Cobra 139XLR problem

[Deleted]

I am restoring a Cobra 139XLR, and yes, this is my radio! Anyway, I have no receive, transmit, or carrier. I do have modulation. I was measuring out components. FET 6 in the schematic has a dead short from gate to source using a multimeter. I found this as R187 (3.3K) was measuring a dead short. Upon looking at the schematic, it was across the FET 6 source and gate. Is this normal? I do not know how to test a FET.

[Sandbagger]

Dec 3, 2009 23:56:56 GMT -5 @vinnies said:

I am restoring a Cobra 139XLR, and yes, this is my radio! Anyway, I have no receive, transmit, or carrier. I do have modulation. I was measuring out components. FET 6 in the schematic has a dead short from gate to source using a multimeter. I found this as R187 (3.3K) was measuring a dead short. Upon looking at the schematic, it was across the FET 6 source and gate. Is this normal? I do not know how to test a FET.

It's not normal for a device to have a dead short. FET's are not as easy to test with a multimeter as bipolar devices.

With the symptoms you are describing, I'd check power supply voltages and then the PLL section. Make sure all the oscillators are running.

[Deleted]

I do have voltage issues. However, I bypassed with a known ext power supply, and it has the same problem. The FET 6 is fine.

[cbrown]

Here's a link to testing FET's:

zone.ni.com/devzone/cda/tut/p/id/6856

[Deleted]

Thanks. I had found that, but the Fet is fine. I have a few problems. One is the problem I mentioned. The other is the power supply voltages are no good. I disconnected the power supply and am using an external. I assume I have a bad power transistor or regulator or zener, but that is nothing. I am geeting the same symptom with the external power supply. I am going to check the oscillialtors and PLL next.

[Deleted]

OK, got the voltages all squared away.

There is definitely a problem with the PLL, VCO, and oscillators. I cannot adjust the VCO to 2V it is stuck on 4.6. The 34.985 oscillators are off. The AM is at 36MHz and the SSB are close to the 34MHz, but are also off and I cannot adjust.

[Deleted]

I check this again. The voltage at TP7 collector of TR35 which controls the voltage for the VCO chip is at 4.68V and cannot adjust. I cannot measure the 10.240 at the PLL.

[Sandbagger]

Dec 4, 2009 22:59:59 GMT -5 @vinnies said:

OK, got the voltages all squared away.

There is definitely a problem with the PLL, VCO, and oscillators. I cannot adjust the VCO to 2V it is stuck on 4.6. The 34.985 oscillators are off. The AM is at 36MHz and the SSB are close to the 34MHz, but are also off and I cannot adjust.

The PLL is running out of lock. Your error voltage driving the VCO is railed at 4.6 V. Since you can't adjust it, something in the chain is broken. You will need to use a scope to see if the VCO is getting mixed to the proper frequency to drive the programmable divider, and that the levels are right. It's possible that either the VCO block or the 858 is bad, but I wouldn't jump to that conclusion before testing a bunch of signals. Also look for "channel mods gone wrong".

[Deleted]

Will do. I do not have an 858. I have several other PLLs, but not that one. As I mentioned, I cannot see the 10.240 at the PLL. I think it is pin 9.

[Sandbagger]

Dec 6, 2009 22:15:50 GMT -5 @vinnies said:

Will do. I do not have an 858. I have several other PLLs, but not that one. As I mentioned, I cannot see the 10.240 at the PLL. I think it is pin 9.

That's a great clue! If the 10.24 oscillator is not running, then the PLL can't function. Now the oscillator for the 10.24 is internal to the PLL chip, so if you have no oscillation, replace the crystal. If there still nothing, then it's likely that the PLL is bad.

[Deleted]

Ugh! I don't have one. What scrap mobile can I get cheap to rampage one from?

[Sandbagger]

Dec 7, 2009 9:35:39 GMT -5 @vinnies said:

Ugh! I don't have one. What scrap mobile can I get cheap to rampage one from?

Practically any 40 channel PLL radio (and some 23 PLL's) will have a 10.240 Mhz crystal. That's the main reference, which is divided by 1024 to give you your 10 khz channel steps.


I think you said that your voltages were ok now, but make sure there is 5V to the PLL chip (pin 12). You should also see 5.12 Mhz (reference divided by 2) on pin 8 of the PLL.

[Deleted]

I have several 10.240 crystals. I don't have the 858 PLL. The TP7 voltage is still locked at 4.8V. I cannot adjust it down. Will follow up after tonight.

[cbrown]

Here's a site with a list of PLL chips used:

www.radiomods.co.nz/integratedcircuits/plllist.html

[Deleted]

OK. Replaced the PLL. Got the 10.240 back (actually 10.239). However, the VCO TP7 voltage still will not adjust and it stuck at 4.6V or so. It won't move in either direction.

[Sandbagger]

Jan 2, 2010 20:17:56 GMT -5 @vinnies said:

OK. Replaced the PLL. Got the 10.240 back (actually 10.239). However, the VCO TP7 voltage still will not adjust and it stuck at 4.6V or so. It won't move in either direction.

PLL problems can be hard to remotely diagnose. Are you sure the PLL chip (I'm assuming the 858) is good? Obviously it's better than the original if the 10.24 is now working. But it's possible that there may be other failures. There could be a problem in the UHIC-005 VCO block. There could be a bad cap in the loop filter, or a crack in the PC board (my TRC-458 had that one) or in one of the film resistors.

Usually when a phase detector rails high, that means that the divided down VCO frequency is lower than the reference frequency (in this case 10 khz), the amplitude feeding the phase detector is too low, or it's missing completely. FET4 is the loop mixer and it feeds the difference between the 34 Mhz VCO against the tripled output of one of the three 11 mhz offset oscillators. If all is working correctly, you should be able to see an approximately 1 Mhz frequency signal (depending on the channel) out of FET4 through TR30 and 31, and on to pin 11 of the 858 PLL with a scope probe.

If that FET or those other transistors are bad and no signal gets back to pin 11, that would cause the problem you are seeing. But those are not the only things.....

[Deleted]

I swapped the VCO from a 520D. Same voltage. Am I supposed to adjust this in receiver? I can adjust it wit the radio in transmit. But in receive, it is stuck at 4.86V or so.

[Sandbagger]

Jan 3, 2010 20:50:41 GMT -5 @vinnies said:

I swapped the VCO from a 520D. Same voltage. Am I supposed to adjust this in receiver? I can adjust it wit the radio in transmit. But in receive, it is stuck at 4.86V or so.

Is the radio transmitting a stable locked frequency? What about receive on SSB? If the problem only shows up on AM receive, one of the 11 Mhz oscillators is out.