Samsung 3 band radio cassette, model: STF55L

[G6ONEDave]

All the components will be soldered, so legs are pushed through the pcb and then soldered to the tracks on the opposite side to the component. However having looked at the pcb layouts there are some tracks and resistors that are printed onto the component side of the pcb. The tracks are silver in colour and the resistors are black rectangular shapes. Try not to damage these because you can't solder them. If damaged you will have to run wires and or resistors on the print side of the pcb to in effect link across them.

Once you have a test meter, one of the checks that you should do is to check the push button switch contacts to confirm that they have continuity when in the FM mode. You will do this check using the low ohms setting, they should read zero ohms or very close to, again you could post the results from these tests.

Dave

[Formby Mal]

Thanks Dave, I will try for the multimeter. Failing that I think I'll post on the other site to see if there is anyone near me who could perhaps try to fix it for me. In the meantime thank you very much for your help. Cheers Malcolm.

[Formby Mal]

Hi Guys, hope you are all well. I'm not sure if you can remember me; I've got a Samsung STF55L and we left it where I was trying to get hold of a multi meter, which I now have. Just wanted to check about the setting on the meter and because there are numerous soldering points on each side of the three 'FM chips' where does one put the testing points? Once again everyone thanks for all your help. Malcolm.

[G6ONEDave]

Forgot to ask in the last PM, what test meter do you have, perhaps you could post a pic of it, this will help with more info on how to use it.

Dave

[Formby Mal]

The Meter is a Splinktch DM2... Cheers Malcolm.

[Nickthedentist]

Nothing wrong with the meter, Malcolm, but the black lead belongs in the central socket!

[Formby Mal]

Hi Dave, and anyone else if looking, I tried what you said earlier (with black lead in centre and power on) and had trouble finding the correct points on the chip, the IC 201. I then tried the ohms test (with the power off) that you said, also earlier, and had trouble getting a reading, just nothing. With the volt test at first I did get a reading of 20 on what I thought was terminal 1. I tried what I thought was terminal 7 and it showed 90, 70, so I've no idea what is happening. I'm having trouble in getting to the chips anyway, I've attached 2 photos showing what I am up against, 1 shows the chips and 2 shows the other side (flipped top to bottom). Thanks Dave, and everyone that tried to help me but I think I'm out of my depth. I will hang on to the set... who knows in the future. I might try the other site on here to see if anyone lives near to me. Once again thank you all very much...especially Dave. Best regards, Malcolm.

[G6ONEDave]

Clip your black meter lead to that long metal switch body this should be at 0v, then touch your red meter probe on each of the IC201 pins in turn, which appear to be getable from the top side. With the meter set as previously PMed to you write down the voltages, remember you want the 20vdc meter range. Once you have done that then compare them with what is printed in the service manual. This will show if that chip has anything amiss on it's pins. Do the same with the other 2 chips but you will have to do those from the print side. Then compare the results, please post thiose results so that forum members can advise further.

After that we'll possibly get into resistance readings. BTW It's a guess but I am taking it that the meter displays 'OL' when it's not measuring, it might display something else but whatever it is it will change when taking a voltage reading.

Dave

[Formby Mal]

Cheers Dave... well, if you're not giving up I won't either, I'll give that a try, fingers, legs, eyes and anyhting else crossed. Malcolm.

[Mr 1936]

Hi Malcolm

I have drawn a little guide to the standard way of numbering the pins on a dual inline IC, see attached.

Nearly all data sheet voltages will be measured relative to circuit ground, so your black meter lead can stay attached to that. Use a clip to keep your hands free.

The red probe needs a fine tip so that you can avoid accidentally bridging from one pin to the next one. A sharp tip is useful, as it helps penetrate any surface corrosion, flux etc and connect properly to the pad. The tracks are usually covered in thin tough varnish of some sort, so it's no good probing the track itself, stay with the IC or component pads.

[Formby Mal]

Thank you for that, it helps. Malcolm.

[Formby Mal]

Well… I did what Dave suggested and with the help of the chip diagram from Mr. 1936, I managed to check all three chips.
IC 201 seems to be OK: the volt check:- Point 1 = .32 / 2 = 1.19 / 3 = 00 / 4 = 2.04 / 5 = 1.34 / 6 = 2.12 / 7 = 2.09 / 8 = 2.45 / 9 = 5.77 / 10 = 5.99 / 11 = .63 / 12 = 2.11 / 13 = 00 / 14 = .90. The Ohms check was a bit intermittent showing 1,2,3 then nothing on all points.
IC 1 chip seems to be the one with the problem as there were no readings at all on Volts or ohms.
IC 251 chip volt check:- point 1 = 3 / 2 = 2 / 3 = 6 and the rest were all showing 0 as for the ohms test they all showed a high reading except for points 6 & 7 that showed 0.
So I’m wondering if it is chip IC 1 at fault as it seems, I think, that the Schematic Diagram is showing IC 201 as the AM chip? Being as it’s only the FM that has stopped working. What do you guys think?
Thank you very much for all your help. Malcolm

[Formby Mal]

Thought I would attach the Schematic Diagram.. Cheers. Malcolm.

