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DX-40 cathode resistor




 
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WA3QLW
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« on: March 07, 2019, 04:50:14 PM »

Have a DX-40 that works for a while with good reports and power then suddenly I see grid drive go negative and the cathode resistor burns open.  Replaced tubes and resistor and worked for a while then did it again.

Anyone see this??

TNX

Leslie

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NW2K
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« Reply #1 on: March 07, 2019, 07:28:14 PM »

Leslie, referencing the schematic, which resistor fails?
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N1BCG
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« Reply #2 on: March 07, 2019, 10:24:17 PM »

Which cathode resistor is burning up? The 6146 only has a meter shunt to ground through normally closed contacts on the key jack.

If the grid current meter goes negative then a positive voltage is present on the 6146 grid. That would cause extremely high Plate current. Itís possible the 0.001 coupling cap in the Plate circuit of the 5763 driver is shorting.

Thereís no fixed bias supply for the 6146, only grid leak, and that relies on the presence of sufficient grid drive. It could also be that the drive is failing.


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WA3QLW
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« Reply #3 on: March 08, 2019, 10:31:18 AM »

Which cathode resistor is burning up? The 6146 only has a meter shunt to ground through normally closed contacts on the key jack.

If the grid current meter goes negative then a positive voltage is present on the 6146 grid. That would cause extremely high Plate current. Itís possible the 0.001 coupling cap in the Plate circuit of the 5763 driver is shorting.

Thereís no fixed bias supply for the 6146, only grid leak, and that relies on the presence of sufficient grid drive. It could also be that the drive is failing.

The driver is a 6CL6 and this is the 0.67 meter resistor. 

When I got this there was a bundle of wire wound on a Popsicle stick where the resistor should be and it did measure close to 0.67 ohms.  I ordered some resistors and installed and pretty quick one blew.  Installed another one and did a little bypass and it seemed to hold and good looking modulation and all on the scopes and on air.

One other thing is whoever had this put silicone diodes in a tube base in place of the 5U4GB and the filament leads were cut off.

Last evening I was digging deep and I think I found the answer, the B+ is way too high near 700VDC which is way past the 6146 ratings so am digging in the junk box for a resistor to put in series with the diodes to lower the voltage.

Another ham suggested modifying the final to be like the DX-60 so that is a possible avenue.

Got to thinking about voltages and back in the 50's home voltage was not as high as it is today plus I am running the 40 off of a constant voltage transformer along with the paired HQ-170A for voltage stability. 

I think this could be the cause as when modulated the peak voltages on the waveform exceed the ICAS and CCS limits of the tube. 

I had it on just steady carrier last evening for nearly an hour and it just sat there happy at 20W on the Bird meter so other than the power transformer getting warm I think I found the problem. 

Got to add a series resistor to drop the voltage down from 700 to the 580 shown in the schematic.

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N1BCG
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« Reply #4 on: March 08, 2019, 11:06:35 AM »

You're right about today's line voltage being higher so ideally you would use a variac to reduce the input voltage, affecting the heaters as well.

The 5U4 has a 44V drop at 250mA but you'll need more than that to compensate for the increased line voltage. It would be good to put high wattage resistors in series with each of the diodes.
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WA3QLW
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« Reply #5 on: March 08, 2019, 11:49:02 AM »

You're right about today's line voltage being higher so ideally you would use a variac to reduce the input voltage, affecting the heaters as well.

The 5U4 has a 44V drop at 250mA but you'll need more than that to compensate for the increased line voltage. It would be good to put high wattage resistors in series with each of the diodes.

And that is what I am digging into the junk box for.  I figure best to drop that down by resistors.  Using the Variac other than bench testing just ties up a good piece of equipment. 

As soon as I find the resistors I think I will get this TX tamed.  Will let you all know here but I suspect to have it working and working good this weekend.

