MODIFICATIONS AND REPAIR OF ICOM IC-R70 AND IC-R71 RECEIVERS



March 19 2000


Almost all modification and service instructions required by a "normal DX listener" have now been added to these pages. Among others, there's more about battery replacement, display problems and 12 V DC connection.

In this article I'll concentrate mainly in the IC-R71, which seems to be much more common than the older R70. Even though the R71 is over 10 years old, completely unmodified units show up now and then.

In this picture, the original memory card is being replaced by an ICM-1024 memory card - probably already difficult to purchase these days. With the new card, the number of station memories is increased significantly and the "muting problem" caused by an empty battery is fixed.

When the battery voltage drops to a critical level, random malfunctions usually show up. The malfunctions are often mistaken for other types of faults, so it is advisable to always check the battery first. For example, in some units, a wrong front-end band filter will be selected.

Some modifications are considered mandatory by DX listeners, and the notch and RF-amplifier have often been modified right after purchase. There seems to be no reason why the notch shouldn't work also in AM mode, and similarly, anybody can switch the RF amp on and off using the front panel switch even on the MW band. Obviously, Icom considers the AM listener to be an idiot, who thinks that the RX is faulty if the receiver overloads on MW or if the notch control is in the wrong position.

During the past years, I have modified and repaired over a hundred R71's, and less than ten R70's. During this time, I have noted some typical problems.

SOME TYPICAL FAULTS IN IC-R7X RECEIVERS

In the Icom IC-R7X many problems have been caused by bad solder connections - always keep that in mind when looking for faults.

BFO PROBLEMS AND ADJUSTMENT

A common problem is a large shift of the BFO's frequency. In many receivers the adjustment range of the BFO's USB trimmer is insufficient. I have carefully cut off a piece of the ceramic trimmer, to get a smaller capacitance. The USB crystal is located too near the af amplifier heatsink, causing a long warm-up time before the frequency stabilizes. A piece of styrox can be added between the af power amp heatsink and the BFO frame to reduce heating of the crystal.

The BFO frequencies should be checked and adjusted when necessary, to place them correctly on both sides of the IF center frequency. If misadjusted, zero beat on AM is not at the correct frequency reading, and the BFO frequency is incorrect with respect to the IF passband, which is especially noticeable on smaller bandwidths.

The BFO is located right behind the audio section when wieved from the front. The BFO unit can be identified by it's two crystals. Let the receiver warm up an hour or two before making adjustments and keep the lid or other object on top of the receiver to reduce heat transfer. This ensures that the adjustments are made as close to normal operating temperature as possible.

To adjust the BFO, you'll need a frequency counter that is accurate at least to the 10 Hz digit at 9 MHz (1 ppm). Connect the counter to R139 and set the receiver to USB mode. If the frequency deviates more than few tens of Hz from 9013.0 kHz, carefully adjust C78 using a non-metallic screwdriver. You should be able to get within a few herz of the correct frequency, at least for short term. LSB frequency is adjusted to 9010.0 kHz with coil L20. If needed, RTTY mode can be adjusted with coil L22 to 9008.475 kHz. CW mode has no adjustment; the frequency should be 9009.8 kHz +/- 200 Hz.

A factory service note promises a 5-fold improvement in BFO stability by gluing a PTC resistor to the BFO crystal and feeding the resistor from the 13.8 volt supply rail.

CHECKING SYNHESIZER FREQUENCIES AND VOLTAGES

In addition to BFO adjustments above, reception quality and frequenqy display accuracy can be improved by correct adjustment of the synthesizer section. In older receivers, which may have been in use for 15 years, the synthesizer adjustments may be off their optimal values. This is easy to check and re-adjust using a frequency counter, digital voltmeter and an RF voltmeter.

In this context it may be noted that if you have an Evdis PLAM board, do not perform adjustments according to the instructions that come with the Edvis board. This will cause SSB carrier frequency to be off by 150 - 300 Hz with respect to the PLAM mode. Instead, you should first adjust the synthesizer in SSB mode according to Icom's instructions, and after that, adjust the PLAM mode using only the trimmer on the PLAM board. When adjusting the PLAM, do not touch trimmer R20 on the matrix board (this trimmer is on the bottom side of the receiver, right under the VFO).

