(1973) Imperial HIFI 2700 stereo decoder and memories

[Richard9025]

Hi!
When I received this table-top 70s radio from Western Germany, I powered it up (all original parts) and on mediumwave and longwave it was working, on FM and shortwave it was completely dead, no static or anything, also the amplifier was very weak and the left output was 10 times louder than the right output.
Then, I replaced every electrolytic capacitor and the radio now works perfectly on FM, it receives a lot of stations without an antenna, the audio is clear and very loud and balanced. But it has some problems, firstly some photos of the unit:







So here are the problems:

1. Stereo Decoding
The stereo decoding is realised by an optional stereo decoder module that this radio is equipped with. It isn't only not decoding the stereo signal, but it is also downgrading the audio quality. If I press the 'MONO' button, or I just pull out the module, the sound is richer, clearer and louder and ... mono .

2. Stereo FM memories
On the right part of the front of the radio, there are some small scales alongside with some pushbuttons that have black sleeves that can be rotated to adjust the mini scales. The mini scales represents the FM band, the numbers next to them represent the channel (2 to 56 = 87,5 to 104MHz) If I rotate the sleeves, it should change the frequency, but it doesn't. (This function is linked to the stereo decoding module, as this memory function can also provide stereo sound.)

3. No shortwave (not really important)
When I press the "KW" (shortwave), nothing happens, no static, maybe contacts in the ganged switches?

The schematics:

-The version edited by me, to highlight which electrolytic capacitors I changed, and which transistors I measured and determined good (solid green rectangle- good transistor/light green rectangle- replaced electrolytic capacitor or good fuse or bulb / yellow rectangle- unchecked electrolytic capacitor) I also put the mV values of the E-B and C-B junctions next to the transistors.

https://photos.app.goo.gl/TGzk3BWwp5egeNqF2 (size too large to upload here)

-The original version, maybe you want to make some illustrations.

https://photos.app.goo.gl/nhnOXqY61o18efC83 (size too large to upload here)

Radiomuseum link: https://www.radiomuseum.org/r/imperial_hifi_2700_1.html

Many thanks!!

 

[kellys_eye]

Selectable tuning - this delivers DC to the varactor-tuned oscillator T101/102. The tuning buttons are interlinked and derived from simple voltage dividers so should be easily traced/repaired.

The stereo decoder may just require re-alignment - you can do this by ear if you're careful.

I can't make out some of the smaller schematic designations but T202 looks to be associated with SW reception and, as you say, the switching may be the issue as the oscillator device is common to all bands.

 

[Richard9025]

I measured the rest of the transistors and they all look good:

https://photos.app.goo.gl/Yy1Y3w9Dg0qWmPIN2

The tuning buttons are interlinked and derived from simple voltage dividers so should be easily traced/repaired.

Some tips for tracing?

Photo with all the modules:

 

[kellys_eye]

T103/104 form a voltage regulator developing 17.5V across P08 (P108?) which is then tapped off and applied to the switched presets in the memory bank. The selected voltage is fed to the oscillator via R185(?) - follow it out from the common switch terminal of the channel selector.

 

[73's de Edd]

Sir Richard9025 . . . . .

See if I'm being correct on my FM tuning operation surmisal just below . . . . .

There are the 5 tuning pots and their pushbutton activated switches associated with the fixed pre-tuned stations aspect of the units FM tuning capabilities.
There is the extreme left knob . . . . . from them . . . . which should be activating the MANUAL hand tuning of the radio to any and all FM stations. Then, in that knobs other option, it reverts to your being able to push in on any one of the 5 LATCHING knobs to the right and being able to use the rear tuning sleeve to be able to tune into a desired "pre-set" FM band station . Then you push in another of the remaining 4 and tune in that desired station.
When finished you have the option of the FAR left knob permitting manual tuning into any desired FM station .
Or pop that FAR left knob to its other option and its then letting you push in any ONE of the 5 already pretuned stations already having been set up.

On the FM Stereo decoder board aspect:
Note that there is a pilot lamp associated with it. When you are able to manually tune between FM stations, and tune into a FM stereo "hot" station, will that lamp come on as you tune "into" the station and drop out as you tune off of the station?
If so 80 % of that circuitry is now working properly.