Leslie

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wb1ead
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« Reply #6 on: March 08, 2019, 02:54:02 PM »

Hi Leslie..congrats on ur DX-40..I have one and they are neat rigs..puzzles me a bit why you didn't consider restoring the rectifier to stock?..when I got mine several years back I thought that wud be a way to "squeeze" out an extra watt or two..I decided against mostly like you and others have found that todays AC line voltage has gone up from the 115VAC common then in the 50's..if you go that route please use a 5U4GB..not a plain 5U4G..there's even mention in the manual on pg 4 'bout this..in my rig there is an actual carbon resistor rated at 1% precision..ain't no popsicle sticks!..BTW in trying out the complete family of 6146..the most consistent output and a watt or maybe more higher the plain ol' 6146 or 6146A beat out brandy new "B or W" versions..one other thing/tip make sure ur 6DE7/12AX7A combo is gud and up to snuff..one last note..DO NOT power ur VF1/HG10 vfo's off the back plug..build ur own supply or find one self powered.."da pwr tranny..she be weak"
               Have fun!..BCG how did you find that schematic anyway?..I'm sure by now you know that's a schematic for the import jobby most widely used in the UK..look at that tube lineup!..and using a 5U4G!..wowsir!
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WA3QLW
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« Reply #7 on: March 08, 2019, 03:58:58 PM »

Hi Leslie..congrats on ur DX-40..I have one and they are neat rigs..puzzles me a bit why you didn't consider restoring the rectifier to stock?..when I got mine several years back I thought that wud be a way to "squeeze" out an extra watt or two..I decided against mostly like you and others have found that todays AC line voltage has gone up from the 115VAC common then in the 50's..if you go that route please use a 5U4GB..not a plain 5U4G..there's even mention in the manual on pg 4 'bout this..in my rig there is an actual carbon resistor rated at 1% precision..ain't no popsicle sticks!..BTW in trying out the complete family of 6146..the most consistent output and a watt or maybe more higher the plain ol' 6146 or 6146A beat out brandy new "B or W" versions..one other thing/tip make sure ur 6DE7/12AX7A combo is gud and up to snuff..one last note..DO NOT power ur VF1/HG10 vfo's off the back plug..build ur own supply or find one self powered.."da pwr tranny..she be weak"
               Have fun!..BCG how did you find that schematic anyway?..I'm sure by now you know that's a schematic for the import jobby most widely used in the UK..look at that tube lineup!..and using a 5U4G!..wowsir!

Cam modified like this and do know about the rare 5U4GB but will keep like this and add in some more resistance about equal to the 5U4GB series resistance at load which ought to work.  Will fudge a little high in value to compensate for the higher voltage.

Am using a digital VFO going into the VFO jack one of the kits on EBAY from Russia and it works real good. 

I'll dig in the junk box tonight and this weekend.  Couple hams are looking for some resistors also so I hope to have it back on the air soon.

All tubes are NOS and tested on testers that test full quality of the tube and the best ones are in the rig now. 

I do have the manual that came with it and of course a sticker on the outside of the manual says untested as is.  Pretty sure the fellow or fellows before were having the same issue so the big hunk of wire wound resistor.  I do have the correct value thanks to EBAY but don't want to burn thru all of them.

73

Leslie

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« Reply #8 on: March 11, 2019, 01:33:55 AM »

The Solid Stated Power Supply listed in the PDF file works great with the DX-40.

SSing it also takes some load off the transformer which is not noted for its robustness.

You might also check Pin 9 of the 6DE7 and make sure the Modulator cathode is not sending too high a voltage to the screen of the Final.


Phil

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WA3QLW
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« Reply #9 on: March 11, 2019, 10:50:05 AM »

The Solid Stated Power Supply listed in the PDF file works great with the DX-40.

SSing it also takes some load off the transformer which is not noted for its robustness.

You might also check Pin 9 of the 6DE7 and make sure the Modulator cathode is not sending too high a voltage to the screen of the Final.


Phil

TNX Phil, as noted the entire B+ is TOO HIGH so I need to tame it down.

For some reason I get an error when trying to download the attachment on the SS power supply.

The previous owner(s) already put silicone diodes in place of the rectifier tube I suspect for the same reason to take the load off the transformer.

