HELP - Marc NR82F1 2nd Mixer IF Problem

[Phantomrose1999]

Hi All,

This Marc NR-82F1 is beautiful but has one problem. Any help greatly appreciated, as its a little baffling.

The Radio has normal superhet for AM bands. But when SW 2-4 is used, it employs a double conversion supher design. The problem happens only on SW 2-4, which is when the double conversion on AM circuit kicks in.

Please see the color coded schematic - David Schematic.jpg

AM Bands work well. RED Signal patch (TR1, TR2 LA1210, etc)

When SW 2-4 is engaged, the double conversion circuit kicks in and the Audio when receiving a signal is HUGELY distorted, almost like the LA1210 is being severley overloaded and thus distorted. BLUE Signal Path

Checks so far:

Using a CRO at at Pin 11 of the LA1210, i can see a HUGELY over modulated signal, compared to AM, which has the normal carrier and small modulations. Something is casing the 2nd IF to over modulate the carrier.

Removed and checked TR5 and TR6. (HFE 90 and 130) In fact the AM plays well, WITHOUT them installed at the moment. But just dead when SW 2-4 is switched in.

TR6 is oscillating at 1.5Mhz. perfect sine wave. which is what is written next to that IF transformer.L15. Measured at the emitter of TR6


Checked the VARACTOR GREEN line, control voltage is good, diode is not shorted.

The amplitude of the IF signal coming into the LA1210 is much larger than when AM is selected, but its also very heavly over modulated as mentioned.


Unfortunately no voltages on the schematic to check.

New 2SC930's should arrive in a day, will check the HFE and install them. Though i doubt its the problem.

Any ideas welcome, as i want to fix it and use it to listen to SSB hams.

David
VK3DMR

[Silicon]

Is it an AM only radio?

Does it have a discriminator for FM demodulation?

[orbanp1]

Hi David,

Interesting radio!

Looks like T44/L14 is the first IF, 1.5MHz higher than 455kHz (1.955MHz).
In that case the 2-nd IF is 455kHz.
It is also not likely that 1-st IF frequency would be 1.045MHz (1.5 - 0.455), in the middle of the AM-band.

I am assuming that you are looking at pin 16 of the IC when you mention input to the AM-part of the IC.
TR5 is a mixer and as such should not contribute too much gain.
Any way for you to check if the input to that mixer is indeed around 1.955MHz, are the oscillators at those bands in the right place? (Would be nice for you to have a spectrum analyzer with a FET-probe!)
You could check the working of that 1-st IF in dual conversion mode by injecting there 1.955MHz AM modulated signal.
Do you get overload in that case?

The circuit is curious as it seems that 2-nd LO is running all the time.
I am wondering if it creates any heterodyne whistles when the radio works in single conversion mode.

You could try a 2N3904 as a sub for the 2SC930, they are quite similar. Watch out for the pin order!

Here is a link to an Italian magazine, it has an article on that radio, though in Italian:
http://www.introni.it/pdf/Onda%20Quadra%201981_04.pdf

Regards, Peter

[Phantomrose1999]

Hi Peter,

Thank you so much for taking the time to respond, the circuit was not easy to understand and your info is greatly appreciated.

I bought a small RF sig generator, LEader LSG-11, old valve unit. pickup on the weekend.

I already have a CRO and the new 2SC930's arrive tomorrow.

Will get lots of pictures from the CRO and post here.


Question: So the main Oscillator TR4 is switching frequencies, during Single and dual conversion operation ? AM + 455Khz and SW + 1.5x Mhz

Tr6 and T16/L16 is confirmed to be running at 1.5x Mhz as its written on the PCB and measured at the collector of TR6. Remember AM works without TR5 or TR6 being installed on the PCB.

David

[orbanp1]

Hi David,

You are correct, TR4 is the 1-st local oscillator (1-st LO), and it tunes with the radio.
TR3 is the buffer amplifier for that oscillator.
When in single conversion mode 1-st LO runs 455kHz higher than the received frequency.
The first mixer is TR1, the dual gate MOS-FET.
When in dual conversion mode the 1-st LO runs 1.955MHz (1-st IF frequency) higher than the received frequency. In that case the 2-nd LO is the fixed 1.5MHz oscillator.
The second mixer in dual conversion mode is TR5, and the second IF frequency is 455kHz.

