Diagnosing Tuner Board questions

Lasonic TRC-920

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Feb 16, 2010
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I have two Lasonic TRC-920's with dead tuner boards.

There are no schematic's for this radio. Does anyone have any idea on the best way to trace my way through a PC board or are there common tuner issues to look for.

What I have figured out:

Both tuners are getting power and have grounds. The switches are correctly switching to the "Radio" setting and the amplifier's are live, there is just no signal coming from the tuner board for the amps to amplify.

This seems to be the case on FM/AM and SW1&2

I do have a third TRC-920 that has a working tuner board, but it is a totally different board layout and wiring set up with different main board as well.

Thanks
 

Tinman

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Mar 4, 2019
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I'm currently dealing with a tuner issue with my Clairtone 7979.
I bought a schematic for it but the components on the board aren't marked like every other box I've worked on so it's not much use to someone like me.
My board will only send one station for the entire dial (88-->108)and even that's spotty.
AM isn't much better and the two shortwave bands are pretty much dead.
After consulting with another member, I've focused on the variable capacitor.
I removed the cap from the board and the 4 separate presets still work and receive FM stations fine.
I'm trying to find a new var cap which is proving very difficult for this box.
I didn't mean to hijack your thread but figured it's nice to know someone else is dealing with a similarly frustrating problem.
 

JVC Floyd

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May 6, 2009
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I have noticed with analog tuners when the voltage drops the reception drops off , so it may well be a voltage issue at the tuner board.
 

Lasonic TRC-920

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Hi guys, let me try to give some more information that I have.

Below is a chart I drew up for myself. This chart shows the tuning boards for the three 920's I have. Two of the boards are the same style, one is completely different (the working one).

Board B is the working tuner. These were tested with A & B on mains and C on batteries, so the voltages of C reflect a slightly lower voltage.

In the chart, you can see the voltage values of each wire, on each board. Board B has voltages on all wires (except ground). The important wires to look at are the LEFT and RIGHT channel of boards A & C. They show nothing and that's where the signal is sent to the main board / amp.

I have connected an audio source to these L/R wires and the amp is receiving a signal, nothing is coming from the tuner boards.

Lasonic Tuner Boards with voltages.png

I suspect what ever the problem is with A & C will be the same issue. I'm hoping there is an "Ah Ha" moment. A common issue with tuning boards that someone else here has experienced.

I have crawled over each board with magnifying glasses to look for any obvious signs of damage like a blown diode, resistor, capacitor, ect. But have found nothing burned. I have also checked for loose wires, bad solder joints and cleaned the switch contacts on the AM/FM/SW selector as well as re-soldered the pins on the switch to guarantee it is getting connection.
 

docs

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Jun 26, 2010
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Hmm, there are several stages in radio circuits that I've learnt as I've gone along. Since there is no audio across the entire radio functions, we need to find something which is common to all output, AM/FM.
Maybe have a little look at the 931 schematic tuner section to see if you can see something after the AM and FM sections which is common to both. Not saying that will bear fruit but I think many tuner sections are very similar in design.
There will likely be a wireless amp which is a transistor. Perhaps take a look at all transistors, record their numbers printed on them and take a google at their datasheets. Then do the same with the IC's.
 

Lasonic TRC-920

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Feb 16, 2010
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docs said:
Hmm, there are several stages in radio circuits that I've learnt as I've gone along. Since there is no audio across the entire radio functions, we need to find something which is common to all output, AM/FM.
Maybe have a little look at the 931 schematic tuner section to see if you can see something after the AM and FM sections which is common to both. Not saying that will bear fruit but I think many tuner sections are very similar in design.
There will likely be a wireless amp which is a transistor. Perhaps take a look at all transistors, record their numbers printed on them and take a google at their datasheets. Then do the same with the IC's.
Thank you, I will have a look at the 931 schematic and see if there are any similarities. Will report back.
 

Lasonic TRC-920

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docs said:
LA1201 is a AM and FM amplifier I think. According to the datasaheet it has 5v going in to pin 14.
FM or AM input is fed via pin 2, determined by the switch.
I think FM goes out on pin 14 while AM is out on pin 1.
This might be a common IC across all 3?

https://datasheetspdf.com/pdf-file/541638/SanyoSemiconductor/LA1201/1

I looked at the diagram on lower page 4.
Having a look now..

I ran page 4 through a translator:

10p SMA typ location 1S188FM, ImA = 0.04 ImA typ AMver loag 100 to IFT4 CONV 25C929cx a02 CS AM bor enns LA1201 CIS 0.01 TCI is anlena 39mAçoltIFTS LAIF-3. Built into LAIF-3. However, the arrow (1) is attached. Specified motion is applied to the radiator's hpB * Select 1S188AM AGC 0.1 to open the lance. Open the lance. P is antenna 8T: 5T, pie filer winding, 5mme air core coil 4T (tap = 1T from ground), 5agg air core M RR coil 125, Smm air core * PM IP trap coil 3T (Tap = 2T from ground) * PM starting coil No. 495-4 / 10

Application example 1, PM / AM radio high-frequency break circuit 1. Circuit diagram: An example configured with CPM-2 CAM-mode is shown. CPM2 AM DUTPUT Voc of the configuration example in the previous section for AM IP stage and FM IP stage] 5.0v Ra 1,5% PAM 2SC0 2SC668c * RF AMP REselt 0.7mA 4,% 23: antenna coll 30sc trop 0.01 Qolu J0.005 150 120 BenA typ 1S188FM F13 ImA shot p IMA typ 2nd year SEBAMver loag book 100 100 op IFTA CONV 2SC929cx 002 CS AM ber mas PT2 coll LA1201 0.01 TCI2 anlna asgA IFTE LAIF-3 It has been incorporated. However, the arrow (-) is attached. 20 3 years Ww Depth 1S188AM AGC book 0.1 rseandsartesuneat HH.
 

docs

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Yeh I think it has a few different layouts, if your working board has the same IC, might be an option to switch it out. it is a common element of both functions though may ofcourse be working perfectly.
 

