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 Post subject: EICO 460 O' Scope pops fuses
PostPosted: Oct Sun 25, 2020 1:10 am 
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Joined: Jun Thu 18, 2020 11:54 am
Posts: 22
Here is the story, The scope is a factory version, not a kit with spray paint markings showing it was a "Philco Ford test Instrument. I recapped the scope and replaced a huge number of resistors that were way off the mark.

When I power the scope on, using a dim bulb tester, I get the initial draw of current (bright) until the tubes start to come into emission then it dims a bit. I get a dot in the CRT...and then the dot rapidly will move off screen as the dim bulb brightens again.

If I leave the scope on it will blow the fuse in a couple minutes max.

The schematic I have (for the kit) is close, but not exactly the same as this factory scope...the schematic is for the kit. I've gone through the kit info and everything seems ok wiring wise...although there are differences...very minor mostly so I chose to accept that they are right.

The resistances and voltage readings in the build manual do not match up to the scope for the most part. Part of this I can attribute to the difference in wiring, but some just don't seem to make sense. For instance the 68U8 pin 6 should read 1.2K ohm, I get 1.6K, the second 68U8 should have the same reading, but I get 1.5K. The two 6CB6 tubes should have 4.3K on pin 7, but I get 22.

All the tubes have one or two pins that are not what the manual says they should be...based on the KIT instructions and schematic.

Any suggestions on what to look at would be appreciated...or suggestions on where to find the schematic that EICO used for in house builds. I looked extensively at all the regular places, but all I find are the build manual and schematic for the kit.

Thanks for any suggestions!
Mike
San Antonio


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 Post subject: Re: EICO 460 O' Scope pops fuses
PostPosted: Oct Sun 25, 2020 1:24 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1797
Mike your first two resistance readings are close enough. It's not a precisions instrument.

The problem may be in the power supply. I don't have a diagram in front of me (you should post one) so I can't be too specific. Pull all the tubes and see if the dim bulb lights. If it does and the fuse blows, you may have a bad power transformer.

After seeing the diagram I can offer help. But poking around tubes like those little ones won't help because they can't cause a fuse to pop. Start with the power supply. If the rectifier is a tube, pull that and see how the unit behaves. You may have miswired a filter capacitor.


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 Post subject: Re: EICO 460 O' Scope pops fuses
PostPosted: Oct Sun 25, 2020 1:26 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16749
Location: Albuquerque, NM 87123
Are you using a 1 amp fuse?

Pull the rectifier tube and leave the scope turned on; if the fuse blows then you may have a bad transformer. If not, then the extra load is on the B+.


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 Post subject: Re: EICO 460 O' Scope pops fuses
PostPosted: Oct Sun 25, 2020 1:31 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1797
I looked at the diagram. Your problem is likely around V9 the 6AX5. Check the connections on C27 A, B, and C. Those capacitors are among the most likely parts that need replacement. V9 may be bad, with a heater to cathode short.


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 Post subject: Re: EICO 460 O' Scope pops fuses
PostPosted: Oct Sun 25, 2020 2:46 am 
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Location: Tucson, Arizona U.S.A.
AFAIK, Eico used the same circuit in their kit and factory wired instruments. So what you have is likely the schematic for a different revision level. So if you can find one for the revision you have, it will be OK whether kit or factory. The revision may be just one sheet referencing the original schematic. That's what they did on the 425 when they changed from a 5BP1 to a 5CP1 CRT. One sheet describing the changes, no new schematic.

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 Post subject: Re: EICO 460 O' Scope pops fuses
PostPosted: Oct Sun 25, 2020 12:48 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 8459
Location: Sunbury, Ohio 43074
Well, I'll offer my usual after the fact observation that by replacing all those parts before turning on the instrument, you have introduced hundreds of variables now to contend with. Don't do that again. ;-)

After the advice by others about pulling the rectifier tube, which will most likely not cause the fuse to blow, I'd start by pulling the V and H deflection tubes, and see if it still blows fuses. If it does, keep pulling tubes, starting with those most likely to put the most load on the power supply until you find out which area of the circuit is causing the problem.

I suspect possibly something going amiss in the HV area after it's warmed up, but that's truly a shot in the dark. (humor intended)

If it is blowing fuses with ONLY the rectifier tube in, and without the other tubes in, then I agree the problem may be the power transformer, but usually that would pop the fuse immediately not after a long warmup. I'd more lean towards a problem with the filter caps and power supply filter circuitry... Please post up the schematic. We're kind of flying blind here.

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 Post subject: Re: EICO 460 O' Scope pops fuses
PostPosted: Oct Sun 25, 2020 6:01 pm 
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Joined: Mar Tue 10, 2020 5:11 am
Posts: 385
Location: Mission Viejo, CA
From your description, it sounds like it's blowing the fuse as, or after, things get warmed up. Thus, you can examine the power supply diagram and find series-connected resistors between electrolytic cap sections, for example. Then connect your VOM or DVM across them and monitor the voltage as it warms up. Where you find large voltage differentials, that's likely connected to the circuit that's drawing too much current and thus blowing fuses. An easy way to know where a voltage diff is too much is to calculate the estimated voltage drop across a resistor to dissipate the power that the resistor is rated for, then assume the designer chose the resistor to dissipate only around 1/2 of that. If your Ohm's Law calculations indicate too much voltage drop on a certain resistor for its power rating, that's likely (but not necessarily) the circuit drawing too much current.

SteveH


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 Post subject: Re: EICO 460 O' Scope pops fuses
PostPosted: Oct Sun 25, 2020 7:39 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1797
I suspect the filter capacitors. Leaky or shorted or installed reversed polarity.


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 Post subject: Re: EICO 460 O' Scope pops fuses
PostPosted: Oct Wed 28, 2020 2:22 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9601
Location: Beaver Falls, PA. USA
Did you replace the high voltage filter capacitors?

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