Loewe TV Chassis Q2500 Startup Failure

[DavetheBari]

I have inherited a beautiful Loewe Xelos TV (Chassis Q2500), which I have been using with a digital TV Set Top Box since the demise of analog TV. However, it recently failed, and I am, for technical interest, trying to repair it.
The failure was in the main switch-mode power supply. The control chip and MOSFET switching transistor blew - bits of plastic case missing in both cases. I replaced these two components and the resistors connecting them - controller output to MOSFET gate. (There are two in parallel. Both had gone very high in resistance). All other components in the switch-mode circuit seemed to be OK. With the base board disconnected from all else, the switch-mode power supply then worked fine. The transformer produced the required secondary AC voltages.
Looking good. The re-assembled set however, misbehaves on startup. The standby power indicator (red LED) shows when mains power is applied. An 'On' command from the remote results in the following display sequence:-
Green LED (main power indicator) On
5 seconds
Red LED (standby power indicator) Off
2 seconds
Green LED Off and red LED On.
Nothing appears on the screen, but speakers provide the usual 'click' as the sound circuit becomes active.
A similar sequence results if the 'On' command is delivered by pressing the power button on the set in after mains power is applied.
I am wondering if there is a history of this type of failure that might indicate the likely problem, or if anyone has some thoughts on what to suspect.
(I have decades of experience in electronics, from valves -YES!- to microprocessors. This is my first look at a Loewe product however.)

 

[kellys_eye]

It's either over voltage or over current shut down - either of which requires you to monitor the outputs accordingly - with dummy loads if necessary - to identify the line supply that is causing the issue.
Once you know which supply is the problem and for what reason you can move on from there.
Do you have the schematics?

 

[DavetheBari]

Many thanks, kellys_eye.
I've now done a check on all the DC power voltages, with the whole set interconnected. They are all OK.
I brief look at some of the measuring points in the Base Board seem to indicate some pretty weird things happening in both the horizontal and vertical sweep generators, but have not had time to do a proper analysis.

That's going to take some careful planning & reading, and will be my next approach, I think. The big problems are that I have only the first 7 seconds of 'power on' to look at, and the information I have is pretty limited - mainly a Q2500 document that contains heaps of circuit diagrams and PCB layouts, plus a section titled 'Technology of Q 2500 colour TV set'. This section appears to have a Loewe 'Part Number' 231-90415.917.

As at least the first 2 pages of the whole document are missing (the first page I have is numbered '1-3'!), I don't know what the whole document's title is, as it seems to be a collection of separate documents not specifically developed for this collection. Whilst it contains heaps of detail, it is not very clear on testing and fault-finding.

Cheers, and thanks again.

 

[73's de Edd]

Sir DavetheBari . . . . . ( Whut's a Bari? )

I am finding Q2500 pages 1-3 as being full schematics in my documentation . . .
To see if our documents are agreeing, see if these part numbers coincide.

The great life resuscitator for the SMPS section should have been, replacement of Q624 . . . a BUZ91A and its two gate drive resistors would be parallelled 22 ohm R630 and R624.

You have MULTIPLE voltages being sourced from the secondary of the T639 SMPS power transformer . .

To wit . . .

Pins 12-13-14 + D686---687---D681 give you your U+NF and U-NF supplies
Pins 16-15 + D671 give you your U7 supply which switches in via Q674 to further give the U5 supply.
Pins 16-17 + D661--663 give you your U14 supply which switches in via Q663 to additionally give the U12, U8 and U9 supplies.
Pins 18-19 + D656 create supply U25.
And now WINNER . ..WINNER ! Chicken dinner ! . . . . that is where I think your problem is most likely to be, it is in the :
Pins 20-22 + D661--663 which creates your UB supply.

