(1973) Imperial HIFI 2700 stereo decoder and memories

[Richard9025]

Have you evaluated the FM stereo audio when listening thru " earmuff " stereo headphones ?

Yea, there is a biiig difference between stereo and mono. No problems there

Now, lemme show you the fm presets:

First, the schematic:


Then the PCB schematic:


Now, how it actually looks:

1. In this image, you can see that the indicator isn't moved by strings, it is moved by something like a stairwell that moves as you rotate the sleeve of the pushbutton, it is like a big awkward screw.



2. The top of that screw is held by a wire that is not connected to anything.



3. the AFC button has 2 options (on or off)



4.The back of the presets alongside the pcb schematic



5. Much clearer photo of the back, alongside the schematic



6. Right side of the fm presets pcb



7. R124 and possibly P106, that's directly hooked to the variable cap, the only writing on it is: Made in West Germany.



8. Top photo of this portion of the radio:



The full schematic (.png) 4MB
https://www.mediafire.com/file/dkgdsrbvmv2tszt/allorg.png

 

[kellys_eye]

P108 sets the main tuning voltage to the switched presets.

The presets are (or at least were) commonly used in TV tuners too - the spiral shaft holding the wiper of the potentiometer and adjustable via the push button (pull it out to adjust?).

You will have a DC voltage at the wiper of P108 - anywhere between 0V and 17.5V as produced by the regulation circuitry around T104/104. This voltage needs to be uber-stable.... if it's not present then check the preset wiper is clean then check the regulating circuitry is working.

This DC voltage is then applied across the complete set of preset tuning controls and individual switches 'tap off' the chosen voltage for application to the varicap tuning circuitry of the FM receiver via R115. This voltage should be present on one end of R115 (orange wire) and will lie between whatever the wiper of P108 is and some other, lower, voltage (the voltage tapped off the tuning presets is developed across a simple divider circuit but the lower divider resistor is not marked for value on the schematic so you'll have to measure the min/max and ensure each preset output falls between these two limits).

The presets will likely be of a linear track so you can guesstimate the change in voltage on the orange wire by the relative position of the wiper on the spiral thread - highest at one end, lowest at the other and proportional in between i.e. if the top of the preset is 10V and the bottom is 2V then expect to see 6V at the orange wire if the preset is set to the half way mark.

 

[kellys_eye]

P106 looks to be a large (physically large - to get a long resistive track) potentiometer coupled to the back of the main tuning capacitor. This would be used to develop the fully-variable tuning voltage required for the normal FM tuned range. The innards may even include some form of reduction gearing. It is switched into circuit by the 'separate' contact to the right of the preset bank (on the schematic) and is closed when 'normal' (i.e. 'U' rather than presets U1 to U5) station tuning is required.

 

[73's de Edd]

Sir Richard9025 . . . . .

Quick Explanative Interjection. . . . . .
( Referencing to your very last photo )

This should have all referencing needed, being all clustered together within a mini schematic .
Will give you full details later tomorrow. in the interim, you see the BLUE RECTANGLE which gives your FM manual tuning, which seems to be excellent, with 20 stations presently coming in. . . . .with half of a plastic paper clip serving as its antenna . . .

The P108 trim pot and its companion P107 are used to establish the tuning extreme high end 108Mhz and the low end 88Mhz alignment ranges. With 'ole R123 in there also, saying "me too " with it
aligning the spread between stations from mid band at 98Mhz. These are all being fine . . .with you ending up with an on the money sensitive alignment that is outputting a PROPER IF frequency at Lo-Mid or Hi
band FM. (No touchee-touchee !)

Your last picture lets me see what I had wondered about in respect to the FM manual tuning. Seems like that yellowed plastic cover is still letting me see a rotary 100K potentiometer . . . your questioned P106 . . .that is about three times the diameter of a conventional potentiometer and its correspondingly longer wiper element and a resultant much greater / higher degree of resistance resolution being capable / produced.

Since it is mechanically coupled onto the back end of the 180 degree tuning capacitor, one would expect that unit to be a CUSTOM build, in its only having 180 degrees of rotation on the resistive element versus a conventional pots larger 270 degrees of rotation.

HOMEWORK . . . .
Since we know that the FM mechanical tuning is working fine, push in the extreme left pushbutton which puts
you in manual FM tuming mode . That will close the switch from RED B to RED D and let whatever voltage that your P106 tuning pit has divided down, and pass up the RED ARROW line to the end of the route down within the FM tuner at the RF and oscillator sections common cathodes of the varator diode sets, to tune, in accordance to the voltage being across them.

