Heathkit DX-100 Transmitter. - UK Vintage Radio Repair and Restoration Discussion Forum

jammy dodger · 25th Sep 2023, 11:27 am

My DX-100 has started to lose grid current. When I go to transmit the meter starts off at 4mA and slowly begins to drop and continues to do so until almost no current is indicated. I have swapped all the valves/tubes except the clamper.
Just wonder if anyone else has had the same problem. Thank You.

Radio Wrangler · 25th Sep 2023, 2:02 pm

There may be a number of differences between the US market DX-100, and the UK market DX100U so be sure you're using the right manual.

4mA doesn't sound like a bad grid current for a pair of 6146. Indeed the meter is scaled for 0-10mA. There are a few possibilities: Something changing in the bias resistors making more negative bias at the 6146 grids moving them further into cutoff so rectifying less of the drive. Ditto happening to grid bias of the 5763 drive, moving it more into cutoff and reducing drive to the 6146 and hence grid current. Or something else reducing drive up the chain. The negative bias chain serves both the 5763 and the pair of 6146.

Over the time while the grid current is falling, what is the driver anode current doing?

You have metering for HT voltages as well. All of these are worth watching to get the most info before opening the cabinet.

David

Aub · 25th Sep 2023, 5:17 pm

You could try removing the clamp valve.

Cheers

Aub

jammy dodger · 25th Sep 2023, 8:28 pm

Hi David and Aub.
Thanks both for your replies, yes I will remove the clamp valve, that is an easy option. I put the post out for ideas ready for when I pick up the courage to get the DX-100 of the desk. My AR88 was already on the bench but the tx is on my operating desk and connected to a variety of other things. I will keep your replies to hand for the day I am able to get it on the bench, unless it is the clamp valve, that would be an easy fix. Thanks again.

25th Sep 2023, 11:27 am	#1
jammy dodger Diode Join Date: Sep 2023 Location: Mold, North Wales, UK. Posts: 8	Heathkit DX-100 Transmitter. My DX-100 has started to lose grid current. When I go to transmit the meter starts off at 4mA and slowly begins to drop and continues to do so until almost no current is indicated. I have swapped all the valves/tubes except the clamper. Just wonder if anyone else has had the same problem. Thank You.

25th Sep 2023, 2:02 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,237	Re: Dx-100 There may be a number of differences between the US market DX-100, and the UK market DX100U so be sure you're using the right manual. 4mA doesn't sound like a bad grid current for a pair of 6146. Indeed the meter is scaled for 0-10mA. There are a few possibilities: Something changing in the bias resistors making more negative bias at the 6146 grids moving them further into cutoff so rectifying less of the drive. Ditto happening to grid bias of the 5763 drive, moving it more into cutoff and reducing drive to the 6146 and hence grid current. Or something else reducing drive up the chain. The negative bias chain serves both the 5763 and the pair of 6146. Over the time while the grid current is falling, what is the driver anode current doing? You have metering for HT voltages as well. All of these are worth watching to get the most info before opening the cabinet. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

25th Sep 2023, 5:17 pm	#3
Aub Octode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 1,971	Re: Dx-100 You could try removing the clamp valve. Cheers Aub __________________ Life's a long song, but the tune ends too soon for us all.

25th Sep 2023, 8:28 pm	#4
jammy dodger Diode Join Date: Sep 2023 Location: Mold, North Wales, UK. Posts: 8	Re: Heathkit DX-100 Transmitter. Hi David and Aub. Thanks both for your replies, yes I will remove the clamp valve, that is an easy option. I put the post out for ideas ready for when I pick up the courage to get the DX-100 of the desk. My AR88 was already on the bench but the tx is on my operating desk and connected to a variety of other things. I will keep your replies to hand for the day I am able to get it on the bench, unless it is the clamp valve, that would be an easy fix. Thanks again.