Toei TC-A252S Woes

[YesAffinity]

I have a Neo 25 w/ Toshiba A59JMZ90X and Toei TC-A252S chassis.

I recapped the power supply a few months back, no issues. I decided to pull the Toei chassis to recap it, due to developing some noise/interference in the image. Otherwise, the monitor/chassis have worked great for as long as I've owned the cab (5 or 6 years now, I believe), and the monitor has always been crisp and bright.

I completed the cap job, connected everything back up, and it ran seemingly normally for about 30 seconds. I saw the Unibios boot screen and Neo Geo startup screen. And then, video disappeared and the cab started producing a high-pitched "squealing" sound.

Video of the squealing, here:

View: https://youtube.com/shorts/dMfFvDcDhQU


Pulling the chassis and inspecting my work, I found C67 reversed. I corrected that, reconnected everything, and no improvement

Quite a bit of poking around, reflowing many things, re-re-checking all of my work, no improvement.

I decided to connect the chassis power to a variac while powering the cab normally. I plugged the monitor/chassis into the isolated socket with the lowest voltage, at just over 100V. The squealing was significantly reduced, almost non-existent. Still no video, but that's something. I then turned the dial on the variac, which would increase that outlet to around 105V, and the squealing increased.

I'm guessing this has something to do with voltage regulation on the chassis.

So, I pulled the chassis and plugged into the variac on the bench. Same result, not connected to a monitor. It's hard to tell exactly where the noise is coming from, but there are 2 components getting extremely warm to the touch (circed in the picture).

I also am able to hear something "turning on and off" when power is turned on and off, sounding like very normal operation - a single switching or clicking sound, one at power on and one at power off. Sounds like that's coming from the flyback. I'm staying hopeful and considering that a good sign for the time being.

I also confirmed the cab plays blind with no chassis.

1) Has anyone experienced this issue before and/or have any insight into the issue?
2) Are there other more common chassis that will work with this tube?
-Being that there are no schematics available, and looking up various part numbers from components on the chassis has produced no success, I'm fearful I'm running into a dead end

 

[YesAffinity]

After a lot of poking, I think I've isolated the issue to this transformer. touching it with a non-conductive pair of tweezers, I can feel it vibrating in conjunction with the squealing. I haven't found any info online on this component. If I knew what the 'Q' logo stood for - assuming that's a manufacturer - I might be able to get closer. Anybody know anything about this device?

 

[nem]

Transformers very rarely go bad.

I would check all components connected to C67. Reversing caps often blows things.

 

[YesAffinity]

Thanks nem. I traced out that circuit. C67 is grounded on one side and goes through pin 2 of the STK79915. There's no data sheet for the STK79915, but the STK79315 looks to be comparable.

https://www.alldatasheet.com/datasheet-pdf/pdf/41668/SANYO/STK79315.html

I make that assumption based on:
1) following this C67 circuit out to the remote board takes me to the vertical centering pot (consistent with pin 2 function of stk79315)
2) grounds consistent with the stk79315
3) b+ adjusment pot in circuit with pin 17, consisten with stk79315 pinout

going through the circuit between c67 and the remote boards, there's a handful of series resistors. I pulled them all and they tested good and consistent with their color band branding. The circuit does branch off somewhere else, which I didn't follow all the way, but just based on all of the above, I don't think this looks to be the cause of the issue with the transformer squealing. I'm happy to hear I'm wrong and should keep focusing here.

I also did a bit of research on "CVT", which in this case appears to be a constant voltage transformer - in effect an isolation transformer built into the chassis.

https://www.aelgroup.co.uk/faq/faq001.php

The below I found to be most interesting.

The CVT is designed to supply an overload of 150% at around 200% overload the output waveform collpases to near 0V, WITH NO HARM TO THE CVT, direct short circuiting for long periods of time is not a problem for the CVT, as soon as the short is removed it will carry on supplying useable power.

This and the below gives me hope that the squealing is the sign of a short, and that no real damage has been done.

I then poked around on the AC input side of the transformer (it's all isolated to the quadrant around AC input). There's a lot of things testing "short" with my multimeter but it seems to be due to a lot of low-value resistors all over the board (both sides of the CVT). But, I got to one resistor at R176 that has both sides connected to separate pins at the CVT, so I decided to pull it. With that resistor removed, the squealing stops. There's of course no other real signs of life, but maybe that's something.

Removing the CVT also causes the squealing to stop...just mentioning that for posterity.

That's about as far as I've gotten. I have a "for parts only" A292 chassis coming, which from the ebay pictures looks to be nearly idential in layout and components. Wondering/hoping the CVT from that board can be swapped in as a direct replacement. Also hoping that the sharpie note written on the metal shield on the A292 stating "shuts off sometimes" is true and the A292 has at least shows some life so I can use it as a comparative test board.

 

[YesAffinity]

I'm happy to report that this chassis is working again. I found the fuse at F1 blown. I had nabbed a parts A292 chassis (29" version), and proved the cab was otherwise working, with the A292 chassis.

These chassis look identical and from everything I compared, all parts are identical. In the process of poking around, I found the fuse at F1 blown on the A252 (original) chassis. I swapped the good one off of the A292, and it fired right up.

I can only assume the reversed cap caused the fuse to blow, and that's what I've been chasing the whole time.

Now then, I also scraped off the glue from the B+ VR, and it is certainly no longer at it's original setting (and probably needs adjustment/verification anyway). Does anyone know what B+ should be set to on these chassis - preferably for the TC-A252S, but if it is known for the TC-A292S, that might get me to the same desired outcome.

 

[YesAffinity]

Spent the morning dialing in this TC-A252S. I set the B+ VR to where it was from the factory. I was able to line it up reasonably well from the hot glue residual left behind, and had a good picture of it to corroborate. Re-hot-glued it and walked away.

Frankly the thing I was chasing - some noise visible in the background - is still there. So that's a little disappointing. I've re-capped the power supply too, so not too sure what else could be causing that. But, I'm happy it's fully working, and recapped and solid for another 5+ years.

As always, phone camera pictures don't do it justice.

Side note: the degauss button in the cab doesn't seem to do anything. I had suspected the degauss thermistor was a problematic part early on, and I swapped it with the one off the A292 that I got, and it made no difference. Can anybody with a NEO 25 or one of these A252S chassis confirm if that's normal or not?