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Johnson Ranger - Problem Child




 
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June 27, 2022, 08:25:21 AM *
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W1GFH
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« on: June 17, 2022, 08:42:53 PM »

Greetings, I used to post here in the early 2000s when I lived in California and ran a Ranger (brown paint scheme, keyer platform) with the 3870 AM gang. Now I live in Massachusetts. The old Ranger is long gone, but I recently obtained another Ranger of the same vintage and run it barefoot, no amp with my old BC779 receiver. An IC-756 pro is the "modern" rig at my rather modest station.

I've replaced all the electrolytics and out of tolerance resistors and caps. Replaced the VFO Chernobyl, the drive pot and R35 with new units. I'd love to start upgrading to solid state power supplies etc. but can't seem to get the damn thing stabilized in its stock form.

The latest: I was testing the Ranger the other day and when I unkeyed -- the mod current suddenly spiked. Angry buzzing, pilot lamps flashing, and then blown fuse.

Replaced fuse and tested. Passed the dim bulb test. And no blown fuse at full voltage with rectifiers in and mod tubes out.

I checked R35, the voltages (top and tap) are in spec.

The driver xfrmer and mod xfrmer resistance measurements are within spec.

With the mod tubes out, I measured the center tap of the driver xfrmer secondary. I get between -8vdc and -10vdc negative bias voltage. It should be at least -28v. I assume this is why the mod current is spiking.

I dismantled the keyer/bias rectifier platform and checked out resistors, they are in spec. I replaced C90A/B with a new 47uf electrolytic, and replaced V14 with a new 6AL5. No luck.

Pulled the 6AL5 and measured 0 vac to ground at pins 5 and 1 tube socket. I am wondering if the power transformer tap that feeds the cathodes of the 6AL5 (marked Y Y on the schematic) has failed.

Bear in mind my test equipment consists of a digital multimeter (sorry, no scopes, signal generators or injectors).

FYI: this Ranger has spent more time on the bench than on the air: more than a bit frustrating. : (

Schematic: https://www.frostburg.edu/personal/latta/ee/ranger/schematic/rangerschematicfullres.jpg
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w4bfs
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« Reply #1 on: June 17, 2022, 10:25:48 PM »

6al5 may still be suspect.    what is the ac voltage between the 6al5 plates and from each plate to ground.  can't remember exactly but 60V from each to ground seems about right.  if you have this then report and will take next step.   73
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W3SLK
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« Reply #2 on: June 18, 2022, 09:22:56 AM »

One problem with my Ranger I that plagued me was every time I would turn the meter switch, the transformer would buzz and I would blow the fuse(sound familiar?) What it boiled down to was the grommet insulators for the meter studs had become so dry rotted that the 600VDC would arc from the terminal to ground of the TVI meter cover. This generally happened when it was in the PLATE position though. A trip to the hardware store 2 new grommets later and all is good. Good luck.
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W1GFH
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« Reply #3 on: June 18, 2022, 09:39:34 AM »

the grommet insulators for the meter studs had become so dry rotted that the 600VDC would arc from the terminal to ground of the TVI meter cover. This generally happened when it was in the PLATE position though. A trip to the hardware store 2 new grommets later and all is good. Good luck.

I'm not blowing fuses with the mod tubes out. The grommets look dry, but intact. But I do measure continuity between each meter terminal and ground when in MOD position (but not (PLATE position).

6al5 may still be suspect.    what is the ac voltage between the 6al5 plates and from each plate to ground.  can't remember exactly but 60V from each to ground seems about right.  if you have this then report and will take next step.   73

Not possible to get at the 6AL5 pins when the tube is in place due to the upside-down-and-crammed-against the panel positioning of the keyer platform. (I guess I need a 7-pin a test socket like this: https://www.ebay.com/p/1434745751?iid=333598791646)

But with the tube out and measuring at the socket, I get...

Pin 1 to 5  = 6VDC (and 0 VAC)
Pin 1 to GND = 6VDC (and 0 VAC)
Pin 5 to GND = 0 VDC (and 0 VAC)

I rechecked the power transformer voltages and I am getting 630 VDC at the HV filter cap, 329 VDC at the R22 center tap, 345 VDC at the LV filter cap, and about 7 V at the tube filaments. So it appears other taps on the power transformer are working.
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w4bfs
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« Reply #4 on: June 18, 2022, 10:52:40 AM »

a strange one indeed!    I forgot yesterday in first reply that this is a negative voltage (bias) supply SO the transformer taps would go to the cathodes of the 6al5.  with it out does your ohmmeter reading of cathode to cathode resistance check out.  if so then next item of intrest is the filter choke.  it could be leaking to ground.  did you connect + terms to ground on your filter caps ?
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W1GFH
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« Reply #5 on: June 18, 2022, 11:21:29 AM »

a strange one indeed!    I forgot yesterday in first reply that this is a negative voltage (bias) supply SO the transformer taps would go to the cathodes of the 6al5.  with it out does your ohmmeter reading of cathode to cathode resistance check out.  if so then next item of intrest is the filter choke.  it could be leaking to ground.  did you connect + terms to ground on your filter caps ?

Yes, the filter cap + side for the 6AL5 rectifier is connected to ground.

