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Apache B+ dropping




 
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kd1nw
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« on: December 30, 2017, 09:38:31 AM »

Hi Folks,

I'm having an issue with my Apache with B+ dropping as the radio warms up. After warm up, I measured HV with both the radio's meter (640 volts) and at the 5R4 around 600V. B+ is around 730 when the radio hasn't fully warmed up. I found this while trying to figure out why the modulator resting current was slowly rising to 250ma over the course of a couple of 1 to 2 sec transmissions. I tried known good 5R4 rectifiers and the same thing. Filament on the rectifiers is fine 5V and I measured about 110V on the primary of the HV transformer. The only thing left is the secondary of the HV transformer which I didn't measure because my meter only handles up to 750VAC. I'm assuming that as the transformer warms it increases it's resistance and the B+ drops? Am I on the right track here? I can swap in a good transformer from a DX100 but wanted to check before I go through the work of swapping.

Thanks 73 - Kevin
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DMOD
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« Reply #1 on: December 30, 2017, 10:18:02 AM »

Some questions for clarification.

Q1: What do mean by "warm-up?" Immediately after Keydown or...?

Q2: Are the Filter caps new or original?

Q3: How old is the 6AQ5 clamper tube and has this stage been adjusted?

Q4: Have the OB2 regulators ever been replaced?

Q5: Have all of the tubes been checked for leakage after warm-up on a tube checker?


Phil - AC0OB

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K4RT
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« Reply #2 on: December 30, 2017, 10:21:00 AM »

What is the B+ dropping to?  Are you measuring the B+ at idle or under load?

The first thing that comes to mind is that as the HV transformer warms up some damaged interior insulation opens up, allowing turns of the secondary to short, effectively reducing the number of secondary turns and lowering the output.  I could be all wet, but that's the first thing that comes to mind.

As the B+ drops, what is happening with the voltages at the modulator tubes?
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kd1nw
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« Reply #3 on: December 30, 2017, 12:09:15 PM »

Thanks Phil and Brad. I tried to answer all your questions below. It does sound like what Brad has described


Q1: What do mean by "warm-up?" Immediately after Keydown or...?
    warm up meaning turning the radio on and letting it warm up. after it warms up a min or so make a 1 to 2 second tx into a dummy load. sorry i wasn't really clear. if i turn the radio on and let it warm about a min i get full B+ then over the course of 2 to 3 mins its starts falling, measuring by briefly transmitting. i also tried just letting the tx warm up for about 5 to 10 mins with no transmission and i never measure normal B+, by that point the radio is fully warmed up and the B+ has already fallen to about 600 volts

Q2: Are the Filter caps new or original?
    All filter caps are new

Q3: How old is the 6AQ5 clamper tube and has this stage been adjusted?
    The 6AQ5 is the one that came with the radio. i did adjust the clamper stage.

Q4: Have the OB2 regulators ever been replaced?
    OB2 are the same that came with the radio

Q5: Have all of the tubes been checked for leakage after warm-up on a tube checker?
    No, i didn't check all the tubes but i can

Q6: What is the B+ dropping to?  Are you measuring the B+ at idle or under load?
    It's dropping from about 730V to about 600V. Im measuring B+ under load. transmitter has been tuned up into a dummy load. it had been previously running okay at least long enough for me to tune the transmitter properly. 

Q7: As the B+ drops, what is happening with the voltages at the modulator tubes?
    All the voltages at the modulator tubes looked okay except for the plate voltage on the EL34. it drops as described above and the resting mod current rises. Screen is 350V, bias is -33V. Both screen and bias voltages stay steady at all times.

Thanks 73
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« Reply #4 on: December 30, 2017, 12:42:52 PM »

With the Finals' removed. at this point I would check the bias voltages right at Pins 5 of the mod control grids.

If one or more of the 1k's (R31,32) are increasing in value as heating occurs, then the bias voltage would go more positive (less negative) increasing mod tube current.

If you have two DMM's, monitor the plate voltage and the grid voltages at the same time.

I have an Apache that is working fine but needs updated caps so I'll measure the Mod transformer winding resistances.

Addendum: After my morning workout,  Shocked I measured the primary resistance of the Modulation transformer and it averages 368.5 ohms.

