Help needed with - Racal Dana 9081

[3pinplug]

Hi all

Part arrived and I replaced the affected components identified earlier. I then decided to look over the PCB and have found even further component leads that are not soldered, this time around IC5 on the FM Loop Assembly Board - see pic of the two pins.

I then started to check all the boards and found a piece of screening braid had ripped some of the top silver ident off the main oscillator board, difficult to show but pic attached. Before switching unit on I need to have checked all boards for good solder joints as not sure whether these are due to someone being in the unit or just how they have come out of factory, probably someone been in I assume.

Replaced the two affected tants with Nichicon Electrolytics so hopefully that's a good move.

Hoping next post to have switched unit on and started checking voltages / for any overheating tants.

Any other known faults I should check for at this time?

[Station X]

The unsoldered joints won't have helped much, as I said I had a similar fault with a 9087. I don't know how this can happen, as I assume the boards are flow soldered then inspected.

Attachment 92817

[3pinplug]

Hi Graham
How weird that's exactly the same type of poor joint I am seeing, maybe a bad batch from the factory or their process wasn't that good and occasional ones slipped through - this will remain one of lifes mysteries I suppose.
Thanks
Mark

[3pinplug]

Hi all
Fixed couple more dry joints and fired the unti up, started by first checking the output of the 3 voltage regulators in the power suply fitted to the back of the unit and got the following readings:-
MC7906CP
Input -10.6V
Output -4.6v?

MC7912CP
Input 19.0V
Output 12.4v

MC7924V
Input 33.8V
Output 23.5V

On the low 6V i desoldered a leg from each of the tants across the pins of the device and still get just -4.5V, I am thinking the MC7806CP may be defective as that is a little low - should i change this and or any others based on the readings?

[dave cox]

Is the MC7806CP hot ?
Can you keep your finger on it ?

Hot, hot, hot - the load is drawing too much current.

If its NOT scorching hot - most likely defective but double check with a scope to see if its oscillating, these regulators can be touchy about the load capacitance.

dc

[frsimen]

In this circuit, the MC7806P is fed by a 47 ohm resistor. The main current path is via Q1, so the regulator itself may not be that hot even when the current is too high.

Try disconnecting some of the load, you may still have something pulling the supply down.
If you have an oscilloscope, check the ripple on the input to the regulator and the output ripple. The input ripple should be less than 1.3V peak to peak. The output ripple should be no more than a few millivolts.

Table 5-1 shows the expected resistances on the outputs of each of the supplies, which may help identify a problem.

The other voltage readings appear normal, so there is no need to change those regulators. The MC7906P may well be fine too, I wouldn't change it until other avenues have been exhausted. Do check that it is a 7906 and that it hasn't been changed by someone to a 7905, although 4.6V is a still rather low for a 7905.

Paula

[3pinplug]

Hi Paula
Thank you for the very comprehensive info, i will run through the checks requested and confirm the findings. ��

[3pinplug]

I can only assume i disturbed something!

Desoldered the MC7906cp wires off the tags and took the IC out, tested it and it appeared OK, put it back, refitted wires and 5.95v output so looks to be OK now. I will replace the two tants across its pins with Nichicon caps.
So back to the diagram to work out where to test next! Pleased that's sorted, even if I have no idea what I did . ����

[3pinplug]

My battle continues!
I started checking some of the +24v, 12v and -6v voltages shown on the overall block diagram for the 9081. Most were ok but the +24v on pin 16 on the FM loop board only read 21v. I have just noticed that R47 is burnt out so assuming the tant C32 (FM loop board)may be the cause, but visually this tant looks ok on this occasion.
So missed that one during my board checks but then there are a lot of components on both sides. So will order / replace those and see what that does, the frequency knob still does nothing when turned, want to try and look at that once I stop finding burnt out components but looking like it could be a while!

[3pinplug]

Hi all
Bit the bullet and thought i would replace all the tants once and for all, but nothing is as easy as it seems. Is there a trick to desoldering them as my god what a faff to get one out, took nearly an hour!!!
If not i may have to call in the heavy guns i.e my vacuum solder desucker.

[frsimen]

I've found that cutting the body of the capacitor off the leads makes the job a little easier,

A solder sucker will remove some of the solder and a pair of tweezers or snipe nosed pliers allow the remains of the lead to be pulled out.

