Sylvania 1-076 arrived!

[bandersen]

That's just a 12AU7 isn't it ? I can send you a few if you need some.

[jeyurkon]

Yes, it's just a 12AU7. I foolishly bought the gold plated version of the ECC-82 from AES thinking that it would be more reliable and have better controlled characteristics if I had to replace it in the future.

JJ doesn't list this version on their web site anymore, which is interesting. AES still carries it.

I'm pretty sure the light bulb effect was because of a heater-cathode short. I looked at the schematic of the Precision 10-12 and when you set it up for a particular tube the first state that it will be in is with the filament return selected and all other pins connected to filament supply. With a cathode-heater short, say in the middle of the filament, then it's like applying 6.3VAC to a 3VAC filament. The cathode that is connected to pin 3 is shorted to the heater, but there's no way I could prove that it didn't happen after the burn-out.

I'm going to check the ECC-82 that I have in the vertical section with a leakage checker first.

Thanks for the offer of the 12AU7s. I just bought four "Baldwin Sylvania" 12AU7s on ebay for less than I paid for a gold ECC-82.

John

[jeyurkon]

The ECC-82 was working even with the shorted heater/cathode because the cathode of that section is at DC ground anyhow. *I meant to also say that the filament supply is isolated.* It just injected some hum, which I had noticed. There had been a very small slow weave of the dot pattern. It would reduce the feedback in the horizontal oscillator a little bit.

I put in a 12AU7 that I had taken out of the set. It's at the low end of the good scale, nearly in the weak when I test it. Using it I get insufficient horizontal width and fold-over on the right. I hope the ones I ordered are strong.

I was surprised that it performed so poorly with this tube so I did some more checking. I found one resistor out of tolerance by 20%. It's a 10% resistor. Probably shouldn't matter too much but I'll replace it. Another one is off by 20% and one is just the wrong value. It looks like someone replaced it using the schematic from the previous production change and not the current one for this chassis. This ends up shifting the grid bias on the Horizontal Discharge half of the 12AU7.

The 6BD5 horizontal output tube seems strong and I didn't find anything wrong in that section.

John

[jeyurkon]

I decided that R151, 470K being high by 20% shouldn't be problem since they used a 2.2meg in the previous version. I was wrong about R152 being the wrong value installed, I misread the bridge number in the schematic. R155 is a bit high, but again shouldn't cause a major problem. I tried various 12AU7's and still had the folding on the right. I decided to take the JJ ECC-82 from the vertical section and try it expecting to have the horizontal back the way it was. No, changer other than improved width.

I then did the horizontal alignment. This was much easier than the first time since there was much less interaction between the adjustments. Everything behaved as it should. I ended up with a nice horizontal alignment. This makes me believe even more that the brand new ECC-82 that I had used originally, had a heater/cathode short to begin with.

If you want to read about how the alignment is done you can find it at:
Service Manual

It uses the following supplements:

Supplement-1

Supplement-2

I have chassis C15 and bridge B17.

While the effects are obvious, I don't understand why the charge peaking resistor R211 needs to be shorted in the first part of the adjustment, or what the ringing coil L60 does.

An oddity is the 20MFD electrolytic paralleling a fuse. It's C219 around F1. Any ideas on that would be appreciated.

John

[jeyurkon]

I was still having a problem with focus on the right hand side of the raster. Jr_tech's suggestion made the most sense so even though I didn't see a ripple in the screen bias I added, I decided to check it again.

I foolishly decided to try a different 6BD5GT HOT at the same time. I didn't get a raster and no HV. I checked the fuses and F3 was blown so no -125V.

I checked the 6BD5 I put in and it was at 100% where a NOS tube should be 70%. I probably should have reduced the horizontal width before turning it on. F3 used to be 1/4amp but they upped it to 0.3amps. Probably still marginal. Replacing it and going back to the first 6BD5 didn't bring back the HV though. I did all the tests I can without removing the HV assembly from the bridge and everything looks o.k. Fortunately the HV RF transformer seems good. That would really be unobtainium.

Maybe the 6Y6 went bad just by coincidence. Except for taking out the fuse, I don't see why the 6BD5 would make the HV go away. The heater of the 6W4 is isolated so a short there shouldn't matter either.

The radio works so the +170 and +180 must be o.k. I'll have to swap the 6Y6 and if that doesn't work, then start troubleshooting.

John

** I forgot, there was something else I did differently too. I had left the TV in the radio position when I shut the AC off. It was still in this position when I turned the AC back on. The HV section still has +170 and -125 supplied to it when the radio is on, but the filament of the 6Y6 is off. Maybe the 6Y6 didn't like waking up this way. Guess I'll find out when I sub it. **

[jeyurkon]

My dad had cut 1/4" off the bottom of the molding in front of the TV section to make room for the cedar boards to fit. You can see the cut in the first photo.

I carefully measured the radius of the bead and bought a beading bit for my router. Although it's obvious in the second photo, I didn't notice that the step isn't perpendicular to a tangent of the arc. I could still use the router bit to make a replacement molding, but I'd have to do it in two steps. First make the bead at the appropriate angle, then go back and make another cut to square up the step. The second cut would have to be precise.

It turns out that the molding goes up 1/2" under the top front trim so all I really have to do is remove the molding and slide it down to the bottom. The top will still be hidden by the trim.

Well, if I break the second molding removing it I at least have a cutter to try to make a replacement.

John

[jeyurkon]

Well, the lack of HV turned out to be the 6Y6. I put in a new one and I got a solid 11KV. Maybe there's something to this cathode stripping theory after all. I hope not, I like to listen to the radio on this set and that biases the 6Y6 with the heater off.

With the higher HV the horizontal width is marginal. I have to turn the width control to the end. I tried the NOS 6BD5 that I thought killed the B- supply. I turned the width to minimum first. It gave me a raster, then as I turned the width up it would soon start to collapse. Eventually, even at a reduced width bad things started to happen. I wasn't sure if it was Barkhausen or breakdown. It caused static in the audio and looked scary enough that I immediately shut it off. The tube tests at 100% when it should test 70% if new.

I can improve the width by going to a 6BQ6. It's an easy change, but it would make it a bridge code B18 instead of B17. I'm not sure if I want the bridge to be different than the version stamped on it.

This and the 12AU7 are two bad tubes in one lot from AES. I can't blame them, but the next time I order any tubes I'll test them in circuit immediately upon receipt. I have a small stock of bottle shaped 6Y6's. I have two really good ones from eBay. Looks like I'll need to obtain a few more.

The focus is better, but when using a dot pattern it's clear that it's an astigmatism. I can correct it with a weak external magnet. The next time I have the CRT out I'll try degaussing the yoke support and play around more with the location of the focus magnet. If that doesn't work I guess I'll resort to mounting a weak magnet.

The photo is the plate of the horizontal discharge 12AU7 tube. The service manual shows a rounded sawtooth and it doesn't have the dip between the teeth that mine shows. Is this a problem?

John