Tranciever Repair  question? - Page 1
Forum section: electronics / repair

author: Crambone
author_rank: Frequent Contributor
timestamp: June 09, 2023, 04:03:18 am
content: On page 58 - 59 I highlighted part of the schematic.I’m not seeing the 7.8MHz at pins 7&8 of IC5 (SO42P) all the voltages are correct. I tried to trace back with a scope looking for a 7.8MHz signal and do see it at TP62 & 63 but not at TP61 & 64??? How is this possible? All the components marked green have been either removed and checked.Am I following the circuit correctly looking for the lost 7.8MHz.The issue I’m having is the radio won’t develop/key without the 7.8MHz and 35MHz present. I have modulation when sniffed with a second radio but no power out. All other functions seem to be working properly.Link to service manual and schematicshttps://cbtricks.org/radios/cobra/2000gtl/graphics/cobra_2000gtl_sm.pdf

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 09, 2023, 01:46:33 pm
content: SuggestionsFirst of all does the radio RX at all???What are you using to check the signals? A scope may load down the circuit at some points and even eliminate the signalCheck at pre driver for the 27 MH signal first.If it is there tune it to max with L47 and L48If this is not there check at TP 14 for carrier 7;8 MHz, this should change with SSB and AM , no signal with SBB without audio inputLet me know

author: Crambone
author_rank: Frequent Contributor
timestamp: June 09, 2023, 02:04:46 pm
content: Thank you, I have great receive in all modes. I also have modulation on all bands & channels using a handheld set to the channel I key on I can hear it. I will check what you suggested.

author: Crambone
author_rank: Frequent Contributor
timestamp: June 09, 2023, 09:20:50 pm
content: Well I went to test TP14 using an Oscilloscope and I was seeing the 7.8 MHz but it slowly drift out so I checked voltages on IC3 and all the voltages low. I decided to check a bunch of other voltage points and everyone I tested was around 1/2 and would drift down. I was getting ready to test anther point and POP went the AC filter before the transformer. The 2.2m resistor let the smoke out. I checked the transformer, rectifier, regulator and found nothing. I replaced the resistor and put on Variac and all seemed good.So I decided to change out all the electrolytic capacitors in the power section before testing again.Update to follow.

author: floobydust
author_rank: Super Contributor
timestamp: June 10, 2023, 03:14:10 am
content: Well that 2.2MEG R415 is the "death resistor" needs to be at least 1/2W and I would consider also replacing the 4 Y-caps 0.001uF C412-C415 as they can also be a safety concern and modern parts are tougher for taking mains overvoltages. Usually the antenna is the chassis earth-ground but... hey where is the mains fuse? I would have at least ~1A SB in there with a polarized line cord.I would carefully check the many power supply regulators are OK first. I will desolder and lift the final's collector lead, TR36 2SC1969-B to disable the transmitter PA until I get more signs of life. Make a fake mic, the aviation connector with a toggle switch for RX or TX to leave it keyed up.I think you can keep troubleshooting by following the PLL signals for 7.8MHz and 34MHz. But so many diodes are switching the 7.8MHz on/off to various stages, so I would instead look if the RX/TX transistor switches are working so the mixer IC5 is getting the right signals.Check the 8.2V regulator IC4 is good, it has 3 (switched) outputs and pin 5 is the TX/RX control. Also at node #12 on the Sams schematic.I would make sure the TX switch is working starting from the mic, I check there because people will short a mic or wire it wrong and damage IC4.

author: Crambone
author_rank: Frequent Contributor
timestamp: June 10, 2023, 03:21:58 am
content: Haha ya that resistor has a nice hole in it now! This radio’s power is weird it switches on the ground?Encountered that when I first started this never ending project and needed to put in a new on/ off switch.

