ka4koe
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« on: March 31, 2013, 02:36:47 PM » |
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I've hooked up my EV368 and I'm not getting any audio output. Getting good level of carrier and modulator current when I advance the mic gain; just no audio while monitoring on a receiver.
The mic element measures around 1200 ohms on a VOM.
Any suggestions as to where I should start looking?
Tnx
Philip
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W2VW
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« Reply #1 on: March 31, 2013, 02:52:20 PM » |
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So when you talk the modulator current increases?
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ka4koe
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« Reply #2 on: March 31, 2013, 02:59:17 PM » |
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No
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W2VW
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« Reply #3 on: March 31, 2013, 03:11:13 PM » |
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What's the static modulator current read?
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ka4koe
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« Reply #4 on: March 31, 2013, 03:24:32 PM » |
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Checked the plug in octal on a lark. Pins 2 and 6 are not even wired together.
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ka4koe
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« Reply #5 on: March 31, 2013, 04:09:17 PM » |
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pin 2 inside the chassis isn't even hooked up. This may be a modded transmitter.
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ka4koe
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« Reply #6 on: March 31, 2013, 04:22:48 PM » |
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Quiescent modulator current is around 55 mA, using the middle scale.
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W2VW
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« Reply #7 on: March 31, 2013, 05:06:44 PM » |
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Sounds like some careful tracing ahead. Maybe they were using the phone patch input for audio feed. More than one problem. Static mod current and no increase with audio applied plus the pin 2 and 6 deal.
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ka4koe
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« Reply #8 on: March 31, 2013, 10:19:44 PM » |
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Pins 2 and 6 jumpered internally after closer examination. Mod resting current rises with advancing the audio control at which point the current will peg the meter. No audio on carrier regardless. The DC Z of the EV368 measured at around 1200 ohms. Seeing some very low level hash on an oscilloscope when the element is hooked up while connected only to the instrument.
Thinking maybe something may have gone awry during transit.
Philip
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VE3AJM
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« Reply #9 on: March 31, 2013, 11:40:36 PM » |
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Do you have another known good microphone that you can try and use with the transmitter?
Al VE3AJM
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ka4koe
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« Reply #10 on: March 31, 2013, 11:46:52 PM » |
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No, but may try feeding audio in via the phone patch input.
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ka4koe
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« Reply #11 on: April 01, 2013, 09:32:30 AM » |
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My EV638 has the impedance transformer inside the head. Now whether this is intended to step up or step down the input Z, I don't know. Likely, its a step DOWN. So, I may be feeding the input of the Valiant with a 150 OHM Z microphone, which MAY explain why I'm not getting audio output on the modulation meter.
I can't find technical documentation telling me one way or the other.
The 638 worked fine on my Flex 5K, so I'm fairly confident its okay.
Opinions?
Philip
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KB2WIG
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« Reply #12 on: April 01, 2013, 01:29:52 PM » |
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FWIW, http://www.coutant.org/ev638/ev638.pdfGot a scope or a VTVM? Check watt happens load and no load on the mic. Then/Or try the mic accross pin 8 of V12. klc |
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Pete, WA2CWA
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« Reply #13 on: April 01, 2013, 02:21:45 PM » |
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As per the specs: If the cable from the mike has two conductors plus shield, it's low impedance. If it has one conductor plus shield, it's high impedance.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
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ka4koe
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« Reply #14 on: April 02, 2013, 12:20:45 AM » |
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Visual Inspection: There is a mica cap C84 hanging loose off of audio clipper V13 pin 1, 0.01 uF. It connects to L45 then onto the grid of third audio V14, pin 6. This cap may have broken loose from one of its anchor points during moving the unit to my domicile. The end of it is just hanging in space. Anyway, this may be the problem. I've been looking closely underneath for things that don't look right and this stood out. No new ZORCH marks.
If I'm reading this schematic properly, the third audio is driven by its grids from the clipper output at the cathode of that tube.
I think this will likely solve my no modulation problem. Opinion?
Philip
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w8fax
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« Reply #15 on: April 02, 2013, 06:06:39 AM » |
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Speaking of the clipper, remember that the control is reverse from what is intuitive. Fully clockwise is actually NO clipper, which is where most ops run. I speak from frustrated experience....lol....Al/W8FAX
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KK4YY
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« Reply #16 on: April 02, 2013, 09:32:08 AM » |
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Phillip,
C84 would be needed for the original circuit to work. In your photo it looks like its lead was cut and there is a white wire, going to the terminal where C84 was removed from, that doesn't look original. A previous owner may have by-passed the clipper circuit. Trace that wire back and see where it goes. Maybe draw a schematic of the circuit "as is" if it differs from the original.
Don
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ka4koe
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« Reply #17 on: April 02, 2013, 10:17:50 AM » |
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yep. Thats what happened. Scratching my head. No zorch marks anywhere. Checked power resistors, pots, etc. in that area and they look fine. Secondary of mod trfmr between pins 2 and 3 check under 100 ohms. I haven't checked the driver transformer yet.
PAN
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ka4koe
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« Reply #18 on: April 03, 2013, 12:08:23 AM » |
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Poked around with the scope tonight. No audio anywhere. I am about 73.22% certain it is indeed the microphone which I had just rewired the other day. I never make mistakes of this kind......Yeah, right. Fed audio in via a haywire arrangement to the phone patch input and did see deflection on the mod meter and heard some audio on a receiver.
So maybe nothing MAJOR is wrong.
About 8.2 on the DUH scale.
Philip KA4KOE
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ka4koe
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« Reply #19 on: April 03, 2013, 12:39:37 AM » |
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This EV368 sounds awful. Listening on a Flex 5K on record.
Philip
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Pete, WA2CWA
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« Reply #20 on: April 03, 2013, 03:32:33 AM » |
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Maybe it's mad at you because you keep calling it EV368.
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VE3AJM
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« Reply #21 on: April 03, 2013, 07:30:07 AM » |
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yep. Thats what happened. Scratching my head. No zorch marks anywhere. Checked power resistors, pots, etc. in that area and they look fine. Secondary of mod trfmr between pins 2 and 3 check under 100 ohms. I haven't checked the driver transformer yet.
PAN
Have you checked all the tubes in the audio section? No audio/modulator current other than the resting current, when you plug the mic into the transmitter going to the 12AX7 first audio stage, but some when audio is injected into the phone patch input? Al VE3AJM |
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ka4koe
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« Reply #22 on: April 03, 2013, 07:43:13 AM » |
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It has 50 mils of resting current. The audio you're hearing is from the mic jack.
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VE3AJM
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« Reply #23 on: April 03, 2013, 07:48:02 AM » |
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Fed audio in via a haywire arrangement to the phone patch input and did see deflection on the mod meter and heard some audio on a receiver.
So maybe nothing MAJOR is wrong.
About 8.2 on the DUH scale.
Philip KA4KOE
OK. I was just going with what you had written. Good luck with it. Al VE3AJM |
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ka4koe
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« Reply #24 on: April 03, 2013, 08:04:02 AM » |
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Measured about 1VDC across the line audio terminals. Don't know if thats enough to be an issue.
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