Driver stage in CB radio - Page 1
Forum section: electronics / rf-microwave

author: Jane
author_rank: Frequent Contributor
timestamp: October 18, 2018, 07:53:00 pm
content: My understanding of a driver circuit is that it provides an input voltage for (a final) power amplifier.In a service manual for a CB radio there is a test point markedTP1 driver.Does it mean that it is an output of that driver stage and an input for a power amplifier?Is there any clue how to find out, in schematic, where such a driver circuit is?Being a newbie with CB radios, can you recommend a website/literature where I can learn more technical details?Thanks

author: tkamiya
author_rank: Super Contributor
timestamp: October 19, 2018, 05:57:05 pm
content: If you can post a schematic diagram, I can try to help you out.TP stands for Touch Point. It's a test point. So it doesn't really mean anything. Since it says driver, it probably belongs to the stage you are wanting, but what the signal entails, there is no telling.A simple CB radio usually has a final stage consist of one transistor. One before that is a driver. Again, with schematic, I and many other here can help you out.

author: Jane
author_rank: Frequent Contributor
timestamp: October 19, 2018, 06:21:50 pm
content: Thank you for your replyI uploaded the schematic.it is on page 5Please check.Thanks

author: tkamiya
author_rank: Super Contributor
timestamp: October 19, 2018, 07:24:48 pm
content: OK, got it.Q4 is the final stage. You can tell this because it comes before output filter.Q6 is the driver stage.Q3 is acting like a switch depending on status of Q13I'm amazed a CB radio comes with such a nice documentation.

author: Jane
author_rank: Frequent Contributor
timestamp: October 19, 2018, 07:53:55 pm
content: Thank you for your explanation of Q4.I also understand that Q3 is acting like a switch depending on status of Q13. In other words when I want to transmit,TX_UNLOCK is 0V ,then Q13 is closed, and Q3 should be open. Am I right?But how do you know that Q6 is the driver stage?

author: tkamiya
author_rank: Super Contributor
timestamp: October 19, 2018, 08:04:36 pm
content: Why Q6 is a driver stage?It is because it is an amplifier circuitIt is because it sits before final amplifier circuitRF goes through it first, amplified, then goes to the final stageI mentioned Q3 because we are talking about "stages". Sometimes, final stage and driver stage can contain multiple transistors, or active devices.I'm not quite sure why Q3 exists though. I didn't read the circuit deeply enough to tell you this. Looks like when certain conditions are met, transmitter drive stage and final stage does not get any RF signal, thus no transmit.

author: Jane
author_rank: Frequent Contributor
timestamp: October 19, 2018, 08:34:31 pm
content: Thank you again for your time and help.But Q9 and Q11 can not be a part of that driver stage as well?

author: tkamiya
author_rank: Super Contributor
timestamp: October 20, 2018, 02:01:14 am
content: Why would you think Q9 and Q11 belongs to a driver stage? They both have more to do with filtering earlier stage than amplification....Q3; however, if you want to include it in driver circuit, you can. I tend to think of it as buffering than driving final, so I tend not to think so. But you can make an argument that it is providing signal for the driving stage.More about Q9 and Q11:What I think they are doing is, read voltage coming from BAND SWITCHES and switch on/off capacitors selectively, thus proving wider pass-band. I would think there are better way of doing this, but that's what designer did.

author: Beamin
author_rank: Super Contributor
timestamp: October 20, 2018, 07:52:37 am
content: How do they stop the Tx RF from going into the rx section on low cost radios like that. Is that the purpose of some those transistors next to each other in the schematic or all those gain stages for the tx? I know in higher quality ham rigs they use a special relay or could they use PIN diodes? If the rx amplifier is powered down by the radio, will that prevent damage since having a TX cm's away from your rx circuit would always leak large millivolts, or more,of rf into it?I always wondered how they can transmit and receive at the same time on the same antenna, unless its digital and they are actually alternating tx/rx many times a second.

author: bd139
author_rank: Super Contributor
timestamp: October 20, 2018, 09:06:40 am
content: That’s more of a problem with VHF/UHF kit where the wavelength and PCB traces are similar. On HF stuff you usually decouple everything heavily and keep traces short and it just works. Very forgiving. You can sometimes see bodges where this didn’t work out and they fix in production.

author: tkamiya
author_rank: Super Contributor
timestamp: October 20, 2018, 03:17:13 pm
content: Looks to be a very crude setup when it comes to TX/RX switching.The way this circuit diagram is drawn, top half is a transmitter train, and bottom half is a receiver train. Now, find a line that connects to a pair of diodes D21 1 and 2. It is located on lower right corner and a line goes from TX section to RX section.Transmitter is always connected to the antenna. Receiver has a pair of diodes to prevent more than 0.7V of RF going into the section. Also, Q47 acts as a shunt to short out the receiver input when transmitter is active.I guess for a 4W RF output, this must work well enough. On larger radio, I've seen using PIN diode as switch on receiver front end. On even larger, I've seen vacuum relay.