TECHNO-AMENDMENT:

The stereo decoder may just require re-alignment - you can do this by ear if you're careful.

" you can do this by ear if you're careful."

" you can never do this optimally by ear if you're using the units speakers "

" you can do this by ear if you're careful and always using headphones to be able to evaluate and optimize the resultant degree of stereo separation.."

73's de Edd

 

[Richard9025]

Then, in that knobs other option, it reverts to your being able to push in on any one of the 5 LATCHING knobs to the right and being able to use the rear tuning sleeve to be able to tune into a desired "pre-set" FM band station .

Firstly, all the little pushbuttons (6) are latched between them, you press one, the other one that has been pressed before pops out, you can't have all the pushbuttons not pressed.



The difference between U and U1 U2 U3 U4 and U5 is that U also has an AFC function and it has a muuch longer scale, the little pushbutton on the far left, just commutes to the much bigger scale and by rotating its sleeve, you enable/disable the AFC (that actually works very very well). U1 U2 U3 U4 and U5 work the same as U but they have much smaller scales and no AFC. By rotating the sleeves on (U1, U2,... U5) you should change the frequency, just like a tuning knob, but it doesn't work, if I press U1 to U5 it just moves on an edge of a station and rotating the sleeve does nothing, it just stays on that same frequency, on all the u1 to u5 .

I think that the problem is a disconnected wire, broken solder trace, that is represented by the red line in the following schematic, because the tuning on the "pre-sets" is made by variable resistors and I think that a disconnected wire can cause a fixed value, that causes this problem in which all the presets are set to a frequency that can't be set.

When you are able to manually tune between FM stations, and tune into a FM stereo "hot" station, will that lamp come on as you tune "into" the station and drop out as you tune off of the station?

This thing I forgot to mention:
The radio receives a lot of stations, ~20 (without an antenna!) , But there is one big catch: That stereo pilot lamp won't lit up, Maybe the bulb is bad, or it is poorly connected, so I hooked up the multimeter on AC with alligator clips on the terminals of the pilot stereo bulb, the result: 0.000V AC when tuned on all the stations: Conclusion - bad stereo decoder.

Also when I press "FM" it automatically puts the radio in stereo mode, to commute it on mono, you press the mono button. One thing that I notice, Is that the sound is louder and clearer when I push the mono button: Conclusion: The stereo decoder isn't only not decoding the stereo signal, it is also deteriorating the sound signal.

Schematic of the Stereo module:





So there is something degrading the sound between the mono input and the right and left outputs
EINGANG= INPUT /// AUSGANG=OUTPUT /// ANZ - TO STEREO PILOT LAMP

The stereo decoder may just require re-alignment - you can do this by ear if you're careful.

I'll try to realign it if it needs it after the stereo decoder is fixed.
Many thanks!!

 

[kellys_eye]

The stereo indicator bulb operates from DC, not AC - one side of the bulb will be -ve 7V (ish) and the other side is switched to ground thru T304 to switch it on.

Check for 6V AC at the pilot light bulbs and the fuse (800mA) if the pilot (scale?) lights don't work.

Check for a change in voltage at the base of T304 as you tune thru a known stereo station signal.

The 'red' line from the preset resistors should show a switched change in voltage as you depress other buttons. If there is no change then there is something wrong in the switching part of it.

Any chosen pushed button should be 'variable' by operating the turning part of the button.

 

[Richard9025]

The stereo indicator bulb operates from DC, not AC - one side of the bulb will be -ve 7V (ish) and the other side is switched to ground thru T304 to switch it on.

So when moving across stations in stereo mode, there are 0 V DC on the stereo indicator bulb...

Check for 6V AC at the pilot light bulbs and the fuse (800mA) if the pilot (scale?) lights don't work.

The pilot scale lights work, 7V on them.

Check for a change in voltage at the base of T304 as you tune thru a known stereo station signal.

Will do.

The 'red' line from the preset resistors should show a switched change in voltage as you depress other buttons. If there is no change then there is something wrong in the switching part of it.

