RCA 9TC240 Alignment...The start of my education (I hope!)

[Penthode]

Originally Posted by bandersen

Looks like it's a 130 ohm (R4?) resistor so that's fine and 20% high is fine too.
Odd half wave power supply design with the center tap being the -6.3 volt rail.

Could be as simple as CR2 being bad. Looks like it's there for protection. Perhaps it is same to disconnect the load form this board and measure the voltages? That way you could isolate the problem to this board or what it is powering.

CR2 is a voltage reference regulator. It is there to provide a -6.3v reference.

[Chris K]

Thank you for all of that work! I’ll see if I can track it down by your process of elimination. How should I go about isolating and disconnecting a specific -6.3V supply trace for a given board? I imagine I identify the numbers of the board tabs that supply a particular circuit board with that voltage and see what happens to the -6.3V reference voltage by isolating and disconnecting a single feeding point but how do I “lift” the single lead for that individual trace?

[Penthode]

Banderson's remark CR2 is for protection got me thinking about the circuit again. The design for for the balanced amplifiers required the negative supply. Because the +20v and -6.3v supplies are derived from the same source without a ground reference within the supply itself, the circuit externally determines it.

I have attached a drawing to outline my reasoning. The power supply is actually stacked 20 plus 6.3. The voltages with respect to ground is determined by the ratio of the +20v load and -6.3v load. Because the same amplifiers are across the +20volts and -6.3 volts, the ratio should remain consistently the same. Therefore the 7.2v zener is for protection. If the =20 rail develops a short, it would push the -6.3v very negative unnecessarily damaging components. The 7.2 volt zener therefore prevent the lower rail dipping below -7.2 volts. It also explains wht the zener appears to be a chassis mount hight powered zener external to the circuit board.

I do not think the zener is faulty. I do however think the negave supply is drawing slightly more current than it should hence the dip to ablout 3 volts.

I am going to reason this through over the next day and if you have any questions on my reasoning, let me know.

I would advise not touching the zener as I think it is okay and removing it may jeopardize other circuitry it was intended to protect.

[Chris K]

We're getting a bit outside my abilities and I hope as the hands on person, I'm providing accurate information regarding the problem solving. I understand solid state stuff even less than tube stuff! If we're talking about relatively low DC voltage circuits affecting one another, I guess I should be much more specific about providing exact values. A few years of measuring and evaluating voltages in the hundreds, and seeing 380V on a circuit that is supposed to have 400V on it thinking 380V isn't an issue, I think I better tighten my error bars. In reality, the +20V line is closer to 22V and the -75V is more like -74.5V (again, these aren't exact...I'm at work and I don't have it in front of me!)

Bottom line...I don't want to keep anyone up late at night chewing and contemplating this trying to problem solve it if I'm not providing accurate data and facts. I'll recheck the voltages this evening and provide accurate information out to 2 decimal places.

[Penthode]

Originally Posted by Chris K

We're getting a bit outside my abilities and I hope as the hands on person, I'm providing accurate information regarding the problem solving. I understand solid state stuff even less than tube stuff! If we're talking about relatively low DC voltage circuits affecting one another, I guess I should be much more specific about providing exact values. A few years of measuring and evaluating voltages in the hundreds, and seeing 380V on a circuit that is supposed to have 400V on it thinking 380V isn't an issue, I think I better tighten my error bars. In reality, the +20V line is closer to 22V and the -75V is more like -74.5V (again, these aren't exact...I'm at work and I don't have it in front of me!)

Bottom line...I don't want to keep anyone up late at night chewing and contemplating this trying to problem solve it if I'm not providing accurate data and facts. I'll recheck the voltages this evening and provide accurate information out to 2 decimal places.

You should not worry about keeping anyone up late. You have provided accurate facts. It is just that sometime the HP engineers were trying to be too clever (my opinion). When troubleshooting, the power supply often provides clues to what is wrong and right now we are only attempting to understand what is wrong. The understanding of the power supply sheds new light and the fact the +20v is spot on and the -6.3v is a bit low to me is not yet a red warning.

Let's review the symtoms again:

"...Have a very low power clean 12.5MHz sine wave. The frequency doesn't change whatever I do to the wheel and the output level has no effect on the amplitude. The meter still doesn't move into the readable zone..."

