Kenwood R-1000 audio output IC kaput & interesting wide/narrow circuit

[Opcom]

The Hitachi HA1368R used as audio power amp IC Q28 in the R-1000 has died.

I suspected it was going a couple years ago due to no audio at power-up then the audio popping on. because I am a miser, I let it fail and got every cent of value from it. 1981-2011. 30 years. That is a quality product.

The part is no longer made and seems to be costly from various brokers who have kept the remaining WW stock safely enshrined. One wanted >$40. Get the incense out.

I went ahead and bought a little Velleman kit from the local surplus establisment, Tanner Electronics. It is the K4001 7-watt amplifier. It uses the STMicroelectronics TDA2003. This IC is also an older type and is NRND, but it's a very good one and having a TO220-5 case it should be able to be mounted by the tab almost anywhere. It does not look like the pins will fit anywhere near correctly in the old Hitachi's place but all that needs done is to mount the board and supply GND, Vcc, audio in, and audio out. I figure I can use a bit of ribbon cable to run from the appropriate points on the main board to wherever the new amp can be conveniently mounted. The kit even comes with about a 1/2" long bolt for the device and included heatsink mounting.

The TDA2003 is a quite higher current IC being able to drive a 1.6 Ohm load to 12W so I hope it will last a long time with the 8 Ohm speaker in the set. besides ST, I also found a datasheet from Contek Semiconductor and one from another competitor, so availability will be OK as long as I don't make a habit of blowing them up.

The small stuff:
The R-1000 gives an audio output rating of 1.8W to 8Ohms at 10% distortion. The raw DC supply is 14V, less 1.2V or so for junctions ought to make more like 2.5W.

The set runs the VCC at 14VDC but through a 5.6 Ohm resistor (possibly this is acting as a flamepoof fuse and current limiter in case of a temporary short). It also seems to be part of an RC filter, having a 1000uF capacitor after it. Whether there is enough hum on the supply to worry about or not is a subject for debate. There is little or no audible hum from the unit at low volumes.

1.8W to 8 Ohms is 10.73 V p-p, or 3.8V RMS @ 475mA. I assume the IC current (less quiescent) is therefore 475mA.

A 2.66V drop would occur across the 5.6 Ohm resistor, bringing the 14V down to 11.34VDC. I'm thinking about getting closer to 2.5W by changing the 5.6 Ohm resistor to something like a 1 Ohm 1/2W flameproof. The average audio ought not pull enough current to upset that size resistor. It may be useful to find an 5-10mH inductor of <2 Ohm DCR to use in place of the resistor and keep the 1000uF bypass cap.

The TDA2003 datasheet from 1995 says it has built-in 40V over-voltage, short circuit, and thermal protection. The HA1368 has over-voltage, reverse voltage, and thermal protection, don't know about shorts. It looks like the TDA2003 is a good fit in most ways. The voltage gain of the TDA2003 is about half that of the HA1368 if the application diagrams are followed

The important stuff:
One odd thing about the R-1000's audio amplifier is the AM wide/narrow switching. The SSB (2.7KC), AMn (6KC), and AMw (12KC) IF filters are all 455KC, and the conversion to AF baseband is done by diode detection for AM or by injecting a 456.6 (USB) or 453.4 (LSB or CW) KC signal into a product detector.

Besides having separate AM wide and narrow 455KC IF filters, the wide/narrow switch is also sent to the voltage feedback circuit of the amplifier. There is an internal 180 Ohm resistor from the (internal) inverting input to HA1368 pin 5.

The HA1368 and the TDA2003 are really both a kind of high power op-amp.
In the Kenwood circuit with the HA1368, there is a 4.7uF cap to ground at pin 5 which seems like it would have an unusually high high reactance at bass frequencies. In a normal circuit this would be larger.

