The Onwa Chassis

The Onwa chassis and its variations has in recent times been used in many different TV models, including Bush, Alba, Matsui and many others.  It has been the cause of much work in the workshop here, but is relatively straightforward once one is familiar with it.

NOTE: This information is intended for use by service personnel with the necessary qualifications
and experience.  VCRs contain dangerous voltages.  Dave Sergeant cannot be held responsible for any damage to equipment or personnel as a result of implementing these modifications.

Although there are many varieties of the Onwa chassis, all made by Onwa but never with the Onwa label anywhere to be seen, they have a number of common features.  There is a discrete component power supply with two electrolytics which dry up (and cause the HT to rise), often but not always 47uF 50V.  The 12V regulator from the line output stage uses a most crude zener regulator which fails when the HT goes high leaving the zener short and its series resistor open circuit.  The lower value fusible protection resistor before the rectifier diode sometimes also goes.

There is a description of this chassis in Television magazine, February 98, which includes the PSU circuit.  A repair kit is available to handle most repairs, but contains far more components than are normally needed so is relatively expensive.

Recently there have been a few Onwa own brand models which use a different configuration to most of the badged varieties.  At present I have limited circuits and information on these and therefore can offer very little advise on them.  If I find more information on these later models I will add to this page.
 
The Onwa 12V 'Regulator'
The 12V regulator consists of a conventional 12V zener diode driven off a rectified winding of the line output transformer.  A series resistor, whose value varies between chassis but is frequently 5.6 ohm defines the current flowing through the zener.

Unfortunately all the 12V current of the set also flows through this series resistor. The value of the resistor is chosen such that with the typical input voltage from the line stage and a typical 12V load current there is a slight excess of current available which flows through the zener.  It doesn't take much of a mathematician to deduce that this zener current, which should be much less than the 12V load current, is very crudely defined, and should there be a small change in the input voltage (from HT variations) or a change in the load current the zener current will rise to a level far in excess of the rating of the zener.

It is rather sad that this design, which should not have got past any quality control vetting office, is now incorporated in all sorts of TVs world wide.  The extra cost of a proper 3 terminal regulator or a pass transistor (with the zener on its base) is minimal and would have saved a lot of repair work in our trade today.
 

 
The Modification
Whenever these sets come into my workshop these days I remove the zener and its series resistor in its entirety and fit a 7812 3 terminal regulator instead.  Depending on the sets PCB layout this is usually fairly straighforward and some of the existing holes may be used.  It is preferable to fit a small clip-on heatsink as it does run fairly hot without.  In some sets there is not much header voltage from the line stage, but I have never had a set where the mod didn't work (yet!). A further advantage is that you don't have to go finding the correct value of the series resistor, which is always difficult to establish and expensive.

This modification has appeared in Television April 98 as 'Mel's Mod' with a further comment in the July 98 issue.

Usually all that needs to be done as well is to replace the two electrolytics (often C909 and C910), 47uF 50V 105C (sometimes 10uF). These should always be replaced in these sets even though they measure correctly.  Occasionally the excess HT has damages other items such as the line transistor or the field chip, but often they are OK.  Some sets use a series transistor switch on the HT to enable standby switching and this is frequently found to be shorted.

The 0.56 ohm protection resistor before the 12V rectifier diode should also be replaced as this sometimes goes high in value rather than open.  Experience shows the set may bounce back if this is not replaced.

Beware with some sets when you disconnect the loudspeaker cable which is via a two pin connector.  There is often an identical 2 pin connector which is used for putting the field stage in its 'Test' mode.  If you plug the loudspeaker here you will get field collapse!
 

 
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