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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 21, 2018 5:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1032
Location: 98802
Frank,

You are apparently not the first guy to have problems with those contacts. Have a look at this:

https://people.ohio.edu/postr/bapix/RME20.htm

Quote:
A contact on the output section was not quite makng a connection. The spring contact was very difficult to get at. Cleaning did not help. The spring flex in the contact was weak and the moving portion of the switch was not quite level. No adjustment would permanently fix the problem. I had no desire to try to remove the bandswitch to replace the contact. Since the contact was near a metal divider/ shield, I took a small piece of hospital tubing, heated the end with a solder gun to make it flat and wedged the piece between the contact and the shield giving it enough additional spring action to solve the problem.


I advertised here on the forum for a db20 (or a db22a or a Meissner signal booster) and did not get a single response. These preselectors are apparently thin on the ground. I wish you the best, but suspect you will need to fix the switch that is currently in your unit.

Do you still need information about how the filter caps are connected? I have since acquired 2 db-20's at hamfests, and could look at those details if you still need. It might take me a few days, as I am moving my hamshack and shop right now and the db-20s are packed.

John


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Thu 22, 2018 4:23 pm 
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Location: Eagan, Minnesota, USA
John,

I think that's the problem! I was starting to wonder why after spending hours comparing the schematic to the set, that if the band switch wasn't the problem, then what else could it be! But I think there is some kind of tension problem within the band switch that is causing the open condition on those two bands. I need to think about this a bit, and see how I'm going to approach it.

I'm ok on everything else. It is now rebuilt. Once I get through this problem, it should be ok, I think!

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Fri 23, 2018 5:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1679
Location: Eagan, Minnesota, USA
Last night, I was able to get the other nut out that holds the wafer section. It was tedious, and I was ready to accept the possibility of breaking that section. As you would imagine, it's at an odd angle, and there are wires in the way. I unsoldered some of the wires to gain access to the nut. I also had to bend back one of the terminals next to the nut. I was able to get just enough pull on a very small 1/4 inch wrench to break the torque on the nut. From there, it was a fractional turn at a time using alternately my wrench and long nose pliers. The nut finally came off and now there are just a few remaining connections to that wafer section. Then I can remove it. I intend to work on this over the weekend.

Once it's out of there, I can then clean and inspect that part. I haven't repaired a switch section before, so if anyone has any
suggestions on doing this, I'm open to hearing from you. Will update my progress next week.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Mon 26, 2018 5:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1679
Location: Eagan, Minnesota, USA
The band switch is not the problem. I tested the wrong wiper and switch terminal set. OK, so I reinstalled that section.

I did find two open coils that relate to band-1 and band-2. It is the final/output coil section that connects to the output terminal of the DB-20. There is continuity on the other four bands. So I know that is why it's dead on those bands. Now, the question is what to do about that. Other than finding a parts unit, I don't see a solution. The coil is found on top of the chassis, and it's the rear most one. I took the cover off of it, and there is a coil insert within an outer coil. It's filled with wax, and made not to be serviced.

Another question I have is that the unit is just about deaf on the other four shortwave bands. I am wondering if this is normal when using an RF probe to a signal tracer, or if this is another problem to deal with. I injected an RF signal into the antenna terminal, and do get signal output for each of those four bands. This is with low signal input into the antenna, so it's amplifying what's coming in to it.

The documentation for this is very incomplete. The Rider info for this unit has a page with voltage and continuity checks. It says to refer to Figure-13 (schematic) for the points to check. But, there are no points to check indicated anywhere on Figure-13 or any other page. There are two versions out there, and RME changed the part reference numbers on them, so that the one parts list that can be found is good for only one version. Lots of confusion for sure, and I am trying to notate what I can determine as correct in a notebook.

So that's where it's at.....

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Frank
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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Mon 26, 2018 7:58 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3422
Location: Monterey California USA
If you have correctly identified the defective windings, the usual process is to remove the coil and see if the broken path is right at the solder lugs. You may have to melt and pour off some of the wax with a heat gun to get a better look. Sometimes you will get lucky and the break is right at the lug, and often there is some green powder there as well. On unlucky occasions the break is deeper in the winding and you have to unwind the thing and count the turns, then rewind it with new wire (or patch some on if you find the break early in the rewinding.) Winding is not that critical unless it's a pi-section thing and then you really need a coil winder machine.

That being said, your difficulty is magnified if this is some sort of coaxially made arrangement or at least one with layers or similar vexation. Additional issues are that the lower band coils may be made with Litz wire, which is a nuisance to work with although I see it seems to be regularly available on eBay.