The AM wide selection adds a 47uF cap in parallel with the 4.7uF cap. This would seem to normalize low frequency response due to shunting more bass away from the inverting input. In this way perhaps the narrow mode shaves off the low bass in the audio amp while limiting the bandwidth at the IF filter, helping avoid an over-bassy sound in AM Narrow. Apparently someone thought of fidelity, as in "lowest freq * highest freq = 400,000" as observed by some audio folks even if the internal speaker could never reproduce it all.

The HA1368 has an internal gain-setting divider made of a 40K resistor and the reviously mentioned 180 Ohm resistor in series tied from the output to pin 5 AC GND. The gain is set at 222.

The TDA2003 uses an external circuit essentially alike, 220 Ohms/2.2 Ohms (gain=100) except there is no capacitor from the inverting input directly to GND, so the transistor switch might not be able to be used, and putting one across a 2.2 Ohm resistor won't make a useful difference. Some experimentation will be necessary to keep the audio bandwidth circuit operational.

[k4kyv]

I have an R-1000.  Been using it for years for SWL purposes, and it can be brute-force fed to receive down to 150 kc/s, even though the nominal frequency range cuts off at 200 kc/s. Using an outboard ant tuner and my 160m beverage, I have picked up some of the European LW AM broadcast stations, at times at good entertainment quality.

For best performance, particularly on the LF band, you need some real rf selectivity ahead of the wide-open band-pass filters in the front end in the receiver.  Otherwise expect it to be full of images, birdies and buzzies. For its class, the audio from mine is quite good.  I hadn't delved into the theory of operation, but I have observed that with the narrow i.f. filter switched in, the audio response is restricted.  Not a bad idea, since it reduces noise outside selectivity passband.

I just wish it had independent selectivity and mode switches.

This is a classic example of what we face with modern "state-of-the-fart" equipment. Replacement IC chips, particularly the manufacturers' proprietary ones, if they don't become outright unobtanium, will be a lot harder to procure than replacement tubes and other components for hollow-state equipment, or even solid-state stuff that uses only discrete components.

In a matter of just a few years, all it will take will be an uneventful lightning surge or some random component failure to turn that $10K Yaecomwood into an expensive door stop or antenna flinging weight.

Wonder how many of those rigs will still be in use 75 years after they were originally purchased?

[Pete, WA2CWA]

The HA-1368 and 1368R are replaced by the NTE-1283. A quick Google turns up several places selling them for $3.99 plus shipping.

[flintstone mop]

GREAT!!!

I forgot that ECG or NTE substitutes stiil around.

How's that Patrick?? $3.99

GOOGLE is my friend toooooooooo

[Opcom]

Well that just sucks all the fun right out of the project! Thanks, but there is a problem. The unit uses the HA1368R, which has the pins in the reverse order. The purpose of that is so two (plain and -R) can be mounted in some symmetrical manner to a heat sink for a stereo amp. The NTE1283 is a cross for the HA1368 as far as I can tell.

[flintstone mop]

oooops
NTE-1283 HA1368 5.3W audio power, 4.95. NTE-1284 HA1368R 5.3W audio power

There you be Patrick NTE 1284 for the HA1368R

Fred

[Opcom]

Cool! I found it at vetco.net for $9, and I also ordered 1/2 lb each #20 magnet wire and #18 hookup wire, as well as de-ox-id and a few other things. They had three of the IC in stock. The magnet wire, 1/2 lb #18, will go to a plate choke project.

That is about 100FT. of wire accdg to the AWG table. #18 is on the large side but an old fart who saw the 3X3000 amp told me #18 instead of #20 for that. I'm sure #20 would work, rated 1.46A, and #18 is rated 2.32
OK my middle name is Overkill but they call me Overload sometimes. Or is it Overlord? I keep getting them mixed up with all the suthern accents around here.

What do you think of the prices at Vetco?
2 x     De-Ox-Id Contact Cleaner     $17.90

1 x    NTE1284 - NTE1284    $8.91

1 x    20 AWG Solid Single Conductor 100' - Color: Grey    $14.65

1 x    18 AWG Magnet Wire 1/2lb    $16.45
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