A light-hearted tussle with an Hitachi CNP 680

Ive had some fun lately with an Hitachi colour set from 1971/2. A Power-supply and Chroma-decoder faults.

This was I think the first colour offering in this country from Hitachi along with its smaller brother the 16 " CFP 470 that shares the same chassis.

Contrary to popular belief these sets have Proper PAL-D Decoder mounted on a separate board at the bottom of the cabinet between the two main blocks, and is not a modded up 'NTSC Nightmare' like Sony and Teleton used in their contemporary offerings, although they do retain a 'tint' control of the NTSC sets....

I picked this set up on Wednesday--from the usual sources, The Tip-- and tried it out yesterday, having given it a little time to dry out as it had been cold/damp in the tin-box overnight.

Experience tells me with these sets and the later 190/192 series to check a few things first before going anywhere near the mains.
The main thing to change is the 3.3uF 350V Cap on the bottom of the right-hand Timebase panel. near the leads connecting the 'B' and 'E' of the field-O/P transistor. This dries up and the field-output transistor suffers.
Also, Fuse, F-903 on the PSU/Sound board can give low HT rail, small picture size and odd colour/brightness effects.

These were taken care of and I confidently fired it up on straight 230V--No bulbing this one!

Well, To my surprise, I had --Low HT, small pic, odd brightness control effects and intermittent but correct colour...
--Just what a blown F-903 will give....
Checked fuse again--No probs. Pull out the power/sound-board check this and that, replace a 10uF 350V that just 'looked' odd although checked OK. For good measure, I replaced the two other leccy-lytics, 330uF 25V too.

Tried again--Still the same. Ah, Now-what!
Set HT Control, R911 doing nothing to alter the voltage. Main regulator transistor TR41 had 160V on its collector, 99V on its emitter and 99.7V on its base.....

Hmmph! Not being driven much I thought.....Diagnosis on this panel is a little difficult as there's loads of leads connecting this board to the mains transformer, the IF panel, the Timebase-generator and Chroma output boards along with all the front controls and timebase O/P stages on the right hand side.

Managed to turn the whole left-hand block on its side and broke an eleccy-lytic off the bottom of the IF board.-
-Eee gads, There must be an easier way...... Tea! I said under my breath--Tea and dig out the Manual!

The dogs looked at me dolefully, wanting their run so I postponed the tea and manual searching for a while--The fresh-air may do my thick tired aging brain good....

While walking, Merlin and Fox--The rather mad whippets ran off on their usual route leaving me with thoughts of error amplifiers and something nagging me about the mains transformer......

Back at the Lab, dogs happy and fed, I actually found the manual, along with one for the 190/192--It was where I put it--for once!
Right!--Now we are ready for war!
The 'lytic was repaired on the bottom of the IF board, the left-hand block positioned where I could get at the PSU/Audio board, and set fired up....
Same nasty small de-focussed picture, this time with a running ref-osc unlocked, giving those characteristic horizontal colour-change effects--Which made Kirsty Allplop look almost Human!....

Nay worry, --That'll be the low HT rail I confidently claimed to myself!

The voltage checks on the error amp transistor TR43 were all low at 90 odd. So were the voltages on the regulator driver, TR42.....

TR42 gets some of its emitter-volts from a diode, CR 36 and two smoothing-caps, C906, C907 from a 5.8V winding on the Mains-transformer, These caps I had replaced earlier.....
The Other end of this winding is connected to the supposed 120V HT Rail......

So, On those caps and that diode, I should have 99V Plus the 9 odd volts of the rectified 5.8. --I didnt! It was only 99V.
Ah--I thought, Dead diode! Great, didn't bother checking, just changed it.....

Did it do the job?--Did it hell! Still the same.....

Penny then dropped--too late to save the diode, and I accused the Mains-Transformer for having an open-circuit secondary 5.8V winding, but nagging doubt returned --Again. I checked at the mains transformer plug into the power-sound board, JA, verdict came back--Guilty as charged!
Ah Ha---Gotcha scumbag!!
Much time was spent pulling the left-hand block apart, the mains transformer is inside the metal block made of the PCB frames....
Checked at the coil in question on the transformer itself--Appeal M'lud, I aint guilty! and he wasnt!

