Heathkit AT-1 Transmitter Question.

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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

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