Topics

Transmitter Adjustment

sv1dpp
 

Hi all,

today I tried to adjust my IC-7000's  transmitter according to the service manual but in step 3 (Final Idle Cur(HF/50M)-2) page 4-3, I found a problem in the regulation of the current so it does not change the current value and remains the same or changes very little (00 value = 1548mA that change to maximum 1610mA)

The problem occurs only in this step, in other steps all are OK

Any help please for what may be the problem?

Many 73

Spiros, SV1DPP


Steve W3AHL
 

It is important that the -1 and -2 Idle Currents match in push-pull amplifiers to minimize even harmonic distortion (even if the circuit will produce its rated power output).  The -1 and -2 adjustments set the ID1V and ID2V bias voltages respectively.  

You could try to measure the bias voltage on Q301's gate while doing the adjustment.  If it varied similar to how Q302's gate voltage varied when doing the -1 voltage, then you know the bias adjustment D/A converter circuit is working.

But usually the problem is a defective FET.  And if the problem is a breakdown between the Gate and S-D channel, then the bias voltage may not vary as it should due to the internal FET failure (even though it might seem that the bias voltage circuit was at fault, which it usually isn't).

With the DC power disconnected, use an ohmmeter to compare the gate to source and gate to drain resistances  of Q301 and Q302,  If they differ significantly, then the FET may be bad.  There's always a chance that other components in the PA circuit have failed, but 90% of the time its the FET.  I usually replace both, even if only one appears to be the problem.  

Does your 7K have a specific problem that caused your to check the adjustments??

Steve, W3AHL

---In ic7000@..., <sv1dpp@...> wrote :

Hi all,

today I tried to adjust my IC-7000's  transmitter according to the service manual but in step 3 (Final Idle Cur(HF/50M)-2) page 4-3, I found a problem in the regulation of the current so it does not change the current value and remains the same or changes very little

 (00 value = 1548mA that change to maximum 1610mA)

The problem occurs only in this step, in other steps all are OK

Any help please for what may be the problem?

Many 73

Spiros, SV1DPP


sv1dpp
 

Thanks w
The problem was a damaged RD70HHF1 and repaired
73
Spiros, SV1DPP

sv1dpp
 

Hi Steve And Paul,
Thanks for the response
yes the Q301 was defective.
I replaced with another one and all are OK
but I have ordered two new matched RD70HHF1, so I will replace both
Steve one day I found that the output power on  160M and ONLY in this band was 70W
in all other bands the power was full 100W, so I went to check
Also the circuit design in 7K does not contain active components around the FETS as in 756, so such damages are obvious and the mind must go straight to the defective FET.
Many 73
Mary XMAS and HNY
Spiros, SV1DPP

Steve W3AHL
 

It's not uncommon for either 160M or 10M to first show low TX power output when the HF FET's start to fail.  Usually the other bands will eventually start to degrade as the gate to channel defect worsens.

The gain of the HF PA seems to be optimized for 14.3 MHz, since typically the Driver stage only supplies about 6.5 Vp-p to drive the output to 100 W CW at that frequency.  For 1.8 and 28.4 MHz the Driver outputs about 12.4 Vp-p for full power.  The gain versus frequency variation is accounted for by the TX Total Gain (HF1)-(HF3) & (50M) adjustments.

Glad it was an easy fix!

Steve, W3AHL


---In ic7000@..., <sv1dpp@...> wrote :

Hi Steve And Paul,
Thanks for the response
yes the Q301 was defective.
I replaced with another one and all are OK
but I have ordered two new matched RD70HHF1, so I will replace both
Steve one day I found that the output power on  160M and ONLY in this band was 70W
in all other bands the power was full 100W, so I went to check
Also the circuit design in 7K does not contain active components around the FETS as in 756, so such damages are obvious and the mind must go straight to the defective FET.
Many 73
Mary XMAS and HNY
Spiros, SV1DPP