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Globe Champ 300A problem




 
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kd1nw
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« on: November 03, 2018, 08:42:27 AM »

Hi Folks,

I'm trying to track down a problem with a Globe Champ 300A and need some advice on where to look next. It all started with choke CH2 overheating and smoking at bit. I pulled the 3B28 rectifiers to  stop that. Looking further I found that resistor R17 had overheated so badly that it had split. I'm able to stop the R17 replacement from overheating by removing capacitor C27. Thinking it must have been shorted, I substituted it with a known good cap and R17 overheats again. I lifted one end of diode SR3 (1N4007) and SR1/SR2 (8V zener) and they test okay. I'm also getting -2V not -8V coming from R16 (4700 ohm resistor). These parts are located on the schematic in the area between the 6CL6 and 2E26. I hope I described that okay and thanks for any help.

Bama link to schematic:
http://bama.edebris.com/manuals/wrl/champ300a/

Thanks and 73
Kevin KD1NW
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w4bfs
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« Reply #1 on: November 03, 2018, 09:19:35 AM »

offhand sounds like ch2 has shorted... if r17 is the bleeder and is overheating itis because the shorted choke will raise the output voltage up to 50% ....  check it and see
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Beefus

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kd1nw
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« Reply #2 on: November 03, 2018, 12:41:33 PM »

Thanks Beefus,

I disconnected ch2/ch2a from the center tap connection and R17 still overheats. I also measured the resistance across the parallel pair of chokes and it read 93 ohms. i'm not sure if that's a useful measurement but it wasn't a short.

Thanks
Kevin
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W1RKW
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« Reply #3 on: November 03, 2018, 02:05:19 PM »

SR3 is a 1N4007??  Wouldn't a selenium rectifier have a larger voltage drop than a silicon rectifier diode?
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Bob
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kd1nw
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« Reply #4 on: November 03, 2018, 02:24:55 PM »

Hi Bob,

I replaced the seleniums a few years back when I repaired the radio. Yes, selenium would have a larger voltage drop. It looks like this area sets the negative bias for the drive buffer and the 809's. I didn't put a dropping resistor after the diode, the radio worked pretty good for a few years. Are you think it was definitely required?

Thanks
Kevin
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Tom WA3KLR
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« Reply #5 on: November 03, 2018, 03:22:57 PM »

I never owned one, but just curious.  I looked at the schematic and the R17 is in the negative bias circuit for a number of tubes including the modulators and finals.  I think this is the source of the problems and the resulting high plate current in the modulator tubes and finals gives the CH2 symptom.  Check the other capacitors in the bias circuitry for low resistance to ground.  There may be some other item pulling the bias towards ground, like a filament to grid short in one of the modulator tubes.
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
kd1nw
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« Reply #6 on: November 03, 2018, 03:35:38 PM »

Okay thanks Tom, I'll do that and report back probably later tomorrow.

Thanks
Kevin
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W1RKW
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« Reply #7 on: November 03, 2018, 03:37:29 PM »

Hi Kevin,
My question was more cursory in nature but depending on the selenium rectifier type and whatever voltage drop it may have had it is a possibility or a factor however, you do state that all was working when the rectifier was replaced with a silicon type.

Tom/KLR makes a good suggestion on a bias issue and excessive current draw.  

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Bob
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Jim, W5JO
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« Reply #8 on: November 03, 2018, 04:23:04 PM »

 Lift the end of R 18 that is tied to voltage and see what happens.  If the voltage is still below -8 then the problem is probably in the interstage transformer.  What is the condition of T4?  Any shorts to the primary or to ground.  What happens when you pull the modulator tubes?
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kd1nw
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« Reply #9 on: November 03, 2018, 05:25:14 PM »

Okay thanks Jim, Ill try R18 tomorrow. Sorry I didn't mention I pulled the modulator tubes already. The secondaries of T4 measure about 265 ohm across the windings and from a winding to center tap measures 127 ohms and 137 ohms respectively. No primary shorts, it looks good.

Thanks
Kevin
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WBear2GCR
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« Reply #10 on: November 03, 2018, 06:42:23 PM »

Looking at the schematic, R17 is a current limiting resistor before the half-wave rectifier.
With the cap to ground, that junction should be over -100vdc.
The cap ought to be a 200vdc rated cap or higher.
The other resistors are too small to draw enough current to over heat the R17.