I suspect that one reason these windings open is that the wax then was natural beeswax which had been processed to some degree. The beeswax is somewhat acidic which eats the wires over the decades, or at least attacks dissimilar metal connections first, such as where the wires are soldered to a lug.

The best test of this preselector is with a receiver. As I recall, mine without restoration back in the 1960's brought the signal levels up on the broadcast and lower two bands, but above that there was little difference. The one I have now, I have rebuilt but not tried yet as the companion RME 69 is still in the queue for total overhaul.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Mon 26, 2018 8:54 pm 
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The somewhat good news is in the frequency range covered by the first two bands the attached receiver shouldn't need any help with gain or image rejection so the benefit from the DB-20 is pretty much nil in that range. If the coils are past repair but you want it to show some semblance of working on those bands you could use a capacitor to directly couple across the defective coils.

The DB-20 was designed primarily for improving image rejection so the RF stages are run with a lot of cathode bias so even running wide open you won't see more than about 20 db of gain and that will drop at higher frequencies. Depending upon the receiver the DB-20 may not result in any increase in usable sensitivity. If the receiver RF stage already has a low enough noise figure and enough gain to overcome the noise added by the mixer then the DB-20 won't improve the sensitivity. In such a receiver the loudness and S meter reading will go up but the noise will go up as much as the signal. In a later set with a low noise RF stage the DB-20 may reduce the usable sensitivity but will reduce image response. Conversely if used with a receiver that has no RF stage (like a Hammarlund Comet Pro) the results from using a preselector like the DB-20 will be stunning on higher bands.

I have a RME-9D and RME-69 each with a DB-20, a RME-70 with a built-in DB-20, and a RME-45 with the later DB-22 and in none of these cases in normal on-the-air usage have I noticed a weak signal that was audible with the preselector in but not audible with the preselector out. I haven't ever made a stringent measurement of the DB-20 (or DB-22) but I suspect neither provides a major increase in sensitivity with its matching receiver but it does provide a very big increase in image rejection. The later DB-22 would likely offer some additional improvement with the early receivers giving its later miniature tube (6BA6) which should provide better RF performance.

The later DB-23 would be one to normally avoid because it only has one set of tuned circuits at the input. It has three stages using three 6J6 twin triodes but except for the antenna input circuit the remaining stages use broadband untuned coupling.

One RME "box" I haven't fooled with yet is their HF 10-20 which looks like their VHF converter that matches the RME-4X series of receivers and it converts the 10/11, 15, and 20 meter bands down to 7 Mhz. creating a dual conversion setup with a decent 6BA6 RF stage. I have one sitting on the shelf but it hasn't made it to the bench yet.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Mon 26, 2018 9:28 pm 
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Location: Eagan, Minnesota, USA
Hi guys,
Thanks for the replies. I will spend some more time with the bad coil, and see if I can identify the exact bad sections. It is built as a coil within a coil. Unless it's something very easy to spot, I think it would be best for me to leave it alone and see if a parts set can be found at some point in the future. In the mean time, I think I might try the suggestion to connect the two bands through a capacitor to the switch. That would mean locating and disconnecting the two wires from the bad coil sections going to the output side of the band switch, and connecting a capacitor from CV-6 to each of the two band switch terminals. I think this would work. CV-6 is located on the tuning capacitor, and is the very last lug on it. It's pictured on the Riders info that shows the parts list and locations of the alignment adjustments.

Anyway, I will probably put more time into this next weekend. I was on this until 2:30 AM over the past weekend. Time sure flies when you're into it.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Mon 26, 2018 10:46 pm 
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Hi Frank,

It is unusual to find the two output coils are the failed units and I thought the problem was with the input side until I re-read your post. You often find primaries of input coils open due to lightning damage or a failed T/R switch when used with a transmitter. You should still be able to use the capacitor trick with these coils. According to the schematic the DB-20 uses an interesting design with no B+ on the primary of the output coil, the DB-22 is more conventional with both stages being fed through the respective coil primary which also acts as an RF choke for that range.

