Sony KV-2781R -- B+ regulator problem

Replace the bad main filter capacitor. 6V ripple is way too much for that set.

The fault may be the main filter capacitor. Also after replacing the main filter cap, if there is a regulation fault, verify if the feedback circuit for the regulation is working correctly. Going from memory, there is an optical coupler IC involved in that area. On a few occasions, I have seen failures in that area, including the optical IC.

--

JANA _____

This 1986, 27 inch TV (chassis SCC552P-A) came in with an open 1.6 A fuse, defective FTB (1-439-372-11), HOT (2SD1497) and IC601 (STR3035 135V regulator) amongst other things. I replaced the components as well as several high ESR electrolytics and a resistor (in the horizontal centering network). All the joints in the horiz. drive cct, as well as several in other areas, were cleaned and resoldered. The horiz. deflection winding rings 8 on my DSE FBT tester. Waveforms in the horiz. drive area and at the HOT collector appear to be normal. Although the HOT heat sink gets quite warm, it stays within the measured temp. range I've seen on other Sonys. The set works, and especially for a 20 year old, has a very good picture. So what's the problem? The 135V B+ output of IC601 is not well regulated. It varies depending on the AC line input.

When powered up at 115V AC in, there's about 157V at the input of the regulator (pin 1) and about 131V at the output (pin 4). The 157V input has about 6V ripple. Pin 2 has a (forward-biased) diode to ground --there's about 0.6V at pin 2. Pin 3 is connected to a network of several resistor and two caps. All were checked individually and found to be good. I find about 133 V at pin 3. The regulator is bypassed (pin 1 to pin 4) by a 120 Ohm 20W resistor -- it's good. The current through the 1.6 A fuse that feeds the regulator is around 500 ma.

I don't have the schematic for this exact model but I do have several others; for example, the Sony SCC552U-A chassis that has the same basic STR3035 regulator layout. All the measurements conform to that indicated in the SCC552U-A schematic (SAMS).

I haven't located a data sheet for the STR3035, but based on my experience with similar regulators, I would expect the 135V output to remain solid. I tried a second new IC as well as changed the diode at pin 2, to no avail.

At this stage I guess I'm looking for confirmation of the measurements around the regulator (based on the proper schematic), and suggestions for (or experience with) this symptom.

(Please reply to group.)

Michael C.E.T.

You're doing what I and many other techs have done on that set. Lose money. That was Sony's first stab at a 27" crt and that monster yoke usually ends up being the culprit, even Sony admitted that. Try pulling the CRT board then fire it up with the yoke disconnected. Yes, you're removing part of the load on that line but, see if it regulates now. The flyback should still have enough load that, if it's a regulation issue, it would still show up. Just my experience with them.

dkuhajda:

Thanks for the reply. You tweaked some memory here -- I just can't isolate where and when I've seen a bad main filter cap before but it was a good while back.

In any event, I checked the cap (560uFd/200V stubby with 90 degree offset pins) out of cct and it tests good for C and ESR (although I appreciate that static testing might not show everything). The cap is connected directly to the main bridge rectifier. There's no RC filter network; it's just the bridge rectifier - filter capacitor - 1.6A fuse - STR3035 regulator. I would have thought the

6 V ripple not all that bad -- the SCC552U-A (SAMS) schematic shows 8.8 V ripple at the same point.

I don't have anything in stock to sub it with so it will take some time to try that out. But I wanted to check with you again on this before I order a replacement. Have you seen these check good yet not be functioning properly?

Michael

Check for open fusible resistors by the STR regulator. Like .47, or 4.7 ohms, & the like. Dani.

Michael wrote:

On Mon, 24 Jul 2006 17:26:42 -0400, "Michael" put finger to keyboard and composed:

The datasheet specifies 135 +/- 0.8.

See

The STR3035 is a linear regulator with a zener reference and a Darlington pass transistor.

It crosses to an NTE1743:

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

I think this might be a case of design, not fault.

First:

dkuhajda: I substituted another cap for the main filter -- there was no change.

JANA: There's no optical feedback in this one -- it's a pretty straightforward linear regulator circuit.

Dani: A possibility but I checked the resistors in the regulator circuitry -- all okay.

Tech Data: it might not be so bad after all.

Franc: Thanks for the data sheet references.

