View Single Post
Old 11th May 2021, 1:07 pm   #28
matspar
Pentode
 
matspar's Avatar
 
Join Date: Aug 2009
Location: Portsmouth, Hampshire, UK.
Posts: 210
Default Re: Philips 663A combined radio/television 1948

I had a big breakthrough followed by a bit of a setback, but hopefully only a minor one.

The new ECH35's arrived and I began by replacing the suspect ones in the line and frame oscillator. Afterwards this produced a cogged but linear test card, with no trace of the previous velocity modulation and a massive improvement to the foldover that existed. EHT and focus was vastly improved also, showing 7.5kv on the meter.

Moving into the sync-seperator the cogging disappeared after replacing a 0.004uf coupling cap and a 75uf decoupling electrolytic, leaving a very respectable picture.
This was sadly short lived though and the width suddenly shot in leaving a narrow 3" raster. EHT is being damped to around 4kv.

HT has risen slightly, the line output valve (EL38) is slightly running into grid current. Examining the line drive waveform from the grid side of R86 is showing a sawtooth with a bit of a flat top.

I think I can rule out valve trouble as substituting the EL38 and even putting the old ECH35's back in are producing the same results.
I originally suspected a problem with the EHT doubler arrangement as such as failing HVR2's. EHT regulation was a bit poor when the Philips was working causing the picture to bloom when advancing the brightness. Could this be a bit of a red herring though?

The EHT transformer doesn't appear to be stressed and is running cool, so I'm hoping this isn't the culprit. Previously the line hold and width pots required critical adjustment. Line coils and componentry read ok.

Suspicion is now falling on C101 (12pf) along with C99 & C121. All low values which I don't have replacements for, so some are on order. Everything else in this area has been checked and/or replaced.

Would anyone tend to agree here?! bit of an unusual one.
Attached Thumbnails
Click image for larger version

Name:	185373921_10165351442055220_433874472476082084_n.jpg
Views:	94
Size:	44.8 KB
ID:	233763   Click image for larger version

Name:	185169002_10165351442445220_2215632861335131136_n.jpg
Views:	94
Size:	22.1 KB
ID:	233764   Click image for larger version

Name:	185531048_10165351442810220_8205556651015665724_n.jpg
Views:	79
Size:	80.9 KB
ID:	233765  
__________________
Matt

Last edited by matspar; 11th May 2021 at 1:23 pm.
matspar is offline   Reply With Quote