The received frequency of the radio is displayed by that frequency counter and display. The counter gets the received frequency information by measuring the 1-st LO frequency, and then doing the math with it, subtracting the 1-st IF frequency.
So that display should indicate if the 1-st LO is running at the correct frequency, even in the dual conversion mode.

If you have your scope, you should measure the 1-st LO on the second gate of that dual-gate 1-st mixer, on that 390 Ohm resistor. You should see a signal of a couple of Vpp.
Compare that 1-st LO signal in the working and not working bands.

Your radio could pick up stray signals, the IF amplification in a radio is quite high, 50dB - 60dB or so.
Do check/clean that S1-4 switch that switches in the dual conversion stage.
You could pick up that noise/stray signal thee if that switch is not working correctly.

Regards, Peter

[Phantomrose1999]

Peter,

Firstly thank you so much...

The Frequency display is correct on AM and ALL SW bands, thus the 1st oscillator is working. The 2nd Oscillator is exactly 1.54Mhz and matches the printing on the PCB next to it. Measured it with the CRO.

The radio actually picks up shortwave stations very strongly. (in dual conversion mode) When tunned on a SW station, the Audio almost sounds like a SSB signal, without the BFO. Turning on the BFO did not make it audible as expected.

When in dual conversion mode, IF signal going into the IC, is VERY high compared to the working AM signal. Almost like an overmodulated AM signal, to the point of distortion.

I though one of the transistors may be being driven into distortion...

Any ideas where the AGC to the 2nd IF is please ?

[orbanp1]

Hi David,

The AGC on AM is internally done in the LA1210, controlled through pin 12.
The AGC is derived from the AM audio detector output, filtered, just like in discrete solution IF-amplifiers.
The detector diode is on L18/T16C, just follow the signal from there.
See how does that signal behaves in the different bands.
Do download the datasheet for the LA1210, I only found in Japanese, still it is useful.

You can tune the second IF by adjusting the 2-nd LO frequency by adjusting VR2.
But also do check the 1-st LO signal levels, as described in my previous reply.

Regards, Peter

[Phantomrose1999]

Funny you mention VR2. Took a while to find it.

Vr2 is called SW calibrate and is used to vary the frequency counter. See photo.

I measure the voltage at the varactor which is varied by VR2 and seems to be max 3 and down to 1.x. Forgot.

Got my new 2SC930. 2x are hfe 129/130. Other two are 7x/102. Will put the 2 good ones in

Picking up a sig gen on the weekend. Have a dual channel cro from work. Will try to get to the bottom of this.

[Phantomrose1999]

Peter,

Please see https://www.youtube.com/watch?v=mwJ2dI-HLkI

Installed new TR5 /6 Hfe of 129/130. Made no Difference as expected.

Connected my dual channel scope as follows

Channel 1, Top Trace to the collector of TR2 Output of 1st Mixer (at the 33K end). Observer:

- Varying VR1 (Which is the RF gain control) did exactly that.

- The 1st oscillator was exactly 2Mhz above the tunned frequency, shown on the digial display of the radio, when in Dual conversion mode.

- The 1st oscillator was exactly 455Khz above the tunned frequency, shown on the the digial display, with in AM MW band.

Thus i concluded the 1st oscillator and mixed are working fine.

Channel 2 Bottom trace, was connected just before the AM detector diode. Measured:

- On AM MW (wihout dual conversion) the input to the detector diode was perfect, large 455khz carrier, with sligh AM modulation seen. Crystal Clear audio.

- On SW (with dual conversion) This signal was VERY heavly modulated, and i believe this is causing the audio distortion on SW.


Summary Thus far:

I cant see what would be causing such distortion when the dual conversion cirtuit is switched in. Perhaps TR5/6 Bias is off.

Why is the 455Khz IF going to the detector diode, so heavily over modulated when dual conversion circuit is switched in

The output from the 1st mixer is fine on SW and looks no different than the workin AM output..

Thus the problem must be in the 2nd Mixer/IF TR5 will be investiagted as its the only amp in that circuit.

[Phantomrose1999]

Finally something of a clue.

See photos.

Collector of TR6 second ID oscillator seems to be clipping the bottom of the sine wave.

What’s weird is the l15 0.0022 cap end is measured and I get a perfect sine wave of the 2nd oscillator. The TR6 collector goes into the centre tap of L15 and the top end goes through a 0.0022 cap. I measured a perfect sine wave after this cap

I can’t measure the if inputs to the main 1210 IC. Cro does. It register.

