Share:
Notifications
Clear all

Bell and Howell Filmosound 202 movie projector  

 
helloekco
(@helloekco)
Trusted V-Ratter Registered

Hi everyone,

I'm not sure whether this is the best area of the forum to post this, or whether it ought to go in the Amplifiers section.

I thought I'd take a little break from my Ekco T344 TV in order to try and get a Bell & Howell 16mm movie projector, that I won at an auction recently, operational. I'm afraid I haven't been taking pictures of it so far (I know...) so I apologise for the lack of these.

The Bell and Howell Filmosound 202 is a 16mm movie projector made in the USA between about 1952 and 1955 (I think). It features a push-pull valve amplifier for the sound, and was the first from B & H to feature both optical and magnetic sound, the latter being recordable too. My example is not the best, looking like it's been stored in a shed for a couple of decades, but it is the cheapest one I've seen sold anywhere!

When I got the projector home and gave it a good inspection, unfortunately one of the valves in its amplifier was broken and there were fragments of glass everywhere, so I removed the amplifier and got to work on the projector itself first of all. The projector is housed in a large wooden case (a "projector in a box" I believe they were known as), so firstly I tried the usual powering through a series lamp, isolation transformer, and variac to just make sure there were no nasty surprises. All looked OK, but with that combination there wasn't enough current available to power the motor, let alone the 750w projection lamp, so next I dispensed with all of these (my iso trans unfortunately isn't big enough to power the projector's lamp), reverting to the large auto transformer that came with the projector (bizarrely they were all made as 110v units and shipped with auto transformers for the foreign markets). This too looked good, although there was initially quite a bit of sparking visible inside the motor housing that gradually subsided. But no burning smells (except of dust from the powerful lamp).

Next, I loaded a movie, and after a bit of adjustment on the feed arm (initially the feed reel was just letting all the film off, faster than the projector consumed it) I got the film to feed smoothly. I powered the lamp on again, focused the lens against a nearby wall, and I had a good stable picture! I couldn't believe my luck.

Next, I removed the projector from its wooden case and followed the service manual I found for a similar model, giving it a good clean and an oil in the correct places. It was now ready to refit into the case, but first onto the amplifier...

The amplifier is a push-pull design using a pair of 6AQ5s - a small 7-pin pentode - in its output. Interestingly, the projector's exciter lamp and the heaters for the valves in the preamp section are run from another 6AQ5 which acts as a 30kHz oscillator, presumably to eliminate hum.

The broken valve was one of the pair of 6AQ5s and it appeared to have been broken by the multi-wire cable that connects the amplifier to its main transformer, which is housed separately in the projector's wooden case, pressing against it. Seeing the prices of matched pairs of 6AQ5s on eBay, an unmatched pair was just going to have to do - I bought a couple of them and thought I would find the best match with those and the remaining two I had by trial and error.

A visual inspection of the amp's capacitors etc didn't reveal anything untoward. Even the paper caps looked good (I think I read somewhere that the American made ones are actually not that bad). I performed the usual series lamp limiter / variac power up and nothing bad happened, so I got it up to 120v on the variac.

It's worth mentioning that the amp is a bit of a pain to work on, because the aforementioned transformer is riveted, not bolted, into the wooden projector case - and the amp's supply is by a two-pin receptacle which has much shorter pins than the modern two pin ones found on many appliances such as VCRs etc - meaning that testing it on a bench requires the large projector box alongside, and the mains connection, in the absence of the projector itself which normally feeds the amp, requires a tenuously located two pin cable connected to a 120v supply (variac), which works loose very easily! Not to mention the way that the amp's point-to-point compoonents are crammed into its little chassis.

When I first ran the amp at its full voltage, I could hear hiss from the speaker that I attached, which was good, but I was concerned about the push-pull valves drawing too much current, not being matched. There was the occasional crackle and then I noticed that there was arcing in ones of the output valves - not good! I turned the power off and swapped the two valves around, same result from the valve that did it before, so maybe a faulty valve? I tried another one in its place and all seemed good, but after leaving the amp running for a few minutes I made the mistake of using my finger to check the temperature of the two valves and burnt myself on one of them, it was so hot. I swapped them about a bit and every combination I tried seemed to cause one of the two to get hotter than the other, so there was something not right.

