make one up.
Chris - Yes, same person, well remembered since I was only there a few months. I can guess from the Herald who you are. I still have the Riley.

It may be that the AT5 is actually performing the best it can. I get 10.6V RMS on the output which is 2.25W into 50 ohms. Its certainly warming the dummy load up nicely. PA cathode current measured with a digital meter is around 50mA as expected. I was expecting 10W out but reading the specs again I see that is input power, not output.

John G7KYH

[MichaelR]

John,

Slowly and methodically put the circuit back to what it should be, at least you know that it works.After each change do a power check. There is not much to the circuitry you should have it fully working in a few hours.

G6T anuki is right , tune up for maximum "smoke" and that may not be the deepest dip that you get.

When it is fully working you will have some fun with it.

Mike

[G6Tanuki]

Another thing to try: swap over the modulator and the PA valves. They're both 6BW6 - and though a rugged valve (essentially a Noval-based glass version of the 6V6) they take quite a beating in these little transmitters.

The PA dissipates more power than the modulator (and since it depends on grid-leak bias can pass unhealthy amounts of current if the drive is low), so it could have suffered a bit.

[Top Cap]

I used to have an AT5 many years ago, it should give you around 7 Watts RF output with a DC input of 10 Watts, assuming at least 70% efficiency in the output stage. Codar really got it right for such a small Transmitter, very little drift due to care in selecting the VFO components and the rig gives nice upward modulation. I miss mine, maybe I will get another when the second hand price becomes a bit more sane, if it ever does.

[John Mann]

An update. It looks like the AT5 is running OK. Having now got an inverted V rigged in a useable state, I have been able to do an on-air trial and got an S9 report over a distance of 180 miles on 80m in the middle of the day. I think my suspicions of low output were due to (a) insensitivity of SWR meters at 3MHz and (b) the rig being designed for 75 ohms and not therefore delivering full power into a 50 ohm dummy load.

Thanks for all advice, Petes point about there being several dip points was useful, once I looked at the output on the scope it was very obvious which was the right one!

John.

[Keith]

Well done, John. Sounds like you've cracked it. If you want some reports, try it on the VMARS net on 3615kHz on Saturday mornings form [from] 08:30. Also the "breakfast club" AM net on every morning on the same frequency from around 07:30. Look forward to hearing it.

73 and HNY
Keith