FACTOIDS . . . .
The voltage taken off with the rotor of P108 will be the highest tuning voltage available.
The voltage at the top of the factory selected limit/trim resistor R124 and its companion P107 trim pot will be the lowest tuning voltage available.

They both end up at the top and the bottom of your P106 main FM tuning pot.
Push in the 1st pushbutton . . . if not aleready done . . . . and DC meter RED D or B and give us the different tuning voltage at the 88 end of the band, then the mid 98 and the final 108 end.

NOW . . . . for the preset test . . . . . I chose the 1st of the preset buttons, so press in that 2nd button over and the same voltage that was at A should ALSO now be present at A prime.
The voltage at C prime should approximate that voltage that was present at C.
And finally the tuning voltage coming from B prime should be adjustable with that pots adjustment, just as the P106 previously produced. and it should show up on that RED ARROW line going to the tuner.
If this still doesn't get tuning and you have basic supply voltage at A prime and a reduced voltage at C prime, better check for connections or possible pushbuttton connectivity . . . but wouldn't expect all 5 !

Thassssit . . . .

Le GRAN SCHEMATIQUE . . . . .

73's de Edd

.

 

[Richard9025]

Seems like that yellowed plastic cover is still letting me see a rotary 100K potentiometer

Yea, if I look closely I can see the big wiper and the resistive track.

one would expect that unit to be a CUSTOM build

That is likely, I also saw a 2200uF electrolytic cap rated at 3V!
Also, with what can I replace the 0,15uF electrolytic caps in this radio? maybe two 1ufs in series?

to tune, in accordance to the voltage being across them.

So the tuning is realised by the voltage on the red arrowed line. Because on a preset the frequency doesn't change, that means that the voltage on the red arrowed line doesn't change.

The voltage at C prime should approximate that voltage that was present at C.

I can measure the voltage on all the points EXCEPT C prime because I can't locate it
Look at this edit that represents the connections of the preset's board:

(if you can't zoom in the photo: https://photos.app.goo.gl/511PWVhDiF1l2Zi03)

As you can see, there are more switches in the presets board than mentioned in the normal schematic, like the AFC (brown) switch, and a switch in the power supply (yellow) (I don't know what it does, there is some text next to it that I translated)

The resistors inside the presets board aren't marked and I can't remove the whole board (I need to remove a lot of things, including the string, bulbs and a big part of the metal chassis), so I can't measure C prime.

I'll measure everything you said tomorrow and keep you up with the results.
Many thanks!

 

[kellys_eye]

You can get 0.15μF capacitors as tantalums. Where are they located in the circuit?

 

[kellys_eye]

So the tuning is realised by the voltage on the red arrowed line. Because on a preset the frequency doesn't change, that means that the voltage on the red arrowed line doesn't change.

The voltage on the presets DO change - they switch between fixed values depending on where the individual tuning preset is set to. It would also change if you adjusted the control manually - like when you set each preset to the required station.

 

[Richard9025]

they switch between fixed values depending on where the individual tuning preset is set to.

Yes, but in this unit, it doesn't! that's the problem that I'm trying to resolve. When I press one of the presets, it just goes to the far-right point (104mhz) and if I rotate the sleeve of the pushbutton, the frequency doesn't change, it just stays on that point ---> the voltage doesn't change.
I'll measure the voltages to see what it actually does

 

[kellys_eye]

When I press one of the presets, it just goes to the far-right point (104mhz)

So pressing a button physically moves the scale pointer too?

 

[73's de Edd]

I really need to know what the variable voltage resulting on the RED ARROW line is at those manually tuned 88-98 and 108 tuned in positions. AND THEN you keep the metering at the same points used and then press in the presets one at a time and see what voltage is then read on the RED ARROW line and if it is staying at the same voltage for all of the pressed preset buttons.

 

[Richard9025]

Ok so I measured the voltages:
First the cropped schematic:



Now, on NORMAL tuning (U), on the RED ARROWED LINE, (same for points D prime, D, B prime, B) I get:
-on 87.5MHz (at the edge of the scale), 2,788 VDC
-on 104MHz (at the edge of the scale), 13,38 VDC

The voltage is gradually increasing as I move to a higher frequency.

Now, on THE PRESETS, (U1, U2,.., U5), on the RED ARROWED LINE, I get:
-on the bottom edge (channel2= 87.5MHz) , 13,38 VDC
-on the top edge (channel 56= 104MHZ) , 13,38 VDC

Rotating the sleeve doesn't change the voltage.

if it is staying at the same voltage for all of the pressed preset buttons.

Yes, on all the presets, the voltage is the same.