To me this again is pointing to the output hybrid amplifier. I think we should step back and reflect how to confirm this.

[bandersen]

I agree with your reasoning about the power supply. It is actually very similar to how the power supply works for the RCA TV.

Perhaps Chris can put his scope on the input to the hybrid amp?

[Chris K]

I could...if you guys know where I should attach it or at least look for it. However it wouldn't be until later today.

[Chris K]

Originally Posted by Penthode

You should not worry about keeping anyone up late. You have provided accurate facts. It is just that sometime the HP engineers were trying to be too clever (my opinion). When troubleshooting, the power supply often provides clues to what is wrong and right now we are only attempting to understand what is wrong. The understanding of the power supply sheds new light and the fact the +20v is spot on and the -6.3v is a bit low to me is not yet a red warning.

Let's review the symtoms again:

"...Have a very low power clean 12.5MHz sine wave. The frequency doesn't change whatever I do to the wheel and the output level has no effect on the amplitude. The meter still doesn't move into the readable zone..."

To me this again is pointing to the output hybrid amplifier. I think we should step back and reflect how to confirm this.

Well...if +22VDC is still considered spot on then we're good!!!!!

[Chris K]

Originally Posted by Penthode

You should not worry about keeping anyone up late. You have provided accurate facts. It is just that sometime the HP engineers were trying to be too clever (my opinion). When troubleshooting, the power supply often provides clues to what is wrong and right now we are only attempting to understand what is wrong. The understanding of the power supply sheds new light and the fact the +20v is spot on and the -6.3v is a bit low to me is not yet a red warning.

Let's review the symtoms again:

"...Have a very low power clean 12.5MHz sine wave. The frequency doesn't change whatever I do to the wheel and the output level has no effect on the amplitude. The meter still doesn't move into the readable zone..."

To me this again is pointing to the output hybrid amplifier. I think we should step back and reflect how to confirm this.

Let me be a bit more specific about the symptoms. The output level DOES have an effect on the amplitude in that whenever you select anything other than a +10dBm level, the signal goes away. The 12.5MHz CW is visible on just that one intensity setting.

[bandersen]

Which makes sense if the output amp is not working correctly. A little bit of signal is getting out on the highest output level setting, but it's so small on the other ranges you can't see it at all.

[Chris K]

Originally Posted by bandersen

Which makes sense if the output amp is not working correctly. A little bit of signal is getting out on the highest output level setting, but it's so small on the other ranges you can't see it at all.

It's not the highest...+20dBm is the highest and there's no signal there. I cleaned the wafer switch as best as contact cleaner and rapid twisting provides.

[bandersen]

Ah, my mistake. Could be a problem in the attenuator then. Wafer switch? The attenuator is shown on page 8.21. It's a cam shaft with specialized switch contacts. Please do NOT open it up and spray cleaner in there.
Cleaning controls in higher end test equipment is a bit of an art.

[Chris K]

I wonder what I remember cleaning then??? I certainly did not open anything up. Anyway, my mistake. Probably one of the sweep toggle switches I remember cleaning but thank you for that piece of knowledge. Part of my education I first mentioned. Precision instrument not to be cleaned like a Motorola volume pot!

[Chris K]

Originally Posted by bandersen

Ah, my mistake. Could be a problem in the attenuator then. Wafer switch? The attenuator is shown on page 8.21. It's a cam shaft with specialized switch contacts. Please do NOT open it up and spray cleaner in there.
Cleaning controls in higher end test equipment is a bit of an art.

The attenuator is that giant bank vault case the knob shaft goes into...yes yes...I remember that. No I didn't mess with it.

[Chris K]

Let me see if I get this and maybe this is what Bob and Penthode have been talking about but in a way I don't understand. I'm trying to keep up with the troubleshooting logic behind what you guys are saying.

I read Penthode's post about the supply voltage being stacked to make the 20 and -6.3 voltages available. Both voltages are across the amplifiers and the draw from those amplifiers is what keeps the voltage ratio the same? The fact that I'm getting a sine wave of 12.5MHz means the preamp is probably working OK and that primary amplification is missing? But if the voltage ratio is off in the preamp and it is getting -3.7 volts, the same as the output amplifier, wouldn't it be affected as well?

I'm giving myself a headache!