Topic: JR 388 Backup Battery

I have a JR 388 Tx which I only use as a spare for slope soaring foamies.
The backup battery has given up, so that whenever the main battery goes flat I lose all my model settings (I believe there is also a capacitor somewhere in the circuit that recharges whenever the tx is switched on, to provide additional capacity to the backup battery).
As I won't remember to switch on every day to keep the capacitor topped up, I would like to replace the backup battery. Don't really want the expense of a proper McGregor repair as it is only a spare (and would probably cost more than the tx is worth!), so any advice on whether it is a DIY job would be appreciated.

You can certainly do it yourself, but it is a bit of a fiddle.  I did mine a couple of years ago.  You need to get the tagged version of the battery and may need to reshape or bend the tags a bit.  I will see if I can remember where I got the battery from and let you know.

Good luck
JWB

Ask mac-man on here. I thought the backup battery also maintained the stick calibrations, so if completely flat wonder if the set might now need re-calibrating.

Steve

I had a chat with a tech support guy at McGregor (not sure if it was Mac-man), he also mentioned the possibility of the set needing to be re-calibrated.
I have re-programmed one model (don't want to set up more than one as I will probably lose them all again until I get the problem sorted) and that seemed to work fine, so maybe re-calibrating will not be an issue.

I did this fix, on my old X347, a few months ago and it has worked fine ever since.
The tagged cell was a CR2032-T19 and fitted perfectly with no adjustments needed.
I got two, just in case I mucked it up. I didn't so I have one going spare.
If you want it then just PM me your address and I'll pop it in the post.   

I also replaced mine in a 3810 without any problems.

HI

Re-programming and re-calibrating are entirely different things.

Re-calibrating refers to the calibration of the stick movement, and possibly other parameters.  Each stick gimble has two potentiometers which are used to determine the stick position. Due to manufacturing tolerances there will be differences in the potentiometers and therefore a calibration has to be carried out to ensure the centre positions and end points of each stick are the same and so provide the correct pulse width.  This can be achieved in one of two ways.  Physically, by means of additional small pre-set potentiometers and possibly some mechanical adjustment or in the case of computer transmitters to store the corrections required for each stick as data in the computer memory.  I am not sure which method the 388 uses, but I would assume it is by stored data.  The question then is where is this calibration data stored.  If it's in volatile memory backed-up by the battery then it will be lost if the battery should fail.  However, it should be possible to change a battery that's low, but not dead without losing this data.  If the calibration data is stored in some other form of non-volatile memory then failure of the back-up battery should not be a problem.  Mac Man can probably answer these questions.

Regards
jwb

The Small Battery Company can supply the CR2032 -T19 Batteries.  I did my X347 and X388 transmitters.  The 347 was completely flat and losing settings but the 388 was still holding them despite a voltage of about 0.5v on the old battery!

All calibration settings (Sticks, Battery Voltage etc) are in volatile memory hence the need for the Lithium.

Saying that, if you use the set on a regular basis then the lithium is rarely used as the stay alive memory uses only a few microAmps and there is a capacitor on board that is charged up very time the set is powered on,  this cap usually allows the set to be turned off for a couple of days without using the lithium cell.

Should the lithium cell drop to a really low level, then you may find you lose things like Stick direction, Swash mix and on occasion the 8th channel.  This is because the yanks wanted it this way so they could flog the 388S as a 7 channel system (think it was called the 387).

Anyways I digress......   Should you decide to send it in, you also get a RF calibration done (if you send the module & xtal).  This way it means that you will be on frequency with the correct deviation.  The sticks get cleaned and the linearity is tested, the pcbs are inspected for corrosion, cleaned and re-varnished if necessary. 

 


But it is entirely up to you


 

My understanding was that the Lithium cell on 347s and 388s only came into play if the main batt pack was disconnected from the Tx. I had not heard about the capacitor secondary back-up.

My 388 lost the entire memory the day before a competition, when I took the main battery pack out to give it a charge on a peak-detect charger. I found out on the comp field when my 6 servo glider didn't work  .

