View Full Version : Drake L4-B Question....
I have been using an L4-B for the past several years. It works great, easy kilowatt out with about 60 watts in from IC746. My question is: Why are my received signals attenuated about 1 1/2 S units when the amp is in operate mode? Does anyone know how to prevent or rectify this situation? Unfortunately, I am not an electronics expert. Thanks ....
Bob, KS5L
WB2WIK
02-21-2011, 03:06 AM
Two possibilities:
1. Your amplifier is really not switching back to "receive," and the internal T-R relay is hanging in the "transmit" condition. When you're receiving, is the transmit lamp still illuminated?
2. The internal T-R relay contacts are pitted, corroded, dirty, oxidized, or misaligned so they don't allow signals to make it through the relay without being attenuated. This wouldn't be unusual after "several years," relay contacts don't last forever. This could be an easy fix (like just wiping a dollar bill or similar through the receive relay contacts to clean them), or a slightly more difficult fix (replacing the T-R relay entirely). If you want to clean the relay contacts, never use a file or an emory cloth or sandpaper or anything like that. I use a dollar bill (or if you're rich, use a $100 bill), which is abrasive enough to do the job fine. A piece of regular paper wetted with WD40 also works. Never use anything abrasive to clean relay contacts!
Thanks for the suggestions. I just recently had some work done on the amp and neglected to mention this little problem. Until now, I have just dealt with it by putting the amp on standby while I am receiving.
WW3QB
02-21-2011, 03:50 AM
Thanks for the suggestions. I just recently had some work done on the amp and neglected to mention this little problem. Until now, I have just dealt with it by putting the amp on standby while I am receiving.
This tells me that the amp is not being keyed correctly. How are you keying the amp with the 746?
Using an Ameritron ARB-704 relay that came with the amp. It appears to be functioning fine.
I agree with Steve... the relay contacts are likely dirty. Clean with paper, or a relay burnishing tool, which can be had for 4 or 5 dollars, or less.
Coating contacts with DeoxIT Gold wouldn't be a bad idea either. If you soak a stiff paper strip or burnishing tool with the DeoxIT Gold, the paper / tool combo would clean and lube the contacts without leaving too much behind.
It works for me... :)
Steve -- NL7W
Two possibilities:
1. Your amplifier is really not switching back to "receive," and the internal T-R relay is hanging in the "transmit" condition. When you're receiving, is the transmit lamp still illuminated?
2. The internal T-R relay contacts are pitted, corroded, dirty, oxidized, or misaligned so they don't allow signals to make it through the relay without being attenuated. This wouldn't be unusual after "several years," relay contacts don't last forever. This could be an easy fix (like just wiping a dollar bill or similar through the receive relay contacts to clean them), or a slightly more difficult fix (replacing the T-R relay entirely). If you want to clean the relay contacts, never use a file or an emory cloth or sandpaper or anything like that. I use a dollar bill (or if you're rich, use a $100 bill), which is abrasive enough to do the job fine. A piece of regular paper wetted with WD40 also works. Never use anything abrasive to clean relay contacts!
WW3QB
02-21-2011, 11:57 PM
He can receive well when he puts the amp in standby. That's switch S4 which opens the circuit to relay RY1. The VOX jack J3 for keying also opens that same circuit (they are in series). So something is keeping the amp keyed that only switching S4 open unkeys it.
Could very well be... But with signals being down 1 to 1.5 S-units (6 to 10 dB), it seems unlikely. One never knows though; we're TS'ing from afar. :)
He can receive well when he puts the amp in standby. That's switch S4 which opens the circuit to relay RY1. The VOX jack J3 for keying also opens that same circuit (they are in series). So something is keeping the amp keyed that only switching S4 open unkeys it.
WW3QB
02-22-2011, 04:37 AM
The think to try is to remove the plug from J3 while receiving and the amp is not in standby. If the signal improves when it is out, we know the problem.
Thank you all for your feedback. WW3QB hit the nail on the head. A little further research on my part informed me that the polarity on the vox jack was likely reversed and would show a rise in plate current while in operate mode. I reversed the plug, and voila, no plate current and no more received signal attenuation. Problem solved.
WB2WIK
02-22-2011, 07:58 PM
Thank you all for your feedback. WW3QB hit the nail on the head. A little further research on my part informed me that the polarity on the vox jack was likely reversed and would show a rise in plate current while in operate mode. I reversed the plug, and voila, no plate current and no more received signal attenuation. Problem solved.
That "rise in plate current" was caused because your amplifier was keyed the whole time it was on and in the OPERATE position, which allows voltage to be provided to the internal T-R relay. Switching to STANDBY breaks that connection.
Glad you found the problem, which was simply that you had the active pin grounded on the RELAY line to the amp; that occurred due to it being plugged in backwards -- the "other" wire is ground.
That's great!
So the PTT (vox) plug polarity was reversed? It was keyed with bias applied to the tubes while in "operate" position -- without the radio actually keying the amp. That threw me for a loop... not seeing it or the schematic.
Best of luck with it. Not following the thread too closely, I take it isn't damaged, being the source RF radio saw the antenna "bypass" when you applied RF to it? Was that the case on transmit? Thanks.
Thank you all for your feedback. WW3QB hit the nail on the head. A little further research on my part informed me that the polarity on the vox jack was likely reversed and would show a rise in plate current while in operate mode. I reversed the plug, and voila, no plate current and no more received signal attenuation. Problem solved.
WB2WIK
02-22-2011, 11:39 PM
That's great!
So the PTT (vox) plug polarity was reversed? It was keyed with bias applied to the tubes while in "operate" position -- without the radio actually keying the amp. That threw me for a loop... not seeing it or the schematic.
Best of luck with it. Not following the thread too closely, I take it isn't damaged, being the source RF radio saw the antenna "bypass" when you applied RF to it? Was that the case on transmit? Thanks.
The rig would see the amplifier input on transmit, and it should work fine. It was just "hung up" in transmit, not allowing the antenna to be connected to the receiver.
So the receiver was connected to the tuned cathode input network in the amp, on receive (and the pair of 3-500Zs were working as a lossy preamp:p).
This wouldn't damage anything unless the 3-500Zs decided to oscillate and blow up the receiver. That would be very exciting.
Thanks. A most weird problem! The things you see...
Yeah, the radio receiver would see the amp's input circuit (a tuned input on the L4-Basher -- didn't know it had one?), because it was continually "keyed". On transmit, it was also keyed... Good grief. ;)
Good catch, Steve!
While he's in there, it would be good to clean the relay contacts... while he's at it.
Of course, before poking any appendages inside, letting it sit for awhile, then shorting out the HV supply after the caps supposedly bleed down -- just to be safe. I assume it has a HV shorting interlock? I don't trust them. :)
I have an old military HV shorting stick I absconded years ago for just that purpose. ;)
The rig would see the amplifier input on transmit, and it should work fine. It was just "hung up" in transmit, not allowing the antenna to be connected to the receiver.
So the receiver was connected to the tuned cathode input network in the amp, on receive (and the pair of 3-500Zs were working as a lossy preamp:p).
This wouldn't damage anything unless the 3-500Zs decided to oscillate and blow up the receiver. That would be very exciting.
