Unitra K100 Magic Eye - UK Vintage Radio Repair and Restoration Discussion Forum

Wanderer · 1st Jan 2018, 5:54 pm

My Unitra K100 tuning indicator does not operate on FM. I cannot see the connection I am expecting to the Ratio Detector from the published circuit. Can anyone cast any light on the mechanism of the tuning indicator operation on the K100? I have checked the EM84 and it closes the columns at about -8 volts, so is fine.

AC/HL · 1st Jan 2018, 6:27 pm

The schematic is available here: http://www.service-data.com/section.php/8379/1/k-100

TonyDuell · 1st Jan 2018, 6:45 pm

Looking at the diagram in the appropriate volume of Radio and Television Servicing (which is what I think is available 'up top'), it would appear that the EM84 is driven by the AGC voltage for both AM and FM (!).

By coincidence, I was given a Unitra K101 yesterday. Similar, but with enough differences to need the right circuit (I have it).

ms660 · 1st Jan 2018, 8:54 pm

I'm looking at a schematic for the receiver and the -ve grid voltage for the tuning indicator on FM is taken from the ratio detector (-ve side) via a 330k then via a 220k then via a 2.5 meg and thence to the control grid of the tuning indicator.

The junction of the 220k and the 2.5 meg is connected to the anode of the AM detector via the last AM IFT's secondary. The AM detector doesn't conduct on FM though because of the -ve voltage provided by the ratio detector via the 220k and 330k resistors.

On FM there's DC feedback (-ve) from the ratio detector to the suppressor grid of the IF valve, which so far as I know is to help prevent overloading of the ratio detector.

.....So far as I can make out.

Lawrence.

ms660 · 1st Jan 2018, 9:31 pm

Correction....Read as 2.2 meg not 2.5 meg.

Lawrence.

ms660 · 2nd Jan 2018, 12:02 pm

So far as I can make out, the 330k in series with the 220k forms the AM detector load, if the 330k went very high or O/C there would be little or no -ve voltage to the grid of the tuning indicator on FM but the AM detector would still have a load resistance (albeit much higher) due to the 2.2meg/47k potential divider feed to the DIN sockets connection.

Maybe check for -ve voltage either side of the 330k?

The schematic source I'm looking at is not from this forum so I won't post it.

Lawrence.

Wanderer · 2nd Jan 2018, 3:01 pm

Thank you for your quick and kind replies. The 330k and 220k resistors are fine and the 100pF C19 is good too (replaced). Careful measurements both on and off tune show a completely inadequate voltage anywhere to drive the EM84. For instance, at the junction of the 330k with the 220k, -290mV on tune, -228mV off. At the negative end of the ratio detector 5uF (new), C18, -187mV on tune, -228mV off. And so on. The signals tune in well and give good and pure audio from the loudspeaker. I have no means of measuring the incoming signal, so do not know whether the required 10 microvolts at the aerial terminals is there, but assume all is well or the radio would not produce a reasonable sound. Is this a reasonable assumption?

Peter

Wanderer · 2nd Jan 2018, 6:05 pm

Sorry folks, meant the two measurements on C18 the other way round. -228mV on tune, -187mV off.

Wanderer · 10th Jan 2018, 6:40 pm

And Finally! Having found no fault with the circuit, I conclude that the weak FM signal strength is the problem. AM with an aerial works the EM84 just fine. So, the signal strength is not going to improve any time soon and the set sounds great. I have derived a power supply, plus and minus around 7 volts from the heater supply using two 1N4007s. These power a 741 which is connected to the ratio detector and provides enough gain to drive the EM84. The output from the 741 connects to the grid of the EM84 via a diode, 1N4148, and another 1N4148 is connected to the grid to connect to the original cable, forming an OR gate for either AM (original signal) or FM (new route from the ratio detector and then some gain. Zut Alors I hear you all cry - originality is lost! The man is a vandal. True, but the tiny extra board can be removed any time and the radio now has full functionality.

