The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

timrim

Try re adjusting the dead key again, perhaps there is a dead spot in the control. Also press around on the board with something plastic or rubber and look for bad soldering. Tap the relay and make sure its properly seated. If this don't help, I can tell you how to start troubleshooting it. An 8 watt...

timrim

That radio has a relay, so you will need to place a cap in series with a resistor across 2 of the relay contacts. I am not at work so right now I am unable to tell you exactly which ones, I will look it up later for you.

timrim

Yes, that's what I have been, the biggest issue is the actual part names like resistor XX will have a different number on the uniden, so I have to cross check part numbers. There are a few other minor things too, but for the most part its not too much of a problem. I would like to get the schematics...

timrim

TC9106

timrim

Thanks Ramblingman. Noise on transmit is now reduced to the point that it is acceptable. Routing the wiring harness was a major factor in fixing this, as well as shortening the mic wire from the jack to the echo board.

timrim

In addition to those 3, you can also use a 2SC2028, that gives you 4 different choices and any of them will work fine, you may find one in a junk CB somewhere. Rf Parts should stock any of those.

timrim

Solder a resistor from the pin of the ext. speaker jack closest to the back of the radio to ground, the resistor should be anywhere from 22 - 150 ohms depending on how loud of a talk back you require.

timrim

PC Board # EPT690010D EL Driver PC Board # EPT91V131Z Radio has buzz on transmit caused by the high voltage unit that lights the face plate. Buzz goes away if I turn off lights, buzz gets worse if I just unplug the face. The buzz sounds almost like alternator whine. I can slightly reduce it by placi...

timrim

Yes, www.rfparts.com , when you do a search there, search for "2SC", It will give you a listing of all the "2SC" series transistors. They can also be ordered directly from Cobra.

timrim

You must have a newer model, most of the older ones are 2SC1957. The newer ones do use the 2314.

timrim

What test equiptment do you have? The easiest way is to look at the output on a scope. Make sure you have a stronger signal on the collector and a weaker signal on the base. If you don't have a scope, then you will need to pull it and either check it with an ohm meter or a transistor checker. If you...

timrim

Yes, it will still receive if the driver is bad. The driver is the amplification stage that supplies the rf signal to the final to be further amplified.

timrim

The main regulator is now putting out 8.9 volts, I put a 6 volt zener in place of the 7.5 volt one to lower the regulators output. The output voltage dropped over 1 volt and the receive is still the same. With the 7.5 volt zener the voltage was just over 10 volts, I have checked all resistors, elect...

timrim

I need schematic, board and parts layout and any other useful data for the Uniden PC-78 Elite, cbtricks had nothing at all on this radio. Does anyone know of a online place where this data is available?

timrim

Glad to hear it, I think perhaps your ammeter was not working, that would explain zero current reading.

timrim

Ramblingman, Thanks for those techniques, especially the one using the 1-2 volt power supply, that will come in very handy in the future. I did open up pin 14 of IC-2 and the receive did not improve, or was this the wrong place to disable the AGC? I did not try shorting the AGC line to ground, I wil...

timrim

There is an integrated circuit (IC-1), you would hook up the S-Meter in this circuit off the cathode of D-14. Everything you need should already be there.

timrim

You don't need anything special, but if you plan to tie the LEDs to the points where the bulbs were, be sure to check polarity. Another thing to remember is that the LEDs may not dim as well as the bulbs did, if that is the case and you wish to fix it, you will need to replace a resistor or two on t...

timrim

Ramblingman, yes, in fact, those overload diodes were among the first things that I had checked. While I did replace all the 10 volt caps I could find in the receive sections, I did not worry about them too much anywhere else, but I have thought about simply replacing them all. I will do that and se...

timrim

Removed and re-seated all coils in receiver sections and the problem is still there. Does anyone have ocilloscope data for the various receive stages for a given signal input? If I knew the peak to peak readings that were normal for the various stages, I think I may be able to isolate this issue. Ca...

timrim

On a the Cobra 29 series radios, all you need to do is install the 2 parts. You can also change a coupling cap or two (not really necessary). It is possible to tweek the output even further by building your own biasing circuit, but installation is much simpler if you just use the EN1230. Yes, some d...

timrim

Ok, just for clarification, make sure: 1.- Your meter is set to measure low amperage 1 amp or 500ma setting if your meter has it, make sure the probe is in the proper socket for amperage, and make sure the protection fuse in your meter has not blown. Make sure your meter is not set to read too high ...

timrim

Well, the mod does indeed work if installed correctly, as for the 10k pot, you should not even need it in the cobra 29 as long as you installed the EN1230 Bias chip. I have done hundreds of various model 29s without any issues. When you turned the pot up and it blew the 5 amp fuse, you are dumping a...

