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Hitachi SR 604 No FM Stereo

T

tonebrain

Active Member
Hi All,

Working on a Hitachi SR 604. All squared away and sounding good with the exception that
there is no FM stereo. FM mono works well and gets lots of reception.

Tuner uses the Hitachi HA 1196 chip. Identified as IC 301 on schematic. All dc voltages on chip are within reason with the exception of post 16 {VCO area} which reads an acceptable 4 volts in mono and dips down to 2.2 v in stereo. Messed with the VCO and Separation variable resistors with no effect. Put them back where they were originally set. C 310 matches the pin 16 readings.

I tested the AC voltages {for signal} on the pins and when switched into stereo both the output and INPUT
voltages went to zero. I have a scope and would think it would show the same results.

I am guessing the signal which is there at the input in mono is getting dumped to ground in stereo. Anyone have an idea where this is happening? Inside or outside the chip? Any info will help.

Thanks
 
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So you get an output when you have it set to Mono-Mute off but no output when set to Stereo-Mute on?

Locate Q301 and Q302. Short the base of either of these to ground. Do you get signal through on stereo now?

And post the voltage on IC201 pin 10 set to mono and then set to stereo.
 
Last edited:
Yes, that is correct. Also, I just noticed when in mono mute off there is signal at the Mono Stereo mute switch but no signal at the switch when in stereo mute on, so plan to go
back to test point 3 and IC 201 and look for signal out and check all pin voltages.

Will also short the transistors you mentioned to ground and post all results. Thanks!
 
Pin 10 of the HA11211 (IC201) is a 'mute' output. Inside the chip it looks for a strong enough signal and for that signal to be tuned near center. If the pin is low (<1v), then it is in 'UNmute', if the pin is high (>2.4v), the the unit is muted. That mute signal is applied both to the output signal of IC201 and the mute transistors on the output of IC301.

You might want to confirm that when tuned to what feels like the center of a signal, the tuning meter centers. If your 'feel' of tuning to the center and the tuning meter disagree, that may be causing the muted condition in stereo.

Taking a second look, rather than short Q301, Q302 base to ground, instead find ZD301 and short it's cathode to ground.
Screenshot 2023-01-17 101059.jpg
 
Will short ZD301 {tested okay on my meter diode setting}. Will also get my best antennae set up and see where the center meter reads and get the pin 10 voltages.

I was guessing mute circuit, but hadn't deciphered the schematic enough yet to know where the mute circuit was, so this helps much.
 
Short the indicated point to ground do not short across the diode itself. I don't suspect ZD301 to be bad, it's just a good point to short the mute line to ground (thereby unmuting the receiver) without creating other issues. Shorting at this point will unmute both the IC201 output stage and Q301/302.
 
When ZD301 is shorted to ground, FM stereo comes in great! What is odd, by both the strength signal meter and by ear,
FM mono signal is much weaker than the FM stereo signal? According to meter {which reads the same whether the zener is shorted
or not}, about half strength.

Will get reading on pin 10, but am checking to be sure I have located the right pin. IC201 has a bit of a confusing pin setup.
 
Took the reading for pin 10 IC201 off of R 219. Mono was 5.1 DC and Stereo was 2.5. Schematic says 5.1 and {4.6} for pin 10.

TP3 read 0 volts DC for stereo and 5.1 for mono.
 
tonebrain said:
Took the reading for pin 10 IC201 off of R 219. Mono was 5.1 DC and Stereo was 2.5. Schematic says 5.1 and {4.6} for pin 10.
Click to expand...
The first listed voltage should be "FM, (Mute on), no signal", the second listed should be in "(AM Mode)".
TP3 read 0 volts DC for stereo and 5.1 for mono.
Click to expand...
Don't understand why you get a DC voltage on that point no matter what mode it is in. That point is capacitively coupled to any place that might produce a DC voltage and is returned to ground through half of R236 and R301. Should be no DC.... Please recheck that. If the voltage is actually there, you might have a bad cap, C228 or C302. Might want to lift each of these and check for leakage.

Is the tuning meter functional? Does it swing left and right as you tune across a station? Assuming yes... With ZD301 cathode shorted to ground, close your eyes and carefully tune to the **Center** of a strong station by ear. Is the tuning meter centered?
 
Thanks Sam, will clean the oscillator trimmer. Don't believe the 604 has a muting trimmer {wish it did!}?

Ylli, sorry about those readings, not sure where they came from? TP3 has zero DC voltage. Pin 10 IC 201 FM no signal DC voltage is 2.4 and AM 5.1.

Center balance meter seems to line up well {centered} with best signal/least distortion {with zd301 strapped to ground}. Both meters work well and seem better than average quality.
Very noticeable is drop in signal with mono FM in comparison to strong stereo FM signal.
 
tonebrain said:
Pin 10 IC 201 FM no signal DC voltage is 2.4 and AM 5.1.
Click to expand...
What is the voltage on pin 10 with a good signal being received? 2.4V is still in the 'muting' range. Would like to see it <1.0V.
Very noticeable is drop in signal with mono FM in comparison to strong stereo FM signal.
Click to expand...
That imbalance may be related to the muting issue, if not we can look at it later.
 
Good signal FM mono pin 10 @4 volts DC
FM Stereo/Mute pin 10 2.4 volts DC
FM Stereo/Mute with grounded diode pin 10 0 volts DC



really appreciate the help!
 
tonebrain said:
Good signal FM mono pin 10 @4 volts DC
FM Stereo/Mute pin 10 2.4 volts DC
Click to expand...
Both those voltags are in the 'Muted' range.
FM Stereo/Mute with grounded diode pin 10 0 volts DC
Click to expand...
The would seem to indicate the voltage is coming from other than IC201 pin 10. I've got to head out tonight, will take a look tomorrow.
 
That would be great, thanks!

Here's the dc voltages off the 6 pins of the stereo/mono switch, taken from the top of the tuner board:

Mono FM setting no volume Stereo/mute FM setting no volume

4 11.1 0 11.8 2 pins toward back of receiver
4 11.1 2.6 2.6 2 middle pins
.250 4.5 2.6 2.6 2 pins toward front of receiver
 
Well, we need to figure out why pin 10 is not going to zero with strong, well tuned signal.
With the unit set to FM, Stereo-Mute on, and tuned to a strong signal:
Confirm you are still seeing about 2.4V at IC201 pin 10.
Lift one leg of R219. Under the same conditions above, measure the voltage at pin 10.
 
Yes, with FM stereo/mute on and good signal, there is 2.4VDC at IC201 pin 10 {or R219}.

With leg of R219 lifted, there is 0V on pin 10 and 2.4VDC on the lifted leg of R219. Stereo does not work with leg lifted, I assume
that is cuz lifting the leg breaks the circuit? mono does work.

my last reply before this one {I had the numbers spread out but AK squeezed them together when I hit the post button}, you can see the @2.4V is at
the mono/stereo switch also {these were taken with R219 connected but I wouldn't think the switch numbers would change with one leg lifted?}.

let me know if you need any voltages along the line downstream from R219 and if we should take those with the R219 leg lifted or in place.
 
Yup, got the data sheets on both the HA11211 and the HA1196.

OK, that tells me the 2.4 volts is coming from somewhere other than IC201.
Let's reconnect R219.
With the unit tuned to a station, and set to stereo measure the voltages at:
IC201 pin 17
The ANODE of ZD301
IC301 pin 12
Locate D302 & D303, Voltage on the ANODE of either of these .
 
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