1972 Baird 8749

BRC 3500 Chassis

Model: Baird 8749

Year:1972

System:625 Line

Original List Price : £000.00 ( to be ascertained)

Valves: None

Transistors:Yes

CRT: A67 120X

Model 8749 Brochure 

Update

Checked the emission of the Mullard A67 120X. After initially reading very low I’m happy to report after 30 mins of running on the CRT tester the Red, Green and Blue guns are all reading up in the 70+%. Probability is high that it will recover even further with extended use, I think we can say the CRT will be a good-un.

Next up is to test the Baird’s supply rails, this has been made a lot simpler with the addition of my recent acquisition, a BRC 3000/3500 test rig. This device can test the rails with visual indication via lamps. The PSU tested and passed so almost at the stage to introduce the whole set to full mains.

The rail voltages mapped and concur with the test jig

The Chroma and Video boards were covered in dust and goo, this had been a smokers set. Once removed and cleaned up they came up a treat. Visual inspection does not reveal anything untoward.

Chroma BeforeChroma AfterVideo BeforeVideo After

Update:

Power Supply Module

The PSU failed due to a dead short killing the 30V rail. The dead short resided on the I.F panel, C132 being the culprit. Whilst testing the PSU I also lost the 60V rail and it quickly deteriorated from there. There were a number of transistor failures, I replaced these along with many caps and resistors, still the rails eluded me. To be honest my head was in the wrong place and once poked in the ribs by a good friend It soon fell into place with the final pieces of the puzzle being a sneaky break in R602 and a track issue on F603.

 Update:

Line Timebase module

Spent some time inspecting and rectifying faults on this module. L502 replaced along with R528 which had burnt up. A few more caps and resistors and dry joints.Once happy the PSU and line timebase modules were connected together and in turn connected to the test jig, the rails remained stable.

Next the collector of VT504 ( line output) was lifted and the scope connected to the collector of VT502 ( Line oscillator. This was to check it was working and at the right frequency. As can be seen from the scope trace it was bang on the money at 64uS 15,625KHz.

Update

Installed the PSU and Line timebase modules into the chassis, hooked them up to the chassis harness and tested the rails were stable. The 60V rail was about 58V, the 30V rail was at 31.2V and the 6.3V rail was spot on. I Replaced R907 ( 1.5R) last night and the voltage to today is spot on at 1.3V

Next the Frame/sound module was installed in to the chassis, hooked up to the harness then connected the tripler, whilst monitoring the 60V, 30V rails and the EHT.

The 60V rails seems to have dropped to around 57V, the 30V rail stays steady at 31.8V . EHT is steady at exactly 20kV, the “SET EHT” preset has no effect whatsoever.

Plugging each module in and checking for shorts to chassis at the fuses. I eventually made it to a raster.

Initial Observations

Poor width ( EHT not set right see above)
Convergence needs adjustment
Brightness and contrast not working correctly.
Wavy edges to both sides of the test card that meander up and down ( smoothing?)

Update

After spending many days and hours rebuilding 3000 PSU modules I finally cracked the supply rails problem, in turn the EHT and subsequently the width. I now have a varicap tuner fault. The decoder is having trouble and the convergences is all out of whack. I need to do a full installation set-up as per the service manual and then address the faults from there.

Update:

Varicap tuner decided to play up CH1 & CH2 both ceased to tune in a signal, CH3, CH4, CH5 & CH6 worked OK. This turned out to the the 100K variable resistor failing. The worm drive rotates but the variable block just travels in one direction, tried cleaning but to no avail. Choices are to substitute working CH5 & CH6 into the CH1 & CH2 positions, moving the broken pair into their old position, or find old stock replacements.

Update

With the tuner fault resolved It was now time to modify the video board. The newer 3500 video board employed the Thick Film unit instead of the more reliable six resistors. The film units were not the most reliable of components, the end product of this would be intermittent Grey-Scale drift.

In an effort to improve this reliability as this set is likely to get used a fair bit, I will be reverting back to the earlier configuration by removing the film unit.

This method employed six individual resistors, three 12K 7W and three 56K 1W.

These are as follows:-

R259, R264 & R277 (12K)
R249, R263 & R276 (56K)

After setting the purity, grey scale and static convergence the resultant test card is looking a whole lot more better and stable. Needs work on dynamic convergence as can be seen.