CTV PCB Repairs

pcbrepban

Welcome to this new section, I intend to feature and deal with some of the boards you are likely to find in early colour televisions. Not only dealing with the standard stock faults you may encounter on the various modules but also dealing with the age related problems such as lifted tracks and how to repair and in extreme cases cracked PCB’s.

There are many ways to do things I not suggesting my way is the only way. I will just detail how I approached the repairs and in doing so hopefully give others ideas and encouragement to do so themselves. Throughout this you will find all sorts of useful documentation such as Stock faults, repair guides, service data, Set-up and diagnostic tips. Hopefully for those interested in vintage colour TV repair will find some if not all of it of use.

The days of ringing up suppliers to get replacement boards or modules are far behind us. Therefore the stock of panels we find at swap-meets or auction sites are our only source. I’m a firm believer that it does not matter their state as in most cases with a few replacement components, time spent repairing tracks, PCB structural faults and a good cleaner, even the most hideous boards can be made to function again.

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Sometimes for lack of a board this is the difference between having a dead TV and one working. For example I recently brought back to life a Thorn 3000 power supply module. It was badly cracked in two places but with careful wire stitching and track repair the module now works flawlessly. If anyone reading this has early colour CTV boards they are thinking of dumping, please consider offering or donating them.


The BRC/Thorn 3000/3500 Power Supply

Lets deal with this one first as its probably the main culprit for a dead set when you get one of these TV’s. First of all we shall go over the start-up sequence and how the various rails are established and what is responsible for what.

  1. At switch On The half-wave rectifier W601 produces the 240V rail.
  2. As a result, the 30V zener W605 comes into operation, biased by R608.
  3. The 30V series regulator transistor VT601 can then come into operation.
  4. Once the 30V rail has been established, the line oscillator will start up supplying the trigger pulses to drive the mono-stable circuit. So the last supply to appear is the ‘Chopper’ regulated rail.
  5. No 30V rail means no ‘Chopper’ drive, therefore the delay switch transistor VT602 makes this doubly certain. Unless W605 conducts, VT602 remains ‘cut-off’ and the mono-stable cannot operate since VT603’s emitter is virtually open circuit.
  6. Therefore if the 30V zener or VT602 or VT601 are o/c or C609 s/c there will be no ‘Chopper’ drive, no ‘EHT’, no ‘Field scan’ or ‘sound’
  7. The 58-65 supply is achieved via the chopper transistor VT604 with its inductive load reservoir L603
  8. VT604 is switched on and off at line frequency by a square-wave generated by a mono-stable multi-vibrator VT603/Vt606
  9. When VT604 is switched ‘ON’, energy is stored in L603
  10. When VT604 is switched ‘OFF’, the efficiency diode W616 switches on and the current flow in the load is maintained.
  11. Stabilisation is achieved by varying the on/off time of the chopper transistor, varying the mark-space ratio of the waveform produced by the mono-stable
  12. The output voltage is sensed by the feedback amp VT608, whose collector voltage is used to adjust the time-constant of the mono-stable circuit
The 240V supply :

W601, C602, C603, R601, R602
(R608) feeds the zener that creates the 30V supply

The 30V Supply:

R606, R615, VT601, C627, R608, W605, VT602, C607, C609

Transistor Functions:
Transistor Function
VT601 30V stabiliser
VT602 Delay Switch
VT603 Mono-Stable
VT604 Chopper
VT605 Chopper Drive
VT606 Mono-Stable
VT608 Feedback Amp
VT609 12V stabiliser

Expected Voltage table of a good PSU

Transistor Base Emitter Collector
VT602 .7 .4
VT603 .85 .4 23.5
VT605 1 .75 11
VT606 1.6 1 6.5
VT608 30.5 31.2 10
What you need to use to diagnose this module

All you need is a meter and an Oscilloscope.

What I use

I use a Thorn PSU module repair Rig, back in the day you could buy the “black box”. This allows you to power the PSU module and see the state of the rails via neons, These indicate if the various rails are present or not. There is also a cut-out which operates the same as the set. You can simulate line pulse by use of the fly-lead which is attached to the W608. Alternatively you can also hook up and power a line timebase module which will generate the pulse and you can test the time-base module.

If find this invaluable as it means I can power these modules independently and work away from the TV. Invariably unless you’re really lucky, you will have to open up the PSU module “Spatch-Cock” style for diagnostics and repair. It would be a PITA to keep assembling and disassembling to test in the TV chassis. Back in the day a field engineer would only go so far with a repair at the customers home, when it became obvious the fault was going to require in depth work, a replacement module would be swapped in. The original PSU then returned to the bench techs at the shop for analysis and repair.

