Sencore VG91 repair part1

[@phays4163]

Mid 90’s nichicon caps were notorious for leaking. They were a promising new design and Denso used them in all their ECU’s from the era which are all failing.

[@uxwbill]

If there is service information out there on reasonable terms, I sure haven't found it. I've reached out to many people who own these, and even asked at an area TV repair shop that's still very much in business. (Turns out they don't even own one.) Sencore, it seems, trashed all of their history when new ownership came along. Maybe you already knew that. I don't use mine very much (and not really for its intended purpose), but I'd probably better deal with those capacitors. Someone should definitely read and save every EPROM in this thing. On to the next video!

[@geirendre]

When a board is burnt that bad, it's best to use a Dremmel or something to carv out all of the burned pcb. You can refill the hole with epoxy, and if needed replace traces. That way you know there's no current leaking trough the board. You can also chech voltages on ICs. Refere to datasheets for expected voltages on them. Good luck with the repair.

[@ischmidt]

SAA1043 is a sync generator. It can output NTSC, PAL, 2 flavors of SECAM (French and Soviet, I think), and even the Brazil-only PAL-M standard. That's PAL color encoding on top of an NTSC 60 Hz black and white frame.

[@sside8]

Bob, back in '94 I bought several Sencore test pieces and they came with a parts list and schematics. I can look up whatever you need.

[@1McMurdoSilver]

By chance, you don't have a schematic or service manual for a CR7000 do you? I'm in need of one. Thanks

[@sside8]

@@1McMurdoSilver sorry, no I don’t. Never had that model. Had the CR70.

[@ElectroRestore]

Being a fellow tube man, I am just shooting in the dark as well. That said, most voltages in logic is 5vdc and 12vdc. It might help to test the other regulators as you mentioned and see if they fall in the ranges that make logical (no pun intended) sense. Also, I would agree @geirendre that it would be wise to cut those ends of the traces completely off and either , take his advice and use epoxy, or cut them out and use jumper wire to bride back to the previous solder node. Heartfelt empathy here Robert! :(

[@mickre-fuses]

Hi Bob. Hasn't Shango got a VG91? Maybe you could ask him to take some reference voltage measurements from the PSU?

[@Timothycan]

I had an LM317 fail short, a few months ago. It was dropping 12v to 9v, so when it failed it let the full 12v through - lucky I spotted it quick. Look at the data sheet for the LM317 and what components you have and from that you should know the intended voltage output.

[@radio-ged4626]

Wish I had some insight for you on this. All I can say is that when I don't have a circuit diagram, like you have suggested, I would cold check the values of the components and further to that maybe disconnect the power output from the rest of the circuit and see if any output voltages change dramatically. Good luck and I hope somebody furnishes you with a schematic.

[@TheFineby]

Hi Bob, I have three VG91’s and only one works. All three have had power supply problems. I’m not good at fixing things this modern, so I’m waiting to see how you make out. The one thing that I do have is the Operation and Application Manual. If I can help, let me know.

[@GrandsonofKong]

Like the Invaders T-Shirt..one of my favorite shows as a kid. Got the whole Series on DVD last Christmas. Looks like they set-up multiple standalone power supplies with a bridge rectifier, filter caps. The 317 is an adjustable and as you stated the output voltage is set by the resistor. If you can read the resistors and go find the App Note to determine ratio and the "planned" output voltage the designers were looking for. the fact they used 1% resistors implies they wanted it pretty close. You can do the same to find the other Output Voltage for the other sections as well.

[@johnsampson1096]

I think, Bob once you get the power supply data, it will expedite the repair as you can check for a missing voltage. An after thought, think about trying to clone those eproms, as they can lose memory after 10 or 15 years. Welcome to the digital world! Good luck at your repair attempt............

[@jimhatch14]

I would really like some advice on buying or maybe I have a prom reader or three around. Haven't got the nerve up to pull the chips out on my several digital Sencores.

[@mikefinn2101]

Great video Bob and Nice to see the inside, I would guess something else went. It seems like the start circuit is not starting do to bad voltage. I hope you find it. Thanks for sharing this very informative video. Mike

[@shango066]

I think I have the service manual for that Or maybe it's for the other one. Good find.I will have to recap mine before it eats itself up. Probably a multi layer board issue

[@bandersentv]

Sencore never produced a VG91 service manual

[@laurdy]

Using the resistors around the LM317 it is possible to calculate the intended voltage output, It should be 1.25*(1 + (R2/R1)). Where R1 goes between pins 1 and 2 of the LM317 and R2 goes between pin 1 and ground.

