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Old 15th Sep 2020, 11:09 pm   #1
crackle
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Default Rotel RVC240

Oh dear I seem to be attracting all sorts of CB's for repair, its a good job this is a hobby I enjoy.

This one is a Rotel RVC 240 which was completely dead on TX. This turned out to be something to do with Q2, or at least the fault disappeared after removing the transistor testing it and refitting it.

I have just been doing some further tests and found that the transmit was off frequency.
I cant get the PLL adjustment correct, the lowest I can get the frequency is 10.30MHz and that is with the trimmer cap CT2 removed completely from the circuit.
I thought that maybe C3 was off but that does not exist on the board.
I have checked and adjusted TP-1 and T1 for 4 volts on Ch40 and can confirm on Ch1 it is 2.3 volts on RX and TX.
I have checked my frequency counter against a known good radio with its own frequency display and it is spot on.

I am at a loss as to how to correct the frequency of the oscillator.

Please can someone advise what else effects the oscillator frequency that I can check to see if it has wandered off spec.

If anyone needs more service information it can be found here on CB tricks. https://www.cbtricks.com/radios/rote...rvc_240_sm.pdf

Thanks
Mike
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Old 16th Sep 2020, 12:03 am   #2
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Default Re: Rotel RVC240

Before getting in too deep about the way this works, the fact that this radio has 'Delta Tune' means that the frequency of the 10.240MHz oscillator (and therefore the channel receive frequencies) are intentionally pulled off frequency when the DT switch is set to either + or -, so any adjustment aimed at setting the frequency of the 10.240MHz oscillator while measuring it MUST be done with the DT switch in the '0' position and in receive mode.

The effect of the DT switch is overridden in TX mode, so the simplest way to set the frequency of the 10.240MHz oscillator is to adjust CT2 so that the TX frequencies are spot on, and don't worry too much about the exact reading you get when measuring the frequency of the 10.240MHz oscillator afterwards.

Then in receive mode, the measured frequency of the 10.240MHz oscillator will shift lower than normal when the DT switch is moved to minus, and higher than normal when the DT switch is shifted to plus.

Has the DT switch wiring been tampered with, or has the DT switch been 'borrowed' for some other purpose? If so, the person who did that may not have strapped the disconnected wires in the right state to hold the DT at '0'.
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Old 16th Sep 2020, 12:37 am   #3
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Default Re: Rotel RVC240

A little bit more detail about the operation of the Delta Tune.

In a 'Normal' 134 chassis without Delta Tune, the frequency of the 10.240MHz crystal is set by CT2 and C3. Once set, the frequency of the crystal does not move.

In a 134 set with 'Delta Tune' C3 is missing and there are two new capacitors which come into play, these are C123, 47pF (switched in or out by Q20) and C124, 18pF (switched in or out by Q21).

With the DT switch at '0' Q20 is turned off and Q21 is turned on so the 'normal' frequency of the 10.240MHz crystal is set by CT2 and C124 in parallel.

With the DT switch at '-', Q20 is turned on and Q21 is turned off so the frequency of the 10.240MHz crystal is set by CT2 and C123 in parallel and is lower than normal.

With the DT switch at '+', Q20 and Q21 are both switched off so the frequency of the 10.240MHz crystal is set by CT2 only, and is higher than normal.

If you go into TX mode, the voltage supplied from the TX-only supply rail through D17 effectively overrides whatever the setting of the DT switch is, turning Q20 off and Q21 on so that it is as though the DT switch is set to '0'.

It may be worth checking that D17 is present, someone may have removed it to make the DT shift active in both RX and TX modes.

