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 Post subject: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 6:15 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
Decided to put this separate from the receiver. I'm trying to determine the value of the electrolytic capacitor that follows the 50K ohm resistor. That resistor comes off one lug of the choke. I cannot find a parts list for the schematic that shows that capacitor. The electrolytic can in my unit has three capacitors in it, with one lead connected to that point. There is another schematic that does not show three leads in the capacitor can; only two, so it doesn't apply to my unit. But, on that schematic, it shows an antenna switch that exists on my unit that is not shown on the other schematic. So what I'm saying here is that my unit has an electrolytic with three sections and an antenna switch. So neither schematic that I know of matches this set perfectly. Somewhat confusing. I'm trying to work between both schematics to get this done.

So, first, I need a value for that electrolytic capacitor that follows the 50K resistor. It looks like it supplies the screens in the 6K7 tubes. Anyone know what it is please let me know. Thanks!

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 6:35 pm 
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Location: Wayside, NJ Monmouth
Try this kink, " http://bama.edebris.com/manuals/rme/db20riders/ ". maybe what you need.


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 6:47 pm 
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Location: Eagan, Minnesota, USA
That's the info I have now. The first schematic shown has a parts list for it, but it doesn't match my unit. This schematic/parts list shows a two part electrolytic, and mine uses three sections. The next schematic down from this is more of a match. It shows a three part electrolytic but has no parts list to get any values. The capacitor I need the value for looks like it might be C6. It's blurry, so I'm guessing this. The 50K resistor is shown as R4 on this schematic, and I mention it only as a way to find the capacitor I'm needing info for. Thanks, though for the reply!

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 7:39 pm 
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This is the other file on B.A.M.A, it may be different " http://bama.edebris.com/manuals/rme/db20/ ".


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 8:17 pm 
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Frank,

The DB-20 has a rather strange power supply setup with an extremely heavy bleeder resistor for a supply where voltage regulation really is of no importance. The supply is already heavily filtered even before it hits additional dropping resistor R4 and since this is well downstream from the choke input you can use any reasonable value capacitor you wish. I would go with a 10uf or 12uf @ 450V but pretty much any value or even no capacitor will have little to no impact upon the operation of the DB-20.

Your original capacitor is probably a triple 10 unit.

This is a place where the voltage rating is important but the capacitance value not so much. Typically this sort of setup is used where further smoothing of the DC is needed, for an oscillator or low level audio stage, but that isn't the case with the screen supply of the DB-20.

I have two DB-20 and I used 10uf@450V replacements for all sections which works fine. The only really critical place is not to increase the value of the input capacitor which would increase current draw and output voltage.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 9:46 pm 
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Location: Eagan, Minnesota, USA
Rodger,
Interesting info! I'll go with a 10 MFD capacitor there. Problem solved.

One other question. There is a 15K 10 watt resistor that I found to be open. That would be R7 on the DB-20 late schematic (first one on the link above). Do you have any idea what may have caused that to fail? I performed a resistance check across the choke (installed in circuit) and get 570 ohms. So I think it's ok. I know the power transformer produced filament voltage prior to the rebuild, so I think it's ok. I'll know once I get the rebuild completed. Just wondering if a failed capacitor somewhere could have caused that 10 watt resistor to fail open.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 10:04 pm 
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Frank,

That resistor is what I was referring to as the heavy bleeder resistor used on the power supply. It would have just failed from heat and age instead of another component causing it to fail. I really don't understand why RME used such a heavy bleeder for this power supply; the preselector doesn't present a widely varying current load nor does it need to be voltage regulated and it already has an input capacitor so there was no concern about a required minimum current load for it to act as a choke input filter.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 10:08 pm 
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Posts: 1707
Location: Eagan, Minnesota, USA
I was just on the Mouser site, and the 10 watt 15K resistors aren't cheap! I normally try to buy a quantity of parts when I make an order, but if I can't find one in my parts stock, then this will be a much smaller order than usual. I'll check tonight to see what I've got.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 10:20 pm 
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These are the cheapest stock items I see: https://www.mouser.com/ProductDetail/TE ... MPkLB30%3d at $1.45

You can of course use a series or parallel combination from your junk box if needed to hit the right resistance/power rating. I wouldn't be concerned at all about going up to 20K if you have one of those and it will also drop the power dissipation a bit.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Wed 07, 2018 10:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
Those would be what I'd order. I should be able to come up with something. There is room for two to make the value if necessary. It is kind of surprising that a lot of what they had was around $4.00 a piece! Must be a reason......


