Toshiba M-9485C VCR

[mike_drz]

Hi folks! This my second post on this fine forum. I've been working on an old VCR that was from my childhood. I've been working at it for 2 weeks now, replacing belts and caps.

I got it to play, but I'm having what I think are some power supply issues. The vacuum fluorescent display is pulsing on the grids that are off. They are coming on and off. Also the picture is pretty bad with hum bars.

I've recapped the main power supply with all new nichicon electrolytics but the problem remains. I measured some voltages and the lower voltages seem stable, but the 32 volt rail is jumping around a lot. I measured it with an oscilloscope and some of the rails have as much as 11 Volts peak to peak AC on them, at 60 Hz. The lower voltage rails have something like 4 VAC on them, but they're not sinusoidal waves.

I can't find the service manual anywhere, I know not many people work on VCR's these days. I'm hoping to get any help I can get with this. I think there's a voltage step up IC somewhere, not sure which one, or how to measure it. Not many of these IC's have data sheets that I can find.

Thanks in advance,

Mike

[dskall]

Re: Toshiba M-9485C VCR

Need some pictures

[mike_drz]

Re: Toshiba M-9485C VCR

Need some pictures

Hey Dskall, thanks for the reply. Here are some pics and videos.

https://youtu.be/68BuT-MloW0

http://vintageelectronics.betamaxcol...delm-9485.html

[dick_barton]

Re: Toshiba M-9485C VCR

Just a thought but with that level of AC and having already changed the capacitors I would think that maybe one of your rectifying diodes may be faulty so I would check all the diodes and bridge rectifiers in the power supply.
It could also be that one of your supplies is heavily loaded and cannot maintain smoothing of the rectified AC.

[mike_drz]

Re: Toshiba M-9485C VCR

Just a thought but with that level of AC and having already changed the capacitors I would think that maybe one of your rectifying diodes may be faulty so I would check all the diodes and bridge rectifiers in the power supply.
It could also be that one of your supplies is heavily loaded and cannot maintain smoothing of the rectified AC.

I was thinking the same thing too, the only caps I didn't change were the ceramics. This circuit is pretty confusing because it doesn't seem to all be contained in the power supply. There are IC's everywhere and shielded coils etc. Maybe the VFD has a short and is loading the supply? This supply won't start up on its own without a load. I don't have the schematics, so I don't want to load it on the wrong pins.

[dskall]

Re: Toshiba M-9485C VCR

I would think with 60hz you still have bad cap somewhere. Maybe you can go around and bridge a cap at various points

[R_J]

Re: Toshiba M-9485C VCR

What regulator ic does it use? (whats its number) I checked my old records and the last time I repaired this model was in 1994, I had the schematics at that time but they have been recycled. check those short wires that connect the transformer board to the regulator board, sometimes they had poor solder connections.

[mike_drz]

Re: Toshiba M-9485C VCR

Wow, 1994, those were the days, haha. I just finished checking the diodes and ceramic caps those seem fine. I'm not sure what IC is what. The one in the regulated board looks like an integrated audio amp(STK7251). Here is a list of IC's I could make out. D1273 P 92, C236, and there is one I cannot see, as it's facing the shielding. The rest in the chassis are: TA7288P dual supply motor driver, TA8675N, TA7347P, 0928G 91, TA75339P, and the one in the picture M50459-109SP. Thanks, I'll check those wires that fold the board over too. When I run it without a load, I only get 6 and 8 or 9 volts.

[R_J]

Re: Toshiba M-9485C VCR

That stk7251 is the regulator ic, I believe it has more than one regulator in it, I can't remember what they are, I will try and find a pdf for it

[mike_drz]

Re: Toshiba M-9485C VCR

That stk7251 is the regulator ic, I believe it has more than one regulator in it, I can't remember what they are, I will try and find a pdf for it

That makes a lot more sense than having an audio IC in the power supply, haha. I'm not good with SMPS'. When I measured around the diode bridge with the scope, I only get sin waves, I don't see any half waves like I'd expect from a rectifier. I may just be measuring from the wrong points though. By the time it gets to the main filter cap there's no AC measuring. There is about 16 VDC across the main filter cap.

I pulled the rectifier diodes and they all pass a diode test. However the resistance across one of them is 800K ohms while the others all measure around 4 Megs. I don't know if that means anything.

[R_J]

Re: Toshiba M-9485C VCR

Its not a smps, that ic is just a regulator ic, but I believe it has more than one in it and if I remember parts of it can be turned on or off. I can't find a pdf for it.
one thing to mention is if you check the 30 volt line to the display you will usually see a lot of noise on it close to the display, that is just the display loading the line, it will look more like a digital modulation than power supply noise.

