Sansui 3900Z Tuner Problem

dcs11173

New Member
Love you people here. So much real knowledge. Hoping for some help maybe

Acquired a 3900Z receiver the other day. It was free so I decided to check it out some.
Had a blown channel and tuner display didn't light up.

So I Decided to through some OP's in it, along with some resistors.
Channel was still noisy and ended up being some pre-drive xstrs.

So on to the tuner display issue. I was sure it was just going to be bad solder on the flourescent readout. WRONG. I actually have no clue what is going on here. If I play with the power switch I get a moment of readout. Tried cleaning the power switch to no avail. I get 30v to the tuner board (3326) and 8v AC going to "FL" board (3321).
After watching youtube video's I realize I am also missing the preset led and there is a red indicator that shows where on the "dial" the station is also missing. As is Signal strength etc.

Any help would be greatly appreciated. Didn't really want to spend this kind of time on this one but I have come this far......
 
The issue you describe is quite a difficult one to work on. You have most/all functions of the display inoperative which probably means that you have a logic fault in one or more of the integrated circuits that runs the display. Added to that the functioning of the display is tightly bound with the tuner section which may not be feeding the correct signals to the display. About the only good news is that you have a total failure of the display, but it does light up briefly when powered on, as you describe.

In the absence of other information, you will need an intimate knowledge of the functioning of the logic circuitry (5/6 IC's) and also potentially of the signals from the tuner that feed it. For example after some work you may be able to get the display to light, and then you may find the tuner needs to be aligned.

Looking at the block diagram on page 3 of the service manual, try to use the tables that follow to check the voltage readings, and functioning of the many signals required for the system to work as it should.

Check the output of the LA5700 IC and change the 3 electrolytic capacitors around it C3, 47µF 16V, C4 4.7µF 25V, and C5 22µF 16V - it should have 9v output on pin 12 - looks like it is adjustable with VR1. Also 8V output on pin 13 appears to be important - check this.

If you conduct a very close examination of the topside components and the bottom side joints of the PCB - you may get lucky and find a problem (cracked component or dry/cracked joint).

NB: Service Manual available from Hi-fi Engine.

Let's see what anyone else has to say about this.
 
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I have not had this problem before but it will require some good detective work to figure it out.

This service manual has a very good description of how the tuner section works.

http://elektrotanya.com/sansui_3900z_4900z_5900z_sm.pdf/download.html

Two things to check before you pull your hair out.

1) Make sure the power switch is working. These type of switches have been known to go bad. I bought a 4900Z on the cheap because of one.

A quick power switch test.

-- turn power switch off.
-- unplug the unit from any power source.
-- using power cord type wire, jumper the two electrical outlets on the back from top bottom on each side. be sure not to leave any wires hanging that will short things out. :no:
-- use a power strip to turn unit on while leaving the front panel switch off.
-- be sure to double check your work before power on. :thmbsp:

You can take a look at the back panel and the schematic to verify this test. This is how I confirmed my power switch was DOA. You can also hear arcing if the switch has gotten really bad.

2) Next thing to do is verify all the voltages from the power supply are stable. If you have a scope you can do this. Check for excess ripple and make sure the zener voltages are clean.

I found excess ripple on the +13V line that feeds the tuner and a dirty zener that needed replacement by doing this.
 
Hello guys!

My name is Mauricio, I write here from Brazil. I created an account here in AudioKarma to report my problem and solution to the Sansui Z4900.

I've been doing some research on the Internet about the Service Manual and came to a stop at this topic. Some people made suggestions to the friend who needed help.

So, I'm going to report my case, maybe it will help somebody in the future.

The digital tuner did not work properly. The display would light but indicate random numbers, which did not match the FM or AM range for example. The auto search only worked decreasingly.

When I studied the service manual, I first checked the zener voltages of the source and consequently switched the four zeners (13V, 2X 18V and 33V), I also changed capacitors LA5700. But it did not help.

When UP or DOWN search button is depressed, Pin 14 or Pin 15 of IC1 is grounded. Then automatic search circuits for Upper or Lower Frequency, and tuning voltage (Voltage of variable capacitance diode) FROM Pin 10 the IC1 icreases for UP or decreases for Down. Also, the Service Manual shows Priority of Functions: 1st: UP SEARCH TUNING!

So my case was in the first order.

So I checked the voltage drop on pin 10 but not the voltage increase on the same pin 10 that matches the search up.

After a time of analysis, I arrived at the capacitor C23 of 33uF, next to the module. I made the replacement and the problem was solved.

Digital tuners are tricky. I particularly prefer analog strings tuners ... lol

Hope it helps someone in the future.

Hugs,
Maurício Chagas, Brazil
 
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