Had I charged the main battery pack in the Tx (in the way I had been doing for 7 years), I would never have found out that the cell was dead. 

Anyway that was a long time ago, and programming a MPX Royal is much more fun. Bought the Royal EVO 9 (now PRO) in frustration due to the JR system letting me down at a competition.

Good Luck,

Jef

When the set is turned off, there is no supply to feed the +5V line that the main electronics work on.  The power supply section also produces a secondary 5V line feeding the memory section of the processor. The capacitor that was charged up by this secondary line starts to discharge through the processor.  The lithium cell is fed into this secondary line via a diode (so as to prevent this line trying to charge the lithium cell).  When the cell voltage exceeds the discharging capacitor voltage, the lithium then starts to supply the memory section.

A very easy circuit and although the manual recommends that the cell is replaced every 5 years, in reality it can last longer.  I have had some sets come in that have never been in since it was bought new and the cell was not fully depleted.

But the recommendation is there for a good reason, some systems (if used infrequently) deplete the cell faster and should be checked.

How much, typically, for the service you describe, mac_man?  I have two 388's, the older of which is about 14 years old 

Mike

Hells bells - throw it away (stick it on Ebay) - you can buy a new 2.4 tx for less money than you'll spend sorting that (good in it's day but now?) 388 out

If you want a 35 Mhz TX then I've a brand new 2720 set doing nothing - will haggle

Hells bells - throw it away (stick it on Ebay) - you can buy a new 2.4 tx for less money than you'll spend sorting that (good in it's day but now?) 388 out

If you want a 35 Mhz TX then I've a brand new 2720 set doing nothing - will haggle

The problem is it's a step down to a 2720, the modern JR replacement is a DSX9, (or a 9X2 if you want to stick to 35MHz)

Quite happy with the 388 thanks and the Assan/FrSky 2.4G modules that go with it.. As CF says, a 2720 is a retrograde step...

Mike,

Best I can suggest is that you speak to Steve at MacGregors, 01628 760430

I used a 388 for ten years - brilliant TX in its day but at the age it is, other things - sticks - pots etc are all getting long in the tooth and they'll start to let go sometime soon!

IF you're just using it for foamies etc then that matters not so much but then again - if you're just using it for foamies then a 2720 would do all you need anyway.

(Not itching to sell mine by the way - just here unused and thought it might find a good home)

Well, this probably tempting fate, but mine both seem to be hanging in there, but thanks for the offer anyway! 

I have been toying with a new Tx but haven't yet decided which way to go.  The new Hitec Aurora looks interesting as does the Spektrum DX8, or JR DSX9. 

There's always Multiplex, but how the hell do I shave if I can't look at myself in a mirror ever again?

@mac_man - I'll give yer man a call, thanks.

 Mike

Mike, could you let me know how you get on with MacGs, (via PM if you'd rather), please.  Mine is about the same age as yours and it may be worth getting it done too.


Mark.

willco

Well, this probably tempting fate, but mine both seem to be hanging in there, but thanks for the offer anyway! 

I have been toying with a new Tx but haven't yet decided which way to go.  The new Hitec Aurora looks interesting as does the Spektrum DX8, or JR DSX9. 

There's always Multiplex, but how the hell do I shave if I can't look at myself in a mirror ever again?

@mac_man - I'll give yer man a call, thanks.

 Mike

Oh don't be intimidated by us Multiplex users - once you've adopted the supercillious all knowing demeanour of someone that knows he's using the best, you can practice your smug expressions whilst shaving 

Spoke to McG's earlier in the week.

They are a bit reluctant to commit to a fixed price for a battery change and service - understandable I suppose if you don't know what to expect when you look inside!

However, they did say that £40-50 should cover it, plus return p & p (and plus parts, I guess).

At that sort of money, I'll probably be looking at a new tx when the battery in the 388 goes phut, but I'd get a new cell and attempt a DIY job on what would become a back-up tx.

HTH,

Mike