[mike_drz]

Re: Toshiba M-9485C VCR

Its not a smps, that ic is just a regulator ic, but I believe it has more than one in it and if I remember parts of it can be turned on or off. I can't find a pdf for it.
one thing to mention is if you check the 30 volt line to the display you will usually see a lot of noise on it close to the display, that is just the display loading the line, it will look more like a digital modulation than power supply noise.

Yeah, I see what looks like RF or noise when I scope near there on most of the voltages, but two of them have a Sin wave. One is 11 vpp the other 6.5vpp. Can I change the rectifier diodes with 1N4004's to test? They are rated for 400V instead of the 600V of the ones that were in there. It's all I got on hand.

Here is a data sheet for an STK7253, only thing I could find. No stk7251

http://www.datasheetbay.com/pdf/489225/STK7253.html

[mike_drz]

Re: Toshiba M-9485C VCR

Changed the diodes and disconnected the front panel containing VFD. There's still AC on the rails, but a lot less without the load. I need to find a way to get the 32 volt rail to come on without being connected to the main board. The voltages are reading low.

That way I can determine if something is dragging down the supply, or if that IC regulator chip is at fault.

[mike_drz]

Re: Toshiba M-9485C VCR

I measured all the pins of the Regulator STK7251, while the VCR was OFF and then while ON. None of the pins put out 32 Volts. The highest voltage is 15.7 V. I've attached a spreadsheet with the pins and measured voltages.

I think the 32 volts is coming from somewhere else, some sort of step up circuit.

[R_J]

Re: Toshiba M-9485C VCR

So the only problem is low 30 volts for the display?, that usually comes from a small dc-dc converter, usually a couple caps get cooked since its on all the time.
On this vcr the dc/dc converter is on the logic/servo board (page 75) It takes the always 6.6 volts and boosts it to 30 volts. You should be able to trace the line back, the converer sometimes looks like a small can 1/2" X 1" and maybe 1/2 inch tall This is for a Toshiba sv-771 I have, the dc/dc converter is in the corner of the board on the botom of the vcr

[mike_drz]

Re: Toshiba M-9485C VCR

So the only problem is low 30 volts for the display?, that usually comes from a small dc-dc converter, usually a couple caps get cooked since its on all the time.
On this vcr the dc/dc converter is on the logic/servo board (page 75) It takes the always 6.6 volts and boosts it to 30 volts

R J... you sir, are awesome! These schematics will make a world of difference. I'm not sure if it's just for the VFD it runs to the reel motor and a few places. Now I'll be able to get an idea of what the hell is going on. THANK YOU!

[R_J]

Re: Toshiba M-9485C VCR

That 30 volts is low current for the display only, look for that can and there should be a couple caps. next to it, I added a picture of one in my other post

[mike_drz]

Re: Toshiba M-9485C VCR

That 30 volts is low current for the display only, look for that can and there should be a couple caps. next to it, I added a picture of one in my other post

Thanks again RJ, mine looks a little different, I have a shit ton of shielding everywhere, it took me forever to get that off, I found one or 2 high ESR caps so far. I have an IC under there too. I'll take my time with this as a side project, there are a ton of caps to go through, and I have to pull them to measure ESR. The meter I have isn't too good at in circuit.

[R_J]

Re: Toshiba M-9485C VCR

The dc/dc converter is usually its own component and there should be only a couple caps (maybe 100uf) next to it. It should'nt be under any of the shields, maybe post a couple pictures of the boards if you can't identify it. If you are replaceing a lot of capacitors, make sure to check the vcr's operation every so often in case there is a problem that way if there is a new problem you can go back and check what you replaced.

[mike_drz]

Re: Toshiba M-9485C VCR

The dc/dc converter is usually its own component and there should be only a couple caps (maybe 100uf) next to it. It should'nt be under any of the shields, maybe post a couple pictures of the boards if you can't identify it. If you are replaceing a lot of capacitors, make sure to check the vcr's operation every so often in case there is a problem that way if there is a new problem you can go back and check what you replaced.

The can I desoldered was from a pcb labled 'power' it has an IC, a lot of transistors and diodes, coils and trim pot. That's where I found a bad 47uF @ 25V it had an ESR of 33 ohms, according to this cheap meter.

There was a 100uF one I replaced too, by another can near by, there's a 10 mH coil bodged onto that can. I'll take a few pictures later on, and test once I get the can back, there's a transistor that uses it as a heat sink.