Is the secondary on the inside or outside of the coil form? If on the outside and the fault is not repairable then you could try rewinding the secondary which isn't that critical.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Tue 27, 2018 3:42 pm 
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Location: Eagan, Minnesota, USA
Rodger,

I think the secondary is on the inside of the coil. That looks really tough to get at. It's got what looks like two pins that run through the outer and inner coil that are heavily soldered, then filled with wax. This morning before I came in to work, I measured the larger outer coil and got 3.8 ohms. That must be the Band-1 primary coil. The lower outer coil read 1.4 ohms, so that must be the Band-2 primary coil. There are a few turns of wire in the space between the top and bottom coil windings. When I removed the cover, those center turns of wire had the insulation falling off of them. The top and bottom coils just mentioned looked normal. So, I don't know what those few turns of wire are for, or why they had what looked like missing enamel insulation with shiny bare wire. Adds to the mystery of it.....

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Tue 27, 2018 5:25 pm 
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Posts: 1679
Location: Eagan, Minnesota, USA
Rodger,

I am thinking of connecting a capacitor from the output switch terminal primary, and connecting the other end to the secondary terminal connection of the switch. I'd do this for Band-1 and Band-2 with each one getting its own capacitor. Since the secondary is open, I think I can just solder the cap to the terminal without disconnecting the wire to the open coil section. Does this make sense? What value of capacitor should I use? I'm thinking of ceramic disk here.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Tue 27, 2018 8:24 pm 
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Frank,

The value isn't critical. RME used .001 discs for DC blocking/coupling caps for the untuned output of their DB23. I would go with .002 discs which provide a capacitive reactance of around 135 ohms at the low end of the broadcast band dropping to below 30 ohms at the high end of the second range.

If the gain drops below unity at the low end of the broadcast band (with the preselector properly aligned and tuned to frequency) then you could increase the band 1 cap but it shouldn't be needed unless the receiver is presenting a very low impedance to the DB-20 at the low end of the band. Neither gain nor selectivity from the DB-20 are important for the first two bands with your RME-69 because it doesn't need any help here and all you want to do is ensure that the DB-20 isn't behaving like a significant signal attenuator.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 28, 2018 6:36 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: 98802
Open coils. Ouch!

It looks like a shorted C8 would put b+ across those coils. I'll bet that happened at some point in it's life. Let us know how it works out with the capacitors!


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 28, 2018 4:45 pm 
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Location: Eagan, Minnesota, USA
I think C8 was a 100PF cap. There is a new silver mica cap in there now. I saved the old parts, and I think I'll check the old one out to see how bad it was. I went through this thing, and replaced every capacitor and resistor. They were junk.

I'm probably going to get at this over the weekend again. I hope installing these capacitors does the trick, and I'll certainly let everyone know. I've learned a lot working on this!

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Frank
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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Mon 02, 2018 5:38 pm 
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Location: Eagan, Minnesota, USA
I made some more progress with this over the weekend. It seems like you can't use the RF probe on a signal tracer to tell if the unit works properly or not. The shortwave bands seemed dead doing it this way. I have a small Hallicrafters S-118 that has a coverage range that will handle the DB-20. This S-118 only has one hot antenna input on it and a ground. So I set the hot lead from the DB-20 to the Antenna terminal on the S-118, and grounded the other wires from the DB-20 to the ground terminal on the S-118.

This is what I observed. Let's say you set the DB-20 to 10 MHz and it's active. What I found is that tuning across the band of the S-118, almost all signals are muted except for those in the general area of 10 MHz on the radio. The signals are much stronger in that active range. This is what happens on all of the bands, so they all must be working. I tried this for many different signals up and down the DB-20 dial, it functions the same way. At this point, I think it must be working properly. I never had a pre-selector before, so please correct me if I'm wrong about this. But it sure seems like that's what it should do.

The ceramic capacitors that were used in the output for Band-1 and Band-2 seem to also be working. I get signal flow through it now.

What I am dong next is working on the antenna switch and it's wiring. I am tracing out every terminal connection to make sure that it's wired correctly. There appeared to have been some prior messing around with it, and I ran into a strange situation where I had to connect the antenna to the center post, and leave the other antenna post and ground not connected. Then, I could not switch between antenna and pre-selector. The antenna was not going through to the receiver. But, never the less, when it was hooked up and operating through the radio, it seemed to work very well.

So this is now where I'm at. At least some hope for this thing exists.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Mon 02, 2018 5:57 pm 
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It sounds like it is working Frank. The typical procedure for tuning the bands while using the preselector is to set it roughly to the range you are tuning, tune the receiver to a signal (or narrow part of the band of interest) and then rock the preselector tuning to peak the signal level. Adjust the preselector gain as necessary to provide the desired level of signal and to avoid overload either in the preselector or receiver.