[@EriksElectronicsWorkbench]

I did a full recap on my VG91 on all boards. Most of the power supply caps were leaking when I got it but the unit had not been powered up so spared any major damage other than corrosion setting in. The power supply heatsinks run pretty hot so I upgraded the larger caps to 105C instead of 85C. Also had some cold solder joints on the regulators from thermal stress. I have the service manual with schematics but it is far too large to scan but if you need some part of it I can try to take a pic with my phone. Side note.. if your LCD display is not backlighting or is dim there is a trimpot on the board near the LCD you can set the backlight intensity. This is not often known on these units. The manual makes no mention of the backlight.

[@bandersentv]

Thanks for the advice. I did just get a scan of the power supply schematic so I'll continue looking there. Huh. I had no idea it was backlit.

[@PracticallyFixed]

@@bandersentv So glad you got the schematic. Looking forward to this repair! Best of luck.

[@THEtechknight]

Hopefully you get this guy fixed and out of sick bay! BTW, your HP Scope will need this same treatment.

[@glennrechnitzer8704]

Check the trace between the LM317 and the output connector. Make shore it didn't damaged when cleaning up the burned spot on the board.

[@stevenlyons4669]

Hello, check your rework around the burnt area of the board. Cold heatsink? Perhaps the rework around the output cap doesnt connect to the output of the LM317 ...no load on the regulator. Also, I have found that those 1990's displacement connectors are unreliable, especially when they have been plugged/unplugged a few times as the wire may have been tugged. May be why the relay doesn't click in. Cheers!

[@suzakule]

Was that Mach GoGoGo (Speed Racer as it was dubbed in the US ) on the monitor in the background? :O

[@bandersentv]

Absolutely followed by Marine Boy

[@classiccomputers6211]

Those Nichicon PL caps are unfortunately one of the low ESR types known to leak pretty bad, like PF, PR, etc. Funnily enough the caps you used (PW) are I believe the recommended modern replacement so they should be just fine :) Some random musings on the power supply - definitely try to figure out the expected outputs, and try checking them on the actual boards they go to. I have had that style of power connector go intermittent in 80s computer equipment. the crimps go bad causing voltage loss - so 12V would fine at the supply, but then I'd read something like 8V at the load. also, those double sided boards can be fragile, especially when damaged by cap leakage - verify your plated through holes are good on the replaced caps. I once spent many hours banging my head against an 80s switching supply that had all voltage rails but one - it turned out when I recapped it, I had invisibly severed the top trace to the through hole, so the voltage wasn't making it past the filter cap. Running a thin wire down the hole fixed the supply. Best of luck!

[@bandersentv]

Yes, I'm discovering the same reading through service forums. Lots of leaky PL caps. Learned my lesson. I'll be more inclined to recap working older test equipment after this experience. I'll pull out the PS board, draw up a schematic and go over everything again. It seemed like it was starting to work after the recap. A flaky PCB trace would explain that.

[@classiccomputers6211]

@@bandersentv Best of luck! The low ESR caps of the 80s and 90s are a real problem in vintage computer circles. Some machines are well known for being badly affected (like the Macintosh Classic, for example) but I have found problems in nearly every machine that crosses the bench. It affects pretty much every manufacturer, as I understand it the dielectrics used in those days to get a very low ESR turned out to be volatile and slowly eat through the rubber seal over the years, eventually spilling out onto the board. I spend more time refurbishing ancient power supplies than anything else these days...

[@bzuidgeest]

In my opinion you are lucky with the fact it's a linear power supply. I have already seen the formulas in the other other comments, you can calculate the supposed output voltages for the 317. Trace out the power supply and at least check that every voltage leaves the board. It's possible that something other then the power board got toasted, but since current goes the way of least resistance i expect most of it went thru the short, leaving everything past that alone, overvoltage would be much more dangerous to the rest of the device. Without schematics it's difficult and some work, but as you said, you do depend on it. After confirming the power supply, you could check where the voltages of the burnt part are routed and check if anything got burned out, or isn't attached anymore. A reason for not drawing power.

[@allisonbarnhardt3647]

Any chance that is a multi-layer PCB, seems unlikely on a power supply PCB but you never know. You are stuck without a schematic unless you take the time to draw the power supply circuit out by hand, Very cool tee shirt, I remember the Invaders being on when I was very young, my mom used to watch it.

