7mm Scale Hymek Overhaul


Way back in the early 1990's before the Heljan 7mm scale Hymek was even heard of, I build this 7mm scale model using the Hornby 'Big Big Train' body and an RJH conversion kit. At the time, RJH was a manufacturer of numerous 7mm scale diesel locomotive kits.
In terms of measurements, I believe that the body of the Hornby 'Big Big Train' Hymek is basically dimensionally correct. The RJH kit, as far as I am aware, is no longer available, but it consisted of all the parts necessary to convert what was basically a toy into a proper scale model. This included:

  • Replacement of everything below the waistline
  • Complete new cast bogie frames with brass internal frames, brass bearings, wheels, motors and mechanism
  • Replacement white metal fuel tanks
  • White metal buffer beams/cowling, cast buffers and buffer beam detailing
  • Numerous body detail fittings
  • Complete roof detail - fan, grill, vents, horns on cab roof
  • Cab internal detailing

At the time, the kit cost nearly £80.00 from the 'Home of O Gauge' (long since closed). It was the most expensive kit I had ever purchased, and unfortunately, it well and truly conformed to the maxim that was prevalent at the time: the quality of instructions was inversely proportional to the cost of the kit.
By today's standards, the white metal castings are fairly crude, but with time and effort, I was able to put together a loco which I thought was pretty good and it did manage to serve on the exhibition circuit for many years.
Some readers may be aware that there was also a similar RJH conversion kit for the 7mm scale Lima class 33.

My very good friend, Steve Rowe, who is a prolific YouTube contributor with his 'West Green' 7mm layout has long since been a 7mm scale modeller and now my father has decided to upgrade from 4mm to 7mm scale, so there is a bit of a motivation for me to do some 7mm scale modelling.

So, after many many years of stirling service, this loco is up for overhaul and some upgrades.

The Work List

The following items need attention:

  • Electrical pickup is being lost on the leading axle of one motor bogie
  • Other pickups have become detatched
  • One brake block has broken off
  • The loco needs re-wiring with an 8 pin DCC socket
  • The loco needs provisioning for DCC sound with a speaker and associated wiring
  • Several of the hoses hanging from the buffer beams have broken

The worklist is being managed by the GPP Software online LocoShed.com software which is a system for managing a database of a model collection, together with a maintenence record.

I decided that the best approach was to sort out all the electrical and mechanical issues first and then do the cosmetics last.

Overhaul Starts

Failed Electrical Pickup

The bogie construction on this loco is the standard RJH brass inside frame with white metal cast outside 'decorative' frames. The inside frames are the structural component and hold the brass bearings. The wheels are insulated on one side (note insulation around pin-point axle on upper wheels of right photo above) such that electrical connectivity is achieved through the brass frame on one side and through additional insulated pickups on the other side. I made the latter using the standard phosphor bronze strip running on the back of the wheels method. These can just about be seen poking down behind the near wheels in the first photo above. At this stage, it will be noted that the motor bogies are completely electrically self-contained and are individually self-propelled - the wiring will be adjusted to be suitable for DCC operation.
As can be seen in the photos above, the motor only drives one axle per bogie. There was an option at the time of purchase to fit a delrin chain drive which would have ensured that the other axle was powered, but as a uni student at the time, expense was an issue, so I didn't fit these. From what I remember, they were very expensive for what they were. In practice, it hasn't really been a major problem because with suitable weight, the loco has been able to pull prototypical length trains without a problem.

The problem at hand is that the non-powered axle had a broken circuit, evidenced by the motor stopping when the motor end of the bogie was lifted off the live workbench test track.
Upon inspection, it was noted that the additional pickups had both become detached from the frames. The one on the motor end was still making contact, but the other end had lost contact. Both were refitted and secured with Superglue. After this repair, there was still a problem. It was found that the bearing was not conducting. It was duly lubricated with an electro-conductive lubricant which resolved the problem.

Brake Blocks Broken Off

A mistake I made in the original build of this loco was to secure the brake blocks using Evo Stick. After 28 years, the Evo-Stick had dried out and was no longer holding things together. As a result, during this overhaul, several of them fell off and the opportunity was taken while the bogies were detatched, to re-attached the brake blocks using Superglue. Once dry, this was repainted to hide the 'glossy' Superglue finish.
As a general rule, I wouldn't recommend Evo Stick for attaching parts over the long term. Certainly not anything structural. I also find that here in Australian climes, glues and paints which may be Ok for use in the cooler European climate sometimes don't stand up well to our heat and humidity.

