Series GSC 100 Chassis
Model: 4415 GB
Original List Price : £000.00 ( to be ascertained)
Intergrated Circuits: Yes
The Grundig has a flat tube.
I connected the B&K up to the Grundigs A51-580X and it confirmed what I already knew, that the emission on all three were lo but what It now allowed me to see was to what degree.
Red is dead! Well hardly registering one division in the bad section.
Green & Blue very low in Yellow region
I ran a clean and balance on G & B, this improved the emission reading but still not acceptable, just outside of the yellow region. I then bopped G & B, this brought both guns well into the green, enough I would think going on past experience, to give a reasonable picture.
What to do about Red though. I decided to give it a bop. This lifted it to about 4 divisions into the bad area, I imagine there’s little or no cathode left to recover.
Feeding in a test signal and the picture is marginally improved (see below) but everything is virtually at max contrast and brightness wise. I did wonder about raising the red drive but I’m unfamiliar with the chassis and I seriously doubt it will help all that much.
So I need to find an A51 580X….
This afternoon I fitted a replacement A51-580X which tested as very good to this rather nice Grundig. The donor CRT came from another identical Grundig 4415, however this other one was lights on but dead, had a seized on/off switch, the cabinet had knocks/veneer chips and generally the chassis in much poorer condition. The tripler looked hideous too so I had no qualms about sacrificing it for the greater good. The boards and other useful parts will go into stock to maintain the one good one.
This was quite a task and required the chassis with control gear to be removed, old tired CRT removed, new donor installed and the chassis re-installed. Once done and checks that everything was connected correctly power was applied. I was greeted with a lovely bright Test Card F with excellent grey-scale, then within about 10-20 seconds Red becomes very prominent.
Now if you remember the old CRT when tested on the B&K, the red gun was totally worn out, hardly any emission left. I suspect that a previous service engineer has increased red drive to compensate or has added or removed some components to achieve the same effect.
Trouble is I don’t have the circuit for this, though I’ve yet to check my Red books. Looking at the video module its not obvious how you would adjust the drive to RGB so its going to take some figuring out. I really need to find the service manual or some data.
Still at least I now have a set with a nice strong CRT, just need to figure out this red drive issue .
Well the data is not in my red books which cease at 1982, however forum member “Cathovisor” located the service manual on e-bay Germany so I’ve purchased it. Despite not being able to read a word of German, I should be able to at least figure how the A1’s are driven.
I swapped over the other now spare RGB board and that has improved things a little but one discovery I’ve made is I have a shed load of brightness at my disposal. So much so that the brightness is way down, if I increase it the screen whites out and sort of pulses I think the beam current trouble as a result.
This morning with a fresh mind I studied the board and figured out how you grey-scale and set the drive bias. Its a combination of R547 on the main PCB and R1911, R1931 on the RGB module. Get it right in Black and White. As I suspected the biasing had been adjusted to compensate for the old tubes failing Red gun, readjusting has returned the raster to correct levels.
The old Valvo CRT was quietly taken to the CRT disposal booth and respectfully put down to air. RIP.