Cobra 148 gtl dx Vco voltage rafting..

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Raverrst
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Cobra 148 gtl dx Vco voltage rafting..

Post by Raverrst »

Hi all,
My recently purchased Cobra 148 gtl dx Mk2 PB-010 AB Board, has what appears to be a Vco locking problem, it’s epromed triple low to uk fm on a six position band switch, Low & Mid bands seem fine, Hi band from channel 17 seems to take awhile to lock when you key the Mic and receive is not locking at all, iv check vco voltage and set it as per manual but when you go to channel 17 and up on hi band the voltage is rafting around, key the mic and it sort of settles, dekey and it’s off again, Any thoughts guys?

Thanks for any help..
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cb4ever104
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by cb4ever104 »

First of all , don't bother using a mic . It might be TX'ing , but not on the right frequency . Put a counter on TP3 and see what the radio's actually doing . Go through all the bands and channels to see where it's locked , and where it's not . I've only ever added the LOLO and HIHI to that chassis , so 5 bands . I'm not sure of the full VCO coverage without doing any more additional mods . One thing I do do though is wire the yellow SFT wire on the band switch in line with the radio . By default it gets +8V on LOW and MID , and GND on HI . I wire it so that it gets +8V on LOLO as well and GND on HIHI just to try and keep the VCO compensated (C98) . I'm not sure how other people do it . Regardless , you may have to adjust L18 slightly to get the radio to cover all your bands (if it can) .
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Raverrst
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by Raverrst »

Thanks for the Info. Using a second radio, the cobra’s channels/bands do all seem to be in order and on frequency once they lock!

Tx on the second radio on say channel 17 hi band and the cobra struggles to Rx lock onto the channel,
Tx on the cobra same channel and again the cobra struggles to Tx lock, keep it in Tx and it will lock on but move to another channel and it wonders again. When it’s wondering a Volt meter on tp2 does indeed indercate the voltage rafting about, not massively but by know means stable, if I adjust L18 I can get part of hi band to stablelise but not all 40 channels. Mid/low/low,low seem ok thus far. It’s only hi and 27/81 bands that seem to be being affected by this..
The eprom itself looks like it’s been installed very well, a very neat ”professional” job.

Is there anything that could be out of adjustment that would directly effect the Vco voltages? and what is the maximum and minimum voltages should I see when moving through the bands/channels?

Again many thanks for any help on this..
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cb4ever104
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by cb4ever104 »

If you wind L18 down then I would expect your HI/UK channels to lock , and you'll possibly lose some of the lower channels depending on the VCO range. You should be able to get the standard HI band to work regardless. What happens to the HI band when you set the radio to 5.4V on TP2 on CH40 MID band ?
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Ex GM0 , now EA5 .
Handle "Fortune Hunter" back in "the day"
Radios ....? Anything that says "TAIWAN R.O.C" on the back.
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Alucard
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by Alucard »

The easiest way I've found to expand the VCO range on the Cobra is to solder another varicap diode in parallel with the existing one.
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Raverrst
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by Raverrst »

If I set it as you have decried it locks mid band fine, hi band seems ok till around channel 17/18 then it starts unlocking. If I increase the voltage via L18 (6.5/7volts) it will lock the rest of the hi channels after a 2 seconds of Txing but it seems as soon as you move up or down a channel it’s off again and when this happens the voltage is rafting about..
Raverrst
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by Raverrst »

Alucard wrote: 24 Nov 2021, 21:27 The easiest way I've found to expand the VCO range on the Cobra is to solder another varicap diode in parallel with the existing one.
In parallel with Diode 32?
Last edited by Raverrst on 24 Nov 2021, 23:44, edited 1 time in total.
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by Alucard »

In parallel with the existing varicap diode.
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cb4ever104
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by cb4ever104 »

Your VCO doesn't need expanding (yet). From the factory it should easily cover the HI band . And HIHI . You've got other problems. Might be VCO related. I'd need to see exactly what's going on with a counter to see where it's failing. With the information you've given I'd replace the varicap diode first. If it was always failing at a specific point regardless of where L18 was then I would have said N code / binary adder / pll first
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Handle "Fortune Hunter" back in "the day"
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Raverrst
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by Raverrst »

Thanks very much for the info guys, certainly gives me something to work on.. Something else iv noticed also is that the Can L17 has been removed and has been jumped across, looks like it was part of the eprom conversion tho, Is this normal for this type of conversion?
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by cb4ever104 »

Raverrst wrote: 25 Nov 2021, 14:11 Can L17 has been removed and has been jumped across
Can't see any reason at all why someone would do that . L17 is used to tune the output of the down mixer (TR25) for a maximum reading . Can't see why it would need to be removed to expand the radio by 40 or 80 channels .
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Handle "Fortune Hunter" back in "the day"
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Raverrst
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by Raverrst »

Yeah pass, it’s beond me.. However I’m pleased to report I believe we’re all now stable and working as it should, I found a couple of dry joints in the area of L18 so reflowed the area and all seem well..

So thanks for help on this..

There is “hopefully” just one more thing which isn’t right with this old beast! Channel read display, on 20 its missing two segments and on channel 40 theses two segment missing from 20 are lit so it reads 90 🙈 any thoughts as to why it’s doing this? Don’t think it’s channel switch as its not making or breaking etc ..
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cb4ever104
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by cb4ever104 »

Sounds like the channel selector. They get worn and dirty a bit over time and need stripping down and cleaning .
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Handle "Fortune Hunter" back in "the day"
Radios ....? Anything that says "TAIWAN R.O.C" on the back.
Raverrst
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by Raverrst »

Yes this was my first thoughts, but normally you can move and rock them about and it’ll at least get the missing segments to flash etc but this is not doing anything and it’s strange it’s only 20 and 40 that effected the rest are fine 🤔
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Re: Cobra 148 gtl dx Vco voltage rafting..

Post by 13EL121 »

Check the electrolytic capacitors in the VCO area. Maybe check all 10v caps. They often fail after 30 years.
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