RCA VCR VR678HF, power supply failure (Thompson Consumer)

[MichaelWR]

Greetings everyone, new user here and as typical I need NFO. I'm looking for information on a VCR. I am saddled with an old '97 RCA (Thompson Consumer) VR678HF VHS VCR to fix (inlaws and such). Unit is DOA, apparently the power supply fails to start... so the oscillator is not running and inhibited from starting up. So... I'm looking for lit, or any thoughts on this failure mode. The SM (Switch Mode) power supply is located on the main board. Even a small scan of the power supply would be wonderful~!~

Thanks in advance,

~Michael

 

[Nigel Goodwin]

Never seen or heard of that model, presumably you're in the USA?.

However, the vast majority of such faults are high-ESR electrolytics, which is why an ESR meter is the most commonly used test instrument these days. If you're lucky you may find one with a bulging top (I found two such in a Panasonic DVD/HDD recorder just yesterday - in the secondary of the PSU).

Next most common would be S/C rectifiers, again secondary side of the PSU, easily found with ohms testing.

 

[MichaelWR]

Good evening Nigel,

Yes, here in Oregon, near the capitol - Salem, currently awash in rain. (I suppose you are too~!~) I do understand your thoughts on the caps and I agree. I recently acquired off EBay a "Working" (wink, wink, wink), Tektronix 2465A along with a Tek 7L13 module for another project, (vintage gear) and both had bad Tantalums and Electrolytic in the power supplies. I suspect there are defective caps here in the VCR too. It just hardly seems worth putting any time into this but it's family... the worse sort of repair. Normally I like to know what has failed (and why) but in this case I may just shotgun all the relative caps. It's always much more difficult without any sort of literature so unless I hear some interesting leads in the forum, I'm just going to go ahead and R&R the caps. None seem inflated but I'm sure high ESR is the culprit. If that doesn't cure it, it may be the control IC or the switching transistor. You think there may be something in the secondary? I can't hear or see the oscillator even trying to start, which led me to think the control circuit is preventing it from starting rather than starting and seeing an overload and shutting down. I was just hoping someone had the key in their pocket for this little pain in my rear. Thanks for the reply too~!~ I know I'm a noob here and I'm sure there are all sorts of newcomers showing up asking for help.


Regards,

~Michael - AF7U

 

[Nigel Goodwin]

As you have a scope then check the ripple on the caps in the PSU secondary, if you have high-ESR ones then you will have excessive ripple on the duff ones.

 

[debe]

Some VCR SMPS have a fairly high resistance resistor feeding a kick start voltage to the osc circuit in the powersuply. This can go high in value & not kick start.

 

[MichaelWR]

The problem I have with checking anything on the secondary is the fact the PS oscillator is NOT oscillating... therefore there is no secondary voltage to check for ripple... damn, this would be so much easier if I had a diagram... At any rate, the feedback idea might be something worth look into. I examined several resistors in the primary and found a couple with physical damage, not burnt, but paint chipping from the bands... which might be nothing. I have not yet pulled the power supply from the chassis because it will require the removal of the entire MOBO assy from the plastic housing. After looking over the process to gain access to the PSU bottom it reminds me of why I changed professions back in `94. The Japanese equipment started looking more and more un-repairable. Anyway, I'm going to wait a few days and see if I get any more feedback before I pull the chassis from the plastic housing to start testing components. I want to thank both you guys for the information, it is appreciated. I'll give this thread a day or so more then I will start yanking boards. I'll keep everyone posted~!~


Regards,
~Michael - AF7U

 

[Nigel Goodwin]

As debe mentioned, if the PSU isn't running at all, then there could be a high value startup resistor O/C in the primary, often something like 330K or 470K.

But check none of the secondary rectifiers are S/C, this commonly completely kills the PSU, they are easily tested in circuit using the ohms or diode test range of your meter.

 

[debe]

It does help if you post pictures of the power supply (these pics taken with a very cheep 2nd hand camera using Macro feature). One pic is a LG VCR other is a SMPS using a ICE2B265 chip. The R5 690K resistor is the kicker resistor that gets the osc going. These pics are just for an example.

 

[vtech]

Been a while since I saw one of those. I used to work on many of the VRxx series. There is a slight possibility that I may have some data on the VR678HF? There was a Toshiba model that used the same transport.
Electrolytics drying up were indeed common with those sets and If I remember correctly, there was one or two low value 'lytics on the primary side (think it used a Telefunken SMPS chip?) that would stop the oscillation all together. I will check and see if I may still have a manual?

Btw, it used a single capstan belt that is more than likely has turned into mush.