Sanyo DC6100K Quadrophonic - FM issue

[lonnyholl]

Hello Forum
After some direction really.
Ive just got this 1970s 4 channel Receiver/Stereo system a Sanyo DC6100K.

It had a blown fuse to start my journey with and I have replaced Rectifier diodes and PS Electrolytics and it is up and speaking again.

All inputs working including AM..... however.

FM is not working - in that - lots of hiss that amplifies...but no tuning of any stations at all can be made....nothing is picked up...even with external antenna...the tuning meter never flickers.

I dont know much about FM.

The Radio Circuit has two IC's Sanyo LA1201 and LA3301

The schematic try as I might is not online- it would have been provided with the user manual originally...the nearest manual is a DC8500K which I have the schematic for...cant be sure but its probably quite similar

Any thoughts from members about where and what to look at will be most appreciated.

[Cruisin Marine]

If you have a 10.7 MHz source, see if that quietens the noise if injected even via a loop held near the area marked as FM 1ST, it doesn't need to be directly connected.

More likely is that the Local Oscillator is not running, and that is marked FM OSC.
A diode probe will indicate oscillation or not. A multimeter checking the voltages on that transistor should also give you an idea of what is going on.
Another clue is , if you have another receiver place that close by and listen for the Local Oscillator signal, that could be either 10.7 MHz Above or below the intended receive frequency.
Look for suspect decoupling capacitors, especially as something took out the fuse.

If your version is a PLL/ Synthesised version of that analog receiver on the supplied diagram, it would probably become much more difficult to track down.
Good luck and Merry Xmas.

[lonnyholl]

Thx Cruisin

I will be investigating later today - Ive got no way currently to create a 10.7 MHz signal at the moment...but I can try bringing another receiver closer as suggested and will check transistor voltages.

[Cruisin Marine]

If you have no 10.7 source to hand, you can also use the 2nd receiver as a poor man's signal generator if you don't have one.
Again, the Local Oscillator will be 10.7 MHz away from your desired receive frequency, you can tune the 2nd portable receiver to give a signal if the mixer stage is working, if it isn't you have a fault there and not necessarily with the local oscillator = FM OSC.

[lonnyholl]

Hello Cruisin

I brought a 2nd receiver and its antenna close to the FM IST section (tuned it to approx approx 10.7mhz either side of the faulty Receivers frequency) ...nothing no difference noticed on the 2nd Receivers output!

With testing the mixer stage (using the 2nd Receiver as a Signal Generator) ..I'm not sure where that is? would we be talking near the FM OSC (at TP1 on the Schematic)? I assume I would have to bring the 2nd receiver physically very close ...does it need to be tuned into a station...or not matter?

I should mention I do have an old dusty 10mhz Trio Oscilloscope, If it could help with this.

Thanks again for your help with this.

[hayerjoe]

Originally Posted by lonnyholl

Hello Forum
After some direction really.
Ive just got this 1970s 4 channel Receiver/Stereo system a Sanyo DC6100K.

It had a blown fuse to start my journey with and I have replaced Rectifier diodes and PS Electrolytics and it is up and speaking again.

All inputs working including AM..... however.

FM is not working - in that - lots of hiss that amplifies...but no tuning of any stations at all can be made....nothing is picked up...even with external antenna...the tuning meter never flickers.

I dont know much about FM.

The Radio Circuit has two IC's Sanyo LA1201 and LA3301

The schematic try as I might is not online- it would have been provided with the user manual originally...the nearest manual is a DC8500K which I have the schematic for...cant be sure but its probably quite similar

Any thoughts from members about where and what to look at will be most appreciated.

Strangely it is really hard finding manuals for a lot of mid 70s stuff especially from the big Japanese manufacturers like Sharp and Sanyo, its very frustrating. Where did you find the DC8500K schematic? I've been looking for Sanyo DC4800k and a Sharp GS-5820 manual for years. Surely Sharp and Sanyo have them in their archives? wish they'd just put them online for people keeping their old systems going

[lonnyholl]

Originally Posted by hayerjoe

Strangely it is really hard finding manuals for a lot of mid 70s stuff especially from the big Japanese manufacturers like Sharp and Sanyo, its very frustrating. Where did you find the DC8500K schematic? I've been looking for Sanyo DC4800k and a Sharp GS-5820 manual for years. Surely Sharp and Sanyo have them in their archives? wish they'd just put them online for people keeping their old systems going

Agree, I did find a guy who had one for the DC6100K but he is not sure where it is..he may have it in storage...made me think to keep an eye out for any that come up for sale and ask the seller if they have the manual. Other than that...if you have a long standing local audio repairer - might just have one. I put a few feelers out ..hope something comes up for us both!