Oscilloscope for waveform testing, Transistor Tester, Thyristor Tester, Capacitor Tester, Thorn CPV (collector Peak Volt) device and finally a Fluke DMM and AVO 9.

Some recently repaired PSU’s

fin

Thorn PSU “Spatch-Cocked” on the rig

put6

If you have anything to add or comment on then please use the comment box below.


Telefunken RGB -PCB repair

A distinct lack of red suspicion first centered on the chroma pcb 3

There is a problem with Red drive, this is confirmed with the scope.

1 2 3 4 c

There was much confusion with service data and my Chroma IA and Chroma IIA boards with regards to component variances especially the IC’s. The only service data I have is the Trader sheets and Radio & Television Servicing, both show different IC’s and number of. A fellow member found me data from another model which appears to be a match for my set up.

The boards varied mine being populated by Telefunken TDA 2140, TDA 2150 and TDA 2160 and are more akin to the 714, the rest of the set matches the 712. Below the Chroma IIA from the 714

From the waveforms above it would suggest IC302 – TDA2160 the Synchronous demodulator and RGB matrix was faulty but after much in-depth testing on the IC it was found to be innocent.

Bread board piggy backed onto Chroma IIA IC output for red transposed to green circuit, if the IC is faulty then Red will work. It didn’t this test confirmed the IC was not faulty and the error condition must exist on the RGB board and the red circuit.

test1maps

The fault turned out to be far simpler but odd, a track in the Red circuit had been deliberately cut, quite why will remain a mystery. Track repairs were performed resulting in the red drive being restored.

Red drive virtually non existent

reddrivelow

Cut track found, why was it cut?

T331cut

Close up under bench microscope

scope1

Now for the repair. Old tracks each side of the breach prepared and ready for new copper.

1 3 4

Red drive restored

t331-1

Bread board and hard-wires removed, IC socket reinstalled along with the IC, low and behold we are in business.

wehavered

Picture centred, to adjust the picture shift, you need to short-circuit two pins ( SV562) on the E-W module. You then adjust R452(Vertical Freq) on the oscillator board for centre. Then adjust R517 ( for full width), R561 ( to correct trapezium distortion) and finally readjust width via R518 for slight over-scan. SV562 on the module fitted to this set was labelled SV502.

Then it was re-grey-scaled, result below.

lastest


BRC 3000 Line Timebase : Rare early two transistor version

he BRC 3000 was launched in 1969 and the first generation sets had a line time-base module that had a few notable differences to the module that followed a year later. This was the use of two line output transistors VT504 & VT505 both R2009’s the later module used a single R2008. The driver transformer T502, is a huge affair compared to the later modules which was much smaller and finally the fly-back tuning was a pair of .056uF 400V caps in series instead of the later singular .028uF 800V. There are some other subtle component differences.

I don’t know about other collectors but I’ve found this module to be extremely rare. I have six 3K sets and 4 spare line time-bases, until recently I had never seen one, all mine and the ones I’ve seen in other collections have the later time-base. Last year I was contacted via this site asking if I wanted a couple of old BRC modules, I gladly accepted and through the post I received a PSU and a time-base module. To my delight the time-base module was the 1st generation as described above.

Its battered, bruised and its been through the wars. Many components and wires are hanging off, there is also some significant damage to one corner. This being a missing piece and along with it a few components, including the interconnecting tracks that completed the circuit. As this to me is such a rare module I plan is to completely overhaul it, repair the damage as best I can replacing the missing PCB section and using my track repair kit rebuild what is missing from the corner.

I had a really battered and broken I/F panel and will use a section of that to rebuild the corner. I will remove any traces that are present and have marked out the exact break to match the missing piece. I will use my Shapercraft scrolling machine to get a precise cut then epoxy them together. Then using a plan view rebuild the tracks, drill the component holes and repopulate.

