Notes above and below this text, is information that is freely viewable thruout the internet and can be viewed by anyone!

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Adjustments:

Adjust L32, L30 for maximum power

CT7; Maximum SSB power

VR8: AM power adjustment

VR6: Mic gain adjustment

VR7: Adjustment for 100% modulation

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10khz jump:
Cut the track that goes to pin 13.
Solder 4.7K across the cut.

Install SPDT center off switch

Wire the top pin to earth.

Wire the middle pin to pin 13 on the PLL.

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Modulation upgrade:
Locate D23 (next to VR7). In the Realistic rigs a 220 ohm resistor is in the D23 location. Replace this resistor with a 1N914 or 1N4148 diode. (observe polarity). This change will make the Realistic 858 mod limiter the same as all of the other brands. Really opens up the audio. Replace C87 with a 10uF 16v electrolytic cap.

Replace C179 if it's still the original tantalum.
Electrolytic 35-50 Volt replacements fine. 

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RF Power MOD:

1. Unsolder banded end of D21 and D22.
2. Add a 4.7K resistor across where D21 and D22 were removed.
3. Change R78 (15K) to 33K.
4. Change R84 (4.7K) to 2.2K.
5. Change C174 (56pf) to 82pf.
6. Change C173 (180pf) to 220pf.
7. Re-tune L32, L30, L29, L28 for maximum Forward Power.

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Open clarifier:
Clip one side of R119 and D30 and move body up out of that way.

The resistor that feeds the clarifier gets removed and moved to a source of continuous 8 Volts DC that stays on for receive and transmit both.

locate the tie point for the 8V positive
- you lift D30 (kill TX) and R119 
- with the lift side being from the 8V RX side 
- that lifted leg is tied to 8V 
- the ground left alone will keep the slide the same.
- The clarifier is regulated by fixed input voltage 
- so R119 is the INPUT to the regulation.
- So if you wish you keep center slot -
- lift R119 from the 8V RX side and solder a wire from your 8V Regulation or PLL's Pin 7 or 12 that 5V power supply.

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Also you can build your own voltage regulator for THAT circuit using a LM7808 with a 330 ohm resistor in series to the ground TO the regulator.. adjust all corresponding varactor variable cap to center 12 o-clock. if need be, use a 10-15 choke in series with the cathode to further your center position.

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Known problems: 

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No AM TX, on SSB TX the relay chattering, Replace C179 with a electrolytic capacitor:; 2.2Mfd/25v; 

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Power supply:
Replace Power Supply Filter capacitor with 3,300 mfd/25v

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RX/TX Relay:

The relay is obsolete.

If the contacts get noisy, a strip of paper moistened with contact cleaner may clean it up. Any kind of abrasive will ruin the electroplated contact surfaces inside it.

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Relay Issues ("Going Deaf" or Stuck Transmit):

The mechanical receive/transmit relay frequently collects dirt, corrosion, or pitting on its contacts, causing the radio to go "deaf" (lose receive audio) after transmitting or fail to switch voltage properly to the transmitter.

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Zener Diodes and Regulators:

Specific Zener and switching diodes (such as D54) around the power regulation and switching circuits are notorious for shorting out, which can burn up adjacent resistors and cause erratic voltage or complete transmit/receive failure.

 

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Dirty Channel Display / Header Contacts: The internal ribbon cable or copper header contacts for the digital channel LED display tarnish or loosen over time, causing a dim, flickering, or non-functional channel readout

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Mode and Function Switches:

Mechanical rotary and push switches (including the mode and PA switches) get oxidized and dirty, causing intermittent audio, frequency jumps, or failure to latch into specific bands. 

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Internal Power Supply Age:

Aging capacitors and original linear transformers often need recapping or replacement with modern switch-mode power supplies to restore clean DC voltages

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Weak Signal / Faulty TR5 Transistor:

A major culprit for a sudden loss of receive sensitivity is a failed TR5 (2SC1730) receiver transistor, which is commonly replaced with a modern 2SC2999 to restore performance

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Fragile RF Interstage Inductors:

In the transmit RF amplifier strip, the tiny toroid L36 (driven by FET6) is physically fragile.

If an internal leg breaks or goes open-circuit, you will see proper 8V DC on the drain of FET6 but an entirely dropped, unnoticeable AC signal on your oscilloscope.

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PLL & VCO Lock Faults

• Free-Running or Stuck VCO: If the radio display becomes wonky, channels skip, or it fails to lock, check the lock detect pin on the uPD858 chip. If the error voltage is pinned entirely high (~5V) or completely low, the Phase-Locked Loop (PLL) has lost lock.

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​ Failed UHIC-001 Oscillator Block:

The companion UHIC-001 oscillator block often breaks internally or drifts out of spec.

Swapping it for a UHIC-003 block is a common repair that also grants a wider frequency range if the radio is modified

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Transmit Failures & Blown Finals

• Shorting Tantalum Capacitors (C179):

• The 2.2µF tantalum capacitor (C179) is arguably the single most common failure point.

• When it ages and shorts out, it instantly destroys the final or driver transmitter transistors.

• recommend replacing all 2.2µF tantalums (specifically C55, C68, C87, C179, and C211). 

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• Failed Power Switches & Regulators:

• The factory-installed mechanical B+ power switch has a high failure rate due to internal arcing.

• Additionally, the stock 2SC1419 voltage regulator (2SD325E in Realistic models) regularly fails.

• This was usually caused by degraded, thick white ceramic/mica insulators and dried-up thermal paste causing the transistor to overheat.

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"Hacked" Modification Damage:

Because the uPD858 is incredibly popular for channel-expansion mods, many circuit faults are actually user-inflicted. Sloppy trace cuts, shorted wires on pins 19, 20, or 21, or ungrounded logic pins frequently cause the chip to behave erratically or fry entirely

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Realistic TRC458 "Upgrades"

Suggested replaceing the following parts:

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C88  4.7uF/10v
C89  33uF/16v
C92  33uF/16v
C94  470uF/16v
C95  470uf/16v
C98  1uF/50v
C99  10uF/16v
C101 100uF/10v
C102 220uF/10v
C104 330uF/10v
C107 1000uF/16v
C110 0.1uF/10v

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D24  CZ092
D25  BZ075
D26  YZ037
D28  CZ092
D31  WZ061
D32  WZ061
D44  CZ092

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R110 470ohm
R111 390ohm
R112 33ohm
R113 150ohm

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TR24 2SC1418B
TR25 2SC1419B

TR26 2SC1419B - Suggest move to main chassis for extra heatsinking

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