On point C I get 2,768 VDC

On A and A prime, on normal and preset, I get 13,38 VDC

I can't get to C prime to measure its voltage.

 

[Richard9025]

You can get 0.15μF capacitors as tantalums

Now I see that the 0,15ufs in this unit are tantalums, not electrolytics. No need to change them.

So pressing a button physically moves the scale pointer too?

No, this isn't some alien technology to do that , each preset has its own little scale. Far-right means the highest frequency (104MHz) that you can get by tuning at the far-right edge of the scale.

 

[kellys_eye]

each preset has its own little scale. Far-right means the highest frequency (104MHz) that you can get by tuning at the far-right edge of the scale.

Gotcha...

Sounds to me like that single resistor at the bottom of the preset bank has gone open circuit or poor solder joint....

 

[73's de Edd]

Sir Richard9025 . . . . .

Those were the exact numbers I needed, and that tells us that we have more tuning voltage than is needed, rather than the other possibility . . . . of being way down at a minimalistic 2.7 voltage level.

Referring to the schematics MANUAL tuning you can see that from " C " to ground that there is that 10K fixed resistor and the P107 25 K variable trimmer which . . . . with smart design protocol . . . . would have that pot adjusted to mid range when being at its optimal aligned point.

Therefore, that would have C being ~22K up from ground . . . . .so o o o o o . . . . . grab an ~ 20 ----22 ? K resistor and two clip leads and get one end of that resistor connected to a good ground.

Now in looking at your first pre tuned pushbutton to the left . . . when you have that pushbutton engaged . . .the " one at a time" interlocked switchbank disconnects from the manual tuning function .
That switch action then makes the B prime and the D prime as being the same connection, so, now since your RED arrow tuning line is connected into circuitry at all times, then connnect your other free end of the resistor to the RED arrow tuning line buss. ( Maintain a DC tuning voltage buss measuring point there, if so desired )
Now with that resistor in circuit, and your now having that first pushbutton engaged, I fully think that your adjustment of that particular preset tuning pot will reveal that you can now cause some tuning change and the capabilitty to tune in some of the nearby 108 Mhz statiions.
If so confirmed pop the next pretune button to the left and check for tuning capability for that pushbutton.

Are you now . . .on a roll !

Then you check out each of the rest of the preset tuning pushbuttons , as also being tunable.

With the very last one working, that would confirm the integrity of that whole shared buss that runs between B prime and C prime.

That then leaves the connectivity down to:

• That unidentified resistor below . . .
• That resistor itself . . .( probably it will be good) . . .
• And . . . . . that resistors connectivity into a good ground . . .

Thaaaassssssit . . .

Aside . . . . .
With this units AF output stage design, it will probably turn out to be the most impressive performing of all of the time aged units that you have come up with . . . .in their restorations.

73's de Edd

.

 

[Richard9025]

Now with that resistor in circuit, and your now having that first pushbutton engaged, I fully think that your adjustment of that particular preset tuning pot will reveal that you can now cause some tuning change and the capability to tune in some of the nearby 108 Mhz stations.

I clipped a 20K resistor between the red arrowed buss and ground and now the presets work, kinda!
But

some of the nearby 108 Mhz stations.

Actually, the frequency decreased, now its ~75 - 90MHz.
I connected a DVM to the ground and the red arrowed buss and I get the following voltages (With the 20k clipped)
NORMAL means manual tuning- U
PRESETS - presets, u1,u2,u3,u4,u5

https://photos.app.goo.gl/Rs5lFOaFT7wQoIXX2
With the 20k resistor unclipped, and the radio powered off, I get the following OHM reading between the ground and red arrowed line (note that the internal ohmage of multimeter is also added up)


https://photos.app.goo.gl/P5rrhRjXC6gKbkiy1
The ohm readings are kinda the same (note that it is same for all the presets), I wonder why the voltage is so strange with the 20k clipped.

Also, I noticed that the black sleeve isn't actually connected to a pot, it is moving a screw that has a little metal thingy that goes along a resistive track as you rotate the sleeve.

https://photos.app.goo.gl/QYnQtdkczQzrlOFM2
If the attachments don't work, click on the links below the pictures

 

[73's de Edd]

Sir Richard9025

Actually, the frequency decreased, now its ~75 - 90MHz.
Plus
I wonder why the voltage is so strange with the 20k clipped.

Two reasons :
BECAUSE we just had to " Guesstimate" as to the actual value of the " Mystery" resistor shown in the YELLOW BOXED area
We are NOT connecting to the actual very BOTTOM contact of a tuning pot, but into its slider contact, but we now know that the top portion of each slider pot is working up and down its range of movement.