The beam limiter sub panel is also missing but I have a spare so that’s not a problem. Once all the loose components have been rechecked and resoldered, the missing plugs etc, then the real fun of power up and testing will begin.

ltb-1 ltb-2 ltb-3 ltb-4 ltb-5

I spent time cutting and shaping the missing piece of PCB. Its slowly getting there, it may not be a perfect colour match but once glued filled at least it will be a lot neater. Once the missing components are installed it should be a little less obvious. The photo below shows the new piece offered up and held in place with blutack. The mark pen on the edge is the file line to match the break, I’m still working on it.

ltb-7

The missing piece has been shaped the best I can do, its held in with two wire stitches and the join filled with milliput and epoxy resin. This was then put to one side to let it all cure. The next task was to two lay down the missing tracks and repopulate the missing components. A little satin varnish will tone down the colour mismatch. The whole board will then get a thorough clean with the PCB cleaner, then onto the wiring and missing beam limiter.

ltb-8 ltb-10

The new rebuilt PCB corner set reasonably well, slight flexing showed it has a good mechanical bond to the rest of the board. The edge pin stitching should help with keeping it all in place. I decided to carefully drill out the PCB mounting post hole and while I’m at it lay down the missing tracks. The tracks are a tricky & fiddly job but got there in the end, having other LTB modules helps with seeing how to replicate the missing layouts. These will need to thoroughly dry before they are drilled for the missing components and circuit posts.

tracks

New tracks were tinned & continuity tested to the rest of the circuit, all passed. Then component holes drilled. The LOPT post J was missing (takes fly lead from point C of LOPT), new one sourced and hole drilled and fly lead fitted from LOPT. At this stage I’m not wasting new components prior to testing so W506, BA148 fitted (fitted from scapper, tested ok) along with C526, 50uF (fitted from scapper, tested ok) C527 270pF (fitted from scapper, tested ok).

ltb-b ltb-a ltb-c

I then went over the entire rear panel addressing loose components and suspect dry joints. The wires that provide connections to the beam limiter had been chopped off quite short so they were extended. I then sourced a replacement beam board soldered up and then fitted and locked down with three new screws. Last preparatory work will be to fit the missing fly-leads and then test. PCB and output transistors looking very clean now.

ltb-d ltb-h ltb-f

Found some spare interconnect leads, one set joins the LTB to the PSU, the other is to the convergence panel. For testing purposes I only really needed the PSU one but thought I might as well add the other. That completes the rebuild and preparatory work, next step is to power up and check my rails. If they are present, then see if the oscillator is running and if I have the 8kV pulse at the EHT nipple.

ltb-1c ltb-1b ltb-1a

Just to be sure the PSU this LTB would be connected to had zero faults, I powered it up on the jig first without the LTB connected. To simulate a line pulse in this scenario to get the 58-65V rail to present itself, you hook the line pulse wire from the test unit to the -ve end of W608. When powered up all rails were present an correct, good, the PSU works and is happy on the jig.

Next I hooked up the early MK1 line timebase module to the PSU, removing the simulated pulse from W608 then powered up.The rails came up but stopped at the 30V one, the 58-65V rail fails to establish. Therefore its fairly safe to say the line pulse must be missing.

likely suspects are :-

  1. C511 25uF short-circuit
  2. VT501, VT502 faulty
  3. C512 .015uF short-circuit
  4. Broken connections on L501
  5. W507

ltb-pwr1

Further investigation revealed that W507 was short-circuit. Replaced and now the oscillator was running……the 8kV pulse is present, nice purple spark can be drawn from the EHT nipple. That conclude the repair.

ltb-work1


Another Thorn 3000 Power Supply

I removed the PSU to give it an inspection prior to the usual Spatch-Cocking for the Test Jig. First observations suggest this ones seen a lot of use, I guess with the smaller cabinet the heat generated and captured cooks things up a tad.

The 60V and 30V fuses are intact, that’s good. I may have found the source of the loud crack, it being C624 a Callins which has shed its coat at one end. R642 looks really burned up too. Both these are in the 30V line. C608 looks poorly as do most of the other Mullard tropicals which are cracked and crumbling.

Just before I open it up I loaded the PSU on the test rig to check the voltages. The immediate thing I notice is the 30V rail light is illuminated and very bright. A quick check of W605 anode and cathode reveals they are both sitting at 40V, this means the 30V zener is short-circuit and has undoubtedly destroyed the delay switch VT602. Junction of R607/C608 was also cooking, its voltage was at 4V no doubt caused by the Chopper drive transistor VT605 being switched hard-on.

Well the next step is to spatch-cock the PSU and replace the failed components, check the burned up R642 and replace a couple of essential resistors then see where we are. Despite the rather worn and tired appearance up top, the print-side of the PCB although dirty will clean up. Unlike the Ultra’s PSU this Fergy’s PSU tracks have not suffered from lifting or abuse due to many past repairs.

More to follow…………..

 

 

 

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