#32 Random ham radio repair Kenwood TM 621A No RX

good evening everyone they own the bench we got a package that came in just received it a few moments ago so we're going to open it up and I think I know what this is and it should be a customer's item before we talk note yep what I thought it was have the fun today the weather's completely crazy last week it was freezing in the day of 70 degrees okay and closes the in the ysk 8:57 box is like in wood TM 621 a 2 meter 20 to 20 Mobile's has a receive issue on the 220 side you know good and talks about please go ahead and fix it let me know listen they hadn't you know and thanks very much for repair on this and I like your VR videos alright this is from Beale with hair on one of these in my prior videos I'll link that one down to life another very nice-looking Kingwood TM 621 hey this is the FM transceiver covers to 2 meter and to 20 megahertz bands and the problem with these radios yes there's another radio I can't remember the model number they it's just like this but it's 2 meter 440 and as a service manager for the 2 meter 440 version 2 problem is there's no service information for the 2 meter 220 so when you got to walk on the 220 board you have no schematics to go back you have no service information you cannot use the 440 schematic because it's a complete different layout so when you're working on these you'll just have to walk it on through you know trial and error there's a few things we can use in the other service manual to find out of this but we'll hook it up more verify what be able to say in and see what we can do with it okay I've got the radio on the 2 meter side connector to the dummy lady and had power and we had a big off service - set to 144 megahertz and we'll turn the unit on headwater yeah the scripture is working and will key the radio work and yes we have power out on the 2 meter side ok we'll switch over to 220 and check that I went down the 220 side so what we'll do is check transmitting 220 we have plenty of power left that looks good test the deviations ready to cast winter Radio check one two one two about 5 kilohertz so that looks good so let's go to back to 2 meters and check the receiver ok the radios back on 2 meters so we're turning down I have for soap dirt won't go to generate and we got received down to about - 118 DBM so that looks good now we own the 20 side and we'll turn the generator on and the radio starts receiving on 220 somewhere around 178 dB and we get full quiet in about minus 50 DVDs okay so we know that the we are verified that the 20 megahertz received is not receiving correctly so we won't take the case off and then we're going to look at it and you know through experience already knows what's wrong with it but let's say I don't have the experience how do we determine what is wrong with this radio we know that there's a problem with the 220 receiver but we have no documentation we have no schematics we have nothing to go on so what's the can we get down into that and figure this out so I have the top and bottom cover off the radio and we're looking at it and we we're lost we don't we don't know which way to start you know which side is which which is the true meter which is just 20 you've got kind of a bit of a giveaway if you look at your antenna connectors one side is listed 144 while the other side has 220 and what this helps you in is that the 144 mega hurt goes on through the topside this is your PA area this is your power amplifier for your 2 meter side there's your coax coming in and you'll see this coax goes down into this sir and it comes in here you can see the color that comes in put it here now your power amplifier is over here on this side so here is where you should fiving the coax jumper that comes from the exciter side into the power amplifier now you can pull the data ship on this m57 7/4 chill and you can get the pin outs for and you can find what you take input so if we follow that piece of coax see it goes right here so we know the inputs over here on one of these two pins so we know that this does not receive cable so we'll look here and you'll see the cause and it comes in to this side and go straight into this connector here that is our a one so you can get that in there so all in one is all receiver input into the bottoms so here you can see our band pass filters and our four stages so you know how can we check the RF amplifier where he is the RF amplifier well if you look over top of the radio we know that since these are the first stages our Orion amplifier has to be closed somewhere here so if you look at this stage you see no transitions on top so we know that our RF has to be underneath so I removed all the screws and disconnected all the cables to this board and what we'll do we're going to pull this forward out and we're gonna start here at this connector and we're going to trace back okay so let's see if we can basically make a block diagram of the circuit of this radio this way might give you a little bit better understanding how things are laid out in it okay start off with the antenna will come in this would be the antenna switch we'll go into a horror story I am this will be our first mixer some crystal builder this will be your main mixer the if' stage and your detector au amp speaker there's four wonderful alcoholics and now let's see what we get here this will be a PLL come back and lean into the mixer this will be our TX amp and buffer stage to our PA we get our amplifier and whenever goes back into the switch so this is basically just a UH a rough block diagram of halwa to ready to work and you would also pick up your own microphone input so you might suck it here so and the VHF is going to be completely identical to it just flipped around and then I'll share the AEF and mic amp circuit so it's very very easy to figure out it's not really complicated at all ok so let's see if we can get a little bit better understanding here only