[Cruisin Marine]

Firstly, let's make sure we are talking about the right devices.
Q101 is the Front end, or RF Amplifier that connects to the antenna, that feeds
Q102 is the mixer converter, the one marked FM 1ST.
Q103 is the Local Oscillator, it is marked FM OSC This too feeds Q102 the mixer stage.
The RF amp signal off air is mixed with the Local Oscillator which is either + 10.7 or - 10.7 MHz away from the wanted receive signal.

Try these tests to start with.
Firstly, see if the Mixer stage is working on the Sanyo.
1. Tune your Sanyo to 96.3 MHz, and tune your nearby 2nd portable receiver to about 107 MHz
2. Tune your Sanyo to 107 MHz and now tune your nearby 2nd portable receiver to about 96.3 MHz
Either way, you should get some sort of signal coming out of the mixer on 10.7 MHz.
You might be able to see this on pin 5 of IC101 on your oscilloscope, or on the collector of Q102, the levels will be quite small and tricky to read though.

Next check that the Local oscillator on your Sanyo is working.
Get your 2nd portable receiver and
3. Tune it to 96.3 MHz, and tune your Sanyo receiver to about 107 MHz.
4. Tune your 2nd portable receiver to 107 MHz and now tune your Sanyo to 2nd portable receiver to about 96.3 MHz

If you can't get results with either test look at the appropriate stage with a voltmeter to see what is happening, it will be worth doing that anyway and report these findings back here.

The next stage would be to identify those devices and try to find replacements or equivelants if necessary.

[lonnyholl]

Originally Posted by Cruisin Marine

Firstly, let's make sure we are talking about the right devices.
Q101 is the Front end, or RF Amplifier that connects to the antenna, that feeds
Q102 is the mixer converter, the one marked FM 1ST.
Q103 is the Local Oscillator, it is marked FM OSC This too feeds Q102 the mixer stage.
The RF amp signal off air is mixed with the Local Oscillator which is either + 10.7 or - 10.7 MHz away from the wanted receive signal.

Try these tests to start with.
Firstly, see if the Mixer stage is working on the Sanyo.
1. Tune your Sanyo to 96.3 MHz, and tune your nearby 2nd portable receiver to about 107 MHz
2. Tune your Sanyo to 107 MHz and now tune your nearby 2nd portable receiver to about 96.3 MHz
Either way, you should get some sort of signal coming out of the mixer on 10.7 MHz.
You might be able to see this on pin 5 of IC101 on your oscilloscope, or on the collector of Q102, the levels will be quite small and tricky to read though.

Next check that the Local oscillator on your Sanyo is working.
Get your 2nd portable receiver and
3. Tune it to 96.3 MHz, and tune your Sanyo receiver to about 107 MHz.
4. Tune your 2nd portable receiver to 107 MHz and now tune your Sanyo to 2nd portable receiver to about 96.3 MHz

If you can't get results with either test look at the appropriate stage with a voltmeter to see what is happening, it will be worth doing that anyway and report these findings back here.

The next stage would be to identify those devices and try to find replacements or equivelants if necessary.

Thx Chris..thanks again for your help!
I tried all 'two receiver tests' - nothing - not a sausage!


Measured Transistor Voltages - I've compared/measured to referenced from DC2700 & DC8500 Schematic's - remembering they are both similar (but not identical to the DC6100K) and the DC2700 Schematic is more readable.

Summary
- TR101 seems ok...TR103 Emitter is 0Vdc should be 1.5-2.2Vdc I believe ?!-
Collectors on TR102 & TR103 a bit high - Reading 9.56Vdc when Manuals suggest 8.6V

Pin5 on IC A1201 reads 1.32Vdc - DC2700 Manual suggests that should be 0.5V

See attached pic for full readings.

