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 Post subject: Solar CBB 1.60 Mod
PostPosted: Nov Wed 22, 2017 10:00 pm 
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It has a bad transformer. I want to swap in one from a Eico 950b but the high voltage winding is 560v instead of 450v. I've looked at many of the kit cap checkers and they seem almost identical to the Solar except for that voltage but they all use the same tubes. Would it be simple swap in or would I need to find a way to get that voltage down to 450v? If so. How?

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 Post subject: Re: Solar CBB 1.60 Mod
PostPosted: Nov Wed 22, 2017 11:20 pm 
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In some way, you need to make the DC voltage supplied to the instrument match what it was originally. That can be done in a variety of ways; dropping the transformer voltage through a resistive divider, modifying the voltage regulator to drop more voltage etc etc etc. Either way needs to take into account that you are going to be generating some heat somewhere. I'd try and find the right transformer. If you try dropping the transformer voltage through a divider, that is in and of itself putting more load on the transformer, heating both it and the divider. So you'd need to make sure that the new transformer has sufficient capacity to handle the new load. It's not simple.

Search more for the proper transformer would be my vote


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 Post subject: Re: Solar CBB 1.60 Mod
PostPosted: Nov Fri 24, 2017 4:01 am 
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It's a bit of a challenge. You need the 55-volt bridge winding in the power transformer in order for the unit to work, but if you put a 560-volt transformer from an Eico 950 in there, the 6X5 will be over its voltage ratings. But this tester does not use the type of voltage multiplier power supply circuit found in Eico, Heathkit, and many other capacitor testers. Instead it is an ordinary half wave circuit with low current requirements. There is a permanent voltage divider resistor connected across it, so a dropping resistor placed in series with the plates of the 6X5 will work fairly well to lower the B+ voltage.

From the circuit http://bama.edebris.com/manuals/solar/cbb it appears that they were hoping to get about 400 VDC out of the power supply circuit across a 36-k ohm divider resistor. This suggests that the load current is around 11-mA. The eye tube would draw perhaps another 2-3 mA, so the total load on the rectifier is estimated at about 14 mA. You need to drop 110 volts to get from 560 VAC down to 450 VAC, so a resistance of 7,857 ohms is called for. This is not a standard value but 7.5-k ohms is. The lower resistance should work out okay because the HV winding voltage of the replacement transformer will likely be less than 560 volts when it is under load. The resistor will be called on to dissipate 1.47 watts, so I would use a 3-watt metal oxide film type (they tolerate high temperatures well).

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 Post subject: Re: Solar CBB 1.60 Mod
PostPosted: Nov Sat 25, 2017 3:00 am 
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Chris108 wrote:
It's a bit of a challenge. You need the 55-volt bridge winding in the power transformer in order for the unit to work, but if you put a 560-volt transformer from an Eico 950 in there, the 6X5 will be over its voltage ratings. But this tester does not use the type of voltage multiplier power supply circuit found in Eico, Heathkit, and many other capacitor testers. Instead it is an ordinary half wave circuit with low current requirements. There is a permanent voltage divider resistor connected across it, so a dropping resistor placed in series with the plates of the 6X5 will work fairly well to lower the B+ voltage. .

Umm. . . The Eico 950 and 950B both use a half wave rectifier, not a voltage multiplier.
http://bama.edebris.com/download/eico/950/eico950.djvu
http://bama.edebris.com/download/eico/950b/eico950b.djvu
Whether they use the same transformer or not is debatable since the schematics don't show the part numbers and the circuits are different. Also, at least the 950B runs the 6X5 considerably over its PIV rating, perhaps 1600 V vs 1250 V rating https://frank.pocnet.net/sheets/127/6/6X5GT.pdf. The 950 runs it over its max heater-cathode rating, >500 V vs 450 V rating. And yet they work and continue to do so.

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 Post subject: Re: Solar CBB 1.60 Mod
PostPosted: Nov Sat 25, 2017 3:18 pm 
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If you follow the Eico 950B circuit, which I believe originated earlier and was used on many other testers, you see that there is a +200 volt output and a -520 volt output derived from a single transformer winding of 560 volts. The total voltage developed is 720 volts but it's not 720 volts above ground, it's 200 above and 520 below. Through clever placement of the capacitors they effectively use both halves of the AC cycle with one half wave rectifier. That's voltage multiplication, though it's not exactly a doubler. They could do this because the voltage polarities are only relative inside the tester, the cap under test doesn't care as long as it is connected the right way.

With about 720 volts on it, the 1250 volt PIV rating of the rectifier tube is not exceeded.

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 Post subject: Re: Solar CBB 1.60 Mod
PostPosted: Nov Sun 26, 2017 2:39 am 
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Chris108 wrote:
If you follow the Eico 950B circuit, which I believe originated earlier and was used on many other testers, you see that there is a +200 volt output and a -520 volt output derived from a single transformer winding of 560 volts. The total voltage developed is 720 volts but it's not 720 volts above ground, it's 200 above and 520 below. Through clever placement of the capacitors they effectively use both halves of the AC cycle with one half wave rectifier. That's voltage multiplication, though it's not exactly a doubler. They could do this because the voltage polarities are only relative inside the tester, the cap under test doesn't care as long as it is connected the right way.

