Hammarlund HX-50 final amp problem

[K9DXL]

About two years ago, I resurrected and recapped an HX-50A. It was working fine until a couple of months ago when it failed to make any power except for about 3-4 watts in the CW mode only. The VFO an low level stages appear to be working, as the signal can be heard in my receiver, and varies in strength with the setting of the transmitter's drive control. In normal operation, with the drive set to 180 MA cathode current, output will be in the neighborhood of 60 watts. Now, however, as the drive control is advanced from the minimum setting (showing 50 MA resting bias, which is normal) only a slight increase in cathode current is possible, and as I noted above, only a few watts output result.

I have swapped the final tube (6DQ5) with a spare and also swapped the driver (12BY7) with no change. Resistance measurements from the final tube socket are in the ballpark for a 60 year old piece of equipment.  Visual inspection found nothing burned, broken loose, or otherwise likely to be a problem.

A major discrepancy was found while checking the voltages at the final, however. Pin 1 and 5, spec. -72 vdc, measured -66. Pins 4 and 8, spec. 180 vdc, measured 187. But pins 3 and 6, spec. 42 vdc., measured 0.

A portion of the schematic is attached. I'm not sure how the expected voltage there is derived. Your suggestions are appreciated!

[WD8BIL]

Jim, from just this snippet, lift C192 and see ifn the voltage comes back.

[Pete, WA2CWA]

A major discrepancy was found while checking the voltages at the final, however. Pin 1 and 5, spec. -72 vdc, measured -66. Pins 4 and 8, spec. 180 vdc, measured 187. But pins 3 and 6, spec. 42 vdc., measured 0.

A portion of the schematic is attached. I'm not sure how the expected voltage there is derived. Your suggestions are appreciated!

I would start by checking the DC resistance from pins (3, 6) to ground. Should be 10 ohms. As Bud mentioned, check C192 and C215 by-pass caps for shorts or leakage.

[WD8BIL]

Also Jim, the schematic on BAMA isn't real clear and my guess is that voltage is actually closer to 4.2VDC. The resistance chart shows 20 ohms at pins 3 & 6. With plate current at 200ma that'll give 4 VDC at pins 3&6.

To get 42VDC there you would need have 4.2 amps thru R183 (10 ohms). The 6DQ6 would croak fast!

The voltage is derived by the cathode current flowing thru that resistor and the choke L114.

[Pete, WA2CWA]

Also Jim, the schematic on BAMA isn't real clear and my guess is that voltage is actually closer to 4.2VDC. The resistance chart shows 20 ohms at pins 3 & 6. With plate current at 200ma that'll give 4 VDC at pins 3&6.

To get 42VDC there you would need have 4.2 amps thru R183 (10 ohms). The 6DQ6 would croak fast!

The voltage is derived by the cathode current flowing thru that resistor and the choke L114.

I saw the 20 ohms listed on the resistance chart. From Pins 3/6, there is a 10 ohm resistor to chassis ground. Is it Hammarlund magic that the measured resistance to chassis ground is 20  ohms?

[K9DXL]

Yes, the cathode to ground resistance is indeed 10 ohms. The 20 ohm discrepancy is probably due to production changes and/or the transition from the HX-50 to the "A" version. Or it might just be some Hammarlund magic.

I lifted one side of C192 and now measure .6v. The output problem is unchanged.  Looking closer at my copy of the manual, what I thought was "42" really must be 4.2. There's a space between the two digits, but the decimal just didn't show up in the copy.

Still poking around in her innards.

73,
Jim

[Pete, WA2CWA]

Yes, the cathode to ground resistance is indeed 10 ohms. The 20 ohm discrepancy is probably due to production changes and/or the transition from the HX-50 to the "A" version. Or it might just be some Hammarlund magic.

I lifted one side of C192 and now measure .6v. The output problem is unchanged.  Looking closer at my copy of the manual, what I thought was "42" really must be 4.2. There's a space between the two digits, but the decimal just didn't show up in the copy.

Still poking around in her innards.

73,
Jim

I have all the early HX-50 docs and schematics and the resistance charts in all of them says 20 ohms.
When the HX-50 was created and hit the market window before it was really ready, there were lots of problems. Hammarlund then created a series of service addenda to correct stuff still on the assembly line and what was already out in the field. When the HX-50A came out, documentation included was the original HX-50 manual (without all the service addenda changes) and another set of sheets to document the changes made to make it a HX-50A. So it's conceivable that all HX-50A's that hit the market had the majority of the service addenda changes that Hammarlund documented for the original HX-50. One would hope.

In looking at the original voltage chart page, it is 4.2V on pins 3 and 6. The perils of looking at copies of copies on the internet. 

[K9DXL]

Pete, I've heard that the HX-50 gave the Hammarlund folks some trials. One thing I've noted, comparing the schematics of the 50 vs. the 50A is that r.f. choke L-114 was eliminated. The updated parts list for the "A" lists L-113 then skips to L-115. Also, the "A" schematic shows only one VR tube, but there are in fact, two.

But I'm still wondering how the cathode to ground path is supposed to measure 20 ohms with just the 10 ohm resistor in the path. None of the documentation was corrected then?  

Jim

[KA2DZT]

That voltage is most likely suppose to be about four volts not 42 volts.  It is cathode bias coming from the 10ohm resistor.  The resistor is there to provide a voltage to the plate current meter circuit through the 4.8K series resistor that is shown on the schematic.

[WD8BIL]

I lifted one side of C192 and now measure .6v. The output problem is unchanged.

There is also C215 off of pin 3 and C198 bypassing the 10 ohm as the schizo shows. Lift those too.

If that does not bring the voltage back it would then appear RF drive is not getting to the grid.

[K9DXL]

The answer!
I put this project aside for a few weeks to do other things. Returning to it yesterday, I started thinking of other areas to look at. While testing the rig, I noticed that I heard a definite "click" from relays K101 and K102 (at the rear of the chassis), but not from K103, which is located near the front. So I pried the plastic cover off, and noted that the relay didn't pull in when the TX was switched from Standby to MOX. Hmmm... voltage at the coil terminals, check. Coil continuity, check.
So... wrapped a plastic wire tie "noose" around the armature and gave it a gentle pull when I switched the rig into transmit--and it came to life! Then I tried several cycles on and off, and the relay worked every time without any help from me.
I did notice when I took the cover off, there was a tiny piece of loose plastic in there, about the size of a grain of rice.
Perhaps, it was the culprit for the stuck relay?  I don't know, but I'll be keeping an eye on it.
I'd been concentrating on the final stage, concerned that I could only get a couple of watts out of the rig. Apparently it was just leaking through from the driver. It just goes to show, that sometimes we can be so sure a problem exists in a certain area, that we overlook other possibilities.
Thanks to all of you who offered suggestions. I wanted to bring this post up to date and let everyone know the problem has been resolved!
73, Jim