Now:

The data sheets Franc directed me to cause me to wonder if the symptom is not a problem at all.

The NTE sheet does not indicate input regulation (i.e, how well the output is held constant with input variations).

The SanKen data sheet (from the B&D Enterprises site), in addition to showing regulation under varying load and temperature, also seems to indicate a possible 2.6 Volt output voltage variation with a change of input from 153 to 170VDC. I say "possible" because the input variation appears to be written in, not typed, and I'm not sure about this. However, it does suggest that input regulation is not as tight as I had expected.

In my case, varying the AC line input so that the DC input to the regulator moves from 153 to 180V leads to an actual change of 5.8V on the regulator B+ line output (from about 130 to 135.8V DC). But there was less than 2.5 V change between 156 and 180 V input.

I'm wondering if my observed variation in the B+ line is, in fact, normal for this TV design.

Incidentally, everything from the power plug to the input of the regulator checks out okay so it's not line input losses.

Is it possible that the regulator is not out of spec after all? I'm tending to think so. Would you?

Michael

the

has

in

I

On Wed, 26 Jul 2006 18:52:00 -0400, "Michael" put finger to keyboard and composed:

The way I read Sanken's datasheet is that the output can vary by 2.6V if the current is kept constant at 0.5A while the input varies from

153V to 180V. Alternatively, if the input is constant while the current is varied between 0.25A and 0.5A, then the output variation is only 1.0V. I also see a temperature gradient of 7.0mV/degC.

If the input is constant and the load is maintained at 7mA, then the output is 135 +/- 0.8V.

The NTE1743 datasheet has similar specs for load regulation and tempco.

Is it possible that your replacement part is made to less stringent specs? For example, what if the zener reference were affected by self heating? An ordinary zener diode with a voltage in excess of about

5.6V will have a positive tempco. This would cause the regulator's output to increase with case temperature.

Is it possible that your replacement part is actually an STR3030 (= NTE1742)? The STR3030 is a 130V regulator.

Try increasing the output voltage by replacing the diode at the common terminal with a 4V7 zener.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

As long as the picture doesn't pull in from the sides during bright scenes, I wouldn't worry about it. As a warranty servicer of this model for many years, I can say that the regulation circuit wasn't one of the best. Chuck

Ditto that. These were not the tightest supplies to start with.

Leonard

-------------------------------------------------------------------------------- I am using the free version of SPAMfighter for private users. It has removed 2352 spam emails to date. Paying users do not have this message in their emails. Try SPAMfighter for free now!

On Thu, 27 Jul 2006 16:19:08 +1000, Franc Zabkar put finger to keyboard and composed:

On further examination, I think the power dissipation in the regulator is fairly constant throughout the 153-180V range of input. This is because the 120 ohm shunt resistor diverts the extra power. Therefore there can be no significant variation in output voltage as a result of self heating.

|-- 120 ohm --| Vin o---|---- Reg ----|---o 135V load

500mA

The power dissipated by the regulator would be ...

V x 0.5 - (V x V)/120 where V = Vin - 135

By my reckoning, the dissipation at 153V, 163V, and 180V is as follows:

153V 6.3W 163V 7.5W 180V 5.6W

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

Thanks to everyone who responded.

I agree with Franc's interpretation of the SanKen data sheet. Moreover, it is possible that the IC's I have were made to slightly different criteria (after all, the data sheet is from the late '80's), or subject to manufacturing variables. (In this regard the IC's I used are new, branded SanKen and from a reliable supplier.)

In addition to Chuck and Leonard's comments, a few additional observations suggest to me that my original results might not be as serious an issue as I first thought. I come to this, in particular, as a result of seeing SanKen (and NTE) data sheet.

The data sheet indicates an output voltage of 135 +/- 0.8V with Iin of 7mA. Iin at 7mA is not much of a load, certainly far less than the real-time situation in which I am measuring (Iin around 500 ma.) The specification for load regulation is 1V between 250 to 500 mA Io. What happens between the no-load (7mA) level and 250mA is not indicated.

Even at the specified Iin of 7mA, the data sheet does not say what the DC input voltage is. The NTE data sheet refers to Vout of 135V +/- 1V at a Vin of

120VAC. With full wave rectification, a good sized filter capacitor, virtually no load, and a nominal RMS AC line of 120V, I would expect the input voltage to be close to 170VDC.