I checked the internals of the amp a bit more closely around the output stage and found that the two output valve coupling caps (which I mistook for large resistors the first time around...) had splits in the outer plastic cases, so these looked like likely suspects. I replaced them, and hey presto - the output valves no longer seemed excessively hot and had no orange glow. I plugged a guitar (the most convenient thing to hand) into the amp's microphone input and I had a reasonably good output level.

Next up was checking that the photocell, the amplifier's main source from the movie, was in good working order. The only test I could think of was to swing the beam from a small LED torch back and forth across it. This produced a clearly audible 'wow' sound a bit like an audio tape being pulled across a tape head by hand, so I thought my work was done, and I refitted the amp back into the projector and the wooden case.

I loaded the projector with film, turned the amp on, and when I started the projector I couldn't hear much from the sound output at all. I turned the volume on the amplifier right up, and there was a lot of electrical interference from the projector's motor, and the sound from the movie was only very faintly audible behind that. The motor still has a few sparks visible inside when running, but the electrical inferference doesn't seem to coincide with these.

One thing I spotted when cleaning and oiling the projector was that the tiny mirror that sits between the exciter lamp and the photocell has lost a lot of its silver, which I believe is a common fault. I don't know whether that is entirely responsible for the low sound output or whether it could be the photocell, or something in the preamp part of the circuit, because there was sound present - it was just very faint. I've tried to find replacement mirrors online to no avail. Also, I don't know what sort of output to expect from the amplifier with any given level of light (e.g. my LED torch) shone at the photocell.

The other issue is the electrical noise - it could be that when I get good enough sound output I can turn the volume down such that this noise is bearable. I really don't know what kind of noise is expected in normal operation.

Does anyone on here have any knowledge of these projectors?

I hope this hasn't been too boring, especially without illustrations!

Thanks.

ReplyQuote
Posted : 07/06/2020 2:00 pm
Alex728, Nuvistor, PYE625 and 3 people liked
crustytv
(@crustytv)
Vrat Founder Admin

For members intrigued by this vintage machine, some brochure extracts to whet the taste buds.

bh 2
bh 3
bh 1

Crusty's Collection: Read the repair blogs
Crustys Youtube Channel: If you want to follow me on Youtube, please consider subscribing

ReplyQuote
Posted : 07/06/2020 4:21 pm
helloekco liked
PYE625
(@pye625)
Famed V-Ratter Registered
Posted by: @helloekco

I hope this hasn't been too boring, especially without illustrations!

Most certainly NOT !  Could do with more like it.

To understand the black art of electronics is to understand witchcraft.
Andrew.

ReplyQuote
Posted : 07/06/2020 8:38 pm
helloekco liked
helloekco
(@helloekco)
Trusted V-Ratter Registered

@crustytv I hadn't seen those anywhere previously, thanks! It's quite hard to find information on this specific model I've found, I have the correct circuit diagrams but that's about it. The only user and service manuals I can find are from different models of a similar era.

ReplyQuote
Posted : 07/06/2020 11:43 pm
helloekco
(@helloekco)
Trusted V-Ratter Registered

Well, sorry it's been a few weeks... I have been busy with the projector when I'd hoped to be using it by now! Since I wrote the above, I think I got to the bottom of the sound issue - but I didn't quite get to find out.

I found a 5.6M resistor feeding HT to the photocell for the optical sound that was almost open circuit - there was about 5V at the anode of the photocell, when it is rated generally for about 90V. I replaced this (with the best I had, two resistors in parallel in the photo below, if you can find them!) and then measured 70V at the photocell, and now, waving a small LED torch across the photocell produced a much healthier "whoosh" sound from the speaker, even with the volume turned down quite a way.

Photo 11 06 2020, 20 05 52

I got the amplifier fitted once again back into the projector and excitedly got the film threaded through, with the amp already switched on and waiting. As I started the film, just as the leader was making its way through the projector, I saw smoke from somewhere inside, and the background hiss from the speaker suddenly got very loud. I knocked off the power straight away, and I could smell something like wood burning. I momentarily powered the projector up again, and then just the amplifier, as I pinpointed the source of the smoke. After getting the amp back out of the projector, I could now see sparking from somewhere near the rectifier, so I wondered at first whether it was just the rectifier valve. It had me baffled where the arc was coming from at first, until my suspicions of the worst were confirmed - it was a short somewhere inside the output transformer. I wonder whether it had been caused by prolonged use with the faulty coupling caps to the output valves.

