Heathkit AT-1 Transmitter Question.

[w5kcm]

I am restoring/rebuilding an old Heathkit AT-1 transmitter. Its not AM but still  boatanchorish. I need a meter. Does anyone know what would be a suitable replacement for the original meter? The partial Heathkit manual that I have shows it to be a 10 ma meter. I assume that it is a 10 millamp full scale deflection..? I am looking at a Triplett Panel Meter Model 327-T DC 0-10 Ma on Ebay and was wondering if this meter will physically fit into the front panel and will function properly? Any ideas on this will be appreciated.
Randy, W5KCM

[AB2EZ]

Randy

Hi!

I put a new meter in a DX-20 a few years ago... so I have been down this path. I used an inexpensive Shurite panel meter (current production) that fit perfectly into the existing meter opening... so I don't think you'll have a problem finding a fit.

Looking at the schematic of the AT-1 (which uses a somewhat less sophisticated metering scheme than the DX-20). you can use any of the following meters

10 mA full scale 750 ohms series resistance*
5 ma full scale 1500 ohms series resistance*
2.5 ma full scale 3000 ohms series resistance*
1 ma full scale 7500 ohms series resistance*

*the series resistance is the total of the intrinsic series resistance of the meter itself (always specified by the manufacturer) + the extra series resistance you need to add, in series with the + terminal of the meter, to make the total equal to the value shown above. The important thing is that the intrinsic series resistance of the meter must be less than or equal to the total resistance specified above.

Any of the above meters will point to full scale when you have 150 mA of plate current flowing in the AT-1. Likewise, any of the above meters will allow you to peak the grid drive in the "grid" position of the meter.

Best regards
Stu

[w5kcm]

Hi Stu,
Thanks for the info. Well I do not have the old meter, it was missing along with a few other parts. I bought this AT-1 in parts in a cardboard box so its going to be some scrounging required.
I have a partial assembly manual that I got from BAMA. It shows the original meter and it looks to be calibrated 0 to 10. I believe that there are two positions, Plate current and Grid current. Grid current is 10 ma and Plate current is 100 ma fsd. I see on the meter switch they switch in a 51 ohm resistor for the Plate current position. I believe this is only correct if I find a meter with the exact internal resistance as the original..?? I have read where some have replaced this 51 ohm with a 43 ohm for more correct plate current. All this is only my understanding at this point and it may be all wrong. There is my disclaimer   .
Randy

[AB2EZ]

Randy

I modified my original comment to your post (please check back to my modified comment).

On the AT-1, the plate current flows (as you note) through a 51 ohm resistor. As a result, 100 mA of plate current will produce 5 volts across this resistor. A 10 mA (full scale) meter with 750 ohms of series resistance will read 2/3 full scale when 5 volts are placed across it. Similarly, in the grid setting of the meter... the meter will read about 20% of full scale when the grid drive is peaked.

Best regards
Stu

[w5kcm]

OK Stu, I have been studying your info. So if I understand it correctly, I really need to find out what the intrinsic resistance of the meter first. Mainly because in the Plate position they are using the meter to measure voltage developed across the 51 ohm resistor. In the Grid position they are measuring current to ground through the 47 K resistor(grid of the 6L6). I remember reading somewhere of a circuit you can build that will allow you to measure the internal resistance of surplus meters. Let me dig into my old hand books it may be in there. So if I find a meter with a 0 to 10 scale (10 MA), I just need to have a total series resistance of about 500 ohms to make it work with the AT-1. If I have to add some series resistance to the meter, I will have to switch it out of the circuit when I switch to the Grid position. So if I need the meter to read full scale (10 MA) across 5 volts across the 50 ohm resistor (100 MA through the 50 ohms), the meter will need to have intrinsic resistance of 500 ohms. Am I on the right track?
Thanks, Randy

[k4kyv]

The original  meter was one of those cheap moving vane meters.  A good d'arsonval movement would be much better, but if you are trying to restore it to stock that might not be the way you want to go.  In years past I have probably thrown out half a dozen of those meters, probably at least one of which was an exact replacement for the AT-1, because I considered them to be worthless junk.  But that was before the days that things like AT-1's became sought-after collectors items.

