Bell & Howell: Model 601 Amplifier Conversion Project

While some people choose to run these stock, I didn’t really like the idea of running something with electrolytic capacitors from 1948 - if one failed, it would be an absolute nightmare to fix.  Actually if anything failed it would be a nightmare.  Apart from that, quite a few resistors were way out of spec.  

But after getting to the end of the process, I completely understand anyone’s reluctance to rip into one of these.  It’s not easy to work on due to the lack of room in the chassis.  This was slow work that required a lot of planning and patience.

Here's the conversion path that I've taken.  Here's a Link to my original post with photos etc

BELL & HOWELL GAUMONT, MODEL 601 REBUILD

I will be using as many components as I can scavenge from the amp, and I'll keeping the schematic similar to the original (minus the weird bits).

The 601 is often said to be the same as the Filmosound 179 - British vs American; they certainly look the same, but there are differences in the schematic.  

The next two images below are two sides of one board - in the image above, the board is jammed into the top left corner.  You can see the positive end of the caps poking out.  The negative ends were soldered directly to the aluminium frame that they're attached to.

 

Perhaps unsurprisingly, a lot of the resistors have drifted out of spec over the last 76 years.  Unfortunately many are too far out of spec to use.

STRIPPED BACK TO THE ESSENTIALS

This is what it looks like clean with a few components and some transformer and heater wiring left in place.  Excuse the dodgy paper masking tape - that's my labelling system for leads.

I can't imagine what it must have been like on the production line making these - so many parts were tucked away in tight locations, and there had to have been a very specific build order to get all the parts in.  You'd really want to get it right the first time.

POWER SUPPLY

Given the first thing that needs testing is the power supply, this was first on the list to finish.  I replaced the strange four pin Jones power connector with a regular IEC.  A little filing on the chassis was required to make it fit, with a couple of new holes drilled.  

The fuse holder will stay for the moment, and perhaps permanently.

The gigantic filter capacitor from 1948 has been replaced with a new F&T 16uf + 16uf 500v capacitor.  I was considering keeping the original in and trying to reform it, but I decided against this, largely due to the size.  It just took up too much space, and I was not confident about leaving it in the power supply, given its age.

REBUILDING THE CIRCUIT BOARDS

A lot of people go the point to point route after they strip these down, I've decided to rebuild on tagboard using my own layout, as I think this will be easier to do (famous last words).  In retrospect, I’m not sure the board made it any easier - it was still difficult.  The style of tagboard that I'm using has smaller spacing than the original, so more components can fit.  

1w carbon film resistors replace the old out of spec carbon composites that looked like a mix of 1/4 watt and 1w based on their size.  I've managed to keep a few of the original Hunts coupling capacitors in the mix, as they are bang on spec, and still test like a new capacitor. 

This is the layout that I used for the boards - I have no idea how close they are to the original, as I didn't spend the time to trace the circuit.

I've intentionally left some of the wiring off the layout to hopefully discourage anyone that isn't sure about what they're doing attempting this.  It's not easy, I went slow and it was still a little frustrating to work on.

Note the dire sounding safety warning on the layout - this should be taken seriously if you are opening up any amp to poke around in.  

I still need to mount the output jacks and tidy up a wire or two, but it's pretty much good to go.  It's less of a rats nest than the original, but this was still slow going for me.