VCR Finlandia , Granada, Hitachi.... PSU

@crustytv If no one has a manual the VT250 is advertised on this link, never used them though.

https://www.owner-manuals.com/index.php?main_page=advanced_search_result&keyword=Vt250

Already seen that Frank, and like you initially I thought it was the one. If you look deeper, you will see the manual offered is not for the VT250 at all, it's another set of models entirely and from 1983. Clickbait link, possibly a scam site.

This VCR I'm attempting to repair is a 4-head NICAM must date from around 1991 onwards.

Oooh that PCB is a bit frazzled around there. I wonder if that is what caused the demise of the diode if it's become conductive? It will be worth running an ESR meter over those caps, especially the one near the diode.

Nice machine by the way.

In the absence of finding a circuit, you might be able to work out the likely voltage from any capacitor connected across it or at least associated with it. 

The diode appears to be straight across the supply so there for protection, designed to go short in the event the rails become too high and might actually be an avalanche diode. C13 is definitely domed and may have also been damaged by excessive voltage on the rail.

Which means a fault in the ref comparator or the opto if it has one.

Posted by: @irob2345

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or the opto if it has one.

It does, and Optocouplers are alien to me, so I had to look it up and see that it transfers electrical signals between two isolated circuits by using light. 😮 

I tested the (Sharp PC111?), in as much as I checked pin 1-2 the diode, it tested OK. I then tested to all other pins, no other pins corresponded to each other. I've no idea if that is a good enough test.

Posted by: @jayceebee

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C13 is definitely domed and may have also been damaged by excessive voltage on the rail.

Five of the electrolytic caps including C13 in that area are toast, one (c18) literally! The 3 NPN Darlington's are OK, but two parts I'm not certain about or what they are, test as short. They're the stubby green L4 & L5, they have 100K written on the top and have a coil symbol on the PCB, but as I say, both test short-circuit.

Posted by: @sideband

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In the absence of finding a circuit, you might be able to work out the likely voltage from any capacitor connected across it or at least associated with it. 

C18 was pretty toasted, also here are all the values and voltages. They've all read over 1000+uF now and high ESR.

Figuring NICAM really got off around 1991, I guess this player must be around that date.

As my VCR service books cover 1987–1994, I thought I must have something. I found in there a 1991 Hitachi F150E, I wondered if this might have used the same PSU as the VT250E, it did! The PSU's appear to be identical.

So here we have the cct for the PSU, with duff components found thus far. The amber ones are what I'm uncertain about, namely L4 & L5

The Zener D9 is a MA2180A, a 1W 18V.

OH, and R36, the fusible resistor, is not open, nor are QF1 & QF2 circuit protectors.

It appears to be a 14V supply rail and judging by the caps on that rail at 16V I would have thought that it would be a 16V zener. The opto coupler IC1 is monitoring the 5V rail...pretty standard sort of circuit. I wonder if it will work if you unplug PG1 so you can monitor the regulated outputs? I'd fit a 1 watt zener for D9 and mount it off the PCB...a neat little bend in the legs to raise it about 10mm off the PCB. 

The 10 micro henry chokes will probably be OK....tell-tale sign is if the outer coating is cracked. If they look OK and have continuity, they probably are OK. Does your component analyser check inductance?

Posted by: @sideband

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I wonder if it will work if you unplug PG1 so you can monitor the regulated outputs

PG1 is not connected, as from the start I removed the PSU entirely from the player for testing. I thought it would be much easier access wise to test than when it's in the machine. I, too, have been wondering if I could power it not connected to the machine to verify repair work, or if it would damage it with no load.

Posted by: @sideband

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The 10 micro henry chokes will probably be OK....tell-tale sign is if the outer coating is cracked. If they look OK and have continuity, they probably are OK. Does your component analyser check inductance?

My Peak does not, but I have another device that does, what's 1uH between friends.

Posted by: @sideband

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I'd fit a 1 watt zener for D9 and mount it off the PCB...a neat little bend in the legs to raise it about 10mm off the PCB. 

Agreed, I have a BZX61C16 1W in stock

Though, the nagging question remaining is what caused the Zener to short.

D9 is there for protection of the 14V rail, judging by the PCB condition it's been getting very hot before it's failure. It may be because the 14V rail has exceeded 18V or C12 & 13 may have been gradually losing capacitance leading to a lot of ripple on the rail and D9 has been chopping off the peaks. 

It looks like the PSU might run standalone, load the 5V rail with a few hundred ohms and measure it to see if it's high. I wouldn't do this for too long as the unloaded rails could be a lot higher than normal and pop a cap or two.

Hi, Chris, I will tell you why your board is burnt, it's a common fault on loads of things, a capacitor has dried out on the rail the opto coupler is monitoring, so the opto coupler is telling the primary side the voltage has fallen and the primary then attempts to push the rail back up again, all this happens over a period of time so the zener just gets hotter and hotter as it attempts to clamp the rail, replace the duff capacitor and use a zener a few bolts over the voltage across the rail when it's working with the correct voltage. 

PSU repaired

The rogues gallery

Powered on, it lives again..... and the tape that was stuck in there ejected.

Reinserted.... it loads and plays

A capstan/loading arrangement with toothed belt, never seen that before.

An £8 bargain and a bit of weekend fun. I'll hook it up to a TV, I'm assuming the heads will be OK.

@michael-dranfield thanks for the explanation as to why the Zener died, that was very much appreciated. 👍 

I still have our old Hitachi VCR, must be similar age to this one, it was still working last time I plugged it in! Maybe I should give the PSU the once over so it doesn’t end up going the same way. Ours has the remote with the LCD on it so you can program the timer on the remote, then press ‘transmit’ and it sends it to the VCR, or you could use videoplus. That machine lasted us ages! I know it replaced a machine that kept getting tapes stuck in it, which wasn’t very old.

Well done getting it going!

I was certainly interesting, glad I looked again at other models in my manuals to find the exact PSU, that was such a help, and I learnt about something new, Opto couplers. Nice to have a win after a week of getting duffed up by the Labgear teletext box.

I still have a few more waiting bench time, one of them with a similar fault as this, dead on power on.

Excellent work Chris! Some of these were real bread-and-butter jobs back in the day. Worth checking the caps that Mike suggested unless you've already done it. Some of these power supplies led quite hard lives and ran quite hot which dried the caps out.

Toothed drive belt....yes several chassis used that notably the Panasonic 'G' deck which I knew very well.

Not bad for 8 quid and a handful of components!

Glad to see you get something of a normal fault for us bench engineer instead of the horrors that seem to come your way, faults like this were the norm which weren't really tackleable in the field.

As for NICAM, we had videorecorders with decoders a long time before transmissions became available. Part of the delay was caused by the chip manufacturers ISTR. Apparently there was some misunderstanding with the translation broadcast specification and I believe it was the far eastern manufacturers who got it wrong leading to the broadcasters having to delay to introduction while they tweaked things to make the chips work. They had to do this as there were so many already out in the wild. The first NICAM machine I remember was the Ferguson FV14T which used a Toshiba IC in the decoder.

Well done Chris, an interesting repair.  😎 I remember a similar fault with Samsung machines that sometimes caused a lot of damage as the Zener didn't always protect the supply lines. 

There were other horrors of power supplies....was it Akai who offered a replacement power supply since a faulty one was almost impossible to fix?

I can remember an Akai machine which used a linear/switch mode combo which caused damage to the VFD. I remember them coming in with the VFD heaters glowing red. I can’t think of anything worse than the PSU in the Thomson designed FV30/1/2, a sick joke on us by the French with it’s MELF components.