author: floobydust
author_rank: Super Contributor
timestamp: June 10, 2023, 04:03:43 am
content: The Line Filter Board I would replace those Y-caps... C412-C415 BoM is "0.001uF 125V" eek that's a bit lowwww. There's many with 0.3" or 0.4" lead spacing, like Kemet C901 seriesC901U102MYVDBA7317.I'm not sure who did Uniden/Dynascan designs, in that era it seemed to be Taiwan pounding out radidios and doing the designs. But this has a MCU TMS1000 so a lot of engineering in it.Lightning is a problem with base stations, could have damaged the 2.2MEG resistor.I have a 29XLR that is a time and money pit lol I still haven't fixed it. Hard to have patience without a test set.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 10, 2023, 01:43:25 pm
content: Good Idea to cut off supply to the final but you can do this by pulling the wire connector at Final VCC TP 7, that is if no one has soldered them in.....TP 8 is the driver VCCThe most common failure resulting in no output is a blown final. If you find this is the case after checking all your voltages, and the signal at the driver, then there are very nice good new transistors available finally. No too expensive at all.Also the AM power regulator is a common fault.We use the ones from HG, they make good transistors. Some people say they are not as "strong" but I have used lots of them and they are fine.https://cqdx11.net/Image_Uploads/2013/07/2SC1969-RF-TX.pdfBuy from a reputable CB supply.As far as test sets, yes, repair would be a lot harder if I did not have a HP 8935 service monitor.Have fixed many 29s. Especially in the past few years when new radios were impossible to get, we fixed some that we normally would not have wasted time on.

author: floobydust
author_rank: Super Contributor
timestamp: June 10, 2023, 09:47:57 pm
content: I usually see a blown final and/or driver causing no TX but shorted would be popping fuses or roasting the AM modulator.For a replacement final, Mitsubishi 2SC1969 long obsolete and tons of chinese junk ones out there. I'm assuming the better ones are HG with fake Mits markings?I was looking at the MOSFET mod with IRF510 or RD16HHF1 but left off getting a decent bias circuit built for it.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 10, 2023, 11:57:47 pm
content: The HG 1969 are good. However some with the HG markings are fake.Let me check with my source and get you the proper source we use. We buy them in maybe lots of 20 and i think they they are $ 6 each.I think we have used maybe 50 of these transistors without one failing.I used to convert lots of radios to 520s but it is cheaper for the customer just to use the new HG 1969.However the 520 in bulk from RFParts is really cheap and is a good part.Best way to convert to 520 is to get a schematic of the radio which has 520 finals. Many radios have models using 1969 originally until they changed production to 520s. You have to change some in and out caps and change the bias circuit to provide adjustable 3 volts for the gate bias thru a choke. There is also a "protection" about 6 volt Zener between the gate and ground, put in where the other diode was but in reverse because it is a Zener.. Some people leave this out.Not very hard to do. You do not even have to drill any new holes....

author: Crambone
author_rank: Frequent Contributor
timestamp: June 11, 2023, 07:58:29 pm
content: Ok, I’m officially lost! Rather than me guess where the 7.8MHz at pins 7&8 of IC5 (SO42P) originates from might someone be able to tell what path it takes? This way I can work my way back from IC5 or even check the originating source to see if the 7.8MHz is even available.Thank, Drew

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 11, 2023, 11:35:00 pm
content: On Page 3 of your manual. TP3 is where you see the 7.8 MHz SSB signal on RX and TX. TP 14 is where you see the AM signal on Tx only. Adjust the cans and cap according to the directions on page 3 / 58. You should see the signals really well with a scope. Should be strong signals. You need some kind of counter to set them accurately. I use a spectrum analyzer. I don't know if your scope has a good counter.They come from the Xtal.Location of TP 3 is pointed out on page 7 / 58. TP 14 is pointed out on Page 6The schematic of the Xtal oscillator is on page 53 at the bottom in the middle of the page. The Xtal is labeled 7.8 MHz on the schematic.You can look at the xtal itself to see if it is labeled.If you have RX in SSB the oscillator is probably working OK.

