RCA model 400 Junior (1345-C) 16mm projector amplifier schematic.

[John Price]

Hi,

I have acquired an RCA model 400 Junior in very nice condition but the 1345-C amplifier needs work! The insulation on the main power cable had completely deteriorated so I've replaced it with a grounded 3 prong plug. I plan to install an isolation transformer in the amplifier as it's the version that runs a pair of 50L6 power tubes, a 12AY7 in the preamp and a 6SL7 phase inverter without a power transformer ie a widowmaker . The service manuals I've found so far lack the pages that contain the schematic for this model. It's on Pages 54-57 of the manual stamped with TITANIC on the film-tech site that ends on page 47! If anyone has the other pages I'd very much appreciate it! I've been restoring tube amplifiers for a few years now and am comfortable working on them so you can be confident that you will not be responsible for my death by supplying the missing pages ... any help with this would be greatly appreciated. Happy new year to all! John.

[Mark Gulbrandsen]

I used to service a lot of 16mm projectors a very long time ago, but the school district did not have any RCA's. There is an RCA 400 amplifier for sale on Ebay for a ludicrous amount, but it appears different than yours. But standard electronic troubleshooting procedures should bring it back to life. My SOP was to replace all the electrolytic and coupling / bypass capacitors, check resistor values and replace any that are out of tolerance, test and replace any bad tubes. Then power it up slowly via a 120 volt variac while watching for smoke and flames. Then see if it's working again. Short of a transformer being bad it ought to work fine. Oh, also check the exciter lamp for continuity.

[John Price]

Thanks Mark!

One of the big issues with the design of this model was that SOME of them had the tube filament heaters connected directly to the 117V wall outlet. The total voltage of all the tube filaments equalled the mains supply... ie. 2x50L6's, 1x6SL7, 1x12AY7 = 118 volts. In this scenario the DC voltage is also tapped directly from the mains voltage through a half wave rectifying diode... I find this design very unsafe so have been adding isolation transformers to amplifiers of this design in order to make them safer. If not and the power plug is plugged in the wrong way it's possible that the projector chassis will have wall voltage on it which could make a mere focus tweek a trip to the ER...

Most models have a power transformer that isolates the mains voltage from the circuit but unfortunately this one does not. I like having the schematic on hand in case the expected voltages are published on them. I find it makes troubleshooting a bit easier. It also takes the guesswork out of replacing any obviously blown or burn out components.

If anyone has the missing pages that would be super.

[Jim Cassedy]

I've always liked the RCA 400 series projectors. They're well built, and the large diameter sprockets &
easy threading path made them very gentle on film. Many spare parts are still somewhat easy to find
or fabricate. They were also very quiet- -especially when compared to all the B&H "projectors-in-a-box"
models (149,185, 285, 385, etc) all of which made only slightly less noise than a power saw.

A year or two ago, I bought an RCA 416 from a guy who was selling them on e-bay. It wasn't cheap,
but the guy cleaned, lubed, and partly rebuilds them, and also modifies them to take one of those
pre-focus reflector type quartz-halogen bulbs like were used in many slide projectors. Now, you're
not going to be able to fill a large auditorium type screen with that bulb, but it's perfectly fine for use
on a typical "home movie" screen (or just the white wall) in my living room, and draws a lot less
power and gives off hella less heat than the 750 or 1000w bulbs those projectors originally used.

It's been awhile since I popped the amp cover off, but the amp uses two a 7025, a 12AX7 & two
6973 tubes. One winding of the power transformer powers the 6.3v filament, & the other winding
feeds a simple rectified voltage-doubler which is filtered to provide the plate voltage. (If my
memory is correct, all those tubes need appx 250~300v on the plate to keep them happy)

A few years back I saw one of these RCA's where the owner kept the 1k bulb, but they gutted
the amp section and somehow managed to shoe-horn an amp from an Eiki in the same space.
The owner said said he had to put in some extra shielding because the Eiki amp was picking up
some hum from the proj motor or someplace, but I heard the proj and it sounded good to me.

I really like the machine I got, & I'm glad I bought it. (Even though the last thing I really needed
was another 16mm projector around here!)

[Mark Gulbrandsen]

Thanks Mark!

