Home |
Search |
Today's Posts |
|
Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
#1
|
|||
|
|||
HP-400D voltmeter
Hi Everyone,
I picked up an old HP-400D voltmeter on a garage sale. Having cleaned most of the grime away from the equipment (it must have set in a basement for years) I plugged it in. The tubes all light up and the power supply voltage is around spec, but the meter moves always past its maximum in every range. The second amplifier chain, after the attenuator, is oscillating at about 1MHz, that is what the meter is showing. What could be the cause of the oscillation? Bad tubes? I am also thinking about bad ground connections, as if (I have to check this) grounds are connected to solder lugs that are riveted to the frame. There could be some corrosion at those rivets. Thanks, Peter -- Peter E. Orban National Research Council of Canada e-mail: |
#2
|
|||
|
|||
HP-400D voltmeter
Check all the capacitors for their ESR specs, and for their values to start
with. In these old units, it is very normally to have many bad caps. Some of the resistors can also be off value from age. -- Greetings, Jerry Greenberg GLG Technologies GLG ========================================= WebPage http://www.zoom-one.com Electronics http://www.zoom-one.com/electron.htm ========================================= "Peter E. Orban" wrote in message ... Hi Everyone, I picked up an old HP-400D voltmeter on a garage sale. Having cleaned most of the grime away from the equipment (it must have set in a basement for years) I plugged it in. The tubes all light up and the power supply voltage is around spec, but the meter moves always past its maximum in every range. The second amplifier chain, after the attenuator, is oscillating at about 1MHz, that is what the meter is showing. What could be the cause of the oscillation? Bad tubes? I am also thinking about bad ground connections, as if (I have to check this) grounds are connected to solder lugs that are riveted to the frame. There could be some corrosion at those rivets. Thanks, Peter -- Peter E. Orban National Research Council of Canada e-mail: |
#3
|
|||
|
|||
HP-400D voltmeter
Peter,
I'd start by cleaning all of the tube pins, as they are likely to have corrosion on them from sitting all these years. Clean the range selector switch too. I suggest leaving the tube pins dry (no contact treatment) after cleaning. Sometimes 400 grit (more or less) sandpaper is the best way to clean tube pins. Regards, Tim Schwartz Bristol Electronics "Peter E. Orban" wrote: Hi Everyone, I picked up an old HP-400D voltmeter on a garage sale. Having cleaned most of the grime away from the equipment (it must have set in a basement for years) I plugged it in. The tubes all light up and the power supply voltage is around spec, but the meter moves always past its maximum in every range. The second amplifier chain, after the attenuator, is oscillating at about 1MHz, that is what the meter is showing. What could be the cause of the oscillation? Bad tubes? I am also thinking about bad ground connections, as if (I have to check this) grounds are connected to solder lugs that are riveted to the frame. There could be some corrosion at those rivets. Thanks, Peter -- Peter E. Orban National Research Council of Canada e-mail: |
#4
|
|||
|
|||
HP-400D voltmeter
"Tim Schwartz" bravely wrote to "All" (11 May 04 12:11:00)
--- on the heady topic of " HP-400D voltmeter" Tim, not to name a brand but I've found the s.brite green plastic scouring pads are great for cleaning tube pins. Just push the tube pins into the pad a few times (one at a time) and it gets cleaned of oxide without removing too much of the gold, silver, or whatever contact enhancing metal coating is used. TS From: Tim Schwartz TS Peter, TS I'd start by cleaning all of the tube pins, as they are likely to TS have corrosion on them from sitting all these years. Clean the range TS selector switch too. I suggest leaving the tube pins dry (no contact TS treatment) after cleaning. Sometimes 400 grit (more or less) TS sandpaper is the best way to clean tube pins. TS Regards, TS Tim Schwartz TS Bristol Electronics TS "Peter E. Orban" wrote: Hi Everyone, I picked up an old HP-400D voltmeter on a garage sale. Having cleaned most of the grime away from the equipment (it must have set in a basement for years) I plugged it in. The tubes all light up and the power supply voltage is around spec, but the meter moves always past its maximum in every range. The second amplifier chain, after the attenuator, is oscillating at about 1MHz, that is what the meter is showing. What could be the cause of the oscillation? Bad tubes? I am also thinking about bad ground connections, as if (I have to check this) grounds are connected to solder lugs that are riveted to the frame. There could be some corrosion at those rivets. Thanks, Peter -- Peter E. Orban National Research Council of Canada e-mail: .... New computer? But I like my vacuum tubes... They keep me warm. |
#5
|
|||
|
|||
HP-400D voltmeter
On Mon, 10 May 2004 14:52:34 -0400 "Peter E. Orban"