The voltage on the red line is 3,2V when on U and 13,34V on U1, U2,..,U5. The voltage doesn't change when I rotate the sleeves of the u1..u5.

The voltage on P106 is 5,34V on all the u's and it doesn't change.

On the emitter of T104- 17.49V

 

[kellys_eye]

The voltage on the red line is 3,2V when on U and 13,34V on U1, U2,..,U5. The voltage doesn't change when I rotate the sleeves of the u1..u5.

Suggests that the resistor that is common to all the switched presets (and is connected to ground) has gone open-circuit (or gone high value).

 

[kellys_eye]

So when moving across stations in stereo mode, there are 0 V DC on the stereo indicator bulb...

0V one side (near enough.... probably a volt or less, depends on how well saturated T304 gets) and 7V DC on the other. You should, with a high impedance test meter, measure 7V DC on either side of the bulb when the bulb is off.

 

[Richard9025]

With the negative probe of the multimeter hooked to the common ground I get the following DC voltages:

The positive hooked up to the BASE of T304: 0.099V on no station (just static from the speakers)
:0.787V on a station (music out of the boxes)

The voltage on the base of T304 alternates as I'm tuning thru the stations, from 0.01V on nothing to 0.7V on a station.

The positive probe hooked up to the COLLECTOR of T304: -8.8V (when on a station and when on static, even on "mono")

The probe hooked on one side of the stereo indicator bulb (and the other the same) -8.8V

 

[kellys_eye]

T304 dud

 

[73's de Edd]

Sir Richard . . . . .

I'm surprised that you haven't come back by now.
On this end, I am ill affected by the after effects of massive tryptophan intake on the
Festive Day of The Turkey.

Now, how about going at it this way . . . initially.
Power up and re- confirm that there is that same collector voltage being present on T304
and that the unit is being tuned off station so that any base drive to T304 will be missing.
Take DVM and switch it to DC current mode at its ~500 ma range.
Place the meter probes across the Coll-Emittter pins of the T304 just for a moment to see if
the STEEEE-REE-OOOOO indicator lamp then does light up.
THUS confirming that there is a valid dynamic voltage across that loop that can light up that
lamp.
If not, there must be a phantom static voltage that just drops when being loaded down.
Then, if the lamp lights, re probe and this time take note of the current reading on the metering
and see if it is about ~100 ma for that units specified lamp. If above that level , confirm that somewhere in its past history that someone put a heavier current lamp in the holder and it either did not work if being excessively above that 100 ma consumption or it did work, but dimly.
If it was then left in the unit, expect that it overtaxed the power ratings of the T304 lamp driver transistor and that it eventually popped open its internal emitter bond / link wire.
If replacing that T304 and finding the proper 100 ma lamp being used in the circuit gets the unit
responding to a FM subcarrier and pilot carrier presence, when being tuned onto a valid FM station.
That would only take care of that minor "indicating " function and you should still fully be expecting the no stereo decoding conversion / / / DISTORTION problem to still be existing.
Looks like you have already replaced the most suspect parts in the respect of those GREEN square marked electrolytic capacitors.

HERE IS A CONVENIENT RE-REFERENCING OF THE FM STEREO DECODER . . . . .

Now, here is a blow by blow of how this particular circuity design works . . . . .

The C302 input capacitor goes right to the origination point of the detected FM audio so that it can get a full spectrum signal BEFORE a high filter network would suppress the FM audio spectrum above 20 khz for the units conventional mono FM.
We definitely need unobstructed, low loss passage at the 19 khz pilot carrier and above for the FM subcarrier frequencies.
Some FM stations also used to sell additional services to customers up on the higher frequencies of their subcarrier.
I remember restaurant and elevator background music services, SUBSCRIBED to, stock market reports and reading of books for the blind, having been used on up in the 78khz regions of the subcarrrier spectrum. Probably none of those services exist now. . . .technology obsoletion.

