Denon Tu-500 , No Stereo

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi, my frined gave me a TU-500 for no stereo operation. Mono signal work for both channels.
FM stereo light work if i bypass HA1156 mpx chip.
Checked front end with my Leader stereo gen for 100MHZ carrier and 19KHz pilot and L+R audio mod.
Front end and IF if fine as I can tune IF stage to compare once demodulate FM carrier versus the composite output of signal stereo generator to be exact minus some usual distortion of signal,
So the front end is a reply of the composite signal that is a 1KhZ L+R audio with a 19KHz pilot all mix together.
I can with my stereo gen have a 67KHz SCA modulation and I can observe it at the input of the MPX HA1156 chip.
Of course replacing, testing all near components near the HA1156 is been done.
All caps are replaced either the coupling one and filter ones.
Did replaced the HA1156 chip, still no success. No Stereo.

So with signal gen at 100MhZ FM carrier with L+R 1 KHz audio and pilot on
I can follow signals on the HA1156 pins.
All DC voltage looks good with a solid 12 V DC .

I have removed the muting signal and the force mono on pin8 to isolate those.
So the HA1156 have now pin2 input which is composite signal, I did injected a direct composite to the pin2 from my stereo gen no difference, still no stereo indication.
Pin6 remains 6,6vdc.
19KHz signal from this HA1156 is solid compared coming from RF front end or pin10 have same frequency of 19KHz compared with frequency counter and a scope.

Outputs pin 4 and 5 have 10Vdc and L+R 1 KHz audio signals mix with 19KHz, this is before the lowpass filtering on the audio signal.

Filter caps direct on pin 4 and 5 are lifted too, so no interference if they are defective.

So to resume:
VR301 cleaned, C303 checked
C304-305 replaced.
C306 checked
C302 replaced
C301 replaced
R308 lifted
R309 lifted
C307-308 lifted
C311-312 lifted
C329-330 replaced
All close resistors measured OK.

Any other ideas where to look for??


  • 20200110_135847.jpg
    985 KB · Views: 231
Ok, some news,

Got my stereo generator in composite level not RF to feed directly the input of PIN2 of HA1156 MPX chip via the coupling cap.
Got a stereo indicator and 1 volt at PIN6.
Done throught the RF stage to demod the same composite with RF stage, it barely locks stereo signal!?
Unstable if you change tuning freq. then back again, no stereo.
Seems the IF stage does not have enough gain even if I tweak T201 and VR204.
signal from demod is there but lower than I can inject from my stereo gen composite output.
Will have to investigate further TR201 a 2SC461C tx and two IC, TA7061AP and TA7060P.
see if I can understand and maybe help.

so if you choose a strong stereo station in your area do you get the high on pin 6 which lights the stereo indicator bulb? If the pin is low but pin 8 is high then I would try to feed a stereo composite signal (that contains the pilot) directly into pin 2. don't send too strong a signal, respect the range of the part in the datasheet.
The other important pin is 8, this is to force mono even if the station is stereo. If you have that pin at ground, no mater what enters the chip it will play mono and keep the lamp off. so monitor that pin in all your testing because it can easily override what you are trying to do or force it high during your testing.

BTW when you listen to a stereo station do you feel you have FM stereo or mono at the speakers regardless of the lamp? Can you also try with a mono station and hear what a true mono station sounds like as reference.

the equivalent part if you can't find the hitachi is the NTE801.
Hi, some more search, all tests with my LEADER stereo generator at 100MHz, center tune and composite is demodulated.
IF stage is sending composite signal of 350mV, so enough from HA1156 specs to turn the pin 6 to ground to pull down the stereo lamp,

But still no stereo.
Hi got oriented to the test circuit suggested from hitachi HA1156 spec sheet.
Changed values to copy test circuit .
Installed same values.
Pin10 oscillator id 19KHz right on with freq. counter and scope looks like a saw tooth waveform.?
disconnected muting, mono switching to pin8, isolated output pins 4 and 5 still having +12V to them but coupling caps removed.
Tried to isolate mostly the IC HA1156 even replaced it with a new from laserdiode supplier in UK.
Lifted stereo lamp out of circuit, pin6 shows 0V.

