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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Wed 02, 2019 5:11 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1690
Use the Rigol with a 10:1 probe while working on any tube gear. That will avert many an annoying and expensive repair of the input circuit.

In fact, use the probe for all measurements except those where you need the extra sensitivity. As mentioned, always be careful to stay within safe signal limits.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 03, 2019 7:04 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
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Location: St. Louis, MO
My scope goes up to 300 Vrms. So if I use the probes set at 10X, I'll be able to measure up to 3000V and if I set that input to 10X, the scope will show the correct voltage?


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 03, 2019 7:16 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1690
Yes and no. I am sure the probe's voltage rating won't permit operation to 3 kV. Most probes are rated at 600 V or so. For higher voltage there are more expensive probes.

Other than that limitation, and perhaps a reduction in bandwidth, the probes will work as you say. Be sure to adjust them with the scope's calibrator.

Many newer scopes allow for probe attenuation factors. Apparently yours is one of those.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 03, 2019 7:21 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
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Location: St. Louis, MO
I found the datasheet for the probes. They go up to 300 VAC in 10X. So I need a higher spec probe? How high do these old tube scopes go in voltage?


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 03, 2019 10:09 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
Some scope probes have a frequency VS voltage de-rating chart. So with safety
and damage concerns, you must state frequency of a sin voltage, and rise time of
a pulse.

Ruining test gear on home TV sets, is one thing. The stunt repeated on commercial
or industrial gear having high energy levels is quite another.

A scope probe rated for 100 mHz, with an adjustment capacitor might be ruined
trying to get the best square wave of the test signal based on a clipped 60 Hz wave.

The fancy imported probe sets costing low money are flimsy,

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Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Thu 03, 2019 11:12 pm 
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Joined: Jan Tue 10, 2012 8:39 am
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Yes it's a conflict between how much to pay and what you want to get. A cheap probe will be fine for a long time if you are an infrequent user. If you are a serious technician or engineer, opt for a higher priced name brand probe.

Probes are available for nearly any voltage. If you have to, build your own. It's not magic, but you must understand the physics well enough to avoid serious mistakes. The prices of high voltage probes are high enough that it gives one pause to think whether it's really needed.

Yes the old tube type scopes can withstand higher signal voltages without permanent damage but they also are very limited in their ability to portray the waveform adequately. If you want to view the high voltage on a CRT you will probably have to build your own probe. For plate circuits a 1 or 2 kV probe will do. I have successfully probed high voltages with the ordinary 10:1 probes but I don't recommend it. If all I want is an idea of what the rf looks like on the plate of a 6146 I can clip the probe to the wire insulation and avoid direct contact. I can't make an accurate measurement this way but it may be good enough.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Fri 04, 2019 12:50 am 
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Location: Lincoln City, OR 97367
Greetings to ZombieElvis and the Forum:

If your probe is good to 300 VAC, then it is good to 424 volts DC. As long as the peak voltage of the waveform you are trying to measure does not exceed that, you should be all right. I am sure that you can read any of the grids in your Eico (with the possible exception of the CRT grids) with your 10X probe. You can measure the DC plate voltages with a meter and then make a guess as to whether or not any peak waveform that you are likely to see exceeds 424 volts. If it is unlikely to do so, then you should be able to measure that point also.

Regards,

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Fri 04, 2019 4:30 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
Bob, a modified old poem.

He who probes the plate of a 6146 by defeating the interlocks may end up in pine box. :shock:

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Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Sat 05, 2019 10:34 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
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Location: St. Louis, MO
Jthorusen wrote:
Greetings to ZombieElvis and the Forum:

OK, now you need to use the Rigol to scope the sweep ramp in the Eico. You might start at the grid of V201A. See what the ramp looks like. Move toward the CRT or away from it until you find the cause of the non-linearity. Do be careful not to allow more voltage to be applied to the Rigol than it can handle. Solid-state scopes are fussy about this!

Good Luck,


Let me see if I got this. Attach my o scope's probe to the grid of 201A and make sure that it's a sawtooth wave? I did that. The slope isn't perfectly straight, but it's fairly sawtooth. The output on the other hand is weird looking.

One thing I did notice is that the SAWTOOTH output on the front is a really ugly sawtooth, which just so happens to be connected to the grid pin of V202.

Did we find the culprit already?


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Sun 06, 2019 1:54 am 
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Greetings to ZombieElvis and the Forum:

It sounds like it. Have you replaced C202? Have you tried another 6SN7 in the V201 socket? You may also wish to check the values of resistors around that tube. You may also wish to try another 6C4 in the V202 position.... 6C4's sometimes exhibit weird behavior.

My bet would be on C202, unless you have already changed it.

Regards,

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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Sun 06, 2019 3:34 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1690
I agree that it's foolish to measure the plate of a 6146 with an oscilloscope. Be that as it may, there are those who would do such a thing and I thought a little comment might be in order.

If the tube is a class C amplifier running at a typical 700 Volts on the plate, the peak voltage will be a few thousand, depending on the Q. If someone tries that measurement with no load, he is definitely asking for a Darwin award.

I suppose there are probes one could buy that will work here but they'd be expensive. And probably of very limited bandwidth.

It's possible and feasible to install a permanent voltage divider to enable such measurements but there is the bandwidth consideration as well as power dissipation. A capacitive divider might be better. If one wants to study Class C amplifiers I would suggest making one with a different tube at a much lower voltage.

