IC-761 ALC / Display Power Issue

[K4NYK]

My 'antique' IC-761 will occasionally drop power to the ALC / Display circuit. The frequency (and everything else) disappears and the S-Meter pegs to the right. The service guy in South Carolina described it as being the fault of a 'small DC to DC power supply' which supplies voltage to both the ALC and Display. He's $75/hr and the part is unavailable. It has been my experience that when components, such as a power supply, die... they pretty much stay dead. Since the problem is intermittent, my suspicion is an interconnecting cable. Short of reseating all of the plugged cables (there are plenty), which cable? I'm going to start with the cable to the display in the front and keep my fingers crossed.

 

[Ham4CW]

Hiya

As soon as I read your description of the symptoms I thought "Ah, DC-DC converter!"

He's correct about them not being available any more (certainly not from Icom anyhow). We have two of those sets in the workshop over here in the UK with similar problems and as yet we've been unable to track down any spares

Although it is possible for them to fail outright, it is also possible for them to develop an intermittent fault. Intermittent faults (in those units) tend to be caused by dry joints, and also by the failure of capacitors, causing the internal oscillator to stop running and hence no out put.

There's a -5V and a -33V output from the unit, the -5V is for the ALC and the -33V for the display (if memory serves!)

I have dismantled the units in the past and repaired them, but you will need a lot of patience/luck/time.

Keep in mind that it might not be the DC-DC converter, it could be one of the inputs to the unit causing the problem.

They are fine radios and well worth fixing if you do it yourself, but to have one repaired by a technician can cost quite a lot due to the time involved.

If you need a copy of the service manual for the IC-761 (in PDF format), I can upload one to the site and send you a link to it. There are no internal details of the module itself, but you can of course see what is connected to what externally, what leads connect to where etc.

The IC-751 used a similar system, but it was built around discrete components so much easier to repair. Again, it is worth having a look at that radio to get a feel for what the converter does, as it's a very similar system indeed.

I can supply the info on the IC-751 also if you need it.

73, Mark.

 

[K4NYK]

My first quess was a fault in an interconnecting cable. I have a small audio PA amplifier that has a board on the front panel for input/control and one on the rear panel which is the power section of the amplifier. It developed problems that could be corrected by turning the unit off then on. I took it apart and cleaned both connection ends of the ribbon cable that connected the front and rear boards with De-Oxit and the problem went away.
I bought the 761 direct from ICOM as they had brought it to the ARRL National, held in Atlanta in 1987, for the "Special Events Station". As the past President and the guy who sold the booth space, I had a "leg up" with ICOM.
I'd appreciate a copy of the Service Manual. I might be able to at least locate the troublesome board inside.
I'll check the forum again in a week or so or you can eMail me at LouisSchubertK4NYK@schubie.com once you have uploaded it.
Thank you so much.

 

[K4NYK]

I have located and downloaded what appears to be a reasonable PDF of the IC-761 Service Manual. Section 4-5( b ) on Page 4-19 describes a DC=DC (IC 4) to supply -5 and -33 VDC from 13.8 VDC. Is this the guy that is now unavailable? Could its function be duplicated off-board?

 

[Ham4CW]

Hiya Louis
Sorry for the delay in replying! I hadn't noticed any new post on the site, and I completely missed your reply.

Yup, that is the beasty. As far as I know they are unobtainable, but having said that someone might have on lurking on a shelf somewhere.

Its outline is shown on page 7-3 of the manual "Display, Keyer and Sensor Units", and appears on page 9-2 "Display Unit" schematic. There's no internal structure shown (because they class it as a 'module'), but a very similar idea was used in the IC-751.

The IC-751 used discrete components though, and appears on page 8-6 of the service manual. Q4+Q5 are the main oscillator section, and Q6+Q7 are to do with control/output (at a quick glance).

Yes (to answer your question), you could build an external circuit to reproduce the functions of the module, the current requirement is very low (a few milliamps if I recall correctly), so maybe one or two 555 timer ICs using diode pumps/multipliers might work as a quick solution. There's most like other off-the-shelf devices available nowadays that would fit the bill too.

I think that the module used a transformer (as per the IC-751), but you could most likely do away with that idea, though you would need to check to see if it's OK to feed the output voltages from ground, or, if they have to be totally isolated from ground i.e. floating.

Hope the info helps, and again, sorry for the delay!

Best 73, Mark.

 

[K4NYK]

Mark.. Has it been your experience that this DC/DC converter's problem can be sporatic? This 761 will operate OK for a couple of days before the problem recurs and then it may be several days and begins to function again only to have the process repeat itself. I've never had a piece of solid state equipment that will work just fine on Monday and Tuesday but goes kaput on Wednesday only to work again on Saturday. My first go will be to locate the connector which supplies the +12VDC and clean it up well.
(QUOTE: "Toto, I don't believe we're in Kansas anymore!")
I have a HamFest coming up next month and I'll try to locate the necessary parts to create my own -5VDC & -33VDC supply. The worst than can happen is that my 'fix' won't work and, so long as I haven't spent a small fortune or time, I haven't lost that much.

 

[Ham4CW]

Hiya Louis

It could be just a dry joint causing all the problems.

An easy way around finding the fault, or rather the result of the fault, would be to solder a few wires to the set at key points i.e. one from the 33V rail, one from the -5V rail, and one to the 12V rail feeding the DC-DC unit.

That way you can make a quick check of the voltages when the fault occurs.

For example if the 12V feed in to the unit was present, but the -5V and -33V rails were missing, it then points to the module being faulty.

If on the other hand all of the Voltage rails are OK, but you still observe fault conditions, you can at least rule out the module and start looking elsewhere.

Sometimes it's worth running a soldering iron over the joints connected with the operation of a unit, as over the years I've had a number of dry joints that were not visible to the naked eye, but never the less caused devices to malfunction.

Sometimes the PCBs have via holes linking top and bottom layers together, and I have known those to fail due to age or poor initial manufacturing techniques.

As mentioned above, very often it's just a dry joint causing the problem, though the down side it that the dry joint may well be within the DC-DC inverter module.

Best 73, Mark.

 

[K4NYK]

Research results via a "Google" search
Symptom: Intermittent display, S-meter may peg and RX/TX may disappear when the unit is in the failure mode.
Probable cause: Cracked solder beneath the DP-6 DC-DC converter on the Display unit.
Cure: Carefully remove the DP-6 converter from the Display unit and resolder the pins of the IC to its circuit board. Then reinstall the DP-6 back on the Display unit.

The DP-6 was a cinch to locate once the cover was off (first time off in 21 years). Only two screws hold the display unit in place. I will remove the DP-6 and see what lies beneath. If I can fix this without resorting to an offboard P/S, so much the better but that offboard guy is my "Ace in the Hole". I will keep you informed and, again, thanks for your help.