I pickup a sig generator sat. Let’s see what I can find

[frsimen]

Hello David,

Your YouTube video shows that the signal level at the detector is much higher on Medium wave than it is under the fault condition in the dual conversion mode. It sounds like you only get output on the modulation peaks and then only a distorted version.

Perhaps there is something wrong with the biasing of TR5. Check the 560k resistor and the 0.047uF capacitor connected between the IF transformer and the base of TR5.

Paula

[Phantomrose1999]

Hi All

Bought an old Ledger RF signal Generator, Valve unit in box..

Ok, spent most of the afternoon / evening on this and not fixed yet.

https://www.youtube.com/watch?v=cWqzRHKApS0

Please see the latest test results, i am not drunk, no sure why i had so much trouble explaing simple things at the start..

1. Injected RF signal, at Aerial, 21Mhz with 400hz modulation tone. Perfect Audio out, and the IF looks good, prior to being detected. Signal Injected at the antenna was quite strong.. The dual circuit was in, on 21Mhz and seems to work perfectly.

2. Then just moved the dial to a SW station and all the distortion was there, looks like the bottom half of the audio is being clipped, when measured after detection.

3. Moved dial back to the 21Mhz test signal, and its perfect.

So, the only clue i can see is my injected signal is very strong, the AM is very strong, and weak SW stations seem to be distorted...

Could the 1st mixer FET be the problem ? perhaps it only like strong signals ?

GREATLY appreciate any ideas, will keep poking around..

[Phantomrose1999]

BIG Thanks to Frismen (Paula) for finding the problem !!

https://www.youtube.com/watch?v=-czlG3f6ISs

Paula suggested the radio may be picking up harmoncis of FM stations. So i took the radio to the roof top, and sure enough, i was able to pick up many short wave stations around the 6Mhz band.

The video shows Radio NZ being received, from Brisbane Australia. As the radio was not screwed together and i was holding the phone in the other i could not tune to other stations..

It defnitely did pick up China, which is my test signal, as they seem to pump out stupid amount of power..ha.

As to why its picking up FM station harmonics, i have no idea, but thats another story to investigate. Schematic of AM and VHF sections attached if anyone has time to take a look.

[Phantomrose1999]

Well, looks like there is no fix. Great article in this link and i have learned something new.

https://www.antiqueradios.com/forums...p?f=1&t=305547

As i mainly restors the big Grundig Tube radios from the 50's, these sets, have never shown this problem, and their SW receiption is legendry..

[Mr 1936]

Hi Phantomrose
I’m not sure about the idea of FM broadcast pickup causing mischief, unless you live next door to a transmitter.
If I have understood the story so far the problem only affects SW bands 2-4 where double conversion is in use. On these bands, TR1 is the first mixer with its LO supplied to one gate by TR4 buffered by TR3. TR2 is a variable gain buffer amp at the first IF (2 MHz). This feeds 2nd mixer TR5 which has a 2.455 MHz LO signal supplied to its emitter from the 2nd LO TR6. The 455 kHz IF from TR5 then feeds into the LA1210 chip which has most of the gain, the AGC function and the demodulation.
On the lower bands TR1 and TR2 (now working at 455 kHz) are in use feeding straight to the LA1210. Since these bands work OK, it would seem that TR1,TR2 and the LA1210 are OK. This only really leaves 2nd mixer TR5 and 2nd LO TR6 as a potential problem area. I notice that both these transistors have base bias supplied by a common 12K resistor which then splits with a 560 K to TR5 and a 270 K to TR6. This bias is shorted to ground by SW 1-5 to disable the 2nd mixer and LO on bands when they aren’t needed.
From your measurements it looks like TR6 is oscillating as it should. I wouldn’t be too concerned about non-sinusoidal waveforms – most simple oscillators distort naturally as a way of controlling their loop gain. I am suspicious that the fault is in the TR5 mixer stage. If it isn’t getting sufficient base bias it will be non-linear to its RF inputs and will not respond properly to small signals. On an AM signal only the modulation peaks may get through. The small amount coming through will make the LA1210’s AGC wind up its gain to near maximum, so you will hear something but with gross distortion.
I suggest you temporarily disable the 2nd LO TR6 e.g. by lifting one leg of its 270 ohm collector feed resistor. Make sure you are switched to SW 2-4 and measure the voltage across the 1K emitter resistor of TR5. I would expect to see very roughly 1 volt DC indicating 1 mA of collector current. Possibly the 560 K base bias resistor has gone high or OC ? This sort of simple (crude) single resistor biasing relies on the gain of the transistor being reasonably well controlled. You may just have been unlucky to have a set fitted with a transistor with a lower gain than the designer assumed, resulting in insufficient base bias to turn it on enough.
The only other thing to check is that the 2 MHz tuned circuit L14 T44 is working OK and not reducing gain on SW 2-4 when it is in use. I think we can assume that the wideband buffer TR2 will work OK at 2 MHz if it works at 455 kHz.
Best of luck !

