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Old 28th Oct 2021, 9:08 pm   #1
filmprojectors
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Default Projector valve amp trouble.

Hello I have just bought a 1954 Bell and Howell 630 with a valve amplifier but the sound output gets quite after about 10 minutes of use, could it be the 5z4 rectifier or it is the one of the two 40mfd capacitors, which one's shall I change first ?
thanks for your advice
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Old 28th Oct 2021, 9:25 pm   #2
kalee20
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Default Re: Projector valve amp trouble.

Almost certainly one of the coupling capacitors from anode of one valve to grid of the next.
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Old 28th Oct 2021, 9:25 pm   #3
vidjoman
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Default Re: Projector valve amp trouble.

Has the amp been serviced recently? Check the voltage from the 5Z4 before and after it fades and check the 40mfd capacitors with a tester. If not serviced also check coupling capacitor to the output valve as if leaky will destroy the output transformer and possibly the valve.
It's not good to just guess what to replace without doing proper fault finding and voltage checks.
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Old 28th Oct 2021, 9:28 pm   #4
GrimJosef
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Default Re: Projector valve amp trouble.

Do you have a reason to believe it's the rectifier or the capacitors ?

The right approach is first to identify the fault which is causing the symptoms (loss of output) and then to fix it.

The best way to identify the fault (assuming some component isn't obviously scorched or exploded or broken) is to measure DC voltages at various places in the circuit and to compare the measured values with what they should be, either read from a service sheet for the amp if you have one or by asking on here (we can help you work out the correct values). This will identify where the fault is occurring and save you from wasting time and money changing things which aren't broken.

Cheers,

GJ
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Old 28th Oct 2021, 9:46 pm   #5
kalee20
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Default Re: Projector valve amp trouble.

I've also known a case where oil had got into the lens assembly following the exciter lamp, and after 10 minutes or so started to vapourise due to lamp heat. This defocussed the narrow slit of light on the film soundtrack, causing muffled, weak, sound.

So, be sure it really is the amplifier before diving in!
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Old 29th Oct 2021, 8:07 am   #6
rontech
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Default Re: Projector valve amp trouble.

Quote:
Originally Posted by kalee20 View Post
I've also known a case where oil had got into the lens assembly following the exciter lamp, and after 10 minutes or so started to vapourise due to lamp heat. This defocussed the narrow slit of light on the film soundtrack, causing muffled, weak, sound.

So, be sure it really is the amplifier before diving in!
I was talking with a projector expert many years ago and he said he had numerous problems with B&H series 6## projectors because of over oiling.
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Old 29th Oct 2021, 8:14 am   #7
wd40addict
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Default Re: Projector valve amp trouble.

On my 640 (successor to the 630, but completely different amplifier) the blimp case has the 3 oiling points on the outside which only connect to short lengths of tube which go nowhere as the 640 has permanent lubrication! Presumably they were using up old cases, I wonder how many owners poured in oil and wondered why it went everywhere

A little bit of history: The 630 was the first UK built model with magnetic sound, based on the US 202. To reduce hum levels it has the amplifier transformer mounted at the top of an enlarged blimp case as far away from the heads as possible. The 640 was the next UK built machine with magnetic sound with a radically redesigned amplifier and standard sized blimp. The new amplifier seems to be a UK only development and uses technology akin to modern switch mode techniques to eliminate the large hum causing transformer.

Last edited by wd40addict; 29th Oct 2021 at 8:34 am.
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Old 29th Oct 2021, 4:27 pm   #8
Techman
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Default Re: Projector valve amp trouble.

Quote:
Originally Posted by vidjoman View Post
It's not good to just guess what to replace without doing proper fault finding and voltage checks.
Exactly! I couldn't have put it better.

While it could be something to do with oil in the optical side it could also, and more likely, be due to an amplifier fault. If you don't know the service history (if any) of the amplifier, then there may well be leaking grid coupling capacitors ready to do some serious damage and these would cause exactly the fault that you have. However, I had this fault symptom with my B&H projector and the grid coupling capacitors had already been replaced by a previous user, but the output valves were (to use an American term) seriously 'red plating' and dragging the HT down after a short time of operation. The fault on this occasion was caused by the cathode bypass capacitor for the 'shared' cathode resistor going totally short circuit. This capacitor was in a block of three, with the other two being the reservoir and smoothing capacitor. Disconnecting this faulty capacitor section and fitting a new replacement in circuit cured the problem.

So a few things for you to investigate, but remember, proper fault finding and no guesswork, as it'll likely end in disaster!
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