Are you asking I measure ohms between pins 5 and 1 *at the tube socket with no tube in it*?...I get around 15K ohms.
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W1GFH
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« Reply #6 on: June 18, 2022, 06:49:56 PM »

OK, well mystery solved, I plunged into the rats next of wires and found the two rectifier tap Y Y leads from the transformer had been cut and badly spliced and soldered and scotch taped by previous owners. So I cut the leads at the splice and tested voltages there:

DC
Lead to lead: cycling between 1.5 and 2.5 VDC.
From each lead to ground: cycling between 1 and 1.5VDC

AC
Lead to lead: 313 VAC.
From each lead to ground: 140VAC.

So I've got 140VAC now. I don't know what the transformer taps "Y Y" are supposed to be putting out, but since they go to the 6AL5's cathodes, 140V sounds like what the tube wants, yes?

If so, I will rewire the "Y Y" tap from the transformer to the tube with new HV silicon wires.
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« Reply #7 on: June 18, 2022, 11:01:03 PM »

OK, well mystery solved, I plunged into the rats next of wires and found the two rectifier tap Y Y leads from the transformer had been cut and badly spliced and soldered and scotch taped by previous owners. So I cut the leads at the splice and tested voltages there:

DC
Lead to lead: cycling between 1.5 and 2.5 VDC.
From each lead to ground: cycling between 1 and 1.5VDC

AC
Lead to lead: 313 VAC.
From each lead to ground: 140VAC.

So I've got 140VAC now. I don't know what the transformer taps "Y Y" are supposed to be putting out, but since they go to the 6AL5's cathodes, 140V sounds like what the tube wants, yes?

If so, I will rewire the "Y Y" tap from the transformer to the tube with new HV silicon wires.

See page 20 of the Ranger Manual which states the typical voltages to be found at the 6AL5 tube pins.

According to the schematic in the same manual, pins 1 and 5 of the 6AL5 should both have 140Vac on them and pin 4 should have 6.3Vac on it (filament). Pins 2 and 7 should have -135Vdc on them.

So the transformer Y-Y  leads should show at least 280 volts since they are the outer winding of the CT transformer for bias. At 120+Vac from the power line, your readings seem ok.

If you don't have at least -28V to the modulator tubes, then yeah, the modulator tubes will sink a lot of current.

I Solid State the bias circuit as per the schematic below because the 6AL5 seems to be a failure point:


Phil - AC0OB


* 6AL5 Solid State Replacement Circuit.pdf (202.75 KB - downloaded 19 times.)
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K6IC
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« Reply #8 on: June 18, 2022, 11:38:32 PM »

Hi Joe, Welcome back to this Forum.

As a complete aside,  had heard that you moved back East.

Used to listen to you, Trevor, and Dave (K6ESE & WD4PLI) --  The Burbank Wrecking Crew  --  on 3870 AM, more than a decade ago, on weekday afternoons.  'Twas fun!   (I was a KF6, at that time).

Back to regular programming.   73, GL,   have FUN on AM,   Vic
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W1GFH
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« Reply #9 on: June 19, 2022, 10:00:47 AM »

Hi Vic! Yes I remember you on 3870, those were fun times, I do miss that gang. Let's hope for 10M AM to open for coast to coast DX soon.

Hi Phil, yes I'd like to solid state the bias rectifier. AD5X has a similar circuit (attached). He says you can simply remove the tapped adjustable mod bias resistor R22 after doing this. It seems to make sense if you increase the HV filter caps and add equalizing resistors, but I'd like to double check.


* AD5X bias mod.jpg (45.58 KB, 557x450 - viewed 26 times.)
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DMOD
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« Reply #10 on: June 19, 2022, 08:44:39 PM »

Hi Vic! Yes I remember you on 3870, those were fun times, I do miss that gang. Let's hope for 10M AM to open for coast to coast DX soon.

Hi Phil, yes I'd like to solid state the bias rectifier. AD5X has a similar circuit (attached). He says you can simply remove the tapped adjustable mod bias resistor R22 after doing this. It seems to make sense if you increase the HV filter caps and add equalizing resistors, but I'd like to double check.

I prefer the adjustable bias option because it allows you to 1) trim the bias for tubes with different gains when replacing, and 2) to adjust the bias for modern AC input voltages.

AD5X's design is good, it's just that I prefer very low impedance power supplies with lots of filtering to keep the ripple down.

Phil  
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« Reply #11 on: June 20, 2022, 12:20:43 PM »

congrats on finding the problem  Cheesy
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Beefus

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It would from many blunders free us.         Robert Burns
W1GFH
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« Reply #12 on: June 20, 2022, 01:45:20 PM »

congrats on finding the problem  Cheesy

Well, the broken/intermittent transformer bias taps was one of the problems I solved. The other was C59B which was in backwards. Mod bias has returned to normal.

However, now...I have no grid drive and the audio stages don't seem to be working Sad The initial "incident" that blew the fuse and prompted this whole thread may have taken something else out. I'll be checking various voltages. So far I have LV and HV supplies at normal voltages, the R35 tap is delivering normal voltages. Maybe I messed up something in the keyer while I had that 6AL5/12AU7 platform apart. Or some other tube got fried  Huh  : ( I'm almost ready to call Chuck, K1TLI.
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