Another RCA CT-100 discovery B8001599

[Penthode]

Still fussing over the power supply.

To accommodate the inherent forward resistance of the selenium rectifiers when replacing them with silicon diodes, I initially inserted a 10 ohm 15W resistor in series with the AC feed to both diodes. I had been powering the chassis and didn't initially pay much attention until I noticed the resistor leads appeared to be blueing! The ac voltage drop across the resistor measured 20 volts. At 2 amperes, the resistor was dissipating a whopping 40 watts!! And it survived!!

So rethinking this I initially determined the forward resistance of the selenium rectifier was 10 ohms at 600mA. That is 600mA per diode whereas my 2 amperes above was a culmulative measurement of both silicon diodes as the circuit is a full wave doubler where each diode conducts only 180 degrees.

I did find that the B voltages were low at an input of 118vac. The higher selenium resistance may be due to aging of the rectifiers? Just to note I left the selenium rectifier running under test at 1 ampere for half and hour and it was only moderately warm. My guess is the fins may help to better radiate the heat?

Nevertheless, I next tried attaching a chassis mount 50W 8 ohm resistor to the chassis upright with a little bit of silicon grease to reduce the thermal resistance. Boy did that heat the chassis! The surrounding metal after five minutes was too hot to touch and after 10 minutes or so the whole upright section was pretty warm.

As the B voltages were a tad low, it made sense to reduce the series resistance. with about 4 ohms and an AC input of between 115 and 120 vac at the set input, the B voltages were close. It is interesting to note that as this beast consumes 475 watts to load down the mains supply!

I had an old pack of 1.5 ohm Ohmite ceramic 11 watt resistors. With three in series, I measured a voltage drop of 8 vac which means that the resistors are dissipating each about 10 to 12 watts. I have mounted them suspended on a tag strip and left the set running half an hour. The surface temperature at the center of the resistors as measured with an infrared thermometer shows 205 degrees celcius! they are mounted directly under the old selenium rectifier (left in place for appearance) and the chassis remains relatively cool (45 degrees celcius and the adjacent electrolytics remain cool: one electrolytic has a cardboard shroud and the other measured 35 degrees celcius after half an hour, I am going to leave it here and it will be a good test to see how these Ohmite resistors last while running at 100% of rated capacity. (If they fail, they will simply go open and they are in seies with the 4.5 ampere fuse).

You will see below photos of the three Ohmite resistor location and the silicon diodes on the opposite side of the chassis upright. I have included a photo of my ballast resistor concoction with the surface mount resistors on the heatsink. The Infrared Thermometer reads about 80 degrees celcius maximum when pointing directly at the resistors from below as mounted. I am happy with this solution as it plugs in to the original ballast socket with no modification.

[Penthode]

The chassis has been running fine on the bench (minus CRT) with it tuned to local programming thru a Zenith DTV box. Listening to just audio all day.

Next is to construct the input head and an IF test block to begin the sweep alignment.

I shall be using my HP8601a Sweep Generator and HP8600a Digital Marker Counter.

[Tom Albrecht]

Originally Posted by Penthode

I had been powering the chassis and didn't initially pay much attention until I noticed the resistor leads appeared to be blueing! The ac voltage drop across the resistor measured 20 volts. At 2 amperes, the resistor was dissipating a whopping 40 watts!! And it survived!!...

I had an old pack of 1.5 ohm Ohmite ceramic 11 watt resistors. With three in series, I measured a voltage drop of 8 vac which means that the resistors are dissipating each about 10 to 12 watts.

I'm not sure I follow the second set of power calculations above, but in general, the actual power dissipation in rectifier series resistors is far higher than what you would calculate based on a voltage drop measurement. Since the diodes only conduct briefly at the peak of the waveform (to recharge what has drained off the first filter capacitor), the current during this short interval is far, far higher than the average current. Likewise, the voltage drop is higher than might be expected based on average or RMS AC current for a sine wave. With the current compressed into such a short interval with very high current, and the power dissipation depending on the square of the current, the actual power dissipation is much higher than expected.

So keep in mind that the usual power, voltage, and current calculations only work for DC current or sinusoidal AC current (with RMS value for the sine wave). Once you go to pulsing current in a rectifier circuit, things are quite different!

The instantaneous power, voltage, and current still follow the usual relationship, but the relationship between instantaneous current and average current is no longer so simple.

[Penthode]

Originally Posted by Tom Albrecht

....the instantaneous power, voltage, and current still follow the usual relationship, but the relationship between instantaneous current and average current is no longer so simple.

Thanks Tom for your input and analysis. You are absolutely correct that my measurements are out the window as far as pulsing voltages are currents are concerned.

I have left the trio of 1.5 ohm 11W resistors in series with the silicon diode rectifiers I installed and they are pretty hot. I loomed at the Ohmite recommendations and the temperature is well within the range Ohmite specifies. And the B plus voltages are near correct. I will leave as us for now.

[Penthode]

Saturday afternoon made the input block attached to the KRK12C tuner using the dimensions from the CT-100 manual. The RF sweep signal is applied to the mixer diode in the tuner and using and alligator clip there is awkward. The input block position is reinforced with a sprung clamp around a transformer can a well as the two phenolic pins with straddle two holes on top of the tuner.

This will be used to perform the IF alignment and chroma channel as well as facilitating full sweep tests as outlined in the RCA Service manual.

Now on to the testing and alignment this week.

[old_tv_nut]

Originally Posted by Penthode

Saturday afternoon made the input block attached to the KRK12C tuner using the dimensions from the CT-100 manual...

Wish they did something similar for the CTC-5. That connection is so sensitive to lead length/dress.

[Penthode]

Originally Posted by old_tv_nut

Wish they did something similar for the CTC-5. That connection is so sensitive to lead length/dress.

Funny you mention the CTC5. A decade ago, when I was using the HP8601a to sweep align my CTC5, the RF alligator clip slipped off and dangling touched the B plus. It blew up the output amplifier of the HP generator. I had used the generator successfully for over a decade and it is a brilliant generator for consumer equipment and was vey saddened by this!

I immediately searched EBay for a replacement part or even generator for parts and found a brilliant and enterprising engineer redesigned the 8601a output amplifier module and offered them for sale. I bought one and the 8601a now has a flatter output response than the original amplifier. Had to be patient on the CTC5 waiting to fix first the HP generator but it was worth it.

I am grateful fortunate that gains do often come after adversity.

The photo below shows the contact spring against stage mixer diode pin.

[Penthode]

Decided to postpone the chassis alignment and bandpass evaluation while I fixed and refinish the cabinet. Peeling veneer relied and front mortised corners were separating. Glued with lot of long clamps all around to pull it back together.

Cabinet was badly banged up. Hoped not to have to do much sanding but had to fix many deep gouges.

Stained to match original finish. Came out very close to the finish removed. Two spray coats of Watco lacquer on the main cabinet to be lightly flattened and polished. Still to do lid and front wood control panel cover.

Seems to be a province wide shortage of lacquer products in Ontario at the moment. Found a hardware store 30 miles away had two litre tins which I collected on the weekend.