Samsung 3 band radio cassette, model: STF55L

[G6ONEDave]

Hi Malcolm,
The end of D6 and D9 is the end nearest to the wavechange switch. My idea was to try and use either of the on board diodes to act as a detector diode. Not forced to work, just trying to think of checks that may or may not give some further clues on the fault. Obviously this would be whole lot easier on a work bench with access to test equipment like oscilloscope, ac millivolt meter and signal generator etc. I would have normally tried a check using ac millivolts but your digimeter does not have any ac volt range below 200v and thus probably would not show signals below half a volt overly well.

Dave

[Formby Mal]

Morning Dave,
Hope you’re well this morning. I went to solder the end 0.1uf Capacitor (from C452) to D6 and then I remembered you saying in a post a while ago that the end is the opposite end to the stripe. So just want to make sure I’m in the right place, please see photo, it’s to the left side of D6 (and the rest of them) next to the long ‘wavechange switch’ that you said? I can get to these on the track side. All the best… Malcolm.

[G6ONEDave]

Yes Malcolm, that's the correct place for the end of the 0.1uf. Remember that the other end of the 0.1uf still needs to be at pin 9 of IC201.

Dave

[Formby Mal]

Hi Dave,
Thanks for coming back…. I tried the 0.1uf soldered to D6 (the other end to pin 9 on C201), but again nothing came out of the speakers, I then tried it with D9 and the same result… just that hiss. The speakers were working as I tried MW and that sounded fine, but at FM nothing, the aerial was connected to TP1, I also remembered to ground the 0.1uf (both times) before soldering to both ‘D’s..... Malcolm.

[G6ONEDave]

Hi Malcolm,
I think that we have now tried everything possible without using more workshop equipment, unless any other forum member can think of another test to help narrow down the fault. To me it's time to get an oscilloscope and possibly a signal generator on the case. Shame that you're so far away from me. Perhaps another forum member that is nearer to you might be preparred to take a look. You would need to start a new thread under the services wanted section.

Dave

[G6ONEDave]

Here is the conclusion to the Samsung radio problem. After trying for help from Forum members in the help wanted section, I decided to offer and Malcolm sent the Samsung to me to look at. This proved to be a strange fault and what follows is my account.

First and with no aspertions cast I did what any other engineer would do and checked all the previous work was effective, which it was (well done Malcom). Next was to check for any further cracked prints on the pcb, which as it happened there weren't any. This was ashame in one sense as it meant that the fault would be more involved.

I then checked continuity of all the tracks on both sides of the pcb and they were all found to be OK. Then I checked all the resistors most of which were the carbon paint variety, and again they all showed sensible readings for in circuit testing. Whilst doing this checking I also checked the plated through holes where the printed resistors made contact to the copper tracks on the other side of the pcb. So far nothing was amiss, which is good but also frustrating as it meant that the fault was still present.

For a while I considered if IC1 and IC201 were responsible due to an internal problem and luckily for me I actually had both types in my stock of components, so I was able to eliminate that possibility. I now knew that the chips were OK.

So what to check next was the question. I decided to check the capacitors in the RF front end, now to do this I lifted one end of each and checked their capacitance with my capacitance meter. The 4,700pf ones all measured at a bit over 5,000pf, so were technically out of spec but not enough to kill reception. Whilst doing this C4 fell out of the pcb after disconnecting one of it's legs. C4 is an 18pf in series with L1 that goes to pin 2 of IC1. So after confirming that the cap measured 18pf, I refitted it and switched the radio back on expecting radio stations to now appear. Well what a disapointment, as there was still no reception. I was thinking about finding a section of brick wall to bang my head on.

So after leaving the set for a little while, whilst sorting some other jobs, I resumed the repair attempt. I downloaded the data sheets for the 2 chips in question types AN7213 and HA12413 to see if there was a detailed description of their operating function, as that seemed to be missing from the service manual. This did point to the fact that the IF chip created feedback to alter the gain of the RF front end (IC1). I also tried injecting a stereo FM RF signal at specific frequencies to no effect. However whilst tuning the Samsung I discovered that at a certain spot it muted out my workshop background music radio (tuned to Lincs FM). So this suggested that the local oscillator in the Samsung was working to some extent.

