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 Post subject: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Mon 26, 2018 12:02 am 
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Posts: 134
Location: Allen, TX
http://bama.edebris.com/download/drake/tr4/sch.gif

I'm here again...confused :?:

My Drake TR4 has developed low filament voltage for the 6.3VAC tubes.

Here's what I'm seeing:
1) it's not the AC4 power supply because when used with a TR 3 I have, the 6 V string of tubes all measure 6.3V and the 12V tubes measure 12.6VAC
2) all of the 12.6 VAC tubes have proper voltage and light up brightly
3) If I remove all of the 6VAC tubes their sockets. the filament voltage rises to 6.3V, but if I insert one tube in the string, the filament voltage drops by about 1 VAC. After I've inserted all of the 6V tubes back in their sockets, the filament voltage for all the tubes goes down to only about 1.4VAC
4) I've disconnected the 6.3VAC wire going to the upper/lower SSB lights to remove them and anything downstream from consideration
5) There is 12.6VAC at the filament fuse wire and at pin 5 of the three final tubes, as well as all of the 12.6V heaters
6) Pin 4 of the final tubes measures the 1.4VAC I'm seeing on all the 6V heaters with all the tubes installed
7) All of the 6.3VAC tubes tested good

I'm confused about just what provides the voltage drop from the 12.6VAC supply to the 6.3VAC needed for the 6V tubes. Is it the filaments for the 3 final tubes? I just don't see what could be causing the apparent inability of the 6.3VAC supply circuit to maintain the necessary voltage as I plug in more filaments in parallel and place the additional load on it.

Again, thanks in advance for your wisdom.

Rick
W5RAA


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Mon 26, 2018 12:16 am 
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Are your 3 final tubes all in place and lit? Are they the proper tube type (same filament current as the stock tubes)? If you are measuring 12.6 volts at the feed to the finals and only 1.4 volt at the feed to the rest of the string there is excessive drop is across the 3 finals and is probably from an open or poorly connecting filament.

NEVER pull tubes in a series/parallel string like this to test or you may end up damaging other tubes. The current draw of the additional 6.3V tubes in parallel has to equal the current of the three final tubes for the proper division of these two strings of tubes across the 12.6 volt supply. Once you start pulling any tubes this current/voltage distribution is upset.

Start by checking all three of the final tubes to make sure the filaments are intact and that the pins are clean and make good contact with their sockets.

Rodger WQ9E


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Mon 26, 2018 4:28 am 
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Joined: Jan Mon 25, 2016 3:13 am
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Location: Allen, TX
Hi Roger,

Before cleaning the 6JB6 tube sockets, the parallel resistance of all three tubes plus the input and output RF chokes measured 0.4 ohms.

The three tube filaments measured 3.2, 3.4 and 3.1 ohms.

After cleaning the tube sockets, the parallel resistance increased to 1.1 ohms, which you'd expect given the resistances of the three heaters.

The pin 4 output voltage however, remains at 1.4VAC versus 6.3 VAC.

Could a partial short to ground somewhere in the 6.3VAC wiring to the various tubes be loading the string down and causing this voltage drop?


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Mon 26, 2018 4:42 am 
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Location: Allen, TX
I ask that because remember, even though I shouldn't have done this, the voltage in the 6.3V string rose to 6.3 with none of the 6V tubes in their sockets, and then gradually decreased as I inserted more tubes.


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Mon 26, 2018 6:22 am 
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Rick,

The 6.3VAC final tubes are in parallel with each other and in series with the other 6.3VAC tubes which are in parallel with each other. So, with only the finals in place (and the sideband lights not in their sockets), you should see 12.6 volts--not 6.3VAC. There would be no current flowing so no voltage drop at the finals. You must have one tube still in place--V6? or a leaky capacitor. Whatever remains in the circuit is still drawing current and is pulling the voltage down as though all the other tubes were in place--so a lot. Look at the schematic. There is one tube, V6, drawn off in the middle of the 12.6VAC string--but it isn't connected there-- instead it is connected to the 6.3VAC string. Weird drawing. There are also some capacitors to ground that may be leaking and drawing down the string.

