Schematic needed for Hitec Focus 4 Transmitter

[flea_flyer]

Hi All
I recently purchased a Hitec Focus 4 Tx that seems to have an unusual (to me) fault. With the fully charged battery installed and the transmitter switched OFF, the battery charge was substantially reduced in about a fortnight. Battery is a new 8-cell pack of 2000mAh Eneloop cells, which should hold their charge for a long time. With the battery out of the transmitter, it holds charge as expected. With the battery fitted and the transmitter turned on, all seems normal (good output on panel meter, etc).
I got a bit suspicious, so I put my DVM in the battery circuit and found that with the Tx switched ON, the battery current was 185mA, which it held nicely while the transmitter was running. This corresponds to about 1.7W at 9.6V, which seems reasonable.
However, when switched OFF, there was a standing current of 1.9mA! In two weeks that would account for about 640mAh or around 30% of nominal capacity.
I'm after a schematic for this radio to see where the battery and switch circuits may be faulty. On the other hand, if anyone on the forum has any ideas, please feel free to respond.
Although this is an Australian version of the Focus 4 and on 36MHz, I doubt that this would make any difference to the root cause of this problem.

[flea_flyer]

A bit more information:
This link is to web site which allows a search of the FCC (USA) database of (among other things) radio control systems that have an FCC compliance ID number. The site is https://fccid.io. From RC groups I found a 2006 post with a partial listing of Hitec equipment, giving the FCC IDs and so put that into 'search'. This entry included a schematic. The only trouble was - it was for a Focus 3 Single Stick Tx and not a Focus 4. I wonder if their battery and switch circuits might be the same? The schematic I found is attached.

[Phil_G]

I cant imagine it would be any different, compliance costs money so once a design is approved it becomes the 'company standard'.
In the diagram the on/off switch is the very first component after the battery. If its 'off' there should be no path anywhere, even to the charging socket has a blocker. Hitecs are notorious for failed switches, I have two Hitecs here that switch themselves on & off at random. Measure the pcb power rails whilst 'off' and draining 1.9mA to prove a path through the 'off' side, the rails should be completely dead. Has it been modified at all? has someone added say a charge indicator LED? or a 'smoothing' capacitor? or a low-battery warning? Is the charge socket in good physical condition (they often fall apart)?
If its unmodified, my immediate suspicion based on previous failures would be the on/off switch itself.
Cheers
Phil

[Martin]

A lot of modern transmitters, the on/off switch isn't a real switch that disconnects the battery: rather it sends a signal to the microprocessor that the user wants to shut down.

My Taranis, and RadioMaster are like that. You can switch them off, but if they're working with a telemetry receiver, they stay on and display/speak the message, "Receiver still connected... are you sure?" You can then force them to switch off by pressing some 'confirm button'. Even when they're working with a 'dumb' non-telemetry receiver, they don't turn off immediately the switch is thrown, and instead display a brief, 'shutting down' animation. You kind-of expect the RadioMaster to be like that because it has a power button, not a switch; but the Taranis has a traditional hefty on-off switch that looks and feels like it should be disconnecting the battery - except that it doesn't!

However, the current draw on those transmitters once they are "off", is in the microamp or less range. 1.9mA is awfully high for a standby/sleep circuit.

I don't know anything about the Hitec radio, so this 'soft power switch' issue may not be applicable to that, at all.

If you don't find an actual fault with the Hitec, and it's "designed to work like that" then I'd fit an auxiliary "true on/off" switch in the battery line, or pull the battery connector if that's easily accessible. Use the built-in switch at the flying field, and the "real switch" or battery disconnect after the session ends: give it a few seconds to truly enter its 1.9mA 'sleep state', and then cut the power.

I have a Sony Vaio Laptop with a similar "design feature" (a.k.a. mistake). It drains its battery when left for a week or two. Sony claim it's a deliberate feature, but that smells like B.S. to me. I read online somewhere that it's because the USB ports aren't really powered down when the laptop is turned off. I always pull the battery out of it now, when I put it back in its case.

[flea_flyer]

Thanks for your comprehensive comments, Phil_G and Martin!
I have looked again at the PCB and it appears that the battery connections do not go straight to the switch. Instead, they go via some sort of filter arrangement made up of two (what I think are) surface-mount shunt capacitors separated by a series-connected choke. If they are capacitors, then the first one is connected directly across the battery leads. At some point, new battery leads have been soldered on to the copper pads with evidence of overheating. The first capacitor is very close to the negative lead and it seems to have been damaged, to the point where it bears little resemblance to the original component (more like a rotten tooth). See my crude attempt at taking a close-in photo, where I have highlighted these areas.
A quick check across these two components showed an open circuit across the good capacitor, but the damaged one shows about 250kohms.
Looking again at the Focus 3 schematic, I have highlighted the components I think are in the Focus 4, but instead of being after the switch as in the Focus 3, they are before the switch (for some unknown reason).
Naturally I really do need to see the schematic for this model to confirm my assumptions, but if they are correct, it may explain the situation.
What do you think?

[Tobe]

To what I can see the switch only cuts +V and not GND-that goes directly to what to me looks like a filter so if switch isn't on there shouldn't be any +V there or better only if switch is set to on.
So if you have PWR there undoubtedly it's a faulty switch, I would start by isolating the switch. I notice from you pictures a lot of "Dirt"-"Corrosion" which I'm quite sure it is also affecting the switch.
I'm missing also the 10 uH choke/inductor shown in the schematics?

Cheers

Tobe

[Phil_G]

You can remove the burnt capacitor to test the set, it won't affect operation. If it works, solder a cap across the inductor tags and you're good to go

[Tobe]

Focus 4.jpg (16.05 KiB) Viewed 1004 times

This is how it should look like, just 2 regular 0.01 caps no polarity! The schematics shows the same.
With the switch off there shouldn't be any voltage/drain here but if one of the caps/remains of a cap is shorted when turned on you should have unnatural power drain as they might act as resistors.
So you might have are 2 failures, one-the switch corroded and leaks and second corrosion issue around the inductor, any chance to get a picture from the component side.
I would suspect that the Tx has been left with a battery installed during storage time....I would not be surprised if more issues will come up eventually like pots, never serviced Hitec but serviced Kraft and Futaba in the early 80's and seen if not all almost all.

[flea_flyer]

Thanks Tobe and Phil_G. It seems we are on the right track. By the way, your pic of the circuit board reminds me that mine is far from pristine as the blobs of plastic used to hold the wires are missing and yes, there is discoloration of the tracks etc.
Taking a pic on the back side of the board might be a bit tricky just now. However, I can confirm the inductor (choke) is between the caps on the reverse (connector) side of the board.
I will make a start by removing the dodgy capacitor and see where we from there. Will post progress as it happens....
Thanks again for your help guys.

[flea_flyer]

Dodgy SM cap removed (unsoldered the ends and a tiny quantity of carbon fell away). New cap soldered in place as per Phil_G's suggestion. Zero uA leakage from battery. Tx bench tested with receiver/servo and all seems OK but I will do a range test outside, tomorrow.
Case closed (I hope).
Now on to the dodgy Hitec Optic 6 with a blank display...
Cheers