OHM out that created UB line, when being ground referenced to the COLD ground presented by the negative terminal of C651 filter cap of that UB supply.
Is there being any LOW ohmage to ground, on that line, that could load down this hundred + volt supply for the HOZ deflection circuit ? ? ?
My next step, in analysis, would be to pull the HOZ output transistor . . . . Q534 . . . . completely out of circuit and then see if the unit will power up this SMPS section and hold your string of supply voltages that I just gave you.

That's it for now, until we see that we are using the same songbook, and I'm not singing bass while you are chiming in, but in tenor . . . . .

Aside . . . . . .
A question for enny uf you astute and intell-e-lectual reeders sitting way ups in the top tiers of the "peanuts gallery"
I koes thet thar tri-angle with an eggs-plane-nation mark, is sig-niffy-eyeing a critical safety related part.
But this schematic has adjunct parts notations of *Best-Var* . . . . .*Typ-Var* . . . . .*Best-Typ-Var* . . . . .placed all around, beside components in this Loews schematic.FIRST time EVER, that I have seen such.
It is being irrelevant, as to whether it is being an inductive, capacitive or resistive element.

73's de Edd
.....

 

[DavetheBari]

Sir 73's deEd (what on Earth...?)
I sing Baritone in a Barbershop chorus. Barbershop is arranged in 4 parts - Tenor, Lead (which normally sings the melody), Baritone and Bass. I also sing Tenor in a Barbershop quartet. So 'DavetheBari' is just short for 'David, the baritone'.)
I found another copy of the Loewe document I started with, in docslide. It has the first pages missing from my original, but not the 'Technology of Q 2500 colour TV set' section. Otherwise, the two documents seem to be the same. Its first page indicates it is 'Servicenleitung' i.e. 'Service Manual'. Other identifications are 'Q 2500 M' and '230-90414.960'. The circuit diagram for the 'Basic Board Q2500' is Page 125. The last page is Page 127, a circuit diagram for 'TVO 3'.
All the part numbers etc. that you quote agree with my documents, so we seem to be singing the same song.
It would be nice to find a Loewe document that describes its nomenclature logic! I have figured out that for all the power supply lines labelled 'Usomething', if the 'something is a figure, it is the nominal voltage. Other 'something's just seem to be identifiers with no hidden meaning. I too have been mystified by these *Best-Var* etc. tags. Perhaps 'Best-Var' components are absent in models (variants) other than the 'best' (probably the most expensive!). Thanks for the info. about the triangle-with-enclosed-explanation-mark.
Thanks for the suggestion to look at the 'UB' line. I found a statement in the 'Technology...' section that to check the horizontal output stage (presumably without generating EHT etc.) you need to break the UB supply at 'L534'. With L534 removed, the UB supply checks out at +160 V. With power removed, the resistance you mention is over 160 Kohms. The resistance into the circuit after L534 (into the T531 driver) is megohms.
Now to re-insert L534 and start looking at the outputs of the stages in the generation of horizontal deflection. (Without the horizontal drive system doing something, the vertical system apparently does nothing, as at least some of its power comes from a secondary winding in T531.)
Cheers
David

 

[DavetheBari]

An update (all this is with no TV signal input):-
1. The standby power system, main switched-mode power supply (SMPS) and all the DC power levels derived from the SMPS are all operating correctly. The return to Standby 7 sec. after power on however always happens.
2. During the 7 second interval, the Signal Board produces opposing sawtooth signals for vertical sweep of correct amplitude and period. It does not produce any pulses on 'HDrive' ( supplied to the Basic Board via connector W511 pin 13), this remaining at 0V.
3. The whole Horizontal output system remains dormant. This means that no power is available on UV+ and UV- supplies which are created from a secondary winding on the Horizontal Output Transformer T531, hence the vertical output stage does nothing with the sawtooth signals being produced for it by the Signal Board.
The documentation I have is a translation from the original German, and is often almost impossible to understand. My guess is that the 'Horizontal Output Driver', consisting of transformer T528, transistor Q534, capacitor C531 and sundry associated components, actually forms an oscillator that synchronises with any pulse train (of the correct period) that it receives on the 'HDrive' signal. Thus it should, I believe, start itself up when its power supplies (U25 and UB) become energised. Because this is not happening, the system detects and error and shuts the SMPS off, returning the system to Standby.
Can anyone comment on these thoughts, before I start the 'orrible task of pulling components looking for a cause? In particular, views on my guess about the Horizontal Output Driver would be very much appreciated.
Cheers
David

 

[73's de Edd]

Sir David . . . . .