[Andrew2]

It's not actually picking up harmonics of FM stations.
What's happening is that harmonics of the local oscillator are mixing with incoming FM stations (which have not been filtered out sufficiently at the front end) and producing a legitimate IF signal. Obviously it's distorted as the AM detector cannot handle it.

[Phantomrose1999]

Mr 1938

Thanks for the time to go through the schematic, i concurr with your comments.

https://www.youtube.com/watch?v=trMWAo06nLw

I took the radio to the top floor and found i could receive SW from 3.8 - 9Mhz, playing in the video is Radio NZ, and i am in Brisbane Australia. I was also able to pick up Radio China on the same band.

However from 9Mhz and up on the next band, the FM signals are heard all over the bands.

I noted that there is a unshielded wire about 3 inches long, taking the signal from the band switch to the input of the Transformer for TR5.. Almost an antenna in the middle of the IF. Surprised to find this.

I did connect a wire to my signal generator with 400Hz modulator and walked the radio around the room and heard the signal clearly, thus i thin the reveiver is receiving.

Just not sure why its picking so much FM from 9Mhz upwards on SW. and NO FM from 3.8 to 9Mhz, which is the 1st Dual convrsion band.

Well, i had enough, put it back together, think its just a bad design, as there is not much in front of the 1st FET mixer.

[Phantomrose1999]

Andrew2. Thanks for the explanation. This must be happening in the 1 st IF ?

The explanation from Mr1936 is how this radio works. Which oscillator are you referring to ?

[Phantomrose1999]

Just cant leave something that does not work alone... I think the radio is actually working as designed, though its a pretty bad design, as there is no front end on it.

I switched it to external antenna, (which disconnecta ALL Bands telescopic aerials) and connected some wire to the SW antenna. Guess what ?

I can still pick up FM stations on ALL dual conversion bands !! Regardless of which VHF band is selected, the SW still picks up the FM stations.

Interestingly, the FM stations are so strong, i can pick them up, without extending the FM telescopic antennas. They seem way stronger than normal.

Anyway, had an old radio chassis, picked it up for $A4, and removed all the coils and a rather large air cap.

Perhaps an external low pass <30Mhz filter before the SW external antenna may solve it.. Still cant believe this was an expensive commercial unit.

PS: bough some DeoxIT D5, will spray the huge main band switch as suggested by Paula. will take 5 days to arrive

[Mr 1936]

Hi again Phantomrose.

I have had another look at the schematic. It does seem that the RF filtering is rather rudimentary i.e. a single tuned circuit. This won't give much out of band rejection to strong signals, especially on the higher bands.
You could well be suffering from harmonic mixing. By this I mean that the 1st mixer is converting down VHF signals due to the harmonics (especially odd harmonics) of the 1st LO. All mixers do this. As an example, say you are tuned to 16 MHz. The 1st LO is at 18 MHz and its 5th Harmonic is at 90 MHz. A broadcast signal at 88 (or 92) MHz will be converted down to the 2 MHz 1st IF. There are many other examples.
To improve rejection of the FM broadcast band, I suggest you add a passive LC lowpass filter between the antenna and the first tuned circuit, i.e. after the internal/external antenna switch but before the 51 pF capacitor. I suggest you try a 3rd order Butterworth PI section lowpass filter with a cutoff of about 30 MHz. This comprises a 100 pF capacitor in shunt to ground, then a 500 nH (0.5 uH) series inductor, then another 100 pF capacitor to ground. Keep the leads really short and find a good ground. You may need to make the inductor yourself, in which case I suggest 14 turns of 1mm diameter enamelled wire close wound on a 5mm diameter mandrel.
Hope this helps, Mr 1936.