UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 18th Mar 2020, 2:11 am   #1
Kyle__B
Banned
 
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
Default New to valves, Dutch Philips TV with 50Hz speaker buzz

Hello I'm new. I'm trying to track down a small sound fault with a telly I've had for a couple of years and I thought it would be best to run my findings past more experienced people.

The telly is a Philips X24T680, from around 1969 or 70. I've got a full circuit diagram and block diagram.

The speaker buzzes at 50 Hz. When I power it off an isolation transformer, the transformer also buzzes at 50Hz. Powering anything else the same tx is silent.

Audio amp diagram:

[img]************************0yqZnTb6/tv-audio-amp.png[/img]

The audio amplifier looks in good condition, it's two electrolytics have been replaced. Valves (PCL 86 and PCL 805) are clean and secure.

The buzz is always the same loudness through the speaker no matter what you do with the volume knob. I've tested it by disconnecting the FM detector and feeding the amplifier from a cassette player, and the buzzing is still there. Other than the buzz, sound is clear and no distortion.

Between that test and it making my isolation transformer buzz, I think it's in the power supply.

Here's the circuit diagram for the power supply:

[img]************************KYGfSt7x/TV-power-supply.png[/img]

[img]************************qvXw5131/tv-symbol-key.png[/img]

Audio amplifier is powered by +1, 280V.

C500 is a big 5uF bipolar for the valve heaters (B404 etc).

C501 and C502 are a pair of X rated 0.1uF safety caps. They've been replaced since the original paper ones burnt up in their old age.

Capacitor C503 is a big aluminium electrolytic can split into multiple sections a b c d. C503a and C503b are 200uF 325v, C503c is 75uF 300v, C503d is 25uF 300V.

C508 is a 700V rated ceramic that's parallel to the rectifier.

Would it be fair to blame the buzz on C503? Or am I barking up the wrong tree entirely?
Kyle__B is offline  
Old 18th Mar 2020, 5:51 pm   #2
simpsons
Octode
 
Join Date: Feb 2007
Location: Harrow, London, UK.
Posts: 1,483
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

We didn't get that quality of design over here, tone controls, negative feedback you're kidding. Few people in the UK would have understood let alone heard any quality improvement over the bog standard and the vast majority could hardly afford the rental never mind buying a quality product.

A buzz? Well 50 Hz doesn't buzz, it hums which should the isolation transformer make a similar sound suggests something else. In the case of the transformer, the laminations are vibrating possibly because the load is near its limit. In the case of the loudspeaker, as this is used in a trasformerless circuit, it can't be that.

But given the loudspeaker is 800 Ohms, are the leads dressed away from the scan coils?

I'd also short pin 8 of B401, PCL86 to ground just to make sure that it isn't from the preceeding stage which has its HT feed from part of the power which should be very quite from 50Hz ripple.

Chris
simpsons is offline  
Old 18th Mar 2020, 6:32 pm   #3
Kyle__B
Banned
 
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

Yes I think it's a well made set, chassis swings out for servicing, has six channel presets with a very complicated bit of clockwork controlling a solid state four band VHF/UHF tuner. The sound is so good a bit of gunfire makes you want to duck.

I wasn't aware that there was a technical difference between a buzz and a hum. I just know it as the 50Hz noise electric stuff makes.

Transformer is 350VA toroidal type, the tv says 150W on the back so not being taxed too hard I think. The speaker wires are kept nice and tidy, they're tied along the outside of the chassis away from anything noisy.

Thank you for your suggestion grounding that pin, I'll try it later and let you know how I got on. If I don't then you know it blew up and killed me.

kyle
Kyle__B is offline  
Old 18th Mar 2020, 7:24 pm   #4
Graham G3ZVT
Dekatron
 
Graham G3ZVT's Avatar
 
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,676
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

I think the hum vs buzz thing is a subjective thing rather than a technical one. I would categorize hum as sinusoidal waveform, becoming a buzz as more harmonics are introduced.

Check if the source is the frame circuitry. Does the pitch of the hum change with the vertical hold? Does the pitch increase slightly when a signal is applied to the aerial socket?
__________________
--
Graham.
G3ZVT
Graham G3ZVT is offline  
Old 18th Mar 2020, 8:20 pm   #5
Kyle__B
Banned
 
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

Not had a chance to poke at it since my last response but from memory, no it doesn't, no difference if there's anything on the aerial or not.