Norm

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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Mon 26, 2018 9:52 am 
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Greetings to Rick and the Forum:

Norm is quite correct.... you should see 12.6 volts on the downstream side of the 6JB6 filaments, not 6.3 (with all of the 6.3 volt tubes pulled). Do the 6JB6 tubes seem abnormally bright when the 6.3 volt tubes are all plugged in? They should, if they are seeing 11 volts across them instead of 6.3.

I think it is time for a current meter. I would suggest that you disconnect RFC10 and place an AC ammeter in series with it. (If you have a clip-on ammeter with a low enough scale, you can simply insert a loop of wire and clip onto that.)

The nominal filament current for the 6JB6 is 1.2 amps, so the total current through RFC10 should be 3.6 amps, or thereabouts.

If it is excessive (with the 6.3 volt tubes in their sockets), you do indeed have a short. One place you might look that is not obvious is around the upper and lower sideband indicator lamps and their sockets. A short internal to the lamp socket or in the lamp wiring could pull down the 6.3 volt filament buss.

If you do have a short (you will need the ammeter to tell you this for sure) then the best location trick I know (unless you are lucky enough to own a current probe for your scope) is called interval halving. Find a point in the circuit that is relatively close to the center of the load (say between V4 and V15) and break the circuit there. If the problem goes away, it is in the left half of the circuit. If it stays, it is in the right half. Restore the connection that you just broke and break the half with the problem roughly at its center. This will tell you which half of the remaining half contains the trouble. Repeat this process until you have a single circuit element that contains the short. The repair should then be obvious.

Of course, if you don't have 3.6 amps or more flowing through RFC10 and the voltage is still low, then you have one or more defective 6JB6's, one or more defective 6JB6 sockets, or bad solder joints or incorrect wiring on the 6JB6 sockets.

Good Luck,

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Jim T.
KB6GM


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Mon 26, 2018 11:18 pm 
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Joined: Jan Mon 25, 2016 3:13 am
Posts: 134
Location: Allen, TX
Thank you for your inputs gentlemen, but the problem persists. Here is my feedback based on your inputs.

There is still 12.6 VAC to the 3 final tube heaters, and 1.0VAC at the output to RFC10 with all of the 6.3VAC tubes in place. If I pull tubes out of the string, this voltage increases at about 1 VAC per tube pulled. All of the TR 4 schematics I've found on the internet show 6JB6 tubes as the finals, but my radio has 12JB6's installed, which makes sense to me because the schematic shows 12.6 VAC feeding the final filaments!?!

With all of the 6.3VAC tubes out, including one 12AX7 which runs on 6VAC in this radio, the RFC 10 voltage rises to 11.5 VAC. The current going through RFC 10 is 1.9A, which is in line with the 3 parallel 12JB6 filament current draw spec sheet of .6A each. So this indicates to me that there isn't addition current drain caused by a short or leak to ground anywhere in the 6.3VAC string.

I've pulled 6.3VAC tubes one at a time to see if the one removed causes the filament voltage to rise to 6.3 but it doesn't.

I did notice this though. There is a nice 6.3VAC reading on the non grounded side of the lamp brightness resistor R 116. I have no clue why!

This radio worked a few weeks ago so that tends to rule out wiring harness issues and makes me think a cap went leaky, but I can't confirm that with the current draw test. I might suspect a power supply problem, but this supply works fine with another TR 3 I have.

And I thought this would be a simple fix!

Rick
W5RAA


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Tue 27, 2018 6:46 pm 
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Rick,

The problem is simple, you need to pull those 12BJ6 tubes out of there because they won't work with the voltage distribution scheme used in the TR-4. Six volt filament finals are REQUIRED.

Rodger WQ9E


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Wed 28, 2018 2:15 am 
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Greetings to Rick and the Forum:

+1 to what Rodger said.... but sometimes, one should leave well enough alone. I couldn't resist creating a spread sheet and doing the math.... and it doesn't come out quite the way it should. Further, there appears to be an error in the schematic. V13's filament connections are drawn correctly. However, as near as I can tell from the schematic, V18 is also a 12AX7... and its filament connections are not drawn correctly.