Back to studying the circuit and trying to check D4, more about that later. Anyway I got back into looking at the circuit and checking the voltages more carefully (probably should have done this sooner). All the voltages on both chips were similar to those shown in the service manual, all that is except for pin 1 of IC201. Pin 1 had 0.3v on it instead of around 1.8v. There are 2 printed resistors (PR203 and PR204) involved, that couple a feedback voltage from pin 4 of IC201 to pin 1. I rechecked these 2 resistors and found that they were OK, I ruled out the chip as this was with the replacement fitted. I checked the ceramic resonator CF202 as I've had issues with these in the past but no it was OK as well. So what else could it be?

Looking at the circuit there is a capacitor from the junction of PR203 and PR204 that goes to chassis/earth. I lifted one end of this cap and tried the radio again, this time I was greeted with the reception of stations. I checked the cap (C202) and it measured 112K ohms but it was not a stable reading. The cap in question is a 0.022uf (22nf) ceramic type, there were another 2 of these caps next to to C202, so checked them as well and found that C211 was also giving a resistance reading, somewhere around 300k ohms and again changing. I replaced both of these along with C201, which tested OK but was of the same manufacturer. The other 0.022uf caps in that circuit are from a different maker and after testing one decided to leave them alone.

D4 according to the circuit is a varicap, this means that the diode has a capacitance that can be varied by reverse biasing it with a dc voltage. The service manual parts list shows it as a 1SS53, which is totally wrong as the 1SS53 is a signal diode and not a varicap. The circuit has a number next to the D4 ref of 3302, so I tried doing a search for 1SS3302 with no luck, I then tried a search for 1N3302 and just got a hit about it being obsolete but to replace it with a diac! I am pleased that the D4 is actually OK as it would would be a complete mare to find a replacement.

The odd thing for me is that these duff ceramic capacitors show no signs of distress and had less than 6v dc across them, although that voltage was variable.

So the Samsung radio now sings again after a long and protracted repair. The radio is currently waiting to go to the courier for return.

I hope that those who had been following the OT find this of interest.

Dave

[Nickthedentist]

Well done indeed, and what a bizarre fault!

I wonder what part the original repair man had condemned and then not been able to source.

[Formby Mal]

Hi Guys,
It's Malcolm here.... well what a fantastic job Dave has done - and Nickthedentist I don't think the 'original' repair man even looked inside the radio?? . I have been communicating with Dave over the course of time and concidering his knowledge, compaired to mine, he was always patient and tolerant, with expert directions. I am so pleased he 'triumphed' in the end. The radio was a gift to my late parents on one of their anniversaries, so it will bring back some memories. Thank you UK Vintage Radio for being there and with the help of some of the members at the beginning and ESPECIALLY Dave who worked very hard; I bet he nearly threw the radio at a brick wall as well as banging his head against it.
May UK Vintage Radio go on to help many other people like myself and other radio repair men/women.
I am so grateful... Thank You, Thank You... One thing has improved for me with all this... my soldering. My best regards to all..Especially Dave.

[agardiner]

Well done Dave. What a fault. One of those that gives a great sense of achievement when you nail it.

[G6ONEDave]

Thanks guys,
It certainly was a challenge and an eye opener on ceramic caps. Like many I originally never even considered the ceramic cap as being a possible cause of trouble, especially at such a low voltage. I have had them blow holes, spark, detonate and smoke before but that was always at 240vac or higher dc voltages, where they were being used as filters.

Dave

[Formby Mal]

Well the Samsung came back, very well packed - better than my packing - and it sounds brilliant. Lovely memories of my parents listening to it in their caravan on the south coast.
Thank you UK Vintage Radio and especially Dave or G6ONEDave to give him his title. He did work hard and I'm very grateful. All the best everybody... Malcolm.