1st Jan 2018, 5:54 pm	#1
Wanderer Triode Join Date: Jan 2018 Location: Maidstone, Kent, UK. Posts: 20	Unitra K100 Magic Eye My Unitra K100 tuning indicator does not operate on FM. I cannot see the connection I am expecting to the Ratio Detector from the published circuit. Can anyone cast any light on the mechanism of the tuning indicator operation on the K100? I have checked the EM84 and it closes the columns at about -8 volts, so is fine. Last edited by AC/HL; 1st Jan 2018 at 6:26 pm. Reason: Forum rule B8

1st Jan 2018, 6:27 pm	#2
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Unitra K100 Magic Eye The schematic is available here: http://www.service-data.com/section.php/8379/1/k-100

1st Jan 2018, 6:45 pm	#3
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Unitra K100 Magic Eye Looking at the diagram in the appropriate volume of Radio and Television Servicing (which is what I think is available 'up top'), it would appear that the EM84 is driven by the AGC voltage for both AM and FM (!). By coincidence, I was given a Unitra K101 yesterday. Similar, but with enough differences to need the right circuit (I have it).

1st Jan 2018, 8:54 pm	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Unitra K100 Magic Eye I'm looking at a schematic for the receiver and the -ve grid voltage for the tuning indicator on FM is taken from the ratio detector (-ve side) via a 330k then via a 220k then via a 2.5 meg and thence to the control grid of the tuning indicator. The junction of the 220k and the 2.5 meg is connected to the anode of the AM detector via the last AM IFT's secondary. The AM detector doesn't conduct on FM though because of the -ve voltage provided by the ratio detector via the 220k and 330k resistors. On FM there's DC feedback (-ve) from the ratio detector to the suppressor grid of the IF valve, which so far as I know is to help prevent overloading of the ratio detector. .....So far as I can make out. Lawrence.

1st Jan 2018, 9:31 pm	#5
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Unitra K100 Magic Eye Correction....Read as 2.2 meg not 2.5 meg. Lawrence.

2nd Jan 2018, 12:02 pm	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Unitra K100 Magic Eye So far as I can make out, the 330k in series with the 220k forms the AM detector load, if the 330k went very high or O/C there would be little or no -ve voltage to the grid of the tuning indicator on FM but the AM detector would still have a load resistance (albeit much higher) due to the 2.2meg/47k potential divider feed to the DIN sockets connection. Maybe check for -ve voltage either side of the 330k? The schematic source I'm looking at is not from this forum so I won't post it. Lawrence. Last edited by ms660; 2nd Jan 2018 at 12:11 pm.

2nd Jan 2018, 3:01 pm	#7
Wanderer Triode Join Date: Jan 2018 Location: Maidstone, Kent, UK. Posts: 20	Re: Unitra K100 Magic Eye Thank you for your quick and kind replies. The 330k and 220k resistors are fine and the 100pF C19 is good too (replaced). Careful measurements both on and off tune show a completely inadequate voltage anywhere to drive the EM84. For instance, at the junction of the 330k with the 220k, -290mV on tune, -228mV off. At the negative end of the ratio detector 5uF (new), C18, -187mV on tune, -228mV off. And so on. The signals tune in well and give good and pure audio from the loudspeaker. I have no means of measuring the incoming signal, so do not know whether the required 10 microvolts at the aerial terminals is there, but assume all is well or the radio would not produce a reasonable sound. Is this a reasonable assumption? Peter

2nd Jan 2018, 6:05 pm	#8
Wanderer Triode Join Date: Jan 2018 Location: Maidstone, Kent, UK. Posts: 20	Re: Unitra K100 Magic Eye Sorry folks, meant the two measurements on C18 the other way round. -228mV on tune, -187mV off.

10th Jan 2018, 6:40 pm	#9
Wanderer Triode Join Date: Jan 2018 Location: Maidstone, Kent, UK. Posts: 20	Re: Unitra K100 Magic Eye And Finally! Having found no fault with the circuit, I conclude that the weak FM signal strength is the problem. AM with an aerial works the EM84 just fine. So, the signal strength is not going to improve any time soon and the set sounds great. I have derived a power supply, plus and minus around 7 volts from the heater supply using two 1N4007s. These power a 741 which is connected to the ratio detector and provides enough gain to drive the EM84. The output from the 741 connects to the grid of the EM84 via a diode, 1N4148, and another 1N4148 is connected to the grid to connect to the original cable, forming an OR gate for either AM (original signal) or FM (new route from the ratio detector and then some gain. Zut Alors I hear you all cry - originality is lost! The man is a vandal. True, but the tiny extra board can be removed any time and the radio now has full functionality.