[w5kcm]

Hi Don,
Yes, I really didn't know the AT-1 was collectible. I was at the Fort Worth Hamfest a couple weeks ago and I found this one in a cardboard box under the guys table for $10. Thought I would give it a go. I mainly just want to get it on the air and make a few contacts with it. Think it will be fun just getting it back on the air. Not really trying to make it all look original and then put it in a museum. I see some on Epay going for big bucks when fully restored to pretty. This one has some extra holes in the front panel and dudnt look all that good but shouldn't take a whole lot to get it going.
Thanks, Randy

[Pete, WA2CWA]

Simple enough to find the specs for the Triplett 327-T and internal resistance. Internal resistance on that specific meter (initial post) is 3.1 ohms.
See:
http://www.meterdistributor.com/TripPages/pg7.htm

[w5kcm]

Thanks Pete, That's good info...

[AB2EZ]

Randy

The is no need to switch out the meter's series resistor when peaking the grid current.

Looking at the manual, the Heath engineers didn't attempt to set the grid current of the AT1 at a particular target value. They just ask you to tune the oscillator to peak the grid current.

They could have designed this circuit to actually measure both grid and plate current... and, if you want to do that, here's how:

I would suggest that you pick a meter whose existing scale makes the most sense for reading the plate current. Since the AT1 manual asks you to adjust the plate current to around 60-80mA, a 10 mA meter seems, at first, like a reasonable choice... because the scale reads 0-10. However, if you pick the series resistance of the meter so that 100 ma of plate current corresponds to full scale ("10") on the meter... then the meter will tend to be "pinned" until the plate current has been "dipped". The required total series resistance is ~500 ohms. [actually 459 ohms, if you do the calculation more accurately]

The above is how the Heath engineers designed the AT-1.

If you pick the series resistance of the 10ma meter to be approximately 750 ohms, then the meter will read approximately 5 (half scale) when the plate current is 75 mA.  [The approximate calculation is: 75mA x 51 ohms ~ 3.83 volts across the plate current shunt resistor. 3.83 volts / 750 ohms ~ 5 ma (half scale on a 10 mA meter). The exact calculation yields a required series resistance of 714 ohms instead of 750 ohms]

Another alternative would be a 15 mA meter, like the Shurite 8304Z.

http://www.primetechnology.com/PartsDetails.aspx?PartsId=77&Parentid=28&ParentType=C

With a 15 mA meter and a 459 ohm* total series resistance (total of meter intrinsic resistance + added external series resistance), the meter will read 7.5 (half scale) when the plate current is 75 mA.

[*This is a more exact calculation than the approximate calculation I used above. With the 459 ohm series resistance of the meter in parallel with the 51 ohm shunt resistor... 90% of the plate current will flow through the 51 ohm shunt resistor, and 10% of the plate current will flow through the meter. Thus, when the plate current is 75 mA, 7.5 mA will flow through the meter]

Now, for the grid current measurement.

Looking at the existing AT1 design... when the meter is switched to the "grid" position, the meter (459 ohms of series resistance) will be in parallel with the lower 47k ohm grid resistor... and thus, most of the grid current will flow through the meter. However, when the meter is switched out of the "grid" position, the actual grid current will drop, because the total resistance from grid to ground will nearly double. Therefore, the original AT1 grid current metering design does not give an accurate measurement of the grid current that flows when the meter is switched out of the "grid" position.

What to do?

Alternative 1: Don't worry about it, just peak the grid current with the meter in the "grid" position", and assume that the grid current that is flowing when the meter is switched out of the "grid" position is about half as much as what the meter read when it was switched into the "grid" position.

Alternative 2:

1. Add a 4130 ohm (3900 ohms would be okay) series resistor between the existing, lower 47k grid resistor and ground.
2. Move the wire that currently goes from the meter switch to the top of the existing 47k ohm grid resistor... so that it now goes to the top of the new 4130 ohm resistor.