At this stage I was a bit gutted and didn't know how to proceed. After thinking it over and searching the internet for a replacement to no avail (all of the appropriately rated audio output transformers, 18 watts or so, were physically much larger than the original for some reason), I decided on 3 possible solutions:

- Use a mains transformer in the place of the original. I found one with an ideal voltage ratio and power rating, and just about the exact same physical size as the original. The downside would be no secondary winding suitable for the negative feedback loop.

- Build a small kit solid state power amplifier on a PCB and fit this inside the projector, wiring the output of the original amp's preamp section into this, the output going straight to the original speaker terminals. This would be a quick way to get the projector working, but I don't know... it felt dirty somehow; I like valves!

- Have a go at rewinding the original transformer - something I've never done nor contemplated before.

So, I went with the third option!

Getting the transformer off the chassis to start with was a bit of a challenge, as like many things on this projector, it's riveted together rather than screwed. I ended up having to leave one of the rivets on the chassis, but looking at the rat's nest layout inside the small chassis I think this will be a blessing in disguise when I come to reassemble it, as I can hopefully somehow reattach the transformer frame to it.

Here is the transformer as it appeared when snipped off the amplifier with the covers and frame removed, and then in various states of disassembly, as I counted the turns and noted how it was constructed:

Photo 14 06 2020, 21 18 26
Photo 15 06 2020, 23 01 04
Photo 15 06 2020, 22 41 15

The results of my counting:

Photo 16 06 2020, 00 03 49

I counted the two secondary coils manually, and looking at the specs of the output valves, 6AQ5s, I estimated the number of primary turns at 2373. I knew that with that much winding to do I really needed some sort of machine, and that's when I came across these lovely Chinese things on eBay:

Photo 20 06 2020, 14 07 23

That is showing my first attempt at the rewind, after first using the machine to accurately count the number of turns in the primary as 2100 - making the impedence of the primary a bit on the low side I would have thought, at about 8.5k.

For my first (unsuccessful) attempt I made the mistake of reusing the original bobbin, basically a box section of thick card - which had burned away at one corner due to the arcing and had to be repaired with glue and fabric tape. The problem with the bobbin was that there were no ends on it - apparently these were machine wound on a long "stick" of card box section in the factory, and then cut down to make individual transformers - and winding to this by hand was very difficult to do without the coil slipping off the ends. As a result, the coil... well, slipped off the ends at times! The rest of the time I couldn't make use of the full width of the bobbin and so it ended up being too fat to fit inside the cores:

Photo 28 06 2020, 15 00 07

What's more, possibly due to slipping off the ends, once I'd got all three coils in place, I kept getting continuity between one or both secondaries and the primary. I came to terms with the fact that I was going to have to try harder...

This time, after a fruitless search for transformer bobbins by size (you would think finding them given required dimensions would be fairly easy) I decided on making my own. The best material I could think of, after reading forum posts, was a couple of old plastic membership cards:

Photo 28 06 2020, 20 45 16

The advantage of this plastic being that its reduced thickness over the original card should hopefully give me plenty of room inside the original cores for winding the coil - I've made the bobbin a very tight fit to maximise the available space.

So that is basically where I'm up to. I'm currently awaiting delivery of some more wire for the primary, as my attempt to reuse the wire from the first winding attempt ended in failure when it all decided to unravel at once and became knotted...

Hopefully I'll be watching a 16mm movie within a week or two!

ReplyQuote
Posted : 01/07/2020 11:57 pm
irob2345
(@irob2345)
Trusted V-Ratter Registered

@helloekco

I have a ready solution to your transformer problem.

This transformer is readily available from Altronics in Australia. I'm sure they will ship to UK, if not, I'm happy to do it. It's actually a PA line transformer but it has a fortuitous set of taps to allow it to work in a PP amp. Here is the link:

https://www.altronics.com.au/p/m1120-20w-100v-line-pa-ei-core-transformer/

I've used it in the posted circuit I'm playing with at the moment. It delivers a good 20 watts RMS into an 8 ohm load from a pair of 6BQ5s.

6BQ5 amp

You don't have to use the screen taps as I have to get ultra-linear operation and the best match to the 6AQ5s might be closer to 6 ohms load but I guess you aren't trying for maximum power.

ReplyQuote
Posted : 02/07/2020 1:04 pm
helloekco liked
irob2345
(@irob2345)
Trusted V-Ratter Registered

Just looking at those taps again, moving the gnd connection from the "4" tap to the "0" would give you a better match to the 6AQ5s. And of course taking the feedback from the "16" tap. You might have to swap anode connections if the feedback becomes positive and you'll have a bit more gain than before.