One of the big issues with the design of this model was that SOME of them had the tube filament heaters connected directly to the 117V wall outlet. The total voltage of all the tube filaments equalled the mains supply... ie. 2x50L6's, 1x6SL7, 1x12AY7 = 118 volts. In this scenario the DC voltage is also tapped directly from the mains voltage through a half wave rectifying diode... I find this design very unsafe so have been adding isolation transformers to amplifiers of this design in order to make them safer. If not and the power plug is plugged in the wrong way it's possible that the projector chassis will have wall voltage on it which could make a mere focus tweek a trip to the ER...

Most models have a power transformer that isolates the mains voltage from the circuit but unfortunately this one does not. I like having the schematic on hand in case the expected voltages are published on them. I find it makes troubleshooting a bit easier. It also takes the guesswork out of replacing any obviously blown or burn out components.

If anyone has the missing pages that would be super.

John,
The direct AC to the chassis was very common post WW-2 in table radios and later in portable TV's, and today in things like hair dryers. Those portable TV's first appeared in the mid 50's. They could only be considered truly portable if the boat anchor power transformer was left out. So that's how the strange tubes like 35W4, 50C5, 25L6 and many others came to be. Most of the tubes already existed in lower 6 and 12 volt filament form anyway. It's also why AC outlets are polarized. It reduces, but doesn't eliminate the possibility of an alectric shock, or electrocution.
Personally, I'd build a new amplifier on the old chassis. This would include a Simple adjustable 3 pin 1.5 amp DC power supply regulator like an LM-317 for the exciter lamp, and a simple amplifier with a mic input fed from the solar cell that would replace the photo tube pick up. Amazon even sells a lot of this stuff, and there is also Digi-Key which is also big in Canada. Anyway, just my 10 cents worth... But if you want to keep it all original, then you'll obviously have to rebuild it...

[Jack Ondracek]

When the "All American Five" was common, I don't know anyone who considered the hazards involved. Back then, educational, experimental and hobby kits from Allied, Lafayette, Heathkit and the like were all powered by household mains, and many of them with series-wired filaments. Yes... you got zapped once in a while. It was part of the learning process.

One of my first memorable circuits was a low-power AM transmitter, built around a single 117N7 tube. No transformer on that one, either.

[Mark Gulbrandsen]

That was the reason polarized outlets and plugs came to be. The theory was that being polarized would keep the chassis at neutral so a person could not get electrocuted. And I stress theory... outlets were not always replaced properly if they cracked or broke. And ground fault breakers and outlets did not come till many years later. That same practice continues today on things like hand held hair dryers and certain small kitchen appliances. I actually knew a person whose wife was electrocuted when using a hand held hair dryer back in the 1980's.

[Tony Bandiera Jr]

John,
The direct AC to the chassis was very common post WW-2 in table radios and later in portable TV's, and today in things like hair dryers. Those portable TV's first appeared in the mid 50's. They could only be considered truly portable if the boat anchor power transformer was left out. So that's how the strange tubes like 35W4, 50C5, 25L6 and many others came to be. Most of the tubes already existed in lower 6 and 12 volt filament form anyway. It's also why AC outlets are polarized. It reduces, but doesn't eliminate the possibility of an alectric shock, or electrocution.
Personally, I'd build a new amplifier on the old chassis. This would include a Simple adjustable 3 pin 1.5 amp DC power supply regulator like an LM-317 for the exciter lamp, and a simple amplifier with a mic input fed from the solar cell that would replace the photo tube pick up. Amazon even sells a lot of this stuff, and there is also Digi-Key which is also big in Canada. Anyway, just my 10 cents worth... But if you want to keep it all original, then you'll obviously have to rebuild it...

See bold text above. WTF does a hair dryer need to have ANYTHING tied to "chassis" other than a ground? I don't know where you get that from. Now they do tend to use a low voltage motor in series with dropper capacitors, resistors, or part of the heating element. But explain how and where any part of a hair dryer "chassis" is tied to the AC line. And every brand of hair dryer I have seen for consumer use these days has an all-plastic case.