Actually I pulled the TV detector out at one point and fed the video amplifier a signal from an external amplifier and an old amiga computer. (improvised test card/crosshatch generator etc)

Doing that I got an incredibly sharp and clear picture, and could use the external amplifier to control the signal strength. weakening the signal slightly would make it lose vertical hold, the weaker signal the faster the roll. Until of course it becomes too weak for horizontal and you end up with a load of rubbish on the screen.

That behavior is totally expected, but more importantly it definately never ever made slightest bit of difference to the unwanted 50hz on the sound.

I'll post the complete circuit diagram and block diagram when I get back to my desk. the main service manual doesn't have much information beyond part catalogue numbers and how to say "voltage tester" in three languages.
Kyle__B is offline  
Old 18th Mar 2020, 10:50 pm   #6
turretslug
Dekatron
 
turretslug's Avatar
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

A power supply like that will be a somewhat peaky and asymmetric load on the supply, and toroidal types are particularly easily affected by waveform distortion and/or DC components- I'd be surprised if it didn't make some sort of mains-related noise under the circumstances, even if well within its ratings. TVs of the era were a bit cavalier about the effect that they had on the public supply, but I think that the utilities later forced the use of less crude designs.

Are you able to try replacement valves in the audio amp? Whilst series-heater valves were designed to give good insulation isolation, they can still develop a heater-to-cathode leak which can cause hum/buzz.
turretslug is offline  
Old 18th Mar 2020, 11:23 pm   #7
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

There is a connection through a 100pfd cap from the video detector to g1 of the PCL86 pentode,.
What’s that for, although only a small capacitor it will mix video and audio together?
__________________
Frank
Nuvistor is offline  
Old 19th Mar 2020, 12:00 am   #8
Kyle__B
Banned
 
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

Nuvistor I'm glad you asked!

I have no idea what it's supposed to do. I've tried both with and without that there, it makes no difference to anything at all. To be honest I thought maybe one of you would know.

As promised, full circuit diagram and block diagram attached.

Took simpsons' suggestion of grounding pin 8, hum is still there but no audio comes though at all. Unless I misinterpreted what you meant, and pin 8 should be connected ONLY to ground? (i.e remove R726?)
Attached Thumbnails
Click image for larger version

Name:	clean-circuit-diagram.jpg
Views:	135
Size:	133.0 KB
ID:	201194   Click image for larger version

Name:	block-diagram.jpg
Views:	108
Size:	77.5 KB
ID:	201195  

Last edited by Station X; 19th Mar 2020 at 9:30 am. Reason: Linked images converted to thumbnails. Disappearing links make threads useless.
Kyle__B is offline  
Old 19th Mar 2020, 2:44 pm   #9
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

Shorting pin 8 will remove the audio, with the hum still there with the short it isolates the fault to the output stage.
__________________
Frank
Nuvistor is offline  
Old 19th Mar 2020, 4:14 pm   #10
Maarten
Dekatron
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

Or the power supply for the output stage. Maybe an o/c electrolytic on +3?
Maarten is offline  
Old 19th Mar 2020, 4:35 pm   #11
simpsons
Octode
 
Join Date: Feb 2007
Location: Harrow, London, UK.
Posts: 1,483
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

Shorting pin 8 of B401p to earth in addition to pointing a fingure of blame at C503a 200uF smoothing electrolytic, also eliminates a possible hum loop with the volume control etc which is at negative feedback poential from B401's cathode.

As mentioned though, a heater to cathode short in either the B402 PCL805 or B401 PCL86 could be worth following up but first I'd bridge C503a with say 32uF and see if this makes a difference before buying new valves. An oscilloscope would be the best way of seeing how well the electrolytic is working and of course finding where the 50Hz "buzz" is comming from.

To replace C503, which is a multiple value unit of a certain size, with say the one on eBay from Switzerland might not be so easy physically so bridging, if sucessful, should be made permanent.

Good luck

Chris
simpsons is offline  
Old 19th Mar 2020, 5:42 pm   #12
Kyle__B
Banned
 
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

I see the moderator Station X turned the schematics I uploaded from links into attachments. While I understand his concern about the images on another website possibly going away in the future, they're now too small to read!

I'm looking right now at the file attachment window. An absolute maximum image size of 800x600. The circuit diagram is about 4000x3000. In order to attach the circuit diagram without it being crushed into a postage stamp, I'd have to cut it into 25 separate pictures, and anyone wanting to read it would have to do a jigsaw puzzle first.