Rewiring for DCC

When I built this loco back in 1991, DCC was barely on the horizon and everyone was still wiring for DC. However, times have moved on. Steve's 'West Green' is a DCC layout and no doubt, when my father gets his 7mm scale model of 'Ashburton' up and running, it will be DCC too, so if I have any intentions of running this loco on either of those layouts, it needs converting to DCC. In fact, rewiring for DCC was the main intention of this overhaul...mainly because I wanted to hear this beast with sound!

At the time of writing, I haven't chosen or purchased a sound decoder for this loco. I decided to 'borrow' the ESU decoder from my 00 scale Heljan Hymek. Running on Steve's and Dad's layout, this loco isn't going to be requiring high power. When I use the loco on a 1 Amp DC controller, it doesn't cut out because it draws under an amp. If I allow the loco to push up to a fixed object and turn the throttle up, the motors are under real stress and can cause a 1 Amp DC controller to cut out. So with gentle use, this loco will probably be OK on a 1.1 Amp ESU decoder. I should note that this is a temporary arrangement and will be replaced in the future with a proper 7mm scale decoder.

In order to rewire the loco for DCC and prepare it for decoder fitting, I needed to install an 8 pin DCC decoder socket. To that end, I made up a 'motherboard', complete with socket, NMRA colour-coded screw connections and extra connections for speaker attachment:

The original Hornby 'Big Big Train' Hymek was battery powered and four 'D' sized batteries were fitted end-to-end through the body length. When I originally rebuilt the loco, these were removed as it uses 12V from the rails. There is an enourmous amount of space inside the loco where the batteries once resided. The remains of the battery retainers can be seen, together with the available space:

Before the new motherboard could be fitted, the motor bogies needed rewiring. This is before (note the wiring for the additional pickups running along the top nearest side edge of the bogie frame):

...and this is after rewiring for DCC:

The change is that the pickups have been disconnected from the motors so that they can be re-routed to the motherboard via red and black wires (NMRA standard).

Once the bogie rewiring was completed, they were refitted to the loco after a few minor repairs and paint touch-ups. The following pictures show the inside of the loco with the wiring connected to the motherboard. Note the 8 pin DCC blanking plug in the DCC socket.

The loco was then tested on DC and operated correctly first time.

Observant readers will probably notice the letters A and B marked on the inside of the loco. This was to assist with the process of wiring to the NMRA standard whereby the positive rail (red wire) is on the right (from the driver perspective) and the negative (black) is on the left. This had to be aligned with the 'front' of the loco. On Hymeks, the 'A' (front) end is the opposite end to the side radiator grills. All of this was about making sure that the loco was wired up correctly and that the 'front' was the correct end.

First test on DCC

Once the loco was operating correctly on DC, I couldn't resist the opportunity to try it on DCC, which, afterall, was one of the major drivers for this overhaul.

I therefore duly borrowed an ESU sound decoder from a 4mm scale version of the same loco. The loco draws about 800-900Ma and the decoder is rated at 1.1A, so there is a couple of hundred Ma to spare. It should also be noted that this is purely for testing and the loco will have a proper 7mm scale decoder fitted. In practice, this decoder runs at the same luke-warm temperature that it does in its home loco, a 4mm scale Heljan Hymek.
Here is the decoder, temporarily fitted - the beauty of 8 pin sockets is that you just take the blanking plug out and plug a decoder in:

Note that going forward, a decoder will be properly fixed and a decent speaker with a good-sized sound chamber will also be fitted - there's plenty of room in this loco! The following video shows the test run. It worked correctly and it ran in the correct direction first time. The 4mm scale Hymek which appears in the foreground is the decoder 'donor' loco:

Installing a speaker and sound chamber

The test above confirmed that the re-wiring of the loco was successful and that a sound decoder would operate correctly.
The next step was to fit a proper speaker and sealed sound chamber. Unlike its 4mm scale counterpart, this loco has masses of space for installing a speaker and sound chamber. Since the decoder is a Loksound 3.5 and only itself a temporary installation, I opted to use a spare DCC Supplies 100 ohm speaker and enclosure and attach it to the roof:

In subsequent testing, it became apparent where the sound was coming from within the loco so I swapped the speaker end-to-end so that the bass sound would come from the 'A' end where the engine is located in the prototype (as shown above). This actually solved the problem. In smaller scales, you don't notice this kind of issue. However, I always feels that sound doesn't sound right if there is no driver in the cab. Weird logic I know! When a suitable driver can be found, he will be seated in the cab. Until then, overhaul is now completed.

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Graham Plowman (4/01/2020)

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