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 Post subject: Looking for troubleshooting ideas for a Hickok 209A VTVM
PostPosted: Sep Sun 02, 2018 8:42 pm 
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Location: Southern Maine - 03909
Hi All,

I have been working on restoring a Hickok 209A. I have been banging my head recently on what I feel is the last issue.. (Yeah, I know, it never is the last issue :) )

I replaced all the caps, and tested most of the resistors (replacing a few mainly during the troubleshooting).

The initial issues were fixed by replacing one of the 90Meg resistors (9 10 meg 1% in series), and replacing the tubes...

I was able to get the meter to function properly on the resistor and capacitance settings, but the AC and DC voltages remained low, with the calibration pots at the end of their range. (9.1 v from a battery only registers around 8v on the 209A. AC 6.3 v shows around 5.5.)

I started looking at voltages, comparing them with the schematic suggested Here is the schematic (copied from an earlier post by another author.
Earlier post: http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=126997&hilit=Hickok+209a
Full Schematic: https://web.archive.org/web/20060323171830if_/http://antiqueradios.com:80/albums/Misc/Hickok_209A_schematic_002.jpg

Detail of interest:
Image


The voltages on the 6SN7 plate (and B+) were low.. (around 87v instead of the expected 100v).

Initially, I thought that the power supply was at fault, and some of that is still unexplained.. but I tried using diodes in place of the 6X5, but no significant difference.

Latest things I tried:

1) Pulled the tubes and checked AC voltage. OK
2) Put 6X5 in the power supply and checked B+. OK (105v)
3) Put in 6SN7 and looked at supply voltages and plate voltages. OK (96v)
4) put 6SJ7 in, and checked voltages (B+ and 6SN7 plate down to 87v, 6SJ7 cathode and grid 35v vs 2v)
5) according to the voltage across the 2.2k resistor (11.1 v) in the power supply, there was only a 5ma draw.

I am pretty much stumped.. One last thought is to use a scope to measure the voltages/signals, and perhaps my VOM is having an effect on the measurements. Also, if there is an oscillation, perhaps I am getting faked out on the voltage measurements.

Any ideas or thoughts greatly appreciated...

George

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 Post subject: Re: Looking for troubleshooting ideas for a Hickok 209A VTVM
PostPosted: Sep Sun 02, 2018 9:37 pm 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3372
Location: Florida
6SN7 cathode voltages? This sets the operating point of the tube and affects circuit gain.

C4, across the meter?

RRM


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 Post subject: Re: Looking for troubleshooting ideas for a Hickok 209A VTVM
PostPosted: Sep Sun 02, 2018 9:46 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 1944
Location: Sunbury, Ohio 43074
Your VOM can be lying to you, actually loading down the circuit. Do you know what your VOM is rated at in the ohms-per-volt spec? If so you can calculate the load the VOM is adding, and adjust your readings for that perhaps.

Try measuring the B+ with your VOM set for AC volts scale, 1 or 10v range, and see if there may be excessive ripple. Sometimes a replacement cap isn't up to the job. Or, if you have a scope, look at the B+ with that. Both filter caps in the supply, as well as the divider resistors, would be my first suspects. Did you replace any of those resistors, and if so, did you put in one that was the same MARKED value as the one you took out ?? Sometimes resistor values are "fudged" at the factory and not reflected in the parts list. I replace resistors only if they are not what they are marked, or if something doesn't work. That, by the way, might be a thing to check in any other resistors you replaced.... that the new ones were the value of the MARKED old ones, not necessarily the parts list or schematic values shown.

I can't see the schematic well enough on BAMA to help you with the DC and AC voltage dividers, but if they are both out of calibration range, also look for something common in those cal circuits. Again, take care when replacing any resistors as noted above. I'll see if I have a better schematic tomorrow when I am up at the shop so I can seeeeeeee :(

luck..........

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 Post subject: Re: Looking for troubleshooting ideas for a Hickok 209A VTVM
PostPosted: Sep Mon 03, 2018 2:41 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7897
Location: Long Island
Due to the high resistances in DC meter amplifier circuits, things like gassy or leaky tubes can be problematic. The gas and/or leakage may be so slight that no ordinary tube tester will show them, and the tubes may be perfectly okay in other circuits with lower impedances. Best way to tell is by tube substitutiion either with NOS tubes or with ones from another meter that works properly.