Using the preselector with an external receiver is very much like using a receiver with separate RF stage tuning and this built in control is often labeled preselector. Once set you can tune a fairly wide segment of the range without changing preselector tuning and the higher you go up in frequency the wider the range before the preselector has to be reset. HOWEVER the tuning of the DB-20 is fairly sharp because it was engineered to greatly enhance the image rejection of a typical single conversion receiver with one or no RF stages and a 455 Khz. IF so it will require retuning with fairly small changes in frequency, especially at the lower end of the shortwave range.

If used with a later high end receiver (i.e. a Drake R4 series) the DB-20 will hurt performance while when used with an early simple superhet (Comet Pro, National FB-7, etc.) the results will be dramatic. It will definitely benefit a RME-69 or similar on 14 Mhz. and higher but receivers like this do quite well without the added preselector at lower frequencies. The DB-20 is also very useful with simple regens that have no RF stage because it will prevent changes in the antenna (i.e. swaying in the wind) from changing the frequency of the regen detector.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Mon 02, 2018 6:26 pm 
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Location: Eagan, Minnesota, USA
Rodger,

It sounds like from what you've described, my DB-20 is working properly. That is a relief. I think I'll probably keep it set up with the RME-69 unless I run across something like you've described where I'd want to see any performance improvements. But it will mostly remain with the RME-69. It is kind of nice to have the radio, and correct speaker and pre-selector for it.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Mon 02, 2018 9:35 pm 
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Frank,

I agree that it goes perfectly with the RME-69 and I have that setup. But don't hesitate to share it with other receivers because the poorer the image rejection (or noise figure and gain) of the radio the more the DB-20 will help.

For typical band cruising you may want to leave the DB-20 in the bypass position so that you don't have to tune it AND the receiver proper. Then bring it into play when needed. It is somewhat like the way I operated CW with the Heathkit SB-102 I got shortly after upgrading from novice to general. I would tune around at the low end of the bands with the SSB filter selected because that made tuning simpler and then brought the 400 hz CW filter into play only when needed.

Much of the time the DB-20 isn't needed by the RME-69 (pretty much 10 Mhz. and lower) so having to peak it while band cruising is a nuisance but once you move up to 20 meters it starts to show off its stuff in greatly reducing images. For best performance select your best performing 6K7 for the first stage in the DB-20 because with it in use this stage pretty much sets the ultimate sensitivity of the combined preselector/receiver setup.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Wed 04, 2018 9:00 pm 
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Glad you got it sorted out!


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Wed 04, 2018 9:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1679
Location: Eagan, Minnesota, USA
Thanks! I just went through the antenna switch wiring, and found that the wires for the antenna posts were reversed. The top antenna post should be going to the first band switch wiper, and they had the middle antenna post going there. That explained what I observed earlier when I finally got this to work. I made the change, and now the top post is going to the wiper on switch 3.3 per schematic.

There is a question though about the middle antenna post lead. The other lead from the antenna switch goes to a coil connection that is grounded. That does not match the schematic. The schematic shows six coils connected together, and that should be connected through the antenna switch to antenna post 2. The wire is factory, and so is the ground, which is just a small piece of bus wire that runs from the coil terminal and is soldered to the chassis. This also matches the coil right across from it, and both are within the same physical area.

The unit is working now. I connect a single antenna to the top antenna post, and nothing connected to the middle post at all. Then have it's output wires connected to the Hallicrafters S-118 with the hot wire connected to the antenna, and the other two wires connected to ground.

I will spend some more time looking at this uncertain wiring. I do find it odd to run a wire from the antenna switch to what ends up as a ground a far distance from the switch when there is a ground at the switch already available. Something doesn't make sense about that.

But, it's working, and for that I'm glad.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Mon 16, 2018 5:13 pm 
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Location: Eagan, Minnesota, USA
All of the repairs have been done to the DB-20, and I finally have connected it with the RME-69 receiver. The most noticeable thing is that when the DB-20 is engaged, the amplification of the signal is very apparent. The meter on the RME-69 has a scale stated in 'DB', and when the DB-20 is on, the reading goes up by 20. For example, if the meter reads 30, then engaging the DB-20 would cause it to read '50'. And so on for all of the other values on the scale. Guess that's where the name DB-20 came from. I haven't had a chance to spend a lot of time with it yet. Mostly a few hours this weekend late at night. Bands five and six are not active late at night, so I'll check them out when I get some time during the day. Also, I'm not sure about how to rate it's performance regarding image rejection. Like I've said, I need to spend more time familiarizing myself with the complete setup. But it works, and seems to work well. Looking forward to spending some time with it.

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