BTW this amp circuit is very well behaved when overdriven, it clips cleanly. That's largely due to the cathode followers that avoid the usual grid current bias pump up issues and allow the output valves to be driven harder into conduction. It follows common guitar amp practice in this respect. I'm not suggesting you modify the existing amp to this circuit! It was just to show the use of the taps on this PA transformer.

Check the link and in particular the dimensions to make sure it will fit. It looks to me as though it will.

ReplyQuote
Posted : 02/07/2020 1:40 pm
helloekco
(@helloekco)
Trusted V-Ratter Registered

Hi @irob2345, thanks for this and the kind offer to ship one to me. I hadn't really paid too much attention to PA line transformers before now, as I wasn't sure exactly what I would be getting in terms of primary / secondary turns.

I'm going to try and succeed with the rewind first though, as I'm kind of invested in it now - with buying the winding machine, several reels of wire, and transformer varnish! But if I fail (again) I will definitely keep this in mind.

One concern though with the line transformer is that on the original on my amp, the negative feedback is from an isolated secondary, it's not a tapping from the speaker output as it is on many amps. I don't know whether this alone matters, however it is of a much higher impedence than the higher 16 ohm speaker output. You're correct about the physical size of that one though, it looks pretty much spot-on.

ReplyQuote
Posted : 04/07/2020 12:07 pm
irob2345
(@irob2345)
Trusted V-Ratter Registered

Re the wire and the winding technique:

  1. Use polyester insulated wire. Most wire you get these days is self-fluxing and will go shorted turns when it gets hot and the "varnish" melts. Polyester wire also generally doesn't need insulation between layers of the same winding. If not sure, see if you can solder to it without scraping the coating off. If you can, don't use it in a transformer that gets warm and/or has high voltages.
  2. Wind a couple of layers of tape onto the cheeks of the former before each winding layer. This stops the wire at the cheeks from running down into the gap that belongs to the layer below.

Good luck!

ReplyQuote
Posted : 06/07/2020 2:07 am
helloekco liked
irob2345
(@irob2345)
Trusted V-Ratter Registered

I can recall seeing a B&H Filmosound in the early 60s when I was in high school. I seem to remember this one was UK made and had a series heater string amplifier with a pair of PL84s, a PY33, a couple of twin triodes and an EF86. The entire unit could be switched to run on 115v or 230v AC. There were warnings that the amplifier chassis was live.

Long time ago and my memory may not be accurate, but I do remember the whole thing got very hot.

ReplyQuote
Posted : 06/07/2020 7:05 am
Alex728 liked
Alex728
(@alex728)
Busy V-Ratter Registered

As recently as 1982 I remember a Filmosound being used at my last junior school for the "birds and the bees" film (special edition of BBC Merry Go Round) we got shown in the final year, and again in 1986 as a teenager in high school it was used for 16mm films about fire safety and some film with Lenny Henry warning against getting into taking drugs (maybe these were seen as too controversial for normal telly as Mary Whitehouse was still quite active then?).

I can't remember whether the projectors had valve or solid state amps, I think they were solid state (and in a grey case rather than a wooden one)

ReplyQuote
Posted : 09/07/2020 11:44 pm
helloekco
(@helloekco)
Trusted V-Ratter Registered
Posted by: @irob2345

Re the wire and the winding technique:

  1. Use polyester insulated wire. Most wire you get these days is self-fluxing and will go shorted turns when it gets hot and the "varnish" melts. Polyester wire also generally doesn't need insulation between layers of the same winding. If not sure, see if you can solder to it without scraping the coating off. If you can, don't use it in a transformer that gets warm and/or has high voltages.
  2. Wind a couple of layers of tape onto the cheeks of the former before each winding layer. This stops the wire at the cheeks from running down into the gap that belongs to the layer below.

Good luck!

Thanks for that - I never got an reply notification and I've only just logged into the forum again.

The wire I used didn't specify whether the insulation was polyester (just 'enamelled') but it's definitely not solderable without scraping it away - in fact the larger gauge for the secondary was damn-near impossible to solder... And yes I was very careful about insulating the sides of the former after my first attempt!

I'm pleased to report that the rewind was a success and I was able to enjoy a movie on Saturday without any mishaps! I'm going to write a full reply when it's not so late at night, I have pictures to include too.