Also, I noticed that the black sleeve isn't actually connected to a pot, it is moving a screw that has a little metal thingy that goes along a resistive track as you rotate the sleeve.

With your forthcoming info and with just a little info coming from the pics, we can see that you have long linear slider resistance elements for each of the presets instead of rotary pots. The insulated little slider contact that is being the moving " center tap " of the long control element is being mechanically connected to the long lead screw that moves from top to bottom, as you rotate its individual front pre set black sleeved knob (gear).

I believe that you told me that you were unable to get access to C prime connection over at the far left end at the mystery value resistor . . . .probably due to the need of some SERIOUS and INVOLVED mechanical disassembly.
NOW I ALSO ASSUMED that you would be unable to get a connection made into the B prime linear/slide pot at ITS VERY bottom connection, which is being wired to the same point to point series wiring buss, that connects into ALL of the bottom terminals of the other tuning pots on its way over to the C prime connection. . . . . .

IF you actually CAN connect into (C prime) via that first preset tuned position which I assigned A prime B prime labeling to, but I had no assignment given to the linear slider pots VERY BOTTOM terminal.

Then you run that 1st pretuned pots indicator bar up to where you receive the closest active station to 108.
If you then remove your 20K fixed resistor and sub in a variable pot, preset to ~ 20K and have that pot connected between the ( ? accessible ? ) very bottom connection of that 1st pretuned linear slider pots and the other lead of the test pot wiper being connected to ground.
You should then be able to exactly replicate the faulty circiutry at the extreme left of the YELLOW box,when you just trim the test pot until the tuning comes in correctly on the bar slider scale up near 108 .
THEN every pushbutton to the left should operate in the same fashion.

If so, then the fault at the MYSTERY left resistor or its connection to ground needs to be seeked out.


73's de Edd

.

 

[kellys_eye]

If so, then the fault at the MYSTERY left resistor or its connection to ground needs to be seeked out.

Sounds to me like that single resistor at the bottom of the preset bank has gone open circuit or poor solder joint....

...not guesswork - the symptoms described pointed straight to it.

 

[Richard9025]

If you then remove your 20K fixed resistor and sub in a variable pot, preset to ~ 20K and have that pot connected between the ( ? accessible ? ) very bottom connection of that 1st pretuned linear slider pots and the other lead of the test pot wiper being connected to ground.
You should then be able to exactly replicate the faulty circiutry at the extreme left of the YELLOW box,when you just trim the test pot until the tuning comes in correctly on the bar slider scale up near 108 .

With the linear resistor being fixed in a position in which B prime = C prime, I added a 20k pot and I was able to tune thru stations just like on normal (U) mode, that led to one conclusion: the problem is on the preset's board.
So with a lot of work I managed to get that board out without the removal of the scale. Then I looked a lot when I found that single resistor (that's actually a resistive track) being disconnected (the trace wasn't conducting in one little section, 2mm wide). As I couldn't solder, I just managed to connect a 20k resistor between the ground and that buss (Cprime), and the presets started working, but on normal the voltage was a littl bit off (0.5V) and the frequency too, but that isn't such a big problem, but the presets range from 6V to 11 so I replaced the 20k resistor with a jumper and now the presets are from 2.3V to 11V.
Now the radio, stereo decoder and presets are working, the only little problem is that the frequency isn't as shown on the scale, but that isn't a problem because this unit receives a loott of harmonics of the stations on 104-108. It receives so many stations that some of them aren't even meant to be there!

When putting the case I noticed something when I was very close to the radio: heat!
After 2 hours of continuously working on low volume (10-15%), the transformer got hot. How hot? Hot on touch. It isn't burning my finger but is hot on touch, ~55 C (130F). Is it normal? The final transistors (4 x 2N3055 ) were cold because the volume was very low. The unit is working perfectly but that transformer is giving me worries.

Schematic with the power supply:


Where should I measure the current and within what parameters should the values be?
The current consumption is rated at max 120W, 15W on standby. (I'm on a 220V line grid)

 

[kellys_eye]

It isn't burning my finger but is hot on touch, ~55 C (130F).

Strangely another thread brings up this issue and I pointed out that transformers have temperature ratings (maximums) that START at 105C and go up to 180-200C for normal operation (depends on their classification).

55C is pretty average under these circumstances.

 

[kellys_eye]

so I replaced the 20k resistor with a jumper

You could fit a potentiometer and adjust it until the scale pointers match the received frequency then measure the pot value and replace the link with the appropriate resistor to get an accurate scale.

Would seem to be a bit of a short cut to NOT get the set working as it should.....