wrote: I picked up an old HP-400D voltmeter on a garage sale. Having cleaned most of the grime away from the equipment (it must have set in a basement for years) I plugged it in. The tubes all light up and the power supply voltage is around spec, but the meter moves always past its maximum in every range. The second amplifier chain, after the attenuator, is oscillating at about 1MHz, that is what the meter is showing. The most common problem with these is the coupling caps between stages. Each stage uses a molded case paper capacitor to couple the signal from the plate of the previous stage to the grid of the next. These caps are almost all leaky by now and this throws the grid bias way off. In the 400D you will find 4 of those caps. One of them is in the power supply and the other three are for interstage coupling. The power supply one isn't a problem, but the other 3 should just be replaced. IIRC, I could hook a DC uAmmeter between the grid end of each cap and chassis and measure the leakage of each one. The resistors in the grid circuit are high enough that it doesn't take much leakage to make for several volts of grid bias change. Getting rid of the leakage will pull down the grid voltages and reduce the plate currents, taking a big load off the tubes and making everything in there run quite a bit cooler, too. I replaced mine (all 4) with nice axial lead mylar caps maybe 15 years ago. It made a world of difference. I found a couple of other problems, too, but I had to get it close before I could even see those problems. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
#6
|
|||
|
|||
HP-400D voltmeter
Asimov,
I'd never thought of using a S-bright for tube pins. It's an interesting idea, I'll give it a try. Regards, Tim Schwartz Bristol Electronics Asimov wrote: "Tim Schwartz" bravely wrote to "All" (11 May 04 12:11:00) --- on the heady topic of " HP-400D voltmeter" Tim, not to name a brand but I've found the s.brite green plastic scouring pads are great for cleaning tube pins. Just push the tube pins into the pad a few times (one at a time) and it gets cleaned of oxide without removing too much of the gold, silver, or whatever contact enhancing metal coating is used. TS From: Tim Schwartz TS Peter, TS I'd start by cleaning all of the tube pins, as they are likely to TS have corrosion on them from sitting all these years. Clean the range TS selector switch too. I suggest leaving the tube pins dry (no contact TS treatment) after cleaning. Sometimes 400 grit (more or less) TS sandpaper is the best way to clean tube pins. TS Regards, TS Tim Schwartz TS Bristol Electronics TS "Peter E. Orban" wrote: Hi Everyone, I picked up an old HP-400D voltmeter on a garage sale. Having cleaned most of the grime away from the equipment (it must have set in a basement for years) I plugged it in. The tubes all light up and the power supply voltage is around spec, but the meter moves always past its maximum in every range. The second amplifier chain, after the attenuator, is oscillating at about 1MHz, that is what the meter is showing. What could be the cause of the oscillation? Bad tubes? I am also thinking about bad ground connections, as if (I have to check this) grounds are connected to solder lugs that are riveted to the frame. There could be some corrosion at those rivets. Thanks, Peter -- Peter E. Orban National Research Council of Canada e-mail: ... New computer? But I like my vacuum tubes... They keep me warm. |
#7
|
|||
|
|||
HP-400D voltmeter
"Asimov" wrote in
: "Tim Schwartz" bravely wrote to "All" (11 May 04 12:11:00) --- on the heady topic of " HP-400D voltmeter" Tim, not to name a brand but I've found the s.brite green plastic scouring pads are great for cleaning tube pins. Just push the tube pins into the pad a few times (one at a time) and it gets cleaned of oxide without removing too much of the gold, silver, or whatever contact enhancing metal coating is used. TS From: Tim Schwartz TS Peter, TS I'd start by cleaning all of the tube pins, as they are likely to TS have corrosion on them from sitting all these years. Clean the range TS selector switch too. I suggest leaving the tube pins dry (no contact TS treatment) after cleaning. Sometimes 400 grit (more or less) TS sandpaper is the best way to clean tube pins. TS Regards, TS Tim Schwartz TS Bristol Electronics TS "Peter E. Orban" wrote: Hi Everyone, I picked up an old HP-400D voltmeter on a garage sale. Having cleaned most of the grime away from the equipment (it must have set in a basement for years) I plugged it in. The tubes all light up and the power supply voltage is around spec, but the meter moves always past its maximum in every range. The second amplifier chain, after the attenuator, is oscillating at about 1MHz, that is what the meter is showing. What could be the cause of the oscillation? Bad tubes? I am also thinking about bad ground connections, as if (I have to check this) grounds are connected to solder lugs that are riveted to the frame. There could be some corrosion at those rivets. Only if they were loose. Use an eraser,the kind for removing ink. TEK used to have a tube-pin cleaning tool made from rubber-abrasive rod like the 'bullets' used for Dremel Moto-Tools,with a small hole in one end.You push it onto the tube pin,and rotate the abrasive rod. You could also have bad electrolytics in the PS or decoupling. -- Jim Yanik jyanik-at-kua.net |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|