After I saw you mentioning the presence of the UB supply . . . .albeit a bit higher than is usually expected . . . .typically being 110- ---130 VDC, as has been encountered on different sets.
But we don't know how much that value is being loaded down to . . . . with a constantly running Hoz deflection stage..
With your qualifying "decades of experience in electronics" this would indicate your owning and having proficiency with a triggered O-scope.
Plus . . . . you just hinted of such use , with the analysis of the vertical circuit circuitry, which is being powered from scavenging windings and their derived power supply sections from THE HOT / LOPT transformer.
HOWEVER that HOZ circuitry seems to try to come up, but then times out and shuts down due to some ill effect related to the HOT or its deflection circuitry.

Testee-Testee . . . . .

NOW . . . use cold ground referencing and move over to the 1st grid of the hoz driver transistor . . .Q526.
(Got you on that one . . .didn't I ? )
There . . . you have your scope probe sampling to see a continual stream of drive pulses . . .just after set power up OR do they stop coming in, after a prescribed timeout period ?

Next move the probe up to either the collector of Q526 driver OR pin 2 of the T528 driver transformer.
Do you now have some FIERCE drive pulses, and do they have the same time of their presence, as was being monitored on the input of Q526 ?

Now move the probe up to the pin 1 of the drive transformer and see if you have VOLTS levels of pulses or just mere 10's of millivolts amplitude ?

Move the probe to the pin 4 of the secondary and inspect for drive pulses, but expect a quantum decrease in amplitude by virtue of the now downward impedance shift. Trading volts for current.
You will then do a dual probing to see if the drive amplitude at the T528 pin 4 is being the same as that of the probing RIGHT AT the base connection to the Q534 hoz output transistor.
With time, and possible marginal factory, flow soldering of the connections to Q534. A resultant ultrasonic current cavitation on its terminal leads , could have developed into intermittent floating solder joint rings .

Waiting for your info . . .up to this stage of initial evaluative testing . . . .

73's de Edd
.....

 

[DavetheBari]

Thanks so much for your interest, 73's de Edd. It is so helpful to have someone to talk to about this technical stuff.
First, the value of UB. My apologies, the figure I quoted was a mis-remember. It is actually approx. 130 volts, but more on that later.
As I have said, the whole line drive system was not doing anything, and there were no pulses coming from the signal board. I think we can agree that my guess about the first stage being an oscillator was wrong. I had originally assumed that it was just an amplifier/shaper, and the signal board should be producing line-timing pulses to drive it. It is a good while since I was involved with TV electronics, but in my experience, the generation of line drive was always done starting with a free-running oscillator that got synched to the incoming signal information when there was an incoming signal. So I started with the view that the signal board was responsible for that bit. Something in the translated 'Technology' document made me wonder, so I created the idea that the oscillator was actually on the Base Board. Silly me!
So I broke the pulse input connection between connector W511 Pin 13 and C524, and fed in a pulse train from a function generator, setting the output of it to approximately match the picture of 'scope trace for MP29 on the circuit diagram of the Base Board. What I found was that after Power On, line output pulses were generated for approximately 5 msec. every 0.5 seconds. In between, nothing. At the same time, UB was at 130 volts while these pulses were being produced, but fell to around 60 volts for the period between sets of pulses. A similar change occurred on U25. (The voltage levels being different, of course.) Pin 6 of the TDA4605 had similar changes with time, The high level on Pin 6 was 11 volts with SMPS pulses to 27 volts superimposed, the low level was approx. 5 volts and flat, well below the 7 volts required to keep the SMPS running.
My deduction from this was that either:-
(a) something was drawing too much current from the line output transformer, or
(b) the line output transformer had an internal fault, or
(c) the line drive output stage was consuming more current than it should.
If (a), perhaps a faulty picture tube? So I disconnected the CRT from the line output transformer, and disconnected all connections to the CRT Board.
With my function generator still supplying line pulses, the line output electronics produced a continuous stream of pulses, and UB remained at 130 volts. However, After a time of between 30 seconds and a minute or two (I was engrossed with what was happening, with no thought of time!!!) A bang announced cessation of everything. The SMPS had blown Q624, R624 and R630. Probably the TDA 4605 as well. Now I am trying to source some replacements, and will be away from home for about a week anyway, so all is held over.
Nothing, as they say, is ever a complete failure. Blowing up the SMPS without the CRT and its Board connected seems to indicate that the problem may be in the Line Output circuits or the Line Output transformer. We shall see....