ReplyQuote
Posted : 16/07/2020 12:19 am
Alex728 liked
crustytv
(@crustytv)
Vrat Founder Admin
Posted by: @helloekco

Thanks for that - I never got an reply notification

Very odd as the forum software is sending out notifications.

As the thread was started by yourself, you should always get notification of replies. However, it has been known for some e-mail clients to unintentionally and randomly decide, an e-mail is spam. It's always worth checking your spam folder, if it's found in there, you may need to 'white-list' the Radios-TV.co.uk domain.

If nothing is found in spam and you didn't accidentally miss or delete the notification, but continue to not receive replies to your thread, use the manual "Subscribe for new replies". This is located at the top of the thread page, see example below. Anyone who is interested but not participating in the thread can also at anytime subscribe to the thread, they will also then receive notifications when a reply occurs. 

Hope that helps.

noti

Crusty's Collection: Read the repair blogs
Crustys Youtube Channel: If you want to follow me on Youtube, please consider subscribing

ReplyQuote
Posted : 16/07/2020 8:10 am
helloekco
(@helloekco)
Trusted V-Ratter Registered

@crustytv Thanks - yes, I've now clicked to subscribe to the topic.

I've checked all of my email folders and there's nothing from the forum since 2nd July, except the notification I received this morning for your reply (after subscribing manually that is).

I don't have any additional spam filtering on my email account itself by the way, it's an ISP-hosted one on which I specifically turned off filtering in order to let my local mail client deal with spam.

ReplyQuote
Posted : 16/07/2020 10:54 pm
helloekco
(@helloekco)
Trusted V-Ratter Registered

My second attempt at winding the transformer was a success - here it is in various stages:

Photo 04 07 2020, 20 38 07
Photo 05 07 2020, 14 25 06
Photo 05 07 2020, 15 41 49

After the cores were inserted I also gave it a thick coat of "transformer varnish" (not a dip in the stuff, as it was too expensive and therefore I didn't have enough of it). I used double quotes there because to me it looked, smelt, and behaved suspiciously like red oxide primer! Certainly not translucent as I'd expected, and to say that it was supposed to be air-drying - well, it took about 48 hours before it was no longer tacky.

Whilst the transformer dried out I took the opportunity to change the amp's cathode bypass capacitor. When I initially checked its capacitance with my DMM, I got a low reading, and subsequent attempts registered nothing at all! So maybe this had a hand in the failure of the transformer.

Here is the transformer mounted back on the amplifier, before tidying the wiring:

Photo 07 07 2020, 21 43 29

Powering the amplifier up again for the first time I was pretty nervous given the work I'd put into the transformer. Initially, I thought I'd failed as there was a very loud hum that was independent of any of the amplifier's controls. But then I remembered that I'd spotted that a soldered earth lead had become detached from the removable bottom cover... surely that couldn't be the cause, given that there was a second wire soldered to the same spot and the cover was removable? When I followed the route of the wire (difficult in this amp), sure enough it appeared to be the only path to earth for the component to which it joined, so I resoldered it and then powered up the amp again - and, success! I had a good volume when connecting a guitar to the mic jack socket.

I left the amp running for 30 minutes or so to give it a good test, monitoring the temperature of the transformer and the output valves, before mounting it back inside the projector.

I previously never got to determine whether I had working optical sound since my earlier fix, however I still had a nice loud 'whoosh' from the speaker when I waved a hand across it, so I was hopeful... and when I loaded up a film, at last I had sound! The projector was now ready for a showing.

Here it is loaded and ready to go:

 DSC 5296 941

It showed the feature film successfully - but - the sound still isn't satisfactory, it struggles to compete with the substantial noise of the projector and the tone control seems to be doing little, the sound is very 'middly'. So I will at some point be having a play around to see whether I can make any improvements, but for now I'm done with it, and reasonably happy.

One thing I will say about these amplifiers is that they're an absolute pig to work on, due to the tightly crammed components and the point-to-point wiring, where components often completely hide others. I really don't get why some people seem to be falling over themselves to even pay good money for them simply to convert to guitar amplifiers. The output transformer seems pretty small for an amp supposedly capable of 18W RMS output too. They'd be much better just building something from scratch. I'd rather work with a PCB than the wiring in this thing.

ReplyQuote
Posted : 16/07/2020 11:50 pm
Alex728, Nuvistor, Doz and 1 people liked