Tooked a LED with 1Kohms resistor to simulate a stereo lamp from +12V to pin6, no, it doesn't want to sink so small current to light the LED ???
Now with a second test with a stereo gen but sending composite signal directly to the HA1156 chip on pin2 with the in line resistor.MUA-01A
It needed near 600mV p.p. to light up the stereo pilot lamp.
Pin6 going down to low volts as lamp is connected to a 6 volts dc.

So from there i'm concluded that IF stage HUX-02A doesn't send enough signal.
I checked and replaced all electrolytics caps, the bypass ones and the filter ones on the board.
Still get at maximum tune IF transformer and output pot a low 500mV p.p. signal, not enough to trig the MPX chip to stereo.
TR207 and 208 will be inspected and tested.


  • Annotation 2020-02-26 130254.png
    Annotation 2020-02-26 130254.png
    131.3 KB · Views: 205
Last edited:
Have you tried feeding just a 19 kHz pilot tone into the MPX input? Datasheet says it should need about 20 mV to lock. If found to be substantially higher, try disconnecting TR311... it is one of the notorious 2SC458s after all.

The post ratio detector amplifier should give a gain of roughly 6 dB, +/- depending on output level adjustment.

Have you checked ratio detector alignment yet?

Do you have any way of generating a 10.7 MHz FM signal that could be fed into (a) the last IF amplifier (b) the ratio detector itself? Together with a ramp (triangle wave) generator for modulation this could be used to sweep the detector and IF to detect any irregularities.
Hi, TR311 is out of the circuit.
My stereo gen does have a pilot output and composite.
I will try direct 19KHz feed to pin2 of HA1156.

On IF pcb HUX-002A, replaced all caps, and TR207-208 and 201 no effect.
I wonder what could be the levels of the waveforms on IF output.
From HA1156 chip pin2 max input is 560mV.
So from there, I suspect something close to this to trigger in stereo.

100MHz FM carrier is tune fine and front end looks to have a clean signal compared to my stereogen. composite output.
Ok, some more tests;
19KHz signal in pin2 of HA1156= stereo
Composite signal, 1KHz L+R and 19KHz pilot in pin2 of HA1156= stereo

Off the air a strong station, getting full signal strengh, no stereo.

Signal at 300mV peak to peak from the IF stage either signal gen or off the air is no stereo.

Knowing max. signal from specs of HA1156 is 560mV, a 300mV should get a stereo signal yes or no??
I think the best test to prove the output of the HUX-002A pcb that the IF stage and etc on it could be to use another FMtuner with a MPX HA1156 stereo demod chip to prove it....
Pioneer TX7500 is one od those FM tuner that uses a HA1156 chip.

And yes I did replaced the HA1156 IC in the Denon....
Hi, this FM tuner was giving to me for fixing a no stereo issue, without knowing if someone as ever touched it, aligned, or else??
Try today to tweak IF signal coming from front end.
Succeed to increase RF level measured at TR201 collector pin.
Got about 100Mv to a 250mV of signal, not modulated, no pilot, just 100MHz, from my stereo generator.
So what I see there is a 10,7MHz.
But at the output TP2 of HUX-002A pcb, still got a 350mV signal that is a 1KHz+19KHz once I reactivate modulation on my stereo generator.

Will have another FM tuner a Pioneer TX7500 based on the same HA1137 and HA1156 topology to compare the IF signal coming from the Denon to check if Pioneer demods this signal or not.
And using the Pioneer FM tuner front end to inject in the Denon at the input of the MPX input.
Everything is in the details......From what i was looking into for many months now, part time, not everyday....
On IF pcb HUX-002A , T201 is the IF demod transformer that have 3 windings inside....
Again this tuner was i'm sure being looked after someone else that maybe loosed some parts, or did not know about how to align a FM tuner,
well have been caught at it.
Theses small transformers do have 2 ferrites on each one,

theses small ferrites one on top and one on bottom.
With schematics i got an idea to look for the bottom ferrite, it wasn't there!
BINGO by installing another one of the bottom and redo fine tuning.
Got the stereo light to come on! With now over 1,2 vp.p. of composite signal.

Now on the muting coil T202 same thing.....
Last edited:
Boy, that was a tough one for sure. Who expects a missing core in the ratio detector IF transformer of all things? No wonder the thing didn't work right. Thankfully your persistence paid off in the end.

I mean, you see wrecked cores occasionally (I've got an Onkyo T-9990 with that kind of problem floating about). But a missing one is the kind of problem nobody expects. Gotta love pre-messed-up units...
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.