This is a very "vintage" problem. Shades of dialup internet and windows 95.

_______________________

Anyway. This is all good advice, thank you all.

I'll get a couple of 32uFs to try and some valves too, don't see any harm in having spares even if they turn out not to be the problem. Buying a scope though is right out. Should have more to say in a few days unless our friend corona shuts the post office down.

Thanks for the hint to look on ebay for replacement C503. It's a shame it's new old stock because the price isn't that bad, definitely a lot cheaper than the services who'll do you a one-off. I just don't trust an elco that's 50 years old, sat in a box or not.

It's a shame C503's values are so strange, otherwise I'd probably already have replaced it with separates. Ever tried finding a 200uf 325V+ capacitor nowadays...
Kyle__B is offline  
Old 19th Mar 2020, 6:23 pm   #13
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

You can upload .pdf documents up to 4.00 MB.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is offline  
Old 20th Mar 2020, 1:08 am   #14
Maarten
Dekatron
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

I see you're Netherlands based. The F6 chassis is not too uncommon in the Dutch collectors community. I'd be surprise if a cap doesn't turn up when requested in the appropriate place. I'm not sure whether I may mention my own transistorforum or it's tube cousin here (as a hybrid set this one is appropriate at both fora) but you can send me a pm and I'll have a few links for you.

As a test, temporarely briding the capacitor with separate ones isn't a problem at all. For the 200u 325V sections, a 220u 400V would be just fine. The 75u section may be bridged or replaced with a 100u one.

Last edited by Maarten; 20th Mar 2020 at 1:17 am.
Maarten is offline  
Old 20th Mar 2020, 3:06 am   #15
Kyle__B
Banned
 
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

I avoided finding a dutch site because I don't know all the vocab in that language. (biuzen is not an obvious leap from valve) Neither does the other half despite being a native speaker!

I'm glad you had a suggestion on the capacitor, I was just about to ask what I could get away with in terms of increasing size. I thought it'd be a bit more strict than some 25% increase. Although I might have an 82uF in the house already.

Can I get away with swapping 25uF for a 33uF? or should I get a 22uF and parallel it with a 3.3u I already have in my box of bits?
Kyle__B is offline  
Old 20th Mar 2020, 10:09 am   #16
Maarten
Dekatron
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

33u will be fine, as will 22u. Making up the exact value isn't needed as original tolerances are often already 20% and for smoothing it doesn't really matter that much. Only in case of a capacitor directly behind a valve rectifier there's usually a strict maximum value.

Most Dutch fora will be okay with English. You're right there's no obvious connection between valve and buis, until you call a valve a tube which is directly translated to buis for both meanings.
Maarten is offline  
Old 20th Mar 2020, 10:50 am   #17
simpsons
Octode
 
Join Date: Feb 2007
Location: Harrow, London, UK.
Posts: 1,483
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

Slightly off topic but I was in Overpelt in February, I know, I know, I should have called in; carrying on, I noticed no TV aerials on the roof of any homes or in the immediate towns. "We live in the 21st Century" I was told, "we're cabled for the Internet, television and radio in every home." Seemed the same when travelling to Eindhoven or even the remote pub.

Looking at Nijmegen, again, no TV aerials or the dreaded satellite dish above the front window a feature we have in Old Blighty.

Does this mean that there are no longer terrestrial television transmitters, it's all on cable?

Chris
simpsons is offline  
Old 20th Mar 2020, 12:12 pm   #18
Maarten
Dekatron
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

While most people have cable or IPTV, there's still a DVB-T2 H.265 network. People tend to use indoor aerials for this, though.
Maarten is offline  
Old 20th Mar 2020, 7:50 pm   #19
Kyle__B
Banned
 
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

Wait, radio is by cable? I thought there was an aerial on the roof of my house somewhere. That wall socket broke a couple of years ago and I never bothered to fix it.
Kyle__B is offline  
Old 21st Mar 2020, 3:07 pm   #20
Maarten
Dekatron
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
Default Re: New to valves, Dutch Philips TV with 50Hz speaker buzz

I was mainly writing about TV, but if you have cable TV from monopolist Ziggo, there's almost always also an analogue FM radio signal on the same cable (for now anyway).
Maarten is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:28 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.