HMV Roadhound TX77 Repair

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thunderc4s
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First Name: John
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HMV Roadhound TX77 Repair

Post by thunderc4s »

Radio Make and Model: HMV RoadHound TX77
PCB Number: PTBM048AOX
PLL Number: PLL020A
Test gear available: DMM, Frequency Counter.
Symptoms:
No RX or TX
What is working:
Everythink looks in place on PCB
What has been tried:
The PLL will not lock - PIN 6 is approx. 1.2 volts and not 5 volts.
I have replaced the PLL020A Chip and both the mixer IC's (IC 2 & IC 3).

How can I check which part of the PLL/VCO circuit is causing the unlocked PLL condition ?

Thanks,
John - Melbourne Australia
zodiac
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Re: HMV Roadhound TX77 Repair

Post by zodiac »

I don't have a schematic for that radio but check the PLL's pin 1 to see if it has 5v.
If it doesn't then find it's zener diode it's close to the PLL and should be at 5v,if not replace it with a 5.1v zener.
Dave.
How far is it.
Twice it's length from halfway.
thunderc4s
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Re: HMV Roadhound TX77 Repair

Post by thunderc4s »

I think the VCO block may be gone - what is the best way to check for a fautly VCO ?
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The Defpom
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Re: HMV Roadhound TX77 Repair

Post by The Defpom »

Check TP1 first, if the voltage is at one extreme (close to 0V or 5V) all of the time then the PLL is trying to compensate for the frequency being incorrect, if the voltage is 0V the first thing you should do it try replacing C3, C4 and C5 (they smooth out the voltage feeding the VCO), if one has shorted it will play havoc.

Because the VCO is mixed with a second frequency, the problem could be with the second frequency, and not just the VCO, can you post the voltages and frequencies of IC2's pins (starting at pin 1) when on CH1 (FCC band).

They easiest way to test a PLL/VCO loop is to disconnect the PLL from the VCO (in this case by lifting R1) and manually inject a control voltage to the VCO (at TP1), if you can control the VCO frequency around the normal frequency range using normal voltages (0-5v) then the VCO is probably OK.

The tuning slug in these VCO's is VERY fragile and easily broken, I have never been able to find replacement slugs for them, so it is best to not try and adjust it unless you are sure it will resolve the problem.
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