This is what caused the problem as the B+ is TOO HIGH due to modern voltages in homes.  Yes I can use a Variac but I don't want to tie up the shop Variac so the sensible thing is to add in some series resistance to equal the internal resistance of the rectifier tube plus a little more resistance to compensate for our higher AC voltages.

Could not find resistors of value and wattage in junk boxes so have them on order.


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« Reply #10 on: March 11, 2019, 12:10:35 PM »



TNX Phil, as noted the entire B+ is TOO HIGH so I need to tame it down.

For some reason I get an error when trying to download the attachment on the SS power supply.

The previous owner(s) already put silicone diodes in place of the rectifier tube I suspect for the same reason to take the load off the transformer.

This is what caused the problem as the B+ is TOO HIGH due to modern voltages in homes.  Yes I can use a Variac but I don't want to tie up the shop Variac so the sensible thing is to add in some series resistance to equal the internal resistance of the rectifier tube plus a little more resistance to compensate for our higher AC voltages.

Could not find resistors of value and wattage in junk boxes so have them on order.


Let's back up a minute.

Some causes of Final tube failure or cathode resistor can be:

1) Insufficient (RF) control grid drive caused by the "exciter" circuit which is composed of the two 6CL6's.

The exciter circuit on both the DX-35 and the DX-40 use a series cascode arrangement where the two 6CL6's act as their own voltage dividers. This is why the filaments of the two 6CL6's have separate transformer winding feeds.

If any one of the two tubes fail or are weak, you're going to have too low an RF drive and too low a negative voltage on the control grid causing high Final current,

2) Bandswitch contacts eroded, corroded, or missing,

3) The L2 coil is mistuned, adjust it for max drive in the middle of the 40 meter band.

4) Too high a screen voltage. If the 6DE7 modulator circuit has a problem and is sending too high a voltage from its Pin 9 to the screen then Final current will be too high.

Remember, these transmitters operate in ICAS service and not CCS. 700 volts on the Final's plate is no big deal.

Since someone else has already been twiddling with the circuit, better trace out the wiring and compare it to the original schematic.

Note: The PS Modifications I am posting will only work IF the 6CL6's have been modified for parallel LV feed. And do not plug in any accessories.  


Phil - AC0OB

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« Reply #11 on: March 11, 2019, 12:11:22 PM »


If you had a filament transformer lyng around, you could try bucking the primary of yer transformer.

KLC
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WA3QLW
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« Reply #12 on: March 11, 2019, 12:57:41 PM »





Let's back up a minute.

Some causes of Final tube failure or cathode resistor can be:

1) Insufficient (RF) control grid drive caused by the "exciter" circuit which is composed of the two 6CL6's.

The exciter circuit on both the DX-35 and the DX-40 use a series cascode arrangement where the two 6CL6's act as their own voltage dividers. This is why the filaments of the two 6CL6's have separate transformer winding feeds.

If any one of the two tubes fail or are weak, you're going to have too low an RF drive and too low a negative voltage on the control grid causing high Final current,

2) Bandswitch contacts eroded, corroded, or missing,

3) The L2 coil is mistuned, adjust it for max drive in the middle of the 40 meter band.

4) Too high a screen voltage. If the 6DE7 modulator circuit has a problem and is sending too high a voltage from its Pin 9 to the screen then Final current will be too high.

Remember, these transmitters operate in ICAS service and not CCS. 700 volts on the Final's plate is no big deal.

Since someone else has already been twiddling with the circuit, better trace out the wiring and compare it to the original schematic.

Note: The PS Modifications I am posting will only work IF the 6CL6's have been modified for parallel LV feed. And do not plug in any accessories.  


Phil - AC0OB

Been thru all of that and tested NOS tubes twice.  The B+ is 700VDC on the plate of the 6146 due to the silicone diode modification that was put in.

Drive more than exceeds the recommended grid drive for a 6146 as per specifications.

All switches have been given DEOXIT treatment several times and all hardware has been taken lose and tightened and all solder connections have been checked and gone over.  There were some connections I would have fired anyone who worked for me making ones like those.