In summary, it's always good to understand what you are doing to avoid serious errors.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Sun 06, 2019 6:49 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
What the Eico 470 presents to the viewer as a linear transition from left to right
of the CRT is a voltage across pins 7 and 8 of V501 (CRT). To see that waveform on your Rigol
scope, would require 2 probes, (tips only) with no ground connection. The Rigol would be in
the differential mode, Both probes would have have to high voltage. Also if the Eico CRT
burps * you really dont want a dainty scope around it.


That CRTs 'burp' (flash over) is why colour TV sets employed gap-caps
(high voltage capacitors with air spark gaps) to protect sensitive solid state
electronic parts.




The Eico 470 horizontal amplifier is AC coupled. Considering this, a triangle waveform
finding it's way over the wide range of sweep frequencies obviously will be compromised
in a scope costing a hundred bucks in kit form.

The best resource on line to dig into scope design in the 1950s is the gigantic Rider &
Uslan encyclopedia... Below is a snip of waveforms of a cathode coupled multivibrator
of the genre the Eico 470 uses.
Attachment:
Rider Uslan c7.jpg
Rider Uslan c7.jpg [ 223.24 KiB | Viewed 1097 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Sun 06, 2019 9:33 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 714
Location: St. Louis, MO
Jthorusen wrote:
Greetings to ZombieElvis and the Forum:

It sounds like it. Have you replaced C202? Have you tried another 6SN7 in the V201 socket? You may also wish to check the values of resistors around that tube. You may also wish to try another 6C4 in the V202 position.... 6C4's sometimes exhibit weird behavior.

My bet would be on C202, unless you have already changed it.

Regards,


Yes, I replaced C202. The original was a double cap, a pair of 8 uF caps in the same case. I replaced it with a pair of 8.2 uF caps sharing a ground. I tried a different 6SN7. It made no difference. I swear I have a spare 6C4 somewhere...

You know what? Let me find my thumb drive. I'll do a capture of the Rigol screen and post it.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Sun 06, 2019 10:34 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 714
Location: St. Louis, MO
OK this is weird. I decided to attach my little function generator back up, started with a triangle wave. If my Rigol is attached to pin 1 of V201, the Eico's screen output is perfect, but as soon as I disconnect the probe, the Eico's screen goes blurry again in the same spot. Just to see what would happen, I unplugged the probe's BNC from my Rigol. That was enough to make the trace on the screen straighten up.

edit: I found my spare 6C4. Swapping it in made no difference.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Sun 06, 2019 11:41 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 714
Location: St. Louis, MO
Here, have some O scope output. Again, merely touching pin 1 with an unplugged probe will make the Eico's screen behave. Yes, I already reflowed all of the pins on that socket.


Attachments:
V201 pin 1.png
V201 pin 1.png [ 43.06 KiB | Viewed 1075 times ]
V201 pin 3.png
V201 pin 3.png [ 50.31 KiB | Viewed 1075 times ]
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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Mon 07, 2019 11:08 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
Make sure R203, S302, and the panel shown below are all tightly grounded .
Attachment:
Eico 470 make the mounting for S302 is solidly grounded.JPG
Eico 470 make the mounting for S302 is solidly grounded.JPG [ 67.3 KiB | Viewed 1066 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Mon 07, 2019 4:46 pm 
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Joined: Nov Mon 02, 2009 7:01 am
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Location: Lincoln City, OR 97367
Greetings to ZombieElvis and the Forum:

If placing the probe on the grid of V201 corrects the problem, look for an out of tolerance R201, or a bad connection between it and the grid or between its other end and ground. In fact, if you haven't already replaced it, I would just go ahead and replace R201 with a modern film resistor. I have seen high value carbon composition resistors do some strange things... even turn into diodes.

Regards,

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KB6GM


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Mon 07, 2019 6:24 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 714
Location: St. Louis, MO
R203 is well grounded. The instructions specified 2 grounds for it. One ground wire goes to a stamped lug surrounding the socket for V201. The other goes all the way to the back, to a screwed on lug down by V202 (reminder: that was corrected in the addendum sheet). As for switch S302, it's not grounded. J201 in the picture is Horizontal Input.

I replaced R201 months ago, but tested it again. I even checked from C201's lead all the way to ground on the far side of the case. It's within 2%.


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Mon 07, 2019 6:53 pm 
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Joined: Nov Mon 02, 2009 7:01 am
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Location: Lincoln City, OR 97367
Greetings to ZombieElvis and the Forum:

I would suggest that you bridge C202A with a .01 disc ceramic cap. Connect it directly from pin 2 of V201's socket to ground using the shortest leads possible. See what happens then. (You can leave the cap in place if it doesn't help; it won't hurt anything.)

Regards,

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Jim T.
KB6GM


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 Post subject: Re: Instruction manual for an Eico 470 oscilloscope?
PostPosted: Oct Mon 07, 2019 9:39 pm 
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Joined: Jul Mon 01, 2019 4:42 pm
Posts: 714
Location: St. Louis, MO
Added that cap to pin 2 like you mentioned. It didn't do anything. In fact, it made the picture worse when I touch a probe to it now. I have several traces overlapped when I touch a probe to pin 1.


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