author: bob225
author_rank: Frequent Contributor
timestamp: October 20, 2018, 04:22:11 pm
content: even with new cb radios the output stages are crude, to reduce costs, channelized fixed frequency (band) are a simple construction - some of the 1980's radios are single sided phenolic pcb's all through hole with a rats nest of wiresif your curious there plenty of sites on the net eg.http://www.cbtricks.com/

author: Jane
author_rank: Frequent Contributor
timestamp: October 20, 2018, 05:59:42 pm
content: As I can see experts are here. I have that CB radio(the schematic is discussed here) but it does not have any output power. ( Measured with a power meter) . When I want to transmit, on the front panel it shows transmitting, yet no output power. During a repair attempt I touched a PCB front panel and it suddenly started to work. Later it stopped working and now it is faulty again.Do you have any clue how to repair it? Thanks.

author: tkamiya
author_rank: Super Contributor
timestamp: October 20, 2018, 06:15:57 pm
content: Take a tooth pick or wooden stick with sharpened end, start tapping everywhere. Also, you can use hair dryer to heat and can-of-air to cool.You have a cold solder joint somewhere.

author: m3vuv
author_rank: Super Contributor
timestamp: October 20, 2018, 10:47:18 pm
content: aA few years ago i used to do warranty work for an importer of cb radios,the fauls on everyone was down to bad solder joints caused by unleaded solder,makes you wonder how many electronic stuff ends up dumped because of crappy lead free solder?,m3vuv.

author: bd139
author_rank: Super Contributor
timestamp: October 20, 2018, 11:04:48 pm
content: It's not just down to lead free solder. Lots of old Thurlby test gear could be fixed just by touching all the joints on it and that was a long time agoCrappy soldering is a major thing really.

author: james_s
author_rank: Super Contributor
timestamp: October 20, 2018, 11:23:02 pm
content: Prior to the huge rash of bad electrolytic capacitors, I'd say that 80% of all failures I saw in consumer electronics started out as cracked solder joints, occasionally that would blow other parts.

author: bob225
author_rank: Frequent Contributor
timestamp: October 21, 2018, 09:45:41 am
content: iirc a lot of cb's used a butt joint between the main board and front panel what is all well and good but add in vibrations if used it a car/truck and you will get some movementedit, also multi way board connectors, some manufactures use surface mount type what end up with cracked joints or cracked tracks under the solder mask what are a pig to find (micro cracks) - just the pressure of probing the joint can make them test 100%

author: German_EE
author_rank: Super Contributor
timestamp: October 21, 2018, 04:19:42 pm
content: The other thing to remember is WHY they have included multiple amplifier stages in a transmitter. You can get devices now that have 30dB of gain but using only one or two stages of amplification would be a bad idea because:a) Too much gain in a small space makes an amplifier very unstable. It could oscillate at your wanted frequency or, even worse, oscillate at VHF or UHF where you wouldn't spot it without a spectrum analyzer.b) By spreading out the gain over a number of devices you help reduce IMD. Operating any semiconductor device right near it's limits is never a good idea and, on CB transceivers especially, you want to avoid having your signal bleed on to the channels either side.Look inside an HF ham radio and on the transmit section you might find a first amplifier, a pre-driver, a driver and a final with an input of about 1mW or 0dBm and an output of 100W or 50dBm. That's a lot of gain, even with some signal lost through the filters.

author: tkamiya
author_rank: Super Contributor
timestamp: October 23, 2018, 04:36:36 pm
content: So.... How is your repair going?

author: Jane
author_rank: Frequent Contributor
timestamp: October 23, 2018, 07:13:19 pm
content: It seems to be repaired. I resoldered some parts on a front PCB board (a cold solder joint seemed to be there) and one more diode was disconnected.Thank you for your advice and explanation.

author: ploughna
author_rank: Newbie
timestamp: October 24, 2018, 11:03:09 am
content: I repaired CB's part-time to fund my way through college many years ago. By far, the most common issues were:- audio chip blown from connecting power the wrong way around- RF outputs blown from transmitting into bad antenna (high SWR)- modification attempts by owner to either increase power and/or extend frequency range

author: bob225
author_rank: Frequent Contributor
timestamp: October 24, 2018, 11:45:04 am
content: Quote from: ploughna on October 24, 2018, 11:03:09 amAll of the above, some rigs use to blow the £"$£ out of the main power track, you would usually find a bit of foil wrapped around the fuse as it had blown for some reasonI had the odd ones with iffy PLL chips - usually after they had been tweaked by a local 'Rig Doctor'

author: tkamiya
author_rank: Super Contributor
timestamp: October 24, 2018, 09:19:29 pm
content: I don't do CB but I repair 80s ham radio rigs for fun. Cold solder joints are very common and so as broken traces. Those phenolic boards absorb moisture and basically rots.... As much as those things cost, I would think manufacturers could afford better material. But single sided phenolic boards are the norm.Hardest one for me is modifications users made. There are no documentation and I have no idea what they are supposed to do. Sometimes I'm not sure if the changes even worked at all. I usually try to undo-them all.