I don't envy you if you are going to change all of them, that's a big undertaking!

Paula

[3pinplug]

Hi Paula,
Thank you for the pointer i will give that a go, yes going to give it a go and attempt to do them all, yes a big task indeed so will let you know how i get on hopefully once i get going it will be a bit easier.

[3pinplug]

Hi all

The project is still ongoing and after a bit more help please, quick update as follows:-

FM Loop Board
C78 burnt (47uf 16V) duff tant, R1 adjacent ok
C27 burnt (47uf 16V) duff tant, R33 & R35 read ok.

Power supply Capacitor C3 6800uf 25v duff, replaced.

The 12 volt regulator is now only putting out 1.5v, was looking to check for a short when a tant on the "Divider & Phase detector Assembly" started smoking. When I look on the layout and schematic this smoking tant doesn't appear to be present. Its a 47uf 16V tant with one leg fitted on the component side fo the board to "pin 13 of IC20" and the other leg going through the board as normal.

The attached pic shows the smoked tant in situ - has anyone else got this part present or has it been added in?, it looks factory as its the same red tant as across the rest of the board. It has one leg soldered to the track running between IC20 Pin 13 and IC23 Pin 3?
Hope someone can confirm if its meant to be there or not, mine is a Racal Dana Model 9081.
Thanks
Mark

[frsimen]

I don't think my 9082 has anything quite like that. Pin 13 of IC20 is connected to 0V, so I expect the other end of the capacitor connects to pin 16, which goes to the 12V supply via R84. It looks like it is just adding a little more decoupling. R84 will be getting hot if that tantalum capacitor has gone short circuit.

There may still be another short somewhere else on the 12V line, you'll know once you have replaced this faulty capacitor.

Paula

[3pinplug]

Thanks Paula i willlook at R84 and replace the cap and when i get 12v back see what else is going on👍

[3pinplug]

Hi
Have replaced the duf tantalum but my board layout is different to the assembly drawing, i enclose some pics.
The legs of the tant in question connect as follows:-
+ Leg of tant goes to Pin 16 of IC23 so as suggested I ent to check R84 but not present and no holes for it to be fitted?
- leg of tant goes to track running between IC20 Pin 13 and IC23 pin 3 component side of board.

Anyone have a layout that matches my board as a bit lost as R84 not there so not sure where to head to for the +12v that should be there?

Pics show R84 missing, assembly drawing showing it fitted and the new tan tant i have fitted and layout of my board.

Its certainly a weird one!

[frsimen]

That there is no R84 is good in this case. If it had been fitted, the +12V line shouldn't have been so badly affected by the tantalum capacitor shorting, hence my comment that there may be another fault. As there is no R84, you will find that the tantalum capacitor is connected directly across the output of the regulator and is probably the only fault that was there.

Just check that there is no short circuit across the 12V regulator's output. If all is well, you should be good to apply power again.

Paula

[3pinplug]

Hi Paula
Many thanks I have the output leg of the 12v regulator disconnected as once the large cap has been replaced I will check its output, it was showing 1.5v when the tant started smoking. Will check and let you know. 👍

[3pinplug]

Pleased to confirm Progress at last - well I hope so!!!

I replaced the 6800uF capacitor in the 12V power supply circuit, checked the 7812 with the output leg not connected and it was fine. Connected it and then checked all the 12V, -6V and 24V points shown on the various boards shown on the overall block diagram drawing. I couldn't get to the -6V (Pin3) and 24V (pin2) on the divider and detector assembly as couldn't see them they were buried.

I now have a frequency display that changes with the selector dial but it is locked on one frequency on each band and that reading is higher than it should be? The thumbwheel is still not functioning so I can't change the frequency, however the channel spacing selector changes the display accordingly.

Here's what the display is showing:-
5-32MHz - displays 0.000
32-64Mhz - displays 67.688
64-128Mhz - displays 135.376
128-256MHz displays 270.754
256-520Mhz - displays 541.358

I also now have full control on the output level meter, however should the "vernier" control next to the output level control move? Mine appears to be locked solid?

Can anyone suggest what I should go after next based on the above progress / current condition , feel like I am getting close now which is really encouraging.

[PJL]

What test equipment do you have? A scope? A signal generator?