BUT they did use that first L301 filter combo to strip off those very highest, unwanted, subarrier frequencies. (In our situation of this being a FM radio reception function only )
All of the combined relevant FM stereo signals are initially amplified by the T301 stage and its emitter follower mode is used to route then amplified signal down to L302 to resonate at the 19khz pilot / FM stereo frequency reference.
Its secondary then couples into the T302 anp stage for a then even STRONGER 19khz signal and a purer enhanced sine waveform by virtue of the tuned L304 resonant circuit at its collector.
Now take note of the treatment of its secondary. There is a center tapped secondary and opposite phase relationships of that now strong 19khz stereo pilot frequency are appearing at the top and the bottom ends of that winding.
Now the magic occurs when they use the configuration of the D301 and D302 diodes such that the positive node configurations are being favored. That result is being a frequency doubled . . . now . . . 38khz signal . . . . coming out of the diodes paired cathodes and being sent over to be amplified by T303.
T303 has a 38khz resonant primary of L306 which further resonates and cleans up the sine waveform.
(Also note that primarys tap off of a sample of sine to pass off thru C316 to your earlier mentioned stereo lamp driver circitry.)
If your lamp is now responding, this complete circuit loop must be operating quite well from transistor to transistor..

Now you have circuit phase two . . . . . where that phantom missing FM stereo audio info magically reappears .
L306 couples into L307 where it has a centertap with one phase of 38khz at the top of that winding and its opposite phase at the bottom of that winding, they are being routed over to the top and bottom of a 4 diode ring demodulator.
Earlier on . . . we had T301 serving a dual purpose with a signal coming out from its emitter, but simultaneously the collector also had its R305 load resistor and C311 coupled off your right and left stereo signals that need to be phase compared to a rock solid 38khz signal (derived from the 19khz pilot reference being transmitted by that same station.) to demodulate into your right and left stereo audio channels.
The mix of multiplexed right and left channels info comes into the center tap of that L307 winding while the 38khz gets coupled / induced from the L306 coil.
There is then being that inter mixed combo that goes into the top and bottom terminals of the ring demod and its diodes and R/C filter combos, which phase compare and sort out the audio right info to be outputted through the C321--R323--C323--C325 and C325 filters with a companion left audio info to be outputted through the C322--R324--C324--C325 and C326 filter networks.
Those high frequency filtering networks now get rid of those now unwanted ultrasonic frequencies.
Your now reconstituted FM stereo audio is taken off at MP305 and MP304.

CONCLUSION ?

I think that your PROBLEM is going to be imbalanced diode action in that ring demodulator from . . .time onset . . . excessive back leakage in the old era AA119 GERMANIUM diodes being used.
With some just slightly less consideration for faults in the D301 D302 pair also.

To avoid / minimize the diode characteristic CHANGE from applied soldering iron heat in pulling up the ONE DIODE LEAD that needs to be lifted out of circuit to test each gemanium diode .
Use a heat sink close up to that lead . . or hemostat tips or needlenose pliers and be QUICK in un soldering.



I think that I remember your also having an analog VOM, so use that in initially finding all of the forward . . . low ohms . . . readings of a diode and then reverse leads and test for the . . .dreaded / unwanted . . . .back resistance readings.

D E E E E E E E E P THOUGHT . . .
HEY ! . . . . . whens the last time . . if ever . . that you read back resistance on a SILICON diode ?

Log all down, so that you can then compare the cluster, for individual evaluation.


Thasssssit . . . . . .coming up next edition . . .the FM preset tuning scenario . . . . .

73's de Edd

.

 

[Richard9025]

Take DVM and switch it to DC current mode at its ~500 ma range.
Place the meter probes across the Coll-Emitter pins of the T304 just for a moment to see if
the STEEEE-REE-OOOOO indicator lamp then does light up.
THUS confirming that there is a valid dynamic voltage across that loop that can light up that
lamp.

With the multimeter on DC mA range, when I connect the probes to the Col-Emi of T304, the stereo indicator bulb lights up! and it reads 86.5mA.

If above that level, confirm that somewhere in its past history that someone put a heavier current lamp in the holder and it either did not work if being excessively above that 100 ma consumption or it did work, but dimly.

