Diagnosing Tuner Board questions

[Lasonic TRC-920]

Right, if you go there you're not thinking about resale value, just for the sake of learning something new or proving to yourself it can be done. There's some truth to getting in too deep with something toshik, but I've found it's a good way to have some fun learning something new that could be useful later if the motivation is there.

I was talking about Hail Mary type stuff, in that zone anything goes. I didn't mean to imply I had a line on tuner boards, but something modern from China might work nicely, like the DJ Techs use with a manual tuning pot. Mount that on the original tuning wheel, maybe use a different pot if the dial scaling isn't 1:1. The band switch on something modern/digital would be way simpler than the monstrosity on the original board, so wouldn't be impossible to mount in place. Using an analog tuner scavenged from another box could work but there are more limitations - the tuning wheel takes precedence over everything else, at least the band switch stands a chance at being jumpered.

Anyway, back to reality, haha
I'd say if you can remove the crushed coil, work out all the kinks and roll it back up on a straw or something of similar original diameter, maybe it will be close enough to at least know if that's the reason it doesn't work at all. I'm suspicious there's something else wrong if you're not even hearing output of any sort at all, even just some quiet white noise on AM.

Good call on unsoldering and recoiling that wire. I was going to use a heat gun I have that focuses the heat to a point and melt the wax, then I can unsolder it. I will count the hoops on the other tuner board, make sure it is the same.

Just waiting on the chips I ordered, will be a few weeks.

 

[BoomboxLover48]

Hopefully you will soon sort out the problem!

 

[Superduper]

You don't know that the lower coil is the same as the upper coil. Without a circuit diagram and/or parts list, will be impossible to know, although I suppose you can test the mh value of it with a meter. The fact that it's squished won't cause the tuner to fail (die), those coils usually only affect FM and are for alignment purposes. After installation, minor adjustments are made by reshaping the coils ever so slightly (expanding, contracting like a slinky) to get it right, then it's set in wax to hold it's form. With it totally flat, then band coverage will definitely be off and you'll never get it right unless you know what you are doing. In general, those coils usually adjust the FM band coverage and are adjusted and tweaked along with the coils in the square 4-coil pack in turn. I'm not going to say don't bother but don't expect the tuner to suddenly work again if you get the coil looking right again. On the other hand, if you replace something (like that IF IC) and it suddenly works again but the FM only tunes from 103mhz to 112mhz, and you are wondering WTF, you can be pretty sure that the flattened coil has something to do with it.

 

[Transistorized]

I'm sure this has already been checked but I always go to the weakest link. Not to say that capacitors, resistors, transistors and diodes can't fail but being completely dead could be a slide Switch or slide control contact. I noticed what almost looks like a record bar in the middle of the tuner board. Is that some kind of selector? If not a connectivity issue then I guess a capacitor/diode being shorted could cause a dead tuner.

Anything getting warm on the board?

 

[hopey]

Those trim pots can fail.

 

[BoomboxLover48]

I'm sure this has already been checked but I always go to the weakest link. Not to say that capacitors, resistors, transistors and diodes can't fail but being completely dead could be a slide Switch or slide control contact. I noticed what almost looks like a record bar in the middle of the tuner board. Is that some kind of selector? If not a connectivity issue then I guess a capacitor/diode being shorted could cause a dead tuner.

Anything getting warm on the board?

The long switch is the band selector.

 

[Lasonic TRC-920]

The tuner chips have arrived, will started with the least damaged tuner board, the one with the coil's intact. See if this chip makes a difference.




Update: This has pretty much played out, as expected in the conversation above...

Installed the new Sanyo A1201 chips into both tuners.

Box A, which had the undamaged coils on the PC Board came to life and is tuning in channels, no problem. TUNER FIXED

Box C, which had damage to one small copper coil on the tuning board came to life, but with limited tuning strength (as predicted by Superduper).

Without a schematic, things are limited. But I do have the schematic for the 931, so I will have a look to see if there are any similarities. I was also having a look at routing too and from that damaged coil and I think I have traced lines to new by adjustment pots, so, since I have little to loose, I may tinker with those.

If not, I will be on the look out for another tuner board. If any one has a 920 tuner board, please hit me up.

Thank you to everyone who had input into this thread, especially DOCS who suggested replacement of that A1201 chip. Can't do these repairs without all of you!

 

[BoomboxLover48]

Good news!


What could have caused the tuner chip to fail? I just wonder!

 

[Lasonic TRC-920]

Good news!

What could have caused the tuner chip to fail? I just wonder!

No idea, but all three of the 920's I am currently working on have all been tinkered with, had boards swapped or otherwise been cobbled together multiple times in the last 35 years. So it could be anything.

 

[Superduper]

Good news!

What could have caused the tuner chip to fail? I just wonder!

No idea, but all three of the 920's I am currently working on have all been tinkered with, had boards swapped or otherwise been cobbled together multiple times in the last 35 years. So it could be anything.

Poor design.

 

[caution]

Indeed - an air core coil right next to a screw hole, that's just asking for trouble!

 

[Superduper]

Hmm. Somebody took a short-cut, cut off the diode from the top side, and replaced it by soldering a new diode by tying the leads to the old snipped leads instead of just installing it right. Diodes in tuners are usually quite specific. Wonder whether it was a compatible replacement since there's no SM, no parts list, and diodes are usually very difficult to identify as they often don't have markings. It's not an uncommon repair technique to snip and tie, but usually only on very difficult or poor access repairs and really on a repair like this, shouldn't have been prohibitively difficult to access the rear. Probably lazy.

 

[hopey]

It makes me wonder if you re-sweated the original IC's on the board would that have fixed the problem?

As this has been worked on before maybe static could have fried the chips?

 

[Lasonic TRC-920]

It makes me wonder if you re-sweated the original IC's on the board would that have fixed the problem?

As this has been worked on before maybe static could have fried the chips?

I had crawled over all three of these boards with magnifying glasses, I did resolder a few points, but these looked ok.

 

[fyj2972]

Common vintage boombox tuner issues
1) Dirty band selecter switch
2) 19 KC adjust pot cause FM no stereo
3) the capacitor in IF transformer change value, FM 10.7 39pf-51pf ,AM 455/465 150-180pf.
I will show you how to deal with those problems later.

 

[goldstar]

I know this is an old thread nobody will read this but it is important on these old boomboxes. Most of them use cheap cheap electrolytic caps in them. I have worked on the Lasonics. If the switches are working you should be getting audio when radio is selected. If not, check the voltage going into the tuner board. If you get a reading of 8 or 9 vdc, you have shorted capacitors. Start with the 10v caps. Most times that will get you up and going.

Side note, if anyone knows of where I can get my hands on a Lasonic trc-931 tuner board let me know, I have one where someone was screwing with the IF coils and who knows what else. I'd be happy to buy one off someone that has one.