Are you referring to the GFI plugs they use now? (They are technically Immersion fault plugs, they trip at higher than the 6ma of a GFCI). Those, as the name implies, detect current from dropping into water. They MIGHT have a shielded cord from the plug to the dryer (as do PTAC units used in hotels/motels) that is used as the sensor for the IFP to trip.

[Jim Cassedy]

<edited> When the "All American Five" was common, - - Back then,
educational, experimental and hobby kits from Allied, Lafayette, Heathkit
and the like were all powered by household mains, and many of them with
series-wired filaments. Yes... you got zapped once in a while. It was part of the
learning process. One of my first memorable circuits was a low-power AM transmitter. . .

In the 'electronics shop' class of my high school in the early 1970's, we all built our own
AA-5 AM radio, with series tube filaments & and a 'hot chassis' from kits provided by
some educational kit supply source. It had a plastic case, so in theory, you were semi
safe from electric shock. But one design flaw was that the plastic volume & tuning knobs
were held onto their shafts by setscrews which, although they were countersunk into
the knob, were still close enough to the knob surface make skin contact if you grabbed
the knob too tightly while turning it. Like you said- - "it was a learning process"
It actually wasn't a bad little radio, and it ran for over 20years before it died.

[Tony Bandiera Jr]

Thanks Mark!

One of the big issues with the design of this model was that SOME of them had the tube filament heaters connected directly to the 117V wall outlet. The total voltage of all the tube filaments equalled the mains supply... ie. 2x50L6's, 1x6SL7, 1x12AY7 = 118 volts. In this scenario the DC voltage is also tapped directly from the mains voltage through a half wave rectifying diode... I find this design very unsafe so have been adding isolation transformers to amplifiers of this design in order to make them safer. If not and the power plug is plugged in the wrong way it's possible that the projector chassis will have wall voltage on it which could make a mere focus tweek a trip to the ER...

Most models have a power transformer that isolates the mains voltage from the circuit but unfortunately this one does not. I like having the schematic on hand in case the expected voltages are published on them. I find it makes troubleshooting a bit easier. It also takes the guesswork out of replacing any obviously blown or burn out components.

If anyone has the missing pages that would be super.

John, most tube gear is very robust and will tolerate out of spec components before having issues. I have recently started restoring tube radios and often can't find a schematic. But that in no way would prevent you from being able to fix your projector. Tube gear will usually tolerate guesswork without causing a meltdown.

You will need a good multimeter (if it can also check capacitance, that would be helpful), a capacitor tester (if your multimeter doesn't), a good 40w soldering iron, and ideally a tube tester (you can get those relatively cheap on fleabay.)

You will need to know, or have a chart for, resistor color codes and capacitor markings (the three digit codes) both available online.

Check each component individually, anything more than 10% off, replace it. Regardless of readings replace ALL wax covered capacitors (and the main electrolytics). Beware of the "Bumblebee" caps, the long black cylindricals with color bands. They often short out or catch fire. Replace ALL of those, regardless of condition. Replace ANY cap that is bulged, cracked, leaking (or melting wax) or looks overheated. . Ceramic disc caps, mylar caps, mica caps (the rectangular blocks with dots) generally don't fail, so a simple test will suffice. ANY caps from AC line to chassis MUST be replaced, regardless of readings. Use modern X or Y rated caps of voltage greater than or equal to the original. Resistors should be no more than 10% out, replace any out of spec, cracked, broken of overheated. If a resistor is discolored, they often will still read close to rated value, use that reading and your best guess on the faded color bands to see what value it is. If it is too far gone, you can safely use the value specified on a similar gear's schematic of the same general tube set.

Clean all tube sockets and tube pins, being careful not to bend or damage contacts or pins. Test the tubes if possible. If not, after you power them up, check tubes for filament glowing. Once they warm up, make sure none of the plates are glowing orange, red or purple, all of which indicate the tube is overdriven, not biased correctly, shorted or gassy. Double check all other components around a glowing tube. Gently wiggle tubes to make sure socket connections are secure and not intermittent.

If you want to check voltages on a given tube, if the manual is not available, a good rough reference is a similar schematic using the same tube compliment (like the other model you found already), or the data sheet for the tube itself which will give operating voltages, usually in a specified range. In an audio-only use, most voltages are not super critical and as long as you are within range (or at the voltages specified on a similar design, within 5-15%) you will be fine. On radios, especially the oscillator and IF stages, tolerances and voltages are very critical.