 

VFO PROBLEMS

Improper operation of the main frequency control (VFO knob) may be caused by malfunctions in the pulse encoder and it's associated electronics. When the frequency control knob is rotated, this encoder outputs signals SV1 and SV2 to connector J6 on the logic board, which resides under a shield plate on the bottom side of the receiver.

When checked with an oscilloscope, these signals must be 5 volt square waves with a 90 degree phase difference. The frequency of the square waves depends on the rotation speed of the frequency dial. The phase difference carries information about the direction of rotation. This same principle applies to almost all modern receivers with a pulse encoder (optical or otherwise) on the VFO shaft.

In one case, a frequency dial problem was caused by a bad solder connection in the tuned circuit between pins CLK1 and CLK2 of IC2 on the logic board. This made the oscillator run on a much lower frequency than normal, causing slow and intermittent operation of the main frequency control.

POWER SUPPLY PROBLEMS

Heat in the power supply has caused some problems. It is worthwhile to check the mains transformer primary voltage setting and set it to the highest voltage available (240 V / 120 V). In the original circuit, voltage after the rectifier bridge is close to the maximum voltage of the filter capacitor. The bridge may also run real hot. It will cause no harm to replace the rectifier bridge with a larger and better cooled one.

It is advisable to install insulation tubing over the leads in the mains primary circuit. These leads run in a confined space between sheet metal and the PCB's foil side. This can produce quite a catastrophe if mains voltage is short circuited to the low voltage parts of the receiver - for example, when a power supply fault causes enough heat to melt the insulation, or a component lead on the PCB punctures the insulation.

Bad solder joints on the power supply PC board have caused various problems - mainly in the power transistor and rectifier bridge. The construction is such that especially the power transistor's solder joints are subject to mechanical stress during transport of the receiver.

Some of the solder mask around the rectifier bridge's solder joints should be scraped off to make the solder connections larger and more reliable.

In the picture above, you can see the rectifier bridge solder joints which do not look too good. You can also see how difficult it is to make the solder flow properly in the small soldering pad area, so the scraping mentioned above is in order.

In later receivers, the external 12 VDC connector on the back plate is not installed at the factory. Installing the connector requires some clever juggling. Obviously, the hole in the back plate is not meant for this kind of connector, or the hole is machined wrong. Using cutters and a file, it is possible to install the connector. Note that when the receiver is operated from the mains supply, a dummy plug must be installed in the connector. It's purpose is to connect the output of the mains power supply to the receiver. If you do not have the original DC supply lead, it is possible to use the dummy plug to construct the supply lead. If you want to do so, first remove the short circuit loop from the plug. Then connect and external 12 DC supply to the plug, with positive terminal on the side with a pointed edge, and negative terminal on the side with a flat edge. The two center pins must remain unconnected.

Install 4 - 6 amp fuses in the DC lead. In mains operation, you must re-install the short-circuit loop.

DISPLAY PROBLEMS

In some receivers the display causes disturbance (false signals) in reception. This problem is often alleviated just by moving around the cables close to the display to minimize display noise. A factory service note advises to to cut a blue lead (W6, connected to pin 6) from the display unit and to insert a choke in series with it. The manufacturer of the choke is Taiyo Yuden, type LAL04NA820K. You may also experiment with other chokes in the 1 mH range.

REPLACING THE MEMORY BACKUP BATTERY

Remove the sheet metal plate on the underside of the receiver. Under it, you can see a PC board with a daughterboard, which contains a RAM chip and the battery, which is soldered in place. Icom has used at least two different kinds of RAM.

Remove the daughterboard by unscrewing the small phillips head screw and pulling the board out of it's connector. Measure the battery voltage - for data retention, it should be over 3 volts. The voltage does not provide an indication of how soon the battery will be completely drained - this kind of battery will have almost full voltage up to the end of it's life, after which the voltage drops quickly.

While replacing the backup battery, connect a 3,5 volt source in parallel with it, so the RAM will have uninterrupted operating power throughout the replacement procedure. It is worthwhile to connect the source far enough from the backup battery, so you won't melt the solder connections while soldering the backup battery.

If you cannot obtain an identical replacement for the battery, you can improvise and use another type of battery, as long as the voltage is correct.

