TES Field Strength Meter MC 661C

[__i4cy__]

I wonder if anyone can help. I have a TES field strength meter MC 661C which doesn't seem to function correctly. As others have done, I have tried connecting a section of wire to the VHF input and plugged in an earpiece to see if I can hear any commercial channel FM broadcasts by setting the range switch and main tuning dial. No channels can be heard other than a quiet click when the aerial is connected.

I then tried connecting a signal generator to the VHF input. The meter does actually deflect only when a 30MHz signal is applied, irrespective of where the range switch is selected or where the main tuning dial is turned to. This leads me to believe that the local oscillator in the unit may be not working.

Does anyone perchance have a schematic diagram for the MC 661C?
Many thanks in advance.

[Chaparal]

Here a front picture

build approx 1969

Peter

[Chaparal]

some specs:

Freq Range VHF, UHF:
41 - 65 MHz
65 - 110 MHz
155 - 280 MHz
470 - 840 MHz

Input impedance:
75 Ohm unbal
300 Ohm bal

Sensibility:
20 µV - 10.000 µV, 0.1 with ext. Attenuator

Freq accuracy:
better than 2%

Sensibility accuracy:
2 dB VHF. 6 dB UHF

No of Transistors = 10

Battery 4,5 V, 100 hours

Weight = 2 kg

[Chaparal]

Schema

[__i4cy__]

Chaparal, thanks for posting the schematic diagram and the additional details.

After reviewing the schematic, it appears that TR5 (AF106A) is responsible for generating the local oscillator output. Measuring with a scope between ground and the collector of TR5 gives no signal and it just sits at 0v. The other DC voltages on the TR5 seem to be ok: Vemitter = 4.0v, Vbase = 4.2v. Replacing TR5 with another AF106A I had lying around, the local oscillator still refuses to oscillate.

Does anyone have a copy of the MC661-C manual?
Or has anyone got any bright ideas?

Thanks.

[Terry_VK5TM]

Assuming I'm reading the resistor values right, there should be ~3.8v on the base.

The collector is grounded for DC, so 0V is right.

Probing directly across the collector and ground may be enough to stop the circuit oscillating.

I also see what looks to be an old grey electrolytic capacitor (10uF?) that is connected between the junction of the 820 and 150 ohm resistors and ground near the AF106. Have had these cause problems in a scanner I fixed recently.

Terry VK5TM

[__i4cy__]

Terry, you're right the base voltage is actually around 3.9v and the emitter is around 4.1v, which is what you'd expect for a germanium transistor. Anyway well done for spotting my mistake.

I did use a x10 oscilloscope probe to try to minimise the probe from preventing oscillation. Also I set the oscillator to its lowest frequency (41MHz) for best chance of oscillation.

The 10u electrolytic capacitor seems to be used to provide supply stability and the supply to TR5 is stable, though I will try replacing it with a new capacitor. If I get it working then I will replace all the electrolytics in the unit.

Thanks for the suggestions.

[Terry_VK5TM]

Just another off the wall thought, is the range switch S1 actually working properly, as in clean contacts?

Terry VK5TM