All new caps all around and all resistors checked and any out of spec replaced.

The coil is tuned as per the manual and years of common sense in working on lots of transmitters.  All wiring has been checked and poor solder corrected right after I first got the TX.

There is arcing from screen grid to grid on high modulation peaks caused by excessive voltages all around.  The 6DE7 pin 9 is too high, again goes back to the silicone modification without proper surge/dropping resistors as noted.

Simple fix is to get the voltages down.  Yes a bucking transformer will work and have tried and yes drops to reasonable ICAS/CCS ratings of the final which is where I want to be.  With key down for a long QSO those ratings are in excess of tube specifications and the engineer in me says never, never run like that for long term reliability.

I have high wattage resistors on order as there were none (0) put in with the diodes and since the plate resistance drop of the 5U4GB was/is not compensated for I need to compensate for that with resistors in series with the diodes which is the proper arrangement I have been aware of how it should be done for over 50-years of work on all types of equipment and devices.

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« Reply #13 on: March 11, 2019, 03:28:16 PM »


...Drive more than exceeds the recommended grid drive for a 6146 as per specifications...

There is arcing from screen grid to grid on high modulation peaks caused by excessive voltages all around.  The 6DE7 pin 9 is too high, again goes back to the silicone modification without proper surge/dropping resistors as noted.


What screen grid pin was arcing to what grid?

What voltage was measured on Pins 2,3 and 9 of the 6DE7?

The DX-40 is a Controlled Carrier, screen grid modulated transmitter that uses the high power section of the 6DE7 as a Cathode Follower.

In my experience, If the Pin 9 voltage is over 100 volts then these are possible causes:

1) HV B+ too high, needing a dropping resistor (10k 5W) to Pin 1 plate of 6DE7 or a series resistor aft of SS (Silicon Diode) string (150 ohms 10W),

2) V4A is open and not conducting, causing a high grid voltage at Pins 2 and 3 of the 6DE7 resulting in a high cathode voltage at Pin 9,

3) I have seen R18 fried and it became a low resistance, causing a high grid voltage at Pins 2 and 3 of the 6DE7 resulting in a high cathode voltage at Pin 9,

4) V4B shorted cathode to plate (unusual but it has happened)

5) Shorted C30

I have worked on many DX-35 and 40's and I hope this helps.


Phil







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WA3QLW
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« Reply #14 on: Yesterday at 12:49:36 PM »


...Drive more than exceeds the recommended grid drive for a 6146 as per specifications...

There is arcing from screen grid to grid on high modulation peaks caused by excessive voltages all around.  The 6DE7 pin 9 is too high, again goes back to the silicone modification without proper surge/dropping resistors as noted.


What screen grid pin was arcing to what grid?

What voltage was measured on Pins 2,3 and 9 of the 6DE7?

The DX-40 is a Controlled Carrier, screen grid modulated transmitter that uses the high power section of the 6DE7 as a Cathode Follower.

In my experience, If the Pin 9 voltage is over 100 volts then these are possible causes:

1) HV B+ too high, needing a dropping resistor (10k 5W) to Pin 1 plate of 6DE7 or a series resistor aft of SS (Silicon Diode) string (150 ohms 10W),

2) V4A is open and not conducting, causing a high grid voltage at Pins 2 and 3 of the 6DE7 resulting in a high cathode voltage at Pin 9,

3) I have seen R18 fried and it became a low resistance, causing a high grid voltage at Pins 2 and 3 of the 6DE7 resulting in a high cathode voltage at Pin 9,

4) V4B shorted cathode to plate (unusual but it has happened)

5) Shorted C30

I have worked on many DX-35 and 40's and I hope this helps.


Phil





TNX everyone,

I got the B+ down to 650V and feel better now so will start on the modifications all of you have posted.  I can see the reasons for the modifications from a technical reason in all that you all attached and they make sense.
At least as of last night, not smoking that resistor so taming the B+ helped and now to move forward.

Leslie



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