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2023, 12:14:52 am
content: The 7.8MHz for TX originates from TR32 Carrier Osc., then gets modulated at IC3. See what you are getting at TP14 keyed up.Then the 7.8MHz goes through the diode-switches D18 to filter FT3 and D53 to the mixer.For transmit, mixer IC5 SiemensS042Ptakes the differential signal between pins 7-8 (7.8MHz) and mixes with pin 11-13 (34MHz) to give you the transmit frequency. Example Ch. 40 PLL at 35.205MHz - 7.8MHz = 27.405 for the RX I thinkedit: this is receiver has dual-conversion so there is an extra 455kHz missing from that for RX, as well as upper and lower SSB. Exact frequencies are a bit of work to figure out.

author: Crambone
author_rank: Frequent Contributor
timestamp: June 12, 2023, 07:02:55 pm
content: I traced the circuit back and see the 7.8 MHz at FT3 from C51 side. When I probe the other side of FT3 at the C52 side it’s weak. I decided To lift R60 and then I see the 7.8MHz no issues to pins 7&8 at IC5. I checked R30 and it’s good and voltage is correct in AM TX and SSB. I put R60 back in and it all falls back to nothing?

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2023, 08:03:10 pm
content: Are you testing in AM or SSB transmit? I would stay in AM, and know SSB does not have a steady carrier like AM, with no modulation.R60 pulled is only switching off DC control voltage for D20 the RX SSB switch.D20 along with D3 switch the Rx between AM and SSB I think. It is confusing because that same circuit block is dual-purpose, used for Rx and Tx.So the RX switch could be stuck on, you could try lift one leg of D20 to unload that, if it's the problem.It's normal to lose a lot of signal in a crystal filter like FT3, which IC5 makes up for. So don't expect a lot of signal at IC5 pins 7-8 and go on a goosechase.Confirm the voltages are good at:Node #2 8.2V rail, feeds Nodes #2-8.Node #4 7.42V SSB/6.13V AM XMTNode #11 7.09V AM sourceNode #12 8.2V XMT(A and B) source, will also switch on TX lamp "On the air".Who is R30?

author: Crambone
author_rank: Frequent Contributor
timestamp: June 12, 2023, 09:01:36 pm
content: Lifting D20 showed no changeNode #2 8.2V rail, feeds Nodes #2-8. Showed 8.3VNode #4 7.42V SSB/6.13V AM XMT. 7.6V SSB / 6.10V AM XMTNode #11 7.09V AM source. 7.15VNode #12 8.2V XMT(A and B) source, will also switch on TX lamp "On the air". 8.3V A&B

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2023, 10:01:28 pm
content: The switched voltages look reasonable.If FT3 is bad, or has high loading resulting in weak output, I would also expect SSB receive to not work as well. Did you check that out?Otherwise, checking IC5 in/out is difficult and I would jump to Tx pre-driver TR-39 and look for signs of life, what signals are there.I found the Uniden 2000GTL schematic:https://www.bensons-funktechnik.de/downloads/ Schaltbilder Service Manuals/Cobra/2000GTL.pdfa bit easier to follow some aspects.

author: Crambone
author_rank: Frequent Contributor
timestamp: June 12, 2023, 11:02:38 pm
content: Ok on TR39B - 1.36V TX. 2.1 to 2.4MHz jumpingC - .6V RX / 8.3V TX. 27.205 MHz as the radio is on AM channel 20E - .6V TX. 5.2 to 6.2 MHz jumpingIC5Pins 1, 4, 6, 9 & 14 groundedPin 2, 3 & 5 8.00V TXPin 7 & 8 2.89V TX NO 7.800MHzPin 10 1.23V TXPin 11 1.43V 34.982MHzPin 12 1.3V 68MHzPin 13 1.42V 35.005MHz

author: floobydust
author_rank: Super Contributor
timestamp: June 13, 2023, 02:26:34 am
content: Those all look reasonable. I'm not sure what the freq. counter jumping around in readings is about. What gear are you using for that.It can be low signal or just a grumpy freq. counter that can't read with DC present or the signal has harmonics that it can't work through.Main thing TR39 E at 0.6V is around 40mA for this pre-driver stage and the collector has 27.205MHz which tells me the mixer is mixing and this stage has gain. That makes me want to look further downstream.How about looking at the next stages driver TR38 and bias TR37 etc. see what DCV readings you get there. TP8 too.Keep in mind sometimes people or gorillas get in here to "boost the power" and do mods or other things that don't pan out, and you can find parts missing/changed/burnt etc.