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Thu 08, 2018 6:05 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
I found a 15K 10 watt resistor. An IRC with a code of 7203 on it. I'm guessing it was from 1972. Anyway, I installed that along with the 10 mfd electrolytic capacitor discussed above. Now, all of the capacitors and resistors have been changed.

Next phase is to sort out the antenna switch. On my unit, this is located on the left side above the on/off switch. I think this is a factory modification. All of the pictures on the web show units with three small knobs and a large tuning knob. My unit has 4 small knobs and a large tuning knob. Judging by the way everything involving this switch has been done, my guess is that this is either a factory change or maybe a custom order. One of the schematics I have for the DB-20 shows a separate antenna switch. There are two wires that are unconnected at the switch, but seem to have been broken off for some reason. There is also a small length of extension wire going from one terminal over to mate with another wire. So, I think I'm going to have to trace all of this out to make sure it's connected properly. All of the soldering joints on the switch look to be undisturbed. With luck, this will only require reconnecting the loose wire, and maybe clean up the short extension wire for neatness sake. Hope to have some time this weekend to deal with this. After this is done, I will then install a new power cord, test the tubes and fire it up for testing.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Thu 08, 2018 9:20 pm 
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Frank,

Is the separate switch for selecting between different antennas connected to the DB-20?

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Thu 08, 2018 10:39 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
Rodger,

I'm going to take a closer look at the wiring tonight. Not sure what's going on with it, but there may be a connection
to the DB-20. I'll verify that. Some of the wiring seems to be sleeved in a faded brown cover. Very thin wire used, all solid.
It is possible that there are connections from this switch into the DB-20. I think I have counted 12 connections coming from the switch, and I think the switch is two position.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Thu 08, 2018 11:00 pm 
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Interesting Frank. It may have been intended for use with switching another converter or similar device in or out of line with the receiving setup. With 12 connections it is a fairly complex setup!

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Thu 08, 2018 11:28 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
Rodger,

I've been looking on the web, and still haven't found any other like it. It sure looks factory done in every way; from the knob to the switch, it looks RME. There are two sets of initials on the chassis. Maybe one when it was built and the other for the switch? Who knows.

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Frank
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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Tue 13, 2018 6:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
I spent some time tracing out the antenna switch. I now have notes on how it is wired, and understand how it works. I was able to connect the two loose wires. Simply connected to the nearest post, and the wire had the correct form in it to go to the right connection. I re-soldered all of the switch connections to assure future reliability.

There are three wires that come from the output plug of the DB-20. One is a wire connecting to a shield in the cable. The other two each have their own plug connection. So there are three wires on my unit vs two shown on the schematic I had to refer to. I'm going to have to figure out the correct way to connect this to the RME-69. Not sure if any antenna jumpers can be used, etc.

The front plate is attached, and knobs reinstalled. I have the small screws that holds the front panel to the cabinet, but am missing all four screws that hold the chassis to the bottom of the cabinet. Need to figure out that size they are. Before it gets installed in the cabinet, I'll make various voltage checks, and verify that things are ok.

I'm close to firing this up for some testing soon. Maybe this weekend.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Mon 19, 2018 3:31 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
I suspect I might have a problem with the band switch. I'm not getting any output on band-1 or band-2. Performing a resistance check shows both as open. If I take my RF probe and go to the connection terminal at the switch, I do get output. So the pre-selector is working.
I unsoldered the wire from the terminal that goes to the output jack from the switch, retested, and have the same results.

This is the rear most wafer, and would be the right most switch on the schematic. The part I'm having trouble with would be the top half shown on the schematic, and that connects to the bottom set of coils shown on the schematic.

The switch does not look damaged as best as I can tell. Not sure what to think at this point.....

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Tue 20, 2018 4:12 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
If anyone has an RME DB-20 for parts with a good band switch, please contact me.
I also posted in the want ad section. Shipping would be to 55122.

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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Tue 20, 2018 4:25 pm 
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Frank,

What is broken with your band switch? Contacts can be replaced if that is the problem.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Mar Tue 20, 2018 4:37 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1707
Location: Eagan, Minnesota, USA
Rodger,

I'm not sure what might be wrong with that section of the band switch. From what I can see, it looks ok. But it seems that on band 1 and band 2, no signal comes out from the terminal that connects to the output socket. It's kind of hard to see the front of that section, because it's very close to the metal shield in front of it. I did remove the outer nut that holds it in place, and I can pull it back a little. Really, at this point I'm just assuming it's bad. I am considering removing it this weekend, but the lower nut is in a very tight place, and it's possible that it might break in the process.

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Frank
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