timrim

Yes, you now require 5 amp fuses, should not blow

timrim

Astatic 636L would be my first choice, I sell lots of them and they sound great. The RK-56B would be my second choice, but the switch isn't quite as good and I have seen alot of RK-56's and RK56B's that need to have a resistor installed so that they do not pick up excessive background noise, somethi...

timrim

What is the current at both test points?
It is possible that either the control burnt out again or there is something else in the bias circuit that is blown.

timrim

Actually its not the voltage you are trying to adjust its the final current. You remove the jumper pc board and connect an amp meter between posts, you need to set the driver current and final current, when doing this go to USB with mic gain fully off. Final Bias - Connect DC Ammeter between TP7 and...

timrim

What size fuses are blowing?

timrim

I think you will find that the coils are out of alignment, I am not at work so I can't tell you which ones to adjust, but it does sound like an alignment issue.

timrim

Glad you got most of it straightened out, I will look at the circuit tomorrow and see if I can offer any further ideas.

timrim

When you say receives fine on AM and sideband, I assume you mean that you can hear sound, if so, the audio chip is good.

timrim

Here is the board layout, I marked where the transistor leads go.

On your last picture, the ball of solder is just to the right of your red circle about even with the bottom of the circle about half inch.

timrim

My god! I see radios come in like that all the time, it just breaks my heart. One guy tried to save money by putting MOSFETs in his radio, the board was butchered so bad, I would not even work on it. Most of the traces in the final area had been completly removed, parts missing etc. His attempt at s...

timrim

At this point I am willing to give that a try, I will unsolder and re-solder all of them and see what happens.

timrim

Interesting, now I would like to see a schematic of the power supply, perhaps I can find one online.

It just dawned on me, is that a switched mode power supply or does it have a transformer?

timrim

Well, that was my last 7.5 volt zener. :( However, I could use a 6 volt Zener just to see if I end up with a lower voltage, if so, I will have to get another 7.5 volt zener with my next parts order. EDIT: I currently have a 6 Volt Zener in there and the output voltage is now 8.9 volts, which still i...

timrim

pgCam, If your Echo Board number is EPT0SSB50I then, Check for bad solder connections in the J-6 & Q-6 area, also make sure that J6 is plugged in all the way, this is the on/off switch connection. You can also remove the plug and short the terminals together and see if the echo comes on, if it d...

timrim

Here are the latest results: Voltages at TR-34 B- 1.95 E- 1.33 C- 12.55 Voltages at TR-501 B- 12.5 E- 13.3 C- 10.15 I have pulled and checked the following resistors: R-205, R-206, R-207 and R-209 EDIT: also pulled and checked C156 D-69 Zener was replaced and D-68 was replaced with 2 1N4148's in ser...

timrim

I will be repairing a 158EDX later today, I will see if I can offer some useful advice.

timrim

If you have no recieve and no transmit, then the final is not the problem. I would check for bad solder connections in PLL VCO sections, we can then proceed from there.

timrim

PM's do not appear to be working, I received an error.

Just email me at (Removed).

timrim

Chevin, I replaced it with a zener. What I am wondering however, is Diode D-68 anything special? I have lots of 1n914's and 1n400x Diodes.

timrim

I have been so busy, I have had to put this radio on hold until today. Well the voltage regulator output (TR-501 collector / D-69 cathode) is 10.1 volts, shouldn't this be 8 volts in this radio? Someone changed the regulator a TIP-30 from a 2SA-473, which the schematic calls for, so this circuit has...

timrim

First I would check TR-21, the transmit switch and the large transistor (TR-23) near the mic jack, I have seen those go bad on several occasions.

timrim

Th LM723 is the Voltage Regulator IC and I have found these do seem to go bad quite often. They are available from many sources and are inexpensive.

timrim

Thanks, I think I will order some and do a little experimenting.

timrim

Has anyone tried using HUF75343P3 Mosfets in place of IRF520s or ERF 2030's in radios? I was just following some posts in another part of these forums where it was stated that these are a stronger Mosfet, and it got me thinking.

timrim

What happens is the buttons do indeed get pushed in, all you have to do is grab the larger outer knob and pull out about 1/8 of an inch, then push the outer back in again and when you operate the switches, you will feel a noticible click. It's just that there is no room for the switch to move curren...

timrim

If you have a spare heat sink, you could add the second final, as long as you tie the finals to +12 volts instead of going through the regulator. I would recommend this on the older ones however, the newer ones just are not built as well.

timrim

I would go with the Superstar 158EDX, I have sold many of those and everyone seems to love them. People that have bought them have recommended them to friends. I personally think it is a very nice radio for the money.

The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

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