[@jimhatch14]

Wasn't this in the Bad Caps period? Many manufactures bought bogus electrolyte. I got my Sencore stuff in 1990, or so. Maybe I'm safe.

[@bandersentv]

Too soon - that was '1997-2007. Also, it was Taiwanese that stole the formula from the Japanese that caused the problem. Nichicons did not suffer from the cap plague. These caps are just about 30 years old and this unit has seen many, many hours of use.

[@1McMurdoSilver]

Often the mini relays in the later Sencore equipment need "exercising" or replacing. Not sure how many are in the VG91

[@bandersentv]

At least 3. You'll hear them in part 2

[@jimhatch14]

@@bandersentv The Sencore relays get really bad, especially if not exercised often. Several times I was able to get the casing off and use deoxit on them on my PA81 and SC61. I think I once thought I had a suitable replacement picked out from Mouser or Digikey.

[@directcurrent5751]

Is it irony that a service gizmo is sans support ?

[@gd2329j]

Top side print under components is never good if something lets go . Missing or damaged PCB barrel ? Top -> bottom connections . PS looking at the print C127 isn't looking so hot !

[@SeekingTheLoveThatGodMeans7648]

You got that microscope, right? If a magnifying glass with a high power spot is not enough to read the values of the power regulator calibration resistors, your microscope might be. It might be worth it at least to see that the resistors are correct. If they have failed so as to send a digital circuit power supply well over their rating, it would be au revoir for said digital circuitry. It might be helpful to look at the supply voltages on those digital chips to see if those are even present, let alone right, however. And trace backward from that relay as well as testing the relay out of circuit as best as possible. I know, a schematic, a schematic, the kingdom for a schematic!

[@bandersentv]

Oh I have one now. Editing part 2 right now

[@directcurrent5751]

Why not a DVD that plays various patterns and set up images?

[@bandersentv]

Watch the first few minutes again. Lots of reasons. It's not just RF. It can output IF frequencies and 4.5 MHz sound carrier. You can also selectively turn on and off parts of the video signal. The companion unit can output up to 200 volts peak to peak and produces drive signals. It can also test flyback and yokes. It will not work without the VG91

[@MrRadar]

You could probably use a modern software defined radio (SDR) to do all of the different types of modulation etc for the test pattern generation, but replacing the amplification functions of the companion box is almost certainly not something you're going to find in an off-the-shelf modern component.

[@bandersentv]

Oh I have another test pattern generator and a bunch of RF modulators. It's the other functions I really miss

[@tonyfulford3175]

It's not always the big caps that cause problems. It could very well be a small 1.0 ufd cap @ 10 volts or less, that is a dead short. Unfortunately you probably need to check a few more. Good video though.

[@johnsenchak]

Burr Brown made some of the best audio chips on the market My guess is a cold solder joint

[@zundfolge1432]

yeah I have the flash drive da converter so I will have to go back to that. leader lcg 400 is displaying an oddball round circle thats shifted.

[@donsurlylyte]

bad solder joing, arcing, scorching?

[@donsurlylyte]

it's always the carburetor bob

[@MrCrystalcranium]

I thought you had a Sencore VA62 that will do almost everything the VG91 will do?

[@bandersentv]

Got rid of it a decade ago. Too clunky and had issues.

[@MrCrystalcranium]

@@bandersentv Ahhh...oh well. There is a schematic for the 62 available. I know Phil of Antique Radio fame posted it on the forum in PDF format. Maybe you'll luck out if the power supplies for the VG and VA are similar in layout.

[@bandersentv]

@@MrCrystalcranium no, radically different. About 15 years apart in technology.

[@MrCrystalcranium]

@@bandersentv Yeah I thought so. Here's the board anyway. See if it has any value. antiqueradio.org/art/temp/Sheet4a.pdf

[@notimetolooz9655]

The VA62 was an earlier version of the VG91-TVA92 pair. It looks like the VG91 has a few more patterns like the big circle and cross that the VA62 doesn't have. The VG91-TVA92 pair also has more advanced circuit design. I don't really know if that translates to better specification numbers however. A real bummer if the service info is not available.

[@daleburrell6273]

...send it back Sencore...(?)

[@bandersentv]

@@daleburrell6273 They are still in business and this was in their 1999 catalog but I don't think they service stuff this old

[@bzuidgeest]

​@@bandersentvthat equipment was expensive and companies often rely on such equipment. I would not be surprised if they could service it. But it would not be cheap.

[@daleburrell6273]

@@bandersentv ...maybe you can find a replacement power supply board...(?)