When the meter is in the "grid" position,  90% of the grid current will go through the meter, and 10% of the grid current will go through the new 4130 ohm series resistor added in steps 1. and 2. above. (If you use a 3900 ohm resistor instead of a 4130 ohm resistor, then 89.5% of the grid current will flow through the meter)

The meter will read 0.9 mA when the actual grid current is 1ma.

When the meter is switched out of the "grid position", the effect on the total resistance from grid to ground will be negligible.


Best regards
Stu

[k4kyv]

I once had an AT-1 that I used as an rf exciter to drive a pair of 807's plate modulated by another pair.

I modified mine to run on 160m.  Found another wafer switch in the junkbox that was of identical construction to the one used for a bandswitch for the AT-1.  I laboriously removed the rivets that held the contacts to the bakelite wafer in the junk switch, and using some tiny screws and nuts, mounted an extra set of contacts on the AT-1 switch, modified the mechanical stop in the switch, and then made up a homebrew 160m output coil, and modified the driver coil IIRC.  It worked FB on topband.  I traded it off to another ham after building a homebrew exciter, since the 6L6 driver was a bit overkill for my 807's.  Unfortunately, he used it for a while, and then stripped it down and parted it out.  At least it wasn't in pristine condx, since the previous owner to me had drilled some extra holes in the front panel to mount a way-too-small loading capacitor in a botched effort to change the link coupled output circuit to a pi-network.

[w5kcm]

Ok Stu, I need a couple days to study your info...sounds like what I need to know. I just bought a 0 to 1 ma meter from Epay:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=280195235401&ssPageName=STRK:MEWN:IT&ih=018
It should be an exact fit into the front panel hole and I will just need to adapt the 1 ma full scale to some value appropriate for the AT-1 plate and grid currents as you spoke about. Here is a few photos of the before project:
http://www.flickr.com/photos/wb5kcm/sets/72157603797664318/
Thanks for the valuable info.
73, Randy

[W1RKW]

You may be able to use the AT-1 as a PW AM xmitter.  If I remember correctly there's a blurb in the original Heath manual for using it as an AM xmitter.

[w5kcm]

Yes, I saw that in the manual. That will be another project to build the modulator for this jewel. 

[Carl WA1KPD]

I modified mine to run on 160m.

Hints and Kinks in the 50s had a mod that (as I recall) that called for just adding 350 pf of fixed capacitance in the 80 meter circuit. I did it and it worked FB OM

[AB2EZ]

Yes... as Carl suggests...

If you increase the tuning capacitance (fixed + variable) by a factor of four (4), you can resonate the 80 meter coil on 160 meters.

Just to be on the safe side (particularly if you decide to operate the rig on AM with an external modulator), I suggest that you also take into account the following:

If you put the rig on 160 by adding the extra tuning capacitance and using the same inductor as is used on 80 meters... then, after you adjust the link coupling to load the AT-1 up on 160, you will end up with a Q in the output circuit of about twice what the Q was on 80 meters. [A little non-intuitive... but that's how the math/physics works out  ]

As a result, you will have twice as much current circulating in the tank circuit as you had on 80 meters (for the same output power).

No problem... provided the tank coil doesn't get too hot.

It is probably worth checking the tank coil (carefully, by turning the power off, unplugging the rig, waiting for the power supply caps to bleed down, and then shorting the 6L6 plate to ground) after about 1 minute of key down operation... to make sure it is not getting too hot... particularly if you plan to use the rig on AM in conjunction with an external modulator.

As an aside... if you decide to use the rig on AM, you may have to make some minor modifications to the crystal oscillator circuit to keep from overheating the oscillator tube (probably running at a little too much plate and screen voltage for "old buzzard" key down operation for 10 minutes or more), and to keep from damaging the crystal (the 100k ohm resistor should probably be reduced to around 15-20k ohms to prevent damaging a typical FT-243 or modern crystal).

In my DX-20, I reduced the plate voltage on the 6CL6 oscillator by adding a 20k ohm power resistor between the B+ and the wire leading (via the 6CL6 plate tank circuit) to the plate of the 6CL6; and I used a 150 volt Zener diode (fed from the B+ with a 50k ohm power resistor) to set the screen voltage on the 6CL6. I also changed the grid resistor from 100k ohms to 18k ohms.

Stu