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Old 27th Aug 2010, 1:12 pm   #1
humorist2751
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Default ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Hi there.

I confess myself baffled; the behaviour of this set reminds me, in spirit at least, of one of the more obscure problems faced by Smithy and Dick in the old Radio Constructor.

See the attached circuit diagram (TBA120U audio IC section only) for information.

The "preset volume" on the control panel is known to vary the DC potential at point K5 (where the potential enters the IF module) and therefore also at pin 5 of the TBA120U. The bizarre bit is that when point K5 is monitored, despite the (presumably correct) variability of the volume control DC potential, no audio is produced; yet if pin 5 of the IC is monitored, uncontrollable audio is produced - the presence of the multimeter right on the IC pin causing it to function (after a fashion).

The circuit is very close to the IC manufacturer's suggested configuration for this device, and presumably it once worked. R320, R322, C319, C323, and C325 have all been replaced, without affecting the nature of the fault.

Might a failure of the TBA120U itself cause this odd behaviour?

Has anyone any ideas - either as to probable cause, or possible avenues for further investigation?

I've never seen anything quite like it - but maybe you have.

All ideas gratefully received!
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Old 28th Aug 2010, 1:16 pm   #2
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Check the earthing. Then try a new chip.

Cheers,

Steve P.
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Old 28th Aug 2010, 8:00 pm   #3
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Looking at the internal circuit of the chip, I wonder if the meter lead is picking up noise, (assuming a digital meter) probably from the line timebase and that's being rectified in the b/e junction when the meter lead is applied directly to pin 5 of the TBA120U and raising the volume control voltage.

Doesn't explain why there's no sound otherwise but the fact that it comes on then suggests that the chip is maybe OK.

Have you tried measuring the resistance from K5 to ground?
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Old 28th Aug 2010, 8:45 pm   #4
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Just a thought - does the voltage at pin 5 of the IC actually vary when the preset is turned? Just wondering if there could be a cracked track or dodgy joint somewhere in this part of the circuit?
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Old 28th Aug 2010, 10:11 pm   #5
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

From memory this is a typical TBA120 fault, also I seem to remember a cap causing the same fault!
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Old 29th Aug 2010, 10:03 am   #6
Brian R Pateman
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

This is almost certainly the TBA120. I have come across it myself a couple of times.

I have a couple of TBA 120Q (same chip but with QIL pin form) in stock. If you'd like one drop me a PM and I'll stick it in the post.
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Old 1st Sep 2010, 7:58 pm   #7
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Hi
I agree with Brian - I think an internal pull-up (or more correctly -down) resistor in the chip goes high and your meter does the honours. I've had this with much newer sets too that use a more sophisticated IC.
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Old 8th Sep 2010, 6:13 pm   #8
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Hi gang.

Speculation vis-a-vis the IC was correct - a new TBA120T (I thought I'd use the one optimised for ceramic filter input, given that this set has one) has brought back the audio. Thanks for their thoughts to all those who assisted in finding this odd and annoying failure.

Unfortunately, the set is still rather limited in volume at full output; this may be because I changed R320 and R322 to their "book values" of 4.7K and 10K, rather than replacing like with like - R320 had been 3.3K, while R322 had been 8.2K. Possibly testing had shown that the original values gave weak maximum audio, production modules were modified, and the documentation - or at least my copy of it - never caught up; be that as it may, I shall try the originally fitted values and see if that improves matters.

On another (yet related matter), while soak testing the set having revived the audio, I noticed fluctuating focus on the picture. I had earlier seen signs of arcing at the focus unit connections, which I had addressed by removing one connection, shortening the wire from the tripler to remove the burnt bit, cleaning up the tag as best I could, and making a new joint onto the focus unit connection.

Apparently this was not good enough to be a complete cure; it would seem that the focus unit is on its way out.

Given that a focus unit of the original type may be hard (or impossible) to obtain, has anyone any ideas of what could be used in its place?

All suggestions gratefully received.
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Old 9th Sep 2010, 3:12 pm   #9
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Hi
Don't give up that easily - it could be the CRT base, or if not I think I might have a focus unit around here somewhere. Any chance of a picture?
Glyn
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Old 9th Sep 2010, 7:29 pm   #10
Jason Pero
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

I occasionally get a intermittent focus or odd intermittent pop in picture traced to CRT socket.

I once or twice had arcing at point of contacts on the wiper for focus or screen pots.

Cheers, Wizard
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Old 13th Sep 2010, 8:01 pm   #11
humorist2751
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Hi.

Fluctuating focus yielded to a replacement focus module - I managed to track down 2 NOS "Welwyn" ones at Donberg in Ireland; they were advertised as for a Decca chassis, but from the online images were clearly "Industry Standard" items as used in several chassis by different makers. On removing the old unit, its centre pin - the one the focus lead connects to - almost fell off, in that part of it came away with the focus lead as I was unsoldering it.

Audio is still puzzling. The originally fitted R320 and R322 values gave no audio; replacing with the "book values" of 4.7K and 10K brought the audio back, but with the necessity of having the "preset volume" at minimum resistance (i.e. shorted out). There is a 1K resistor in series with this pot, so I'll try bridging that with a wire link - maybe I'll get more audio that way.

There is sibilance on the sound, so L304 the quadrature coil needs a tweak.

There are also striations at the left of the picture - any ideas here? - and the convergence in the bottom left-hand corner is pretty bad, though it seems not too bad elsewhere.

The set ran happily for two episodes of "Mapp and Lucia" - the best part of two hours? - so seems stable, even if there are remaining issues.

Thanks for all ideas - keep 'em coming!
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Old 13th Sep 2010, 8:30 pm   #12
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Be careful when subbing the TBA120 series with slightly different types. There were quite a few alternatives made and optimised for different circuits. I know there was a 'U' type, a 'T' type, an 'A' type and an 'S' type from memory. Some won't work when swapped with another type and others may give poor results or buzzing. Just because your set has a SAW filter, doesn't mean the 'T' type will work better, or at all, when replacing a 'U' type. When I worked at Philips, they sent a technical note around regarding this very subject. Sometimes the TBA120x was optimised for a particular manufacturer and you had to order the correct type. I seem to remember many years ago trying to sub a TBA120 in a Hitachi portable and brought home half a dozen different types from Philips, none of which worked properly, some not at all. I ended up ordering the correct one from CPC and curing the problem.


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Old 14th Sep 2010, 9:52 am   #13
humorist2751
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Sideband:

Thanks for the information vis-a-vis the various flavours of the TBA120; how useful it is to have access to insider knowledge! I'll obtain a TBA120U and give that a try, though I shall stick to the book values of R320 & R322, at least initially.

Incidentally, there is 50 Hz buzz, but only at high volume control settings (which are currently necessary to get decent volume output); if the TBA120U gives more "oomph" at a lower volume control threshold, that issue may disappear.
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Old 20th Sep 2010, 10:29 pm   #14
humorist2751
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Default Re: ITT CD 652 (CVC 32/3 chassis) bizarre audio fault

Stop press

TBA120U a complete cure with bags of volume (and to spare!). That'll teach me to "improve" things.

Only minor issues left, having solved the sound and the focus; thanks to all who contributed - where would we all be without the forum, eh?
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