Admiral 17T1: Never Give Up, Never Surrender

[Winky Dink]

The voltage difference between pins 5 and 6 of V-9 is dependent on the setting of the contrast control. When contrast is set at midrange as directed in the instructions, the voltages are consistent with the specifications in the voltage chart.

The 80-volt difference is correct when the contrast control is at its max setting.

[jr_tech]

Have C-37 and C-39 been replaced with mica caps yet?

jr

[Winky Dink]

Yup. C37, C39 are now .001pF mica, but no improvement. Reassessing the problem. Set worked before I started restoration. Intended to do nothing but replace paper, cardboard, electrolytics, and defective components. But after I started work, the set didn't work. Two possibilities are (1) I inadvertently changed something, or (2) There are one or more failing components which have not be identified. So, my current plan is to restart the project using the 200 chassis photos I took before I touched anything. I think I can reproduce its original condition (from the previous restoration), and recheck every connection and component. This could take a few weeks. Hope you all will be there when I start posting again.

Thanks for all the help so far.
Winky.

[Winky Dink]

Hello again. I've spent a lot of time ensuring that this set is in exactly the same condition as when I acquired it. Also checked the values of 20 original ceramic caps and replaced one disk that showed more than twice the specified capacitance and one with possibly overworked leads. Also repaired and reinstalled the original horizontal size pot. Other than replacing defective parts and upgrading parts to comply with advice from this forum, the set is exactly the same as when I bought it.

Now, to address one issue at a time, please consider V15, 6SL7, Balanced Vertical Output. I have inconsistent voltage readings going back to when I first started working on the set. Now, rather than trying to describe my difficulty in reading the voltage, here's a quick video of the multimeter on one plate and one grid.

Adm 17T12 9015 V15 Jitters by H Boyars, on Flickr

I checked it with another meter and with another tube, but both grids, both plates, and one cathode still give the same result. What might cause this phenomenon?

Here's a link to the 17T1 schematic:
https://flic.kr/p/2mv3Wrm

[bandersen]

The plate voltage for the 6SL7 comes from the HV supply thought a string of voltage dividing resistors. If the RF HV supply isn't working properly or any of those resistors are off, you can get strange results. Like the voltage may fluctuate with the screen brightness if the HV rectifier tube is weak.

[Winky Dink]

Thanks. I've been looking things over. Retested the 1B3 and even though it's quite strong, I tried switching out another strong 1B3. To no avail. I'm rechecking the resistor chain(s) and everything connected as if I've never seen it before.

[jr_tech]

Possibly your dmm is measuring the dc value plus the instantaneous value of ac vertical sweep voltage present on the tube elements mentioned.

Do you have an old meter with a needle (slower response time) such as they likely used when making the voltage measurements in these ancient service documents?

jr

[Winky Dink]

1. Regarding the HV supply to the 6SL7, the circuitry is correct and all components are the right values. I followed the voltage from the tube back through the resistor chain until I reached 500V at R73. The voltage readings were still fluctuating.

2. Regarding "instantaneous value of ac vertical sweep voltage" (not sure what or how that happens): I have no analog meter. But wait--I do have a 15 volt panel meter! I set the needle at 1 volt so I could see negative deflection, checked the grids and cathodes, then reversed the leads and repeated the readings. The readings were perfectly steady: G1 1.5V, G2 zero, K1 zero, K2 4.5V. So, does that mean the vertical sweep voltage is the cause of this issue, and I can ignore the irregularity of the voltages?

[jr_tech]

“-I do have a 15 volt panel meter! I set the needle at 1 volt so I could see negative deflection, checked the grids and cathodes, then reversed the leads and repeated the readings. The readings were perfectly steady: G1 1.5V, G2 zero, K1 zero, K2 4.5V. So, does that mean the vertical sweep voltage is the cause of this issue, and I can ignore the irregularity of the voltages?”

Perhaps... the ac voltage (vertical sweep signal) may be disrupting the measurement of the dc voltage, as your fast response dmms may be sampling the dc voltage as well as the value of the ac signal at the instant(s) the samples are taken.

Or perhaps the dmm is attempting to auto-range to track the varying voltage .

On the other hand, the jumping could be caused by a noisy resistor in the hv circuit (popcorn noise), but it seems that your panel meter would have indicated fluctuations.

Do you have a high voltage probe?

jr

[bandersen]

Yes, I would think it's the sweep voltage causing those readings. It's a pretty large sweep sawtooth wave on the plate and grid.

[Winky Dink]

High voltage probe-no. I should mention that with the dmm set for AC, when reading what should be 300VDC, then I get the erratic reading in the neighborhood of 30 volts-whereas I would usually expect less than one volt.

Popcorn noise? Is that audio? I have been getting some crackling noise (with no signal) which I didn't notice previously. To rule out noisy resistor, I could do a temporary switch of ALL suspects if you could tell me which ones to do.

Thanks. I'd better leave it that. I can't deal with much more than one thing at a time.

[jr_tech]

30 volts fluctuations, or 30 volts signal?

Can’t point out a particular resistor, any can become noisy/erratic.... high meg ohm resistors are likely the worst offenders.

je

[Winky Dink]

30 volts signal--fluctuating within the 15 to 40 volt range. Nearly all of the resistors in the megohms have been replaced with new. I may start replacing the old ones to see if that helps. Beyond that, I have no direction to go with this project.

[jr_tech]

30 volts signal??? is this measured on pin 5 and/or 2 of the 6SL7? I woud expect hundreds of volts of vertical sweep signal.

jr

[Winky Dink]

Sorry. I wrote this reply earlier but apparently failed to post it.
Repeated those readings several times with similar results for the plates, pins 2 & 5. The readings are from numbers erratically flashing on the meter for less than half a second. Always ranging from 10V to 65V when the meter is set for AC. (Actually, one time I got steady readings of .8V and 1.0V, but the set wasn't turned on.)