[G6ONEDave]

Hi Malcolm,
There is definately something wrong with most of those voltages when compared to the voltage charts for those chips. IC201 is the IF amp and detector for both AM and FM, so is very relavent in both modes.

The fact that a lot of those voltages are either low or missing suggests that there is a supply missing or has gone very low. Examples are IC201 pin 1 should be between 1.3v and 1.85v but you only have 0.32v. All the measured voltages should be within probably more or less 0.2v of the voltages printed in the charts at the right of the circuit diagram.

I would study that circuit again, you are looking for supplies common to all 3 chips. This is probably a positive voltage issue. Check that the band change switch is OK first. At the lever end of the switch section 5 (S1-5), look at the following page 10 which shows the switch labelling and track pattern. The contacts to check are shown on the right of the switch but you will have to measure volts from the track side. There appears to be a supply coming through the switch contacts and it might be missing. You should have about 9v at this point. There are 5 contacts on the switch but the switch is a 4 way. The fifth contact is the common which is the 9v input to the FM and AM stages.

To check further you could do voltage checks on S1-5 for all the ranges and let us know what you have. Remember it's just section S1-5 that we need voltages for and not the rest of the switch sections at this time.

We'll look at what comes next once we know the switch voltages.

Dave

[Formby Mal]

Hi Dave,
Thanks for your reply. You’ve lost me a ‘wee’ bit, could you explain what you want me to do. Do I still attach the black tester to the outside of that switch – component side – and then I’m not sure where to put the red tester when you say “The contacts to check are shown on the right of the switch but you will have to measure volts from the track side” I’ve looked on the right and cannot make out where. I’ve sent a photo of S1-5 track side in the hope that you can show me… (the fluff is where I cleaned the PCB with photographic lens cleaner and cotton wool buds) Cheers Dave. Malcolm.

[G6ONEDave]

OK Malcolm here goes. Conect the black meter lead to the switch body again and set the meter to DC volts 20v range. Now looking at the trackside of the pcb under the switch nearest to the lever end of this switch. You will see a track going between the common of S1-5 and S1-6. These 2 switch contact sections are side to side with each other. You want page 10 of the manual and it's the lefthand drawing labelled PCB layout. Now the band switch has 8 sections and each section has a reference these being S1-1, S1-2, S1-3, S1-4 etc upto S1-8. The end of this switch that you want is near the pcb edge where the other switches are mounted to the pcb and just below the variable capacitor. The capacitor is the square line drawing with the white circle centre.

Now what you need to do is measure the voltage on that track that joins the S1-5 and S1-6 contacts with the red meter lead, you should have 9v dc or very close to that. I suspect that it will be there, so next move your red up by one solder contact on the same switch towards the pcb edge. The voltage hear will depend on what is selected by the switch. One of the switch selected positions should display the 9v again. Once you've done that do the same for the next solder pad for that switch, you should get the 9v reading again. This should be true for all four of the switch positions of S1-5.

Please report back with your results, of course this requires the radio to be switched on for these voltage checks. Then we can go to the next stage of testing. Hope this helps to clarify.

Dave

[G6ONEDave]

I hope this image helps to make life easier for you. I have highlighted S1-5 in yellow and have drawn a red line to the common track (your starting point). The pcb drawing shows the components from the component side with the tracks underneath. The tracks are on the solder side of the pcb.

Dave

[Formby Mal]

Hi Dave, and anyone else who may be looking. Thanks for the info and diagram which was a great help. I did the checks and have attached the results, they all seem to be roughly 6.81 volts...

[G6ONEDave]

Hi Malcolm,
After re-studying the circuit, those 6.8v readings might be OK as they are on the output of R102 a 100 ohm resistor in series with the 9v supply line. Now if you look carefully at the component side of the pcb there is a row of 3 IF cans, they have a central adjuster in the their tops. The can nearest to the band change switch (S1) has a printed carbon resistor PR206. It's the black rectangle between a silver strip at each end. It's next to the IF can T101 at the side nearest to the pcb edge where all the controls are located.

With the radio turned off use your resistance meter range (200 ohm) and measure it. It should give a reading of 100 ohms or less. Next again on ohms measure from pin 11 of IC201 to the end of this resistor. You want the resistor end furthest from S1 and you should get a zero ohms reading. If either of these resistances are not as described, it suggests that that is where the fault lies. My guess for now is the resistor having gone high value but it could also be the silver paint track between it and the radio stages.

I'll wait for your next measurement results.

Dave

[Formby Mal]

Hi Dave, thanks again for your help. I thought about what I said on the PM and looked at the PCB with the components and realised that yes I need to use both leads on the PR206. The results are…
The PR206 ohms reading is.. 97.2 and the ohms from IC201 pin 11 to the end of the PR206 is showing 0.9. The points either side (as there are 16 points on this) start off high and drop to 0. I did the volts test on the PR206 and the point nearest to the SI switch showed 6.8 and the other showed 5.96.

Hope all this helps. Thanks Dave. Malcolm.