Checked at the plug to the board, O/C. Check again at the transformer, Fine. Follow wires from the transformer----Ah, whats this?
A Fuse?--That Is Not On The Schematic, has no identification on the set itself, and no mention is made of at all--anywhere--, and hiding behind the chroma output and timebase generator board, on a small paxolin panel, with another fuse, which was also nameless...

--It was O/C of course, and jailed for the duration in the bin....

Replacing this, I was now confident I could set the HT rail to 120.5V,--and I did! Ah--something going right for once!
My happiness was short-lived however.

I was greeted with a nice--Monochrome--picture, somewhat mis-converged and grey-scale that would make Kermit proud.....
Oh-Poo! What have I gone and broken now? Much time was wasted checking for broken wires, mis-placed plugs and other Engine, eng....-idiot....-made faults....
Nothing doing....

Then I got the Scope Out!

Now, To most, this is a nice easy procedure, all you need to do is find the plug hidden under the half-dismantled DAC90 chassis, plug it in, grab the probe and away to go...
--Not in my case! Suffice to say after complete household upheaval, cross words, whinging Whippets and Partner and much waste time, it was located and It was set up.....

After all that, I had almost forgotten just What I wanted to check with the darned thing!
--Ah, yes--No chroma at all. Tint-control,--tinting, but not a spot of colour anywhere--Except the Kermit green of the poor grey-scale....
--For something to do--to postpone the inevitable delving into the bottom PAL Decoder board, I set the grey-scale roughly and was greeted with a fairly nice monochrome picture--with blue-droop of the Convergence each side.

That'll be the Grey-Cap I thought!-
-On the convergence panel, by the side of the coils on the right-hand side,--It was too! a 10uF 25V device, someone had replaced it--Badly--and it had broken off one side, they hadnt put it through the board, but had tacked it to the lead-out of the coil next to it, and another connection. C853 on the Convergence panel duly replaced--Properly this time, and now I had no droop at all!--Things are looking up!

Couldn't put it off anymore, I carefully removed the chroma decoder, unplugged the board to clean the huge carpet of dust that had accumulated on it over the last 40 years--It was quite amazing, It was not possible to make out even one component part on that board, the dust was that thick, just odd shapes in the crud....

'You'll take That Thing outside right now! said SHMBO, having just come in from the cold outside......

I complied sheepishly, thinking of the discord I had caused earlier with the scope debarcle, and looking forward to the thoughts of a possible supper that evening.

Board cleaned and re-connected, I set about checking for the chroma fault. There was burst getting to the decoder from the IF board on plug C and also chroma getting to and from the user-colour-control to the decoder at plug HF2...
Checked for reference-oscillator function--There was None! Not a spot of 4.433Mhz anywhere on that crystal or the ref-osc. transistor, TR1003.
Voltage checks suggested the transistor was O/C,-- 2.3V on Base, 0.002V on its emitter and 11.8V collector....
It was evicted and tested. It was O/C on all connections and was replaced with a BC337--My' use for most small low-volts things' transistor....

Sure enough, Now we had colour--well, Sort of. The osc was unlocked again, but a quick tweak of the Collector coil, L1005 restored a solid lock to the oscillator. Great....

Kermit however had returned. Green faces, Hmm--locked in the wrong phase....Time for More Tea!

I left the set running, made the requisite cuppa and pondered the colour, as I savoured the brew.
The Sky was blue, the grass was green, but still I had green faces.

Its not locked out-of-phase--the blue is right, and so is the green. Checked the tube connections to the chroma-timebase generator board, they were fine...

'Ive Lost My R-Y' I burbled, disturbing SHMBO who glared at me curiously over the paper--'You lost that years ago dear' was the sharp quip.....

Looking at the schematic, I located the R-Y Demodulator stages, and its associated Transistor, TR1005, and demod diodes, CR1004 and CR1005. I checked for R-Y signal at the diodes--There was None! Going back to the transistor TR 1005 collector there was nothing there either.
Hmm-A Dual suicide of Transistors possibly? Voltage readings indicated this to be the case with 2.8V at the Base, nothing at the Emitter and 11.8V on the Collector....

Another 'fix-ya-all' BC337 was fitted in the casualty's place and the set fired up. At Last, We got colour, although Kirsty Alplopp had been replaced by Bruce-Forskin--who looks like an animated fossil on any telly!

This really IS an odd hobby!