The AM Forum

The RX has worked for years.  It sat unused for 5 months and on power up has problems..The BITE test won't give me any codes.  It will receive on a keypad  entered frequency, but a mode change will shut it down..  The LOCK is stuck ON... I got this from Gary Wingerd years ago.  Is he still around?  I have the service manual but I'm not  equipped for receiver service.  Any suggestions?   Thanks   Steve

Hi,
 
It's possible that the memory battery for the Racal's RAM failed.
It's a 2.4V NiCad in a black, half-inch square by one and a half-inch long
rectangular package, mounted on the CPU board.
My battery is a Varta 'Mempac', part number 55615 702 012. 

The CPU board is attached to the right-hand side, near the front panel.
The battery is soldered onto the board, but it's easily unsoldered
using old-school PC board desoldering tools.

73,
Ed

Check the filter caps.  i've had several of those come thru that had very strange computer problems and in both cases it was one or both of the large can filter caps.

Check ripple on the DC supply.

Gary Wingerd is <gewingerd@netzero.net> - I got an LCD panel from him last year.

The battery shows 2.68V.  A little hi for a 15 yr old 2.4V..Filter caps check OK.. Power supplies run 11.4V, 21.5V, and 22.4V measured at the caps.   I left it plugged in for a while and then did a BITE and a reset..... it worked normally for a few minutes then stopped and the freq readout says P4OLOLOL...Maybe the battery is in failure mode? Im gonna replace it.

Did you check the ripple on the supplies?  Is the 5V to the computer board in spec and clean of ripple?

If you have not done it already unseat and then reseat all the multi-pin connectors. You may want to give them a shot of Deoxit while your at it. If it works for a few minutes and then dies it may be a component thermal problem so hunting for the bad part with judicious use of freeze spray may locate the problem, otherwise you may be forced to do some good old fashion trouble shooting with a schematic and a scope.

I checked for ripple with a scope across each of the filter caps.  There is a little ripple in the DC but not much..I havent checked the 5V on the computer board....I did the R & R on all the multi pins and the coaxial interconnects.  It ran as it should for about 15 min this morning after I left it plugged in all nite.(power switch off)  Does the memory battery charge with the power switch off?

No, the memory battery is only charged if the receiver power switch is on.
(page 7-33 of the manual, upper right part of the schematic).

When the power supply is on, the memory battery is normally charged
via the +5V regulated supply.  If the +5V regulated supply quit,
memory voltage comes into play from the +12V  regulated
and the +5V unregulated supplies.

73

If the battery has corroded the board, you may have a solder joint that has gone flaky. You might try reflowing all the IC pins on the microprocessor board.

Checking the output of the power supplies at the vertical row of 7 solder pins at the supply output.( from the top down) 19.66V, 5.29V, 21.6V,     11.5V, 15V,  GND, -15V.  The radio actually played as it should for about 10 minutes and then I touched the dial and it went into shutdown....I have the BAMA manual but no schematic...(I'm looking)  New battery is on the way..It was inexpensive and easy to buy from  Amazon. 

This manual has the schematics http://blackradios.terryo.org/documents/manufacturers/Racal/manuals/Racal-RA6790-manual.pdf

For checking ripple on DC lines, I set the Fluke 77 to AC and measure the RMS AC voltage.

You may have a failed/failing/intermittent tantalum capacitor - this thing is full of them.

The 6790 GM is working!  I narrowed the problem to the A6A2 board.  I replaced all the caps (14) on the board...Some of the ceramic caps that were bad had black spots on them and one of the tantalum caps was shorted.  I had to get the caps from 3 separate sources as those tantalums in the values I needed were out of stock at my usual sources.  Thanks for the Forum help and for the PMs with suggestions...  I wanted to report that this project is a success.....Thanks again...  Steve

W3JN knows his receiver scheißen!

OK...Now on to Racal # 2..It is also a 6790GM.  The RX works but with very low sensitivity.   The A2 board is different from Racal#1.  The sealed mixer box contains a card with 4 FETs that I suspect one or more has failed due to overload from nearby transmitter.   In 1988 Racal made a replacement card with a siliconix 5000I chip to replace the card with the 4 FETs in the first mixer box assy.  I have one of the chip cards with a bad chip and one of the 4 FET cards with bad FET(s).  I ordered a new chip(obsolete)  and was able to find the (obsolete) FETs  from UTsource in china.  If I can desolder and solder this thing back together, I may have another working rx.. Has anyone else repaired the sealed mixer box assy on A2 card?   The pics show the FET card (pencil) and the chip card. 
Suggestions welcome.   Steve