Icom IC-720A / MCTR 200

[Sparky67]

Hi All,

I have just got an IC-720A (which has been in storage for a long time and which only lit the meter lamp) running again (R115 on Main board o/c), but now have about 20dB low Rx sensitivity and poor Tx RF output.

Reading various forums it appears the rotary relay in the filter box is often suspect, so I have opened that up to have a look. Sure enough it looks very tarnished so it has been left soaked in switch cleaner overnight. I need to do more work on it but the moving contacts look very fragile and the copper track looks very thin. The forums say 'clean the relay' or 'replace it with the conventional bank-of-relays mod' but so far I haven't found any suggestion as to the best way to clean the relay so the set can be proved to be OK!

Has anyone sucessfully cleaned one of these? If so, what method did you use?

Cheers,

Martin
G4NCE

[g4wim_tim]

Many years ago I had a 720A with the same problem - no amount of cleaning provided a satisfactory fix so I opted for the nuclear option and completely removed the rotary switch and replaced it with relays driven from a ring counter - effectively replicating the function of the rotary relay.

Worked great and was a lot quieter as well !

Maybe yours is not as bad as mine was and will respond to a good cleaning.

[Sparky67]

After the switch cleaner I ran some 1200 grit over the contacts in-situ to clean them up. The filter switchery seems to be working OK at the moment but I have yet to find out how long it will last. Suspect I will have to bite the bullet at some stage and do the filter selection (and the associated switch position feedback) relay mod... Did you follow one of the published designs or design your own mod?

The Tx seems to be healthy but I am chasing about 10dB of sensitivity compared to my TS-430S and the S-meter won't calibrate as per the service manual. USB reception works fine but I have noticed LSB is quite edgy and distorted, something else to look into. This set could be on the bench for a while!

Cheers,

Martin
G4NCE

A PCB maker (surprisingly quite a few left here in the UK) will hard gold plate the contact ring for a few quid. The only problem I had with my old 720 was the TR relay, went a bit high resistance on RX, a quick poke of the transmit key cleared it for a bit. A very good set and fixable. The tuning encoder is one of the best I have encountered, almost a real one! It was my first self contained rig, bought second hand in 1986 and given away in about 2000 to the local radio club, it is still working well.

10dB down, is the RF pre-amp working?

[Sparky67]

Hmmm, that’s something worth considering. A quick Google threw up quite a few companies offering a gold-plating service within a few miles of here...

The pre-amp has been taken out, cleaned and a few solder joints reworked. Injecting a signal at the input and output via a cap (there’s a filter-diode-switching voltage on the output) shows it’s working OK.

The 720 has a stack of three boards (IF, RF and PLL), all in tin boxes and screwed together, which were all loose when I got it, so maybe a suspected fault in there has been investigated, or the alignment has been adjusted?

This particular radio is an MCL-modified Icom 720A (MCTR 200) with a wider SSB filter and different BFO xtals. The mod sheet says the freq displayed has to be offset by +700Hz for USB and -700Hz for LSB. It actually receives OK with the carrier freq displayed on USB and LSB is difficult to resolve at any displayed freq! That’s the next thing to look into...

Cheers

Martin
G4NCE

[Sparky67]

Originally Posted by Sparky67

This particular radio is an MCL-modified Icom 720A (MCTR 200) with a wider SSB filter and different BFO xtals.

Now I have looked closer at this Marlborough Communications-modified radio the BFO xtals are the standard IC-720A fit although the USB / LSB injection freqs (and freq on the the main tuning display) are set 700Hz different to cater for the wider SSB filter, according to the MCL notes. They won't pull that far on this radio, for some reason.

The MCL notes also say the SSB bw is now <300Hz to >3kHz at -4dB. The track under the xtal filter has certainly been worked on (although standard Icom 2.3k FL-30 filter label is still on it) so I'm thinking it has been taken out and modified internally. I plan to revert that to a 'normal' spec filter at some stage.

Second fault found yesterday, 2.2 Ohm resistor (R15) in the RFPA driver quiescent Ic control cct was oc. The Ic setting pot was fully clockwise and the quoted 100mA was more like 20mA at best. Hmmm...

Next job is that rotary relay...!

Cheers

Martin
G4NCE

[Sparky67]

Before I look at the rotary relay I decided to buy a *normal* Icom FL-30 SSB filter (from Israel) and fit it. The set is transformed, USB and LSB both now resolve normally, the S-meter has burst into life and it sounds a whole lot better too.

The sensitivity of this set is now about 6dB lower than my TS-430S. Comparing the two, I think the latter is too sensitive for my suburban location, with a lot of man-made noise coming in via the antenna. So I will at some stage add a 6dB pad to the Kenwood radio Rx input and see what effect that has...

Cheers

Martin
G4NCE