Gas in tubes that have sat around for years can sometimes be reabsorbed by their getters if you let them operate a day or two at normal temperature. Since some meters apply low DC voltages to tube plates to cut down on noise, you may need to put the tubes into a radio or something else that runs full plate volage to really get them hot enough. Leakage, on the other hand, usually doesn’t improve with time or temperature; in fact, it tends to get worse.

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 Post subject: Re: Looking for troubleshooting ideas for a Hickok 209A VTVM
PostPosted: Sep Fri 14, 2018 7:31 pm 
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Joined: Mar Sun 16, 2008 5:05 pm
Posts: 376
Location: Southern Maine - 03909
UPDATE.. (corrected)...

I resurrected a VTVM to be sure that the voltage measurements are more accurate.

I had pulled the 6x5 and 6sj7 tubes out, leaving the 6x5 power and the 6sn7 tubes in the circuit.

The voltages seemed to be reasonable. The 6sn7 plates were 74v, and the cathodes were at 2v. B+ was about 95v.

I then added the 6sj7 into the circuit, and the voltages go off. The 6sn7 plates dropped to around 65v, and the cathodes to around 1.5v. The voltage on the 6sj7 was way off. The plate was at 75v, and the cathode was around 50v (correction].

Something wierd is going on. A 50v (correction) drop across a 1 meg resistor is only 50 (correction) microamps. That difference in the B+ load should not cause a 10v drop with that little of an extra load. I tested the voltage on the other side of C2, and it was 0, and disconnected it just to be sure. I also tried several new 6sj7 tubes with no change.

UPDATE: I disconnected the 1 meg cathode resistor from the 6sj7, and the plate returned back to the 95v. The cathode however remained at 50v. The 6sn7 voltages returned to normal.

The B+ has a 7.6v p-p ripple (asymetric) with respect tp ground. The only filter is between the B+ and B-, with a voltage divider to ground.

Still scratching my head.

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Last edited by gkaczowka on Sep Fri 14, 2018 9:06 pm, edited 1 time in total.

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 Post subject: Re: Looking for troubleshooting ideas for a Hickok 209A VTVM
PostPosted: Sep Fri 14, 2018 9:01 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 1944
Location: Sunbury, Ohio 43074
All other things being equal, this would indicate that the 6SJ7 and it's circuitry is drawing too much current. I have a pair of 209A's but I have not had time, nor I do right now, to get them on the bench unfortunately to help you out from afar.

Why is the 6SJ7 drawing too much current? The usual suspect is a leaky cap somewhere. Check the 1M cathode resistor to see if it's gone open, but I doubt that would cause this issue anyway.

I suppose the other possibility is that the power supply cannot supply the required current. The reasons for that are literally limitless, from a bad tube, power transformer, cap, resistor, anything really. Do you have a scope to look at the power supply output? Excessive ripple there could indicate one of your replacement electrolytics is bad or the rectifier tube perhaps.

At this point I'd go back to the beginning. Perhaps you replaced a resistor with the wrong value, going by the schematic instead of what was in the instrument. They made mods and updates to these things all along the manufacturing cycle, and schematics usually didn't keep up.

hang in there :) It's a cool VTVM. I'll restore my 209C black face first, then get back to the 209A's

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 Post subject: Re: Looking for troubleshooting ideas for a Hickok 209A VTVM
PostPosted: Sep Sat 15, 2018 1:09 am 
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Joined: Mar Sun 16, 2008 5:05 pm
Posts: 376
Location: Southern Maine - 03909
Thanks for the thoughts Barry. I updated my post while you were writing your post. The most interesting thing i found was that if i opened up the 1 meg resistor on the cathode of the 6sj7, the voltages returned to normal everywhere except the cathode of the 6sj7 remained at 50v.

Earlier, i measured the voltage across the 2.2k resistor off of the 6x5 power tube, and it was 10.5 without the 6sj7, and 11.1 with it in the circuit. I thought it may be the 6x5 or transformer.. i tried several 6x5s, and even built a solid state rectifier with a transformer from another 209a that a member got for me in the classified. Everything acted the same.

Going back to the beginning makes sense.. i am thinking or replacing everything that touches the 6sj7. Go down to the bare socket, clean it, and rewire.

I appreciate everyone's input so far.. i will keep plugging!

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http://www.oldradios.com


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