If V18 is not a 12AX7, then this might account for the discrepancy that popped up in the spread sheet.... or perhaps, Drake engineers figured it was close enough. (There is a precedent for this; in my Galaxy 5, I had to add a parallel resistor to prevent the dial lamp voltage from being too high.)

Anyway, the spreadsheet numbers come out such that the 6JB6's have only 5.88 volts across them, while the 6.3 volt filament string has 6.72 volts.

This is probably close enough to work, although if it were my radio, I'd add some resistance to the 6.3 volt buss to try to warm up the finals a bit more.

Anyway, here's the spread sheet:

Attachment:
TR4 Filament Budget.xlsx [10.5 KiB]
Downloaded 18 times


Can anyone spot my error? Or is this the way the rig really works? (Corrections about my assumptions about V18 and the schematic would be welcome.)

Regards,

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Jim T.
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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Wed 28, 2018 2:31 am 
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Jim,

Thanks for putting together the spreadsheet.

My manual show V18 is a 12AX7 but it is fed directly from 12.6 volts and isn't part of the series/parallel string involving the finals. Instead V3 (12BY7) is part of that circuit and with the 12AX7 (.3 for both segments) removed and the .6 amp added in then you end up with 3.45 A which is pretty close to a perfect balance. If you want to make it perfect add another #47 lamp :)

Rodger WQ9E


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Wed 28, 2018 5:13 am 
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The TR-3, which does use 12JB6's for finals, has a completely different configuration. It places the 6.3 volt filament tubes in pairs and drops the 12.6 volts across two tubes. Can the TR-4 run with 12JB6's instead of the correct tubes? I am definitely not taking one of my TR-4's and trying it that way.

My manual shows it the way Jim has it--except V6 should be included with the 6.3 volt tubes. Look after RFC10 there is a wire drawn going to the right where V6 is in the 12.6 volt string. If I look close, the 12.6 volt string starts with V5 and then there is a capacitor and then V6 but no tube filaments. The tube was moved to the right and then added to the 6.3 volt string with a wire--because they didn't want to have to redraw it and they forgot to erase the V6 label. V6 adds .6 amps so it helps even out the string. Throw in some chokes and you probably have it pretty close.

Norm

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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Wed 28, 2018 9:30 am 
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Greetings to the Forum:

Thanks Rodger and Norm for the corrections. I do not own any Drake radios and so I don't have any manuals for them.... I was going by the .gif file in the original post. If you look there, I am pretty sure that that is the source of my errors. Must be an early drawing... since corrected in your manuals.

Anyway, if we have convinced Rick that 6JB6's are the correct tubes, then we can all rest content in a job well done. :D

Regards,

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Jim T.
KB6GM


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 Post subject: Re: Drake TR 4 Low 6.3V Filament Voltage
PostPosted: Feb Wed 28, 2018 1:43 pm 
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I believe that using V1, V3, V4, V6, V13 (both filament segments in parallel), V16, V17, and V19 as a 6.3V group in series with the three parallel 6JB6 finals was the "stable" design for the TR-4 series and that is how my TR-4CW is set up and that is the latest TR-4 that I own. I doubt if the infall TR-4CW + RIT model changed the filament distribution.

The GIF version of the schematic from edebris clearly has an error because it shows the 12.6V filament of V18 (12AX7) with the two filament segments in series across the 6.3 volt filament bus and I also notice that bypass capacitor C58 has the V6 designation next to it although the V6 parallel filament segments are correctly shown elsewhere in the diagram. The schematic on edebris was probably the earliest version after the transition from the TR-3 and some mistakes crept in.

In any case a TR-4 using 12JB6 tubes would require complete redesign of the filament bus to make it work. A lot of the transceivers from this era have this sort of convoluted series/parallel filament bus so that the filaments could be powered directly from the car battery for mobile usage. It isn't a bad idea with these rigs to put an inrush limiter in the filament circuit to reduce turn-on stress because although the voltage/current distribution is pretty good once the filaments are at operating temperature that isn't true during warm up. Tube manufacturers created a line of controlled warm up time tubes with filaments specifically designed for series string operation but in sets like these there is often a mix of "regular" and controlled warm up time tubes and providing additional inrush protection at turn on will reduce the chance of "blown" filaments at startup.

Rodger WQ9E


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