Phil - AC0OB
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« Reply #5 on: December 30, 2017, 01:15:03 PM »

Have you checked or replaced the HV rectifiers??
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kd1nw
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« Reply #6 on: December 30, 2017, 03:30:08 PM »

Hi Guys,

The voltage at pin 5 is -33V and never changes, and I tested with known good rectifiers with no change.

Thanks 73 Kevin
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kd1nw
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« Reply #7 on: December 30, 2017, 03:45:37 PM »

Oh yeah sorry Phil, my mod primary measured 359 ohms
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« Reply #8 on: December 30, 2017, 04:07:36 PM »

What is the B+ suppose to be under normal load?? Do you know??

Fully warmed up means what??  As the tubes warm up they draw more current, but if the voltage is dropping without the xmtr being in transmit probably means something is not right.

One possibility is if one side of the HV xfmr secondary is open, you would see normal HV with a light load and would drop under heavier loads.   The HV rectifier would only be rectifying half of the sine wave.
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kd1nw
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« Reply #9 on: December 30, 2017, 04:40:34 PM »

Hi Fred,

The manual says 750V at the 5R4 with plate power on. fully warmed means the point after initially turning on any power to the transmitter and there are no other changes in plate voltage. unfortunately i don't have a thermometer to measure transformer temps etc, so i can't plot change of voltage versus temp.

Thanks 73 Kevin
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« Reply #10 on: December 30, 2017, 04:59:53 PM »

Turn the xmtr OFF.  Measure the DC resistance of the HV xfmr secondary winding.  Pull the 5R4.  See if the xfmr secondary winding is not open.  Should measure some DC resistance from each 5R4 plate pin to ground.  The DC resistance on each half of the winding will NOT be exactly the same but close to each other.  Probably each half should measure something less than 100 ohms.
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kd1nw
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« Reply #11 on: December 30, 2017, 05:34:25 PM »

they measure 49.2 and 52.8 ohms
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« Reply #12 on: December 30, 2017, 05:42:27 PM »

Quote
I found this while trying to figure out why the modulator resting current was slowly rising to 250ma over the course of a couple of 1 to 2 sec transmissions.

So the modulator resting current climbs as the Measured B+ drops from 750V to 600V?

Resolder the 0.2 ohm Mod shunt connections in case you have a high resistance and crystallized connection.


Phil
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« Reply #13 on: December 30, 2017, 06:32:25 PM »

they measure 49.2 and 52.8 ohms

Looks normal,  problem is not the HV xfmr.  I'll have to take a look at the schematic if I can find one online.  Last time I saw an Apache was in high school over 50+ years ago.
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« Reply #14 on: December 30, 2017, 07:54:51 PM »

they measure 49.2 and 52.8 ohms

Just measured the Apache Power Transformer secondary on the bench and it measured 60 ohms across and 30 ohms to CT both sides.


Phil

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« Reply #15 on: December 30, 2017, 08:06:12 PM »

they measure 49.2 and 52.8 ohms

Just measured the Apache Power Transformer secondary on the bench and it measured 60 ohms across and 30 ohms to CT both sides.


Phil



Maybe he has a different version of the HV xfmr.  His measurements also look about right.  I'll have to check further to see what his problem could be.
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« Reply #16 on: December 30, 2017, 09:12:19 PM »

Looked at the schematic.  Two 5R4s,  should be about 730 volts DC at the filter caps.  Not sure if that voltage is under transmit conditions or on standby.

If you are only getting around 630 volts on standby conditions, then something is not right.  I guess it would be under transmit conditions since you have to close the plate switch to get any HV.

Can only be a few things.  You may have two weak 5R4s.  Something is loading down the HV way too much.  You're not getting the full 115vac reaching the HV xfmr primary.

Could be a bad filter choke, but bad chokes are rare.  Filter caps maybe.
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« Reply #17 on: December 31, 2017, 02:10:10 AM »

they measure 49.2 and 52.8 ohms

Looks normal,  problem is not the HV xfmr.  I'll have to take a look at the schematic if I can find one online.  Last time I saw an Apache was in high school over 50+ years ago.

He needs to remeasure because that is not correct.

If he is getting 52.8 ohms across the full secondary then he should get approx. 26 ohms from each side of the power transformer to ground.