[lonnyholl]

A couple of photo's of the Radio Board for reference.

[lonnyholl]

For Reference Set of Measured Voltages currently across IC A1201
Pin
1 2.5Vdc
2 3Vdc
3 0.73Vdc
4 1Vdc
5 1.32Vdc
6 0.6Vdc
7 0Vdc

8 7.1Vdc
9 6.2Vdc
10 0.27Vdc
11 0Vdc
12 2Vdc
13 0Vdc
14 6.66Vdc

[lonnyholl]

Posting the Schematic of the DC2700 unit for reference - again similar to DC8500 and DC6100 but a clearer schematic Scan

[Cruisin Marine]

TR103 Emitter is 0Vdc should be 1.5-2.2Vdc I believe ?!-

That is most strange?
That would indicate a short from emitter to ground, I can't see how that is happening unless there is a mechanical short, a solder splash or something.

I suggest you check the transistors with a multimeter next. Base to collector and base to emitter, they should only flow one way, make sure the power is OFF when you do it though.
Also test for a short on Q103 emitter to ground.
You won't be able to test Q101 this way, you would need a curve tracer or a special tester for that as it is a FET.

[lonnyholl]

I rechecked TR103 and its def at 0v
Also checked either side of R104 and its consistent at 0v with power on.

C165 on this unit has one side to Earth and the other to the Emitter..do you think I should try lifting its Earth leg to try..would that cause any issue in of itself do you think? ..saying that... I cant measure any short to ground on the Emitter...most odd...is it possible the transistor is at fault ?

I tested transistors for flow in circuit with power off..however I could not get any definitive readings from either ..maybe affected by other components?

I also measured resistance of junctions in circuit fwiw

Both 2SC668 were identical
BC 1.8K
BE 3K
CE 1.8K

[Cruisin Marine]

C165 on this unit has one side to Earth and the other to the Emitter..do you think I should try lifting its Earth leg to try..would that cause any issue in of itself do you think? ..saying that... I cant measure any short to ground on the Emitter...most odd...is it possible the transistor is at fault ?

Yeah, lift that cap leg up and see what happens.
It might be worth taking TR103 off the board and test it, so there is no loading of measurements from other components.
It should definitely have some voltage on the emitter.

[lonnyholl]

TR103 lookslike its defunct!
Removed and Tested first on my cheap chinese MTester - Failed the test.
Then tested with Fluke meter on Diode Test setting - Base Emitter, Base Collector...no Voltage drop at all...reversed leads/tried all combinations...nought.

Made sure my Meter and Mtester were working - tested with another transistor.

Have ordered some 2SC668 'express delivery' ...now we have to wait...

Hoping that's the only problem...!

[G6ONEDave]

Given the fun and games that I had with a Sanyo portable earlier in the year, I would now be inclined to check all the unbranded ceramic capacitors in the LA1201 circuit. Especially any that go to earth with a low voltage on the non earthy side. You will probably need to lift one end from the pcb and check on your meters resistance range. You're looking for anything that shows a resistance, these capacitors should give an open circuit.. I could be wrong but I'm tipping that one or two of them will be showing resistance in the tens of K ohm.

Dave

[lonnyholl]

Originally Posted by G6ONEDave

Given the fun and games that I had with a Sanyo portable earlier in the year, I would now be inclined to check all the unbranded ceramic capacitors in the LA1201 circuit. Especially any that go to earth with a low voltage on the non earthy side. You will probably need to lift one end from the pcb and check on your meters resistance range. You're looking for anything that shows a resistance, these capacitors should give an open circuit.. I could be wrong but I'm tipping that one or two of them will be showing resistance in the tens of K ohm.

Dave

Thanks for sharing that Dave, Will def do that!

[lonnyholl]

FM is now working on this... TR103 has been replaced....stations coming loud and lovely once again.!

Thank to all who responded...and especially Chris for his guidance!

Cheers All

[lonnyholl]

One FM query on this receiver

( I don't have the manual for this one)

has a 'FM Mode' RCA Output Socket on the back panel....How were these used? Is it like an FM mono Pre-out ?


Cheers