Lasonic TRC-920

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docs said:
Yeh I think it has a few different layouts, if your working board has the same IC, might be an option to switch it out. it is a common element of both functions though may ofcourse be working perfectly.
The one working board I have is now in the finished 920 I just buttoned up, so I don't want to pull it all apart and unsolder the IC.

I do have various other radio boards, some with tuners. I will look for that IC on them.
 

docs

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Ahh got ya, yeh don't be messing with the shelf queen.
It's a possible starting point but others may have a better option of where to start.
 

Lasonic TRC-920

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Feb 16, 2010
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docs said:
Ahh got ya, yeh don't be messing with the shelf queen.
It's a possible starting point but others may have a better option of where to start.
Haven't been home all day, going to dig deep into this tomorrow and see if I can identify and test some of these IC's and maybe locate them on other old boards I have.

Will up date tomorrow
 

Superduper

Member (SA)
It won't cause failure of tuner, but THIS lower coil looks disturbing to me. It probably should have looked more like the upper coil, and if it did, it means it probably won't be able to tune worth a dang in, even if it weren't dead.

Lasonic TRC-920 tuner coils.jpg

The voltage measurements of the wires are interesting but of no real value. They will vary and change depending upon whether you have tuner set to AM/FM, stereo/mono, or depending on where you have your tuner tuned, the tuning strength of dialed location, and whether it is locked into a stereo channel.

If your DMM has a diode test function, you should probably test and verify all of the diodes, as they are essential in tuner function. Does your tuner have any noise at all such as white-noise that gets louder as you turn up the volume, or completely silent? If it's completely silent, you may in fact want to look into the LA1201 chip. Don't go looking for old ones, you can still get NOS ones, probably $5-$10. Or get NTE1003 or ECG1003 which may be compatible.
 

caution

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Mar 25, 2014
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Keep in mind you can always use a different (and better performing) tuner board should these fail to come back to life. The FM stereo light and stereo mode wires are standard features of the MPX chip, Even if it doesn't have a tuner indicator light, it could be added with a little effort.

Aligning the tuning wheel and the band switch could be tough but I've seen some impressive hacks here over the years.
 

Lasonic TRC-920

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Feb 16, 2010
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Superduper said:
It won't cause failure of tuner, but THIS lower coil looks disturbing to me. It probably should have looked more like the upper coil, and if it did, it means it probably won't be able to tune worth a dang in, even if it weren't dead.

Lasonic TRC-920 tuner coils.jpg

The voltage measurements of the wires are interesting but of no real value. They will vary and change depending upon whether you have tuner set to AM/FM, stereo/mono, or depending on where you have your tuner tuned, the tuning strength of dialed location, and whether it is locked into a stereo channel.

If your DMM has a diode test function, you should probably test and verify all of the diodes, as they are essential in tuner function. Does your tuner have any noise at all such as white-noise that gets louder as you turn up the volume, or completely silent? If it's completely silent, you may in fact want to look into the LA1201 chip. Don't go looking for old ones, you can still get NOS ones, probably $5-$10. Or get NTE1003 or ECG1003 which may be compatible.
I did see that (I had looked over the board completely with jewelers glasses) and figured that wasn't good. All of the 920's I currently own have been rebuilt multiple times over the course of their life, so I'm not surprised there is some damage here. Here are the coils on the other board.

2020-01-23.jpg

DOCS said the same thing about that LA1201 chip. I ordered two, I will swap them out. At least it would check that off the list.

As for static. Both tuners are totally dead. No fuzz, static of shifting of noise when scrolling through the dial range.

I just went through all my old PC boards and don't have any LA1201 chips, so I will have to order them. Was hoping I could just have one to test, but that's OK.



caution said:
Keep in mind you can always use a different (and better performing) tuner board should these fail to come back to life. The FM stereo light and stereo mode wires are standard features of the MPX chip, Even if it doesn't have a tuner indicator light, it could be added with a little effort.

Aligning the tuning wheel and the band switch could be tough but I've seen some impressive hacks here over the years.
That is interesting. I guess that is an option in a last ditch effort "Better than nothing" situation. Not that these boxes have a ton of value, but as we know, most collectors that I may sell or swap these radios with down the line aren't typically into "modified" / "non stock" boxes.

BUT, I would like to look into that. Do you have any more information on these stand along tuners?
 

caution

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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.
 

BoomboxLover48

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toshik said:
Everything can be fixed but without knoledge and experience you might be wasting your time.
News to me ... :w00t: :w00t: :w00t:


You remind me of Super glue commercial in the 60s or so..

Can fix anything except a broken heart!