Kevin needs to put one probe lead on the chassis and the other probe on Pin 4 (measure result) and Pin 6 (measure result) at the 5R4.

The CT of the power transformer goes to ground. Pins 4 and 6 of the 5R4 socket should show about 26.4 ohms each.


Phil - AC0OB

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« Reply #18 on: December 31, 2017, 07:13:24 AM »

I believe he said 52.8 ohms was one side of secondary. I see no one has picked up on the fact that the ac voltage on the secondary would change because of the primary voltage. One of the statements says 110 volts on the primary. That would bring down the secondary alot. Although I dont believe he has a bad transformer as shorted turns in the secondary would most likley cause a fuse to blow. Quick check on the transformer. Disconect all secondary leads, put 100vac, using a variac, on one side of hv winding to center tap. Now measure the other side. It should be the same. My next step would be to lift the B+ lead on the output of the supply. Toss the dc volt meter onto the the supply output and turn on the rig. See if voltage drops. If not, reconnect the lifted lead and lift the B+ at another point. Continue til you find the stage the problem is in. Good luck.
Regards,
TRS
Gary
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« Reply #19 on: December 31, 2017, 09:29:32 AM »

Thanks for the help everyone. I broke my multimeter when I turned off the plate switch. I'll have to get back to it when I get a new meter.

Thanks and 73 Kevin
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« Reply #20 on: December 31, 2017, 12:21:43 PM »

I'm voting for gassy modulators, el34 I believe ... seems as their Q point is varying
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DMOD
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« Reply #21 on: December 31, 2017, 12:23:18 PM »

Hi Folks,

I'm having an issue with my Apache with B+ dropping as the radio warms up. After warm up, I measured HV with both the radio's meter (640 volts) and at the 5R4 around 600V. B+ is around 730 when the radio hasn't fully warmed up. I found this while trying to figure out why the modulator resting current was slowly rising to 250ma over the course of a couple of 1 to 2 sec transmissions. I tried known good 5R4 rectifiers and the same thing. Filament on the rectifiers is fine 5V and I measured about 110V on the primary of the HV transformer....

Thanks 73 - Kevin

If the Modulator resting current rises this would pull down the B+ since in Phone mode he now has two loads on the HV B+, the Final and Modulator circuits.

So far it appears the Mod. transformer primary resistance is ok, and possibly the HV transformer's secondary is ok.

If the transmitter's HV B+ does not drop significantly in CW, but only in Phone, then the problem is in the rising of the resting current of the Modulator.

Kevin also needs to tell us what the voltage drop is in CW mode, and test or substitute the EL34's.


Phil - AC0OB
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« Reply #22 on: December 31, 2017, 02:49:57 PM »

Hi Phil,

when i blew the meter i had pulled the el34 and was testing in cw. i was seeing 730v key up and 630v key down. it seemed like the radio was working okay so maybe it is the el34. i wont be able to do more until i get a new multimeter though. the EL34 are new from JJ but maybe there is something wrong with one or both.

Thanks 73 Kevin
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« Reply #23 on: December 31, 2017, 03:22:55 PM »

Not to worry, as I have frapped more than one meter when testing.

For good tubes, try Dave Whitham, K9DQ at:

http://www.electricgurupartshouse.com/


Phil - AC0OB
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« Reply #24 on: December 31, 2017, 03:48:06 PM »

No clue as to whether this will help or not, but I had an sb220 that started doing this.  The voltage drop from rx to TX was getting worse and worse.

It also had a slight buzzing sound.

Was my first commercial amp, so I just thought, as is!

Over years, it got worse.  I also noticed my voltage drop from idle to TX was getting worse and worse.  Must be time to recap it.

Sure was.  Couple days later, turned it in, plate current meter pegs!

Ended up being the ceramic doorknob bypass cap at the base of the plate choke.

That freaking cap STILL tests good at any cap  Hecker voltage that I have around here.

Get it near a kilo volt, and it instantly avalanches and shorts!



I'm glad I didn't lose an xformer.   Upon seeing my delemna posted here, k1mh had sent me a package of 10 kv caps to fix my amp, gratis.

Hopefully this might help?

--Shane
KD6VXI
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