I should say that C1 also influences the frequency of the crystal but since this is fixed and always in circuit it can be ignored for the purposes of the above description.
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Old 16th Sep 2020, 7:16 am   #4
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Default Re: Rotel RVC240

Hi
OK I will check for D17, but I am sure that my tests showed that in TX the DT switch had no effect, and all my tests were done with the DT switch at 0.
thanks
Mike
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Old 16th Sep 2020, 7:48 am   #5
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Default Re: Rotel RVC240

D17 is present and correct.
The TX frequency on Ch1 is still 1.4kHz high, and seems to be consistent across the band.
And that is with the CT2 trimmer removed, to ensure absolute minimum capacitance.
The next stage is to check C1, ill Get round to that shortly.

thanks
Mike
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Last edited by crackle; 16th Sep 2020 at 7:56 am.
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Old 16th Sep 2020, 8:58 am   #6
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Default Re: Rotel RVC240

Reducing or removing capacitance will almost certainly send the frequency higher, not lower, so you're probably going in the wrong direction with that. You could try fitting the missing C3, but if that works it is only masking the real cause as C3 was never fitted.

Did you actually think of just changing the crystal? I imagine you have a few old corpses lying around which you can pull one from temporarily. Fit a replacement crystal and then see if you can set the TX frequencies to be correct using CT2.
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Old 16th Sep 2020, 10:30 am   #7
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Default Re: Rotel RVC240

"Reducing or removing capacitance will almost certainly send the frequency higher"

Oh that's interesting, even though it is going to ground?
I know the TX frequency was closer to the correct one when I removed the trimmer cap. But maybe I made a mistake with the direction. Ill check that later.
Thanks
Mike
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Old 16th Sep 2020, 12:37 pm   #8
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Default Re: Rotel RVC240

Quote:
Originally Posted by SiriusHardware View Post
Reducing or removing capacitance will almost certainly send the frequency higher, not lower
Absolutely!

I would add that capacitor and see what happens.

Crystals usually go down with age, not up, but it's not impossible for them to rise.

You want one of those Chinese crystal testers that you can get in kit form for a fiver or so from the usual place. I was thinking of getting one a month or so back and when I mentioned it to a CB operator who had boxed up a load of old electronic bits and pieces for me to go and collect, he happened to mention that he'd built one and it didn't do what he wanted it to do, so he'd chucked it in one of the boxes of bits for me - it's a wonderful little thing and I'm still unsure why it didn't work for him or why he didn't want it after going to the trouble of building it up.
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Old 16th Sep 2020, 1:24 pm   #9
SiriusHardware
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Default Re: Rotel RVC240

For evidence, you just have to look again at how the Delta tune works.

To shift the crystal downwards, a larger than usual capacitance (47pF) is switched in in parallel with CT2

With the crystal on-frequency, there is a smaller capacitor (18pF) in parallel with CT2. This is probably the value that the missing C3 would normally have in a set without the Delta Tune feature.

To shift the crystal high, both the 47pF and 18pF capacitors are switched out, leaving only CT2 down to GND.

Therefore more capacitance = greater downward shift.
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Old 16th Sep 2020, 7:16 pm   #10
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Default Re: Rotel RVC240

I am thinking there may have been a fault with the original red CT2 trimmer cap. I was trying to test it to measure the value and could not get a steady reading.
Anyway I have taken a green trimmer from an old chassis and fitted that.
The frequency can now be set precisely to the correct value for the channel, so all good there.
The signal on my adjacent radio is now a lot stronger, with no aerial plugged in before it was reading S9 it is now reading S30.
Just need someone now to pop up on the band so I can give the radio a final check for quality.

Thanks for your advice and help, I know a bit more about the process of Delta Tune now.

Mike
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Old 16th Sep 2020, 7:21 pm   #11
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Default Re: Rotel RVC240

That was going to be my next question, does CT2 actually vary the frequency at all? If not that would suggest CT2=open circuit, which certainly would send the frequency high.

Glad you have it sorted.
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Old 16th Sep 2020, 7:32 pm   #12
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Default Re: Rotel RVC240

PLL crystal ageing is not unknown. Many 27/81 radios use a 10.240460 crystal and
sometimes a 10.240 crystal can't be moved up enough on the trimmer.
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Old 17th Sep 2020, 6:47 am   #13
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Default Re: Rotel RVC240

Yep all good, reports of the radio sounding more natural now.
Thanks
Mike
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