Assuming the static (no voltage present) resistance reading for the two other resistors,
R16, R18 are ok and not shorted or very very low resistance, the circuit must work to
produce the ~-100vdc bias voltage.

The zeners, IF they work will force the -8vdc, UNLESS too much current is being drawn on
the other end of that line. C23 though, could be leaky and drop that voltage down also.
Observe the "reversed" polarity of that filter cap if replacing.

This biases the modulator tubes - which will work ok even with -2vdc on the grids, but will
draw a bit more quiescent current.

You may have more than one problem going on at the same time.

Also in my Glub Chump 350, all the "couplates" were NG.

                                _-_-bear
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kd1nw
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« Reply #11 on: November 04, 2018, 01:16:12 AM »

Okay Bear, thanks,

Voltage at the point before SR3 is 96 VAC and 92 VDC. Voltage after after SR3 and before R16 is 72 VAC and -30VDC. C27 before i took it out is 22u/f @ 450V.   Other side of R16 is -2 volts which is too high.  Also, I replaced C23 with known good 1000uF because I removed the 22uF to test. No Difference.

Thanks Kevin
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kd1nw
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« Reply #12 on: November 04, 2018, 07:00:13 AM »

Hi,

I lifted R18 and the same problem. With C27 in circuit R17 cooks. Out of circuit R17 is fine

Thanks
Kevin
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kd1nw
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« Reply #13 on: November 04, 2018, 07:31:45 AM »

one by one i removed 6AQ5, capacitor C61 and center tap of T4 and no change, the same issue
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WBear2GCR
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« Reply #14 on: November 04, 2018, 01:00:21 PM »

Maybe the zeners are toast?

So, anyhow, remove both the resistors (one end) going to the zeners, and the other side.
Get the -100vdc (nominal) running free of ripple (the cap is good and proper polarity and works).
Then connect the side to the zeners, with the modulator tubes removed. The voltage should
be exactly the voltage of the zener voltage. If not repair.

Plug in modulator tubes - if they are shorted, for example, the voltage will go "wrong".
If the voltage holds...

Then do the other side.

Regardless the only thing that can possibly smoke R17 would be a shorted cap or the equivalent
low resistance path.

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Tom WA3KLR
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« Reply #15 on: November 04, 2018, 01:09:57 PM »


With C27 in circuit R17 cooks. Out of circuit R17 is fine

Are you skipping past the bad C27?
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
kd1nw
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« Reply #16 on: November 04, 2018, 01:48:43 PM »

Okay thanks Bear Ill work on that.

Hi Tom, Yes, I removed C27 to keep R17 from cooking. Thinking C27 was bad, I put in another cap and R17 overheats. So for now I'm just leaving it disconnected to be able to work on the radio

Thanks
Kevin
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WBear2GCR
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« Reply #17 on: November 04, 2018, 05:24:36 PM »

You put the cap in with reverse polarity??
It's a 250vdc rated cap or better?

If it's just the resistor, diode and the cap, what's there to draw current??
Something. Maybe mechanical, as in a solder glob/short??
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kd1nw
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« Reply #18 on: November 10, 2018, 05:20:46 PM »

Hi All,

Problem fixed. It was SR3. Even though it tests good on the diode tester, I replaced it and all problems went away. I have -7.4 VDC bias, R17 is nice a cool and CH2 is cool and not smoking. Now I need to get a 12AX7 and 12AU7 and I'll be good to go. Thank you everyone for taking the time to help me!

73 - Kevin KD1NW 
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W1RKW
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« Reply #19 on: November 10, 2018, 06:17:13 PM »

Nice work Kevin.
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Bob
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His fear was when I turned it on for the first time life on earth would come to a stand still.
kd1nw
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« Reply #20 on: November 10, 2018, 06:27:34 PM »

Thanks Bob, not sure if you remember me. A few years back I came over and got that S40A, the VHF RX and SB301. I still got them, I use the S40 all the time as a bedside RX  Smiley
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W1RKW
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« Reply #21 on: November 10, 2018, 06:49:36 PM »

Hi Kevin,
Yes, I do remember.  Glad the radios are working good for you and they found a good home.  I never was able to find anything about that VHF receiver. I'm happy you got it working. 

Bob
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Bob
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Home of GORT. A buddy of mine named the 813 rig GORT.
His fear was when I turned it on for the first time life on earth would come to a stand still.
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