or a fam let's throw it over here it's all one you you'll find a capacitor to school on the ground and we have another feed capacitor okay and you just a lot of girls users that'll bill output of the FE T and this will go through an adjustable call you should have a resistor going to go in capacitor so that's basically the first circuit so we see right here on one we'll have all input three will be our ground so that's how we can fall off single front that's just a rough sketch of it not to scale and all this is basically gonna be about the same and just about all these lines of chem with radios so you could probably take the Kim wood TM 721 400 megahertz come on it can pretty much follow through just the same way it's not gonna be very much difference it's just in already our front end it's going to have course where the 440s come we'll have helical resonators okay we're looking at the bottom of the board and this is our input connector for the in telling you see the two grounds on it the connector will have two grounds and then your center conductor will be on this end just say you got a ground here ground here then your center conductor so we'll follow this out we'll find the two crayons and I'll send a conductor we come through here through this chip capacitor and we'll follow right on down a little bit and you'll see there's the bypass cap here going to ground and then there's another capacitor that it goes through into this trace when we follow this trace we find this 14 transistor here and this will be your first or EF ampere so at this point we know that since this is a four pin device this is going to be an FPT and that's your RF amplifier so you can take the time and take your voltmeter put it on continuity test go to the input and check the other pins and you can see I hope you can hear the meter beep I'm doing one time just in case you didn't we only input for this transistor and we're showing short on three things so we can pretty much rule that that transistor is no more Guinness so it looks like the radio has took an overload and hits two transitions out this is very common on these 220 boards I don't know why but it's things like real strong stations can wipe out this transistor very quickly our static electricity I've seen one a guy have drove under a drive through port and his antenna raked across the the metal roofing and took out his 220 receiver so what wouldn't even do is remove this transistor and put a new one in so as you can see I've got the connect dress over to this side so I got just how I got the board or unit under the microscope and I got the microscope real close so that you can see the orientation since this transistor has four pins you got to really make sure you put it in the right way you don't want to put it in upside down or you gonna have all kinds of problems so you can see the transistor has three RS written on it so we're going to remove this transistor and install those the new way but we're use hot air but you got to be very careful when doing this because you've got all these other components around here so you don't want to blow them off the board this is a good time if you got some metal tape or some capstan tape that you can put on the board around the RF and this way you don't plug those components off the ball with so here you can see I have covered the board and capstan tape and just to hold the audio components in place when we heat this up and remove these old transistor all right we have a hard out gun may we have it you know Solan girl he putting cutting their own mm-hmm you know that old sorter off of it I don't know I moved back then they didn't usually it free sort of us they like to put fresh solder on it and walk around our for leads and that'll pull all that old contaminated sort of roll okay people have a new transistor okay I mean transistor on the board this is where I would really really really love to have I try knocking off microscope but we don't have that so we got to do it under this USB microscope and when you're doing like this you're looking up and you're not looking directly at your work so there's little confusion at this work quickly I want to see a coin do you there we go and we have a new oil plant installed okay I had so I had to uh get on the cam and look at that real business to make sure Walter wants to do this you're not seeing nothing wrong with that all right he's reinstalled and see what it does okay the boys reinstalled just turn the power on and I got the generator set at minus 78 DVM that's what it's ready a started receiving before when we get our test on 220 so we'll switch over to generate and if you can see even now we have a full scale on s meter we got minus 78 DBM turn it down and she's receiving it about my Muslim 118 okay so just a quick little repair as you see no schematics so we were able to pour the board Turman which board it was you know the problem was in was over the port aboard followed the path to the forest RF amp and change it out and that solved the problem so you know sometimes you have to bite the bullet and do it the old-fashioned way and just you know sort of reverse engineer what you have cause you can't always find documentation on these receivers now if you were to went on Google and searched Kenwood TM 621 a no to 20 receive you would have found some information online and that information is stuff that I posted and also on my other YouTube video of repair one of these so you know there is some documentation out there but we don't always have the access to it so you know there's fifteen minutes and this radio is back ready to go back to the customer anyway hope you enjoyed this quick will repay if you did give it a thumbs up and if you have it place this graph and we'll catch you in the next video thanks

Channel: The Radio Shop

Views: 19,880

Keywords: ham radio repair tm-621a

Id: ifHua5aHoFQ

Length: 31min 55sec (1915 seconds)

Published: Wed Mar 02 2016