If the battery (and therefore, also the RAM) is already empty, the radio will not be operative. The RAM board must be sent to a repair shop for replacement of the battery and reloading of the software. This shouldn't be very expensive, but it is advisable to check the cost before sending the board. The board should be back with you in a few days, equipped with a new program that (at least in theory) enables you to receive from 0 to 50 MHz.

AUDIO MODIFICATIONS

The audio quality of IC-R71 leaves something to be desired. While not an expert on this subject, I have in some receivers changed the loudspeaker coupling capacitor value. Kiwa Electronics (US) has a modification kit for improving the audio.

THE MOST COMMON - AND MOST NECESSARY - MODIFICATIONS

The simplest IC-R71 modifications in short:
1. The RF-amplifier can be forced on by cutting diode D23 in the RF-unit, which is located on the right hand side of the receiver.
2. The low band attenuator can be disabled by shorting R11 and R12 and cutting R13 in the aforementioned RF unit.
3. The notch can be made operative in AM mode by changing the circuit as in the following diagram.

 

ADDING MEMORY CARD ICM-1024

Although the Willco Electronics memory card is no longer available as new, I have attached a few pictures of the connections. In one case, it turned out that an uninstalled memory module was at hand but without any documentation.

 

In the following picture you can see how the memory module is connected

 

In this picture, an example of switch wiring

 

Finally, a list of connections to the IC-R71 receiver

 

More complete documentation is available from the webmaster upon request.

FILTER MODIFICATIONS

I have designed several alternatives for switching the IF filters. In the original circuit, although there's a filter selection switch on the front panel, filter selection is also controlled by the mode selection. This is somewhat limiting.

To have more options in selecting filters, some modifications around IC3 on tha main board are necessary. All filter combinations are not useful in DX-listening, so the modification presented here is limited to three choices, which should be quite sufficient. Four choices could be made available, but this would not provide additional value unless actual filters were also replaced.

The modification is probably best described as a series of steps as follows. In addition to tools and wire, two diodes are needed - 1N4148 or 1N914 will do.

The modification is performed on the main board, in the vicinity of IC3 (4051)

1. cut diodes D41, D42, D47, D49, D50, D51 and D52
2. cut the cathode leads of D44 and D43, and solder the cathode of D44 to where cathode of D43 was
3. solder a new diode from pin 13 of IC3 to the cathode of D44
4. cut diode D40 and solder it's anode to IC3 pin 13
5. cut cathode of D39 and solder a new diode from pin 1 of IC3 to where the cathode of D39 was
6. cut diode D33 and solder it's anode to pin 1 of IC3 using an extension wire
7. turn the filter switch to position "on"

You should now have three combinations of filters available. The functioning of this modification is explained in articles found in the list of magazine articles (DX-Kuuntelija and Radiomaailma, in finnish).

Many users have not been satisfied with the performance of the original IF filters. Relatively high-priced replacement filters are available for both of the two last IF's. The wide filter can be considerably improved by replacing it with a narrower one of higher quality. A suitable bandwidth would be 3-4 kHz, depending on personal preference. Space is provided on the PCB for a FL-44A crystal filter for improving the SSB mode. Before installation, the original filter must be removed. An old Herz brand filter will fit, if you can locate one; please note that in this case, isolation capacitors must be installed due to the different internal construction of the Herz filter. The PCB foils leading to the filter must be cut and bridged with 10 nF capacitors. The value of these capacitors is not critical.

When replacing the wide filter with a new and preferably metal-cased filter, the larger filter will not fit in the original filter's space. It can be mounted by soldering carefully at both ends to the shield plate adjacent to the filter area. You'll need to bend some components to make the filter leads as short as possible. Connect together the ground pins of the filter and the pin connected to the filter's case. Use insulated tubing where the filter leads pass close to other component's leads. You can also install the filter on the foil side of the PCB, provided that it does not hinder the board's installation back into the receiver.

Suitable filters are Murata CFS- and CFK-455 series models "I" (4 kHz) and "J" (3 kHz). The CFS-series filters may be difficult to obtain as new, but they, as well as the Herz filter, can be found in some modified Trios.

BACK TO THE TECHNICAL INDEX