 

[73's de Edd]

Sir Dave . . . . .


" free-running oscillator that got synced to the incoming signal "

That dates, all the way back to day one of tube technology of the handling of hoz circuitry. Black and white difference in solid state.

OH . . .NOOOOOOoooooooooo . . .
To err is human and you certainly erred in the choice of using a function generator.
The normal drive duration of a pulse to the Hoz output is but a mere ~18 % of a duty cycle, and here you were hitting it with a full 50% duty cycle . . . . . that is . . . .up to that point where both the Hoz output circuit proper and the UB power supply circuitry TOLD you . . . I CAN'T TAKE IT ANYMORE ! and then responded accordingly with the end result..

The plan was just to monitor the different points that were given and see how the SETS own generated hoz pulse was traveling thru the proceeding points that I gave you.
Never did hear back . . . to initially see if the HOT . . . transistor Q534 . . . was just sitting there with a C-E short across it.
Getting power back . . .
In testing . . . . again . . . .and finding drive all the way to the base of the HOT.
The next logical procedure would be to tie on DC metered monitoring:
Meter referencing to the cold ground at pin 9 of the secondary of the flyback
Then the metered voltage can be at either C660 for the MINUS UV supply voltage
or
at C558 for the POSITIVE UV supply voltage.
or
at C548 for the POSITIVE USVM voltage.

THEN, you power up for just long enough to see if any of hose voltages are trying to come up.

73's de Edd
.....

 

[DavetheBari]

Thanks, 73's de Ed, but...
(a) So in 'solid state' TV, there is no horizontal drive if no TV signal? How come our current TVs show dialogs saying 'no signal' or similar if you disconnect the antenna? How is that done if there is nothing to drive the steps from pixel to pixel in the horizontal direction? Surely that is done by something equivalent to a free-running oscillator that can sync. to information from the incoming information. (By the term 'free-running' I mean not dependent on a signal input. Of course with the facilities available in digital technology, the pulse train would probably be derived from a system clock. What is a system clock? A free-running oscillator!)

(b) Regarding pulse length, you are telling me that the Loewe information on the input to the horizontal drive is wrong? There are two pics that tell me that the pulse coming from the Signal Board (on pin 13 of the connector W511) is approx. 18 microsec. long with a period between pulses of approx. 32 microsec. That is a pulse length of 56% of the duty cycle. See the plots for MP29 and MP16 - that is, input from Signal Board and output from the transistor Q526 respectively. My function generator provided pulses exactly as shown in the Loewe pic. for MP29.
The resulting output to the HOT is, according to Loewe, a pulse with a max. width of 6 micosec. i.e. 19% of the period between pulses, as shown by Loewe for MP20. (I did not get to view this.)

(c) The problem with your plan, 73's de Ed, was that without any pulses from the Signal Board, there WAS NOTHING TO SEE in the horizontal drive system. I thought that I had indicated that.

As I said, I have to leave all this alone now for a week or two.
Cheers
David