The bulb that's inside is brand new, the original 7V 0,1A, it hasn't been lit all these years because this radio was used as an amplifier at a nightclub,
but look at the blown old scale bulb that I replaced with some blue LEDs:

If replacing that T304

T304 is an AC 151-7, the only germaniums that I readily have are an AC 180 K, EFT 373 and EFT 323, I'll look for a replacement. I'll remove and check T304.

If your lamp is now responding, this complete circuit loop must be operating quite well from transistor to transistor...

It's responding only when I put the DMM in DC mA between the collector and emitter of T304, also if I measure the voltage on the base of T304 when I'm on a stereo station, I get 700mV (the bulb still doesn't light up) and when I'm on nothing, static, I get 100mV.

I think that I remember your also having an analog VOM, so use that in initially finding all of the forward . . . low ohms . . . readings of a diode and then reverse leads and test for the . . .dreaded / unwanted . . . .back resistance readings.

Measured them, they look good (red-diode check, blue-ohms).

HEY ! . . . . . whens the last time . . if ever . . that you read back resistance on a SILICON diode ?

Some months ago, I can't remember

DISTORTION problem to still be existing.

Yes, it is not decoding, not indicating and distorting the audio.

Thasssssit . . . . . .coming up next edition . . .the FM preset tuning scenario . . . . .

Many thanks!

 

[kellys_eye]

The voltage at the base of the transistor is also limited by the clamping diode beside the transistor which is why it never rises past 700mV.

The lamp current of 80-odd mA fits with the spec (they were never made that accurately anyway).

 

[73's de Edd]

Initially lift one lead of D303 out of circuit and see if the lamp activates on a STEREO station tune in.
Looks like either of your AC180 or EFT373 should sub in as being a lamp driver function . . . if you properly . .and correctly . . identify their proper CBE lead assignments.

Lastly, if your failure mode could possibly have been created by a problem of low impedance loading down of L305, by either that D303 diode or the base of the T304 transistor, their loading effect to the related L305 tap from its primary could cause a loading and Q shift such that the phase relationship between the primary winding of L305 is upset. That in turn would skew the tuning of the secondary L307 winding and cause a resultant phase imbalance into the ring demodulator that would have shifted from a proper normal alignment, to an imbalanced phase relationship then feeding into that ring demodulators input.
To answer to that situation, merely lift T304 base out of circuit as well as D303's anode. Therefore they could no longer cause that possible effect, due to the other end of their related coupling cap
. . . .C316 . . . .now floating out of circuit.

 

[Richard9025]

Initially lift one lead of D303 out of circuit and see if the lamp activates on a STEREO station tune in.

With one lead of D303 lifted, the lamp doesn't activate, -80mV on the base of T304 (when on station and when on static)

Looks like either of your AC180 or EFT373 should sub in as being a lamp driver function . . . if you properly . .and correctly . . identify their proper CBE lead assignments.

I replaced T304 with an AC180K, but the result is still the same, and I checked the old transistor that I replaced and it measures very good.

To answer to that situation, merely lift T304 base out of circuit as well as D303's anode.

With the anode of D303 and the base of T304 lifted, the stereo bulb still doesn't light up. The only way to light up that bulb is to measure its current via the collector and emitter of T304

Now, if I measure the voltage on the base of T304 I get 80mV when on station and when on static too, it doesn't go up to 700mV when on station and 100mV when on nothing, maybe I messed something up?

Also I measured C316, C306 and C309 and they check good.

 

[Richard9025]

I've discovered something:
D303 is completely dead! half of the glass is broken, and it is open. I'm searching for a german diode to substitute

With another german diode as D303 (i don't know what diode) and with the original T304 (checked with the AC180K too) still 100mV on the base.

 

[Richard9025]

So, I replaced D303 with an exact romanian replacement and now the stereo bulb lights up when on a stereo station!

Now, when I press the "mono" pushbutton, the sounds gets a little louder and richer, the difference is notable. The stereo bulb still lights up when on mono.

Actually, on stereo it sounds much better, I can notice the stereo separation.

And the last issue are the memories.
Many thanks!!!

 

[73's de Edd]

Have you evaluated the FM stereo audio when listening thru " earmuff " stereo headphones ?
Now in process of replotting the portion of the schematic of the pushbutton / vs / manual tuning modes.