With about 720 volts on it, the 1250 volt PIV rating of the rectifier tube is not exceeded.

Whoa there. It's still a half wave rectifier; they just put a voltage divider on the output to get two different voltages. They happened to put the ground point at the tap. That's the same as back bias in a radio. It's not a voltage multiplier and it doesn't use both halves of the AC cycle. The rectifier blocks one half cycle. Note that the transformer secondary has no ground connection.

Back to basics. When the AC input to a half wave rectifier is negative, the peak voltage on the plate of the tube can go as high as the peak output of the transformer. The schematic says the RMS voltage is 560 so the peak is 1.414 times that or 792 volts. The filter capacitors are holding the cathode positive 720 volts. Since the plate is negative and the cathode positive, the voltages add. 792 plus 720 is 1512 volts. Since the transformer is unloaded at this time, it's output will be higher. That's how I got "perhaps 1600 V".

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 Post subject: Re: Solar CBB 1.60 Mod
PostPosted: Nov Sun 26, 2017 3:15 pm 
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So much for a write-up from an old electronics magazine I read years ago and never questioned or debunked. I had to sit down with the Eico 950B schematic and a second cup of coffee over the long weekend and figure out what is really going on in its power supply circuit. It's a bit off topic for the thread but may prove useful for somebody who has this type of capacitor tester.

With reference to the 950B diagram (http://bama.edebris.com/manuals/eico/950b/), let's take the eye tube and the filter capacitors out of the circuit for a moment. We then have the voltage adjust pot, P3, in series with R7, 3.3-k ohms, connected to the upper end of the HV transformer. The lower end of the transformer is connected to the plates of the 6X5. Its cathode is connected to R8, 68-k ohms. The lower ends of the pot and R8 are connected to chassis ground which completes the circuit.

Now on AC half cycles when the lower end of the transformer winding is positive, the rectifier will conduct and we get a little less than 560 volts across 171,000 ohms. (The rectifier drop will only be a few volts since the load current is very low). Ohm's Law tells us the current will be 3.27 mA. This will produce about +222 volts across R8 and -327 volts across the pot. About 10 volts drops across R7. When the lower end of the transformer winding is negative, the rectifier will not conduct and no current flows.

So how do they get the leakage test voltage to -520 volts with +200 volts on the other side of the supply? Put the eye tube and the capacitors back into the circuit. The capacitors are small but the shunt resistors have relatively high values. So the voltages will not rise to peak value but they will be above RMS. The eye tube draws 2-3 mA's of target current, and since everything is in series with P3, the target current increases the voltage drop across the pot to about 500 volts. This leads to the somewhat unexpected conclusion that if the eye tube is weak, the tester will not develop the full leakage test voltage. It also demonstrates why the filter caps have to be in perfect condition in order for these testers to work properly.

On the AC half cycles where the rectifier doesn't conduct, the upper end of the HV winding is held at -520 volts DC by C1 (8-uF). The upper end of the winding is the positive end during these half-cycles, so the peak voltage applied to the plate of the 6X5 at the negative end of the winding is -520 volts + (-790 volts) = -1290 volts. C9 will hold the cathode at +200 volts so the total PIV is 1490 volts. At first glance this appears to be bad engineering since every tube manual in the world tells us that the 6X5 is only rated for 1250 PIV. But that PIV rating is for full wave service which assumes that one diode in the tube is conducting while the other one is blocking. Used as a half wave rectifier, the PIV is higher. Eico would have had to call the applications engineers at their tube supplier for this data as it is not a documented mode of operation for the 6X5, but it does turn up in other things like HV supplies for oscilloscope CRTs. It appears that it was an acceptable practice as the tubes seem to work well and last a long time in this application.

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 Post subject: Re: Solar CBB 1.60 Mod
PostPosted: Nov Wed 29, 2017 2:10 pm 
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The previous owner had tacked on an outboard transformer to provide the HV. It turns out that it also has a 6.3v tap. So I think for now I'll use it and install a pair jacks for the 55v and use my PR57 to supply voltage for power factor tests. I'll put off building a new box until I find the right transformer.



Thanks for your input guys. Your knowledge is phenomenal. I only possess a small fraction of a clue. :) So didn't understand all of what was posted but I know more now then I did before.

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 Post subject: Re: Solar CBB 1.60 Mod
PostPosted: Nov Thu 30, 2017 8:40 am 
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I think part of the problem with understanding the 950B power supply is that it is drawn with the negative on top and the positive on the bottom. That's upside down from what we usually see in tube circuits. Add the tapped voltage divider on the output and it can look pretty weird.

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