Bottom line is, don't let the lack of the specific schematic hold you back on this. Dive in, do all of the visual and power off checks I outlined above, and move forward. You can most certainly install a 1:1 isolation transformer on the incoming AC, it is a good idea. Be aware that it won't totally eliminate a shock risk (due to capacitive coupling you can still get a small tingle) it will be much safer.

Tube equipment is much easier to repair than solid state in many ways, and it will forgive mistakes in repair and testing that would completely destroy most solid state gear.

[Mark Gulbrandsen]

Or better yet... Send it to Tony for repair!

Tony... My friends wife died due to moisture that built up in the hair dryer handle. This was in the early 1980's when ground fault was finally becoming standard. My wife's hair dryer has the ground fault built in, so no worries. I never had enough hair to justify owning one myself. So I can't speak from experience there. I only know that she was killed because she was holding the hair dryer by the handle... where the power switch is at... then at the same time went to turn the water off in the sink. That was the inspectors findings.
The Cook County electrical inspector was able to tell exactly what happened, based on how corroded the metal parts were inside the hair dryer. And marks on her skin. Plus it was an old post war home that only had old two conductor wiring, and no exhaust fan in that bathroom. The moral of the story is... Don't necessarily trust plastic, only trust ground fault breakers. If one had been there, she'd still be alive.

[Mark Gulbrandsen]

In the 'electronics shop' class of my high school in the early 1970's, we all built our own
AA-5 AM radio, with series tube filaments & and a 'hot chassis' from kits provided by
some educational kit supply source. It had a plastic case, so in theory, you were semi
safe from electric shock. But one design flaw was that the plastic volume & tuning knobs
were held onto their shafts by setscrews which, although they were countersunk into
the knob, were still close enough to the knob surface make skin contact if you grabbed
the knob too tightly while turning it. Like you said- - "it was a learning process"
It actually wasn't a bad little radio, and it ran for over 20years before it died.

I only built Grief Kits, at least that's what my High School buddies called them. By the time I was building them, everything out of there was Solid State which may explain Jim's radio kit coming from another kit source. The first one was an AA22 integrated amplifier I bought in 1969. Then I built 20 more, some for myself, including the 15 inch color TV, and one stereo receiver for my High School Counselor. The amazing part is they all worked, even the TV at first turn on. Good Grief!

[Stefan Scholz]

When the "All American Five" was common, I don't know anyone who considered the hazards involved. Back then, educational, experimental and hobby kits from Allied, Lafayette, Heathkit and the like were all powered by household mains, and many of them with series-wired filaments. Yes... you got zapped once in a while. It was part of the learning process.

One of my first memorable circuits was a low-power AM transmitter, built around a single 117N7 tube. No transformer on that one, either.

Voltages up to 250 V AC in respect to earth were chosen early, as they "under normal circumstances are not lethal". These projectors/radios/TVs were used indoors, on isolated floors made of wood with rug, vinyl, or linoleum flooring atop. This acted as a layer of insulation, so the remaining EC hazard become very small, and at 100 to 127 V mains it is even more unlikely to be electrocuted under normal circumstances.
Not that I consider series heating and direct mains operation it a good or acceptable design. I always hated it. Proper isolation transformer!

THe repair practice is similar to all tube gear of the time. I have repaired a couple of RCAs for friends, and the faults were similar. Some used bumble bee capacitors, known to fail over time. So my procedure was new electrolytics, new capacitors. Than see, if it works. I also found a dead high resistance feed to a PEC making sound low.
I always fed these amps using a variable insulation transformer in the desk panel.
John seems to be knowledgeable in tube circuits, so what tips to give? Actually follow the sense used in all other tube repair projects. Once capacitors and faulty resistors are cleared, feed signal into output tubes and then go further up to determine another faulty stage. In my case, I didn't have a circuit schematic, but managed to get it back to life.

[Rick Raskin]

Not to rain on anyone's parade, there was a reason we called these designs AC/DC death traps.

[John Price]

Yep! I’ve installed isolation transformers on all of the units I’ve worked on that lack a power transformer!