Here are my measurements, with the TV running and Iin around 500mA.

AC Line input VDC output VDC

118 153 130 119 155 132.5 123 160 134 126 165 134.4 130 170 134.9 134 175 135.4 138 180 135.8

(The normal line voltage here is 120VAC.)

There could be measuring differences: The TV's circuit and that of the data sheet are not exactly the same. The data sheet has R3 of 12K Ohms and R4 of

220k Ohms whereas in the TV the equivalent resistances are 15k and 470k. Also, the DC input to the regulator has about 6V p-p sawtooth ripple (there's no RC filter as in the test circuit, only a capacitor). I don't know how well the IC handles this. Moreover, I don't know how my digital meters handle the ripple, on top of their normal accuracy tolerances. If it is a simple averaging of the ripple, the DC input voltages that I have recorded at different line levels might be incorrect, say, within a 3V range.

All to say that I tend to agree with Chuck and Leonard that my observation of input regulation is probably not a defect. I would add that the raster is quite stable with varying brightness.

Michael

(snip)

I do not know if this applies to the regulator in question, but I worked on a Sony that had the regulator and the audio output in the same hybrid package. It had horrible regulation at low A.C. input voltages (108 volts or less). I found out that at low voltages there was still 15v or so drop across the regulator. I finally removed a resistor (120K or 220K) between the Darlington base pin on the package and A.C. ground. Regulation at low voltages improved dramatically. As to why it was in the circuit is a mystery to me as it basically has no effect at higher input voltages.

David

On Thu, 27 Jul 2006 17:17:01 -0400, "Michael" put finger to keyboard and composed:

Applying I = C x dV/dt, and assuming a ripple frequency of 120Hz and a load current of 0.5A, this would imply that the main filter capacitor is of the order of 680uF.

Allowing for a 1.2V drop through the full wave rectifier, and a drop of 3V to account for ripple, I'd expect that your DMM should read about 166VDC at the input to the regulator. Instead you seem to be losing about 10V somewhere. If this loss is resistive, then something must be dissipating around 5W.

The fact that the datasheet specifies a test range of 153-180 suggests that the nominal expected input is 166.5V. Is it possible that your variac is affecting the DMM reading? Are you measuring the AC at the bridge?

I notice that the regulator's performance is specified in reference to a particular test circuit. IMO a regulator's specs should not be dependent on the choice of external support components. In fact the test circuit produces absurd results.

For example, Io is the load current, not the regulator current. It is the sum of the regulator current plus the current through the bypass resistor, R5. If the entire load current of 0.5A were to be drawn by R5 (47 ohm), then the voltage drop across it would be 23.5V. This would give a maximum input voltage of only ...

Vin = 135V + 47 x 0.5 = 158.5V

Conversely, an input voltage of 180V would require a current of approximately 1A through R5.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

Franc:

The capacitor is 560uFd (nominal) .

I was measuring the AC input on a separate power meter at the Variac output. It allows me to verify the line voltage and current at the TV plug. (I have checked it against my DMMs.)

As for losses between the plug itself and the bridge rectifier, there is an inductive filter on the line and a 1.8 Ohm 10W resistor between the relay and the bridge rectifier. (I've checked all connections and the relay contacts.) Also, after the bridge rectifier, in addition to the filter cap there is a 68K Ohm to ground (good idea for techs -- it discharges the filter cap in the event the 1.6A fuse has blown).

I just measured the AC at the bridge itself. There's a drop of less than

2VAC between the plug and the bridge. This is as expected, largely because of the 1.8 Ohm resistor.

The SAMS schematics I have for similar Sonys with the 3035 regulator circuitry show 120VAC at the plug and about 156VDC at the input of the

3035 regulator. They also show similar levels of ripple at the output of the bridge rectifier. This would seem to support my measurements of DC input to the regulator.

On the subject of the "absurd" 3035 specs, is the bypass resistor in the test circuit only 47 Ohms?

data

of

On Fri, 28 Jul 2006 11:32:30 -0400, "Michael" put finger to keyboard and composed:

See the test ciruit on page 3 of Sanken's datasheet. I don't understand Japanese, but the second and third parameters of table 6 (on page 2) appear to refer to test circuits 1 and 2, respectively.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.