GE Adventurer II / 15SF not locking sync

[ieure]

Hi folks. I have a Computer Space arcade game which uses a modified GE Adventurer II (model SF3101BK) television to display the game. These were inexpensive hybrid sets, mostly tubes, with a handful of solid state components. They were modified to inject signal directly into the video amp, by connecting it to pin 11 of V2.

My symptom is that vsync doesn't lock, so the image rolls. You can get it to roll very slowly by adjusting the vertical hold/size/linearity pots, but it never locks, and will begin creeping up (or down, depending on the pot position) after a few seconds. This set has very low hours on it, maybe 100 total over 50 years, so the failures are more likely to be from age than wear.

I replaced C402, which is a multivalued electrolytic cap, and looks to be the only lytic in the set. This maybe helped a little, but didn't solve the problem.

I kind of suspect the vertical tube, a 17JZ8 @V4, but it seems unlikely that it would have gone bad over time.

Am I off base? What else should I be looking at?

The schematic is in the back of the manual for the game: https://www.computerspacefan.com/ComputerSpace.pdf

Note that some copies of this game came with an S3 chassis set, and others with an SF, so the manual has both drawings. Mine has the SF chassis.

[Electronic M]

It's most likely in the resistors and caps between the plate of the sync separator and the vertical tube grids or possibly in the parts between the vertical output plate and the vertical osc grid. Resistors of this era tend to drift far higher than tolerance and caps can short, leak DC current or go open. If the tube was bad you'd probably have no deflection or little deflection... Though odd ball failures like a heater cathode short introducing AC could cause it. (Granted if it was a heater cathode short the set would loose sync strongly for an instant every ~11 seconds, assuming NTSC color vertical rate ,then relock on its own and continuously repeat the cycle).

Resistors you can check. Caps you can't properly check for leakage without an eye type capacitor tester that can perform leakage testing at full rated voltage...So substitution with new caps is usually the best test.

If you can make it roll up or down it's almost certainly a sync issue. Horizontal locking range will also tell you something about where the issue is...If horizontal hold is stable for almost a whole turn then it's the components between the separator plate and the vertical, but if hold just barely stably locks the whole sync separator stage, its tube, the components that feed it video and the level of that video become suspects.

[ieure]

Originally Posted by Electronic M

It's most likely in the resistors and caps between the plate of the sync separator and the vertical tube grids or possibly in the parts between the vertical output plate and the vertical osc grid. Resistors of this era tend to drift far higher than tolerance and caps can short, leak DC current or go open. If the tube was bad you'd probably have no deflection or little deflection... Though odd ball failures like a heater cathode short introducing AC could cause it. (Granted if it was a heater cathode short the set would loose sync strongly for an instant every ~11 seconds, assuming NTSC color vertical rate ,then relock on its own and continuously repeat the cycle).

Resistors you can check. Caps you can't properly check for leakage without an eye type capacitor tester that can perform leakage testing at full rated voltage...So substitution with new caps is usually the best test.

Thank you for the reply. I checked all the resistors from V2C down into the vertical oscillator and output stages (R170, R171, and R201-R215), and everything other than R209 looks good. That's the variable resistor which controls vertical linearity. The manual says it's a 2MΩ part, but mine peaks around 500kΩ. The image does exhibit non-linearity (the top of the image is compressed), but since this is a very cheap set and I haven't worked on a similar one, I'm not sure if that's a symptom of my problem, or just how these models are. The vertical hold, linearity, and size pots all seems to influence each other -- if I increase the size or change the linearity, the speed of the image roll changes.

I used some contact cleaner on the pots, and R209 improved, going up to around 1MΩ. Unfortunately, this made the overall problem worse -- the roll is much faster, and while I could get it to roll slow enough to show a clear image, I no longer can. Not sure what to make of that. This does at least track with my feeling of things being age-related. This set sat in a warehouse in Vancoucer, BC for decades, and while it looks very clean, I'm not surprised that some stuff may have oxidized.

I checked the caps in that area, and they don't seem to be short, at least. I'll put together a parts order with what I need and hope that helps.

Originally Posted by Electronic M

If you can make it roll up or down it's almost certainly a sync issue. Horizontal locking range will also tell you something about where the issue is...If horizontal hold is stable for almost a whole turn then it's the components between the separator plate and the vertical, but if hold just barely stably locks the whole sync separator stage, its tube, the components that feed it video and the level of that video become suspects.

Horizontal sync seems normal, and I have a large range where it'll lock hsync.

The schematic for this set seems to call the separator a "sync clipper" -- is that what they call the sync separator?

[ieure]

More clues: I pulled the bank of vertical adjustment pots, and they measure good out of circuit -- the vertical linearity pot tops out around 1.7m ohm. In circuit, it reads around 520. I don't read any continuity between its traces with it pulled, which suggests the current is finding an easier path to ground.

I swapped C208 with one from an S-3 parts chassis. The one in my problem set had some fluctuating readings on my meter, and the replacement didn't.

The way these sets were modified, the antenna screw terminals were repurposed to carry audio and video. I noticed that the solder on the terminals inside the set had failed, and the wires were flopping around the terminal holes. Resoldered those.

I'm not sure if it was the cap or terminals, but the problem has lessened. It still doesn't seem like it properly locks sync, but it drifts a lot less.

I think the next things I'll try are swapping C201 and C210, both those are close to the problem and one end connects to ground, so current could be escaping that way.

The other thing I'm wondering about is the trapezoid-indicated +135v supply, which is connected to one side of the vertical linearity pot. I observed that R403 (130 ohm, 5W ceramic resistor) gets very hot, and that's on the +135v line. It measures good on my meter, though, I suppose a 5W resistor is going to get hot. It's not glowing, just uncomfortably hot to the touch.

Any other suggestions for me?