CRT Hercule display board

[Tech5]

Hi and first post here!

I have a CRT Hercule on my workbench that has had the display slightly modified. Someone has replaced the background illumination from the original light bulbs to leds, but it does not work as intended. I found a service manual on the web but the display schematic is not the same. By chance, does anyone have or know if there are several models of the display board?

Thanks in advance!

 

[AudioShockwav]

I think there are several clones of the radio built by Ranger, the Galaxy Saturn Turbo, 2990, 2995dx
What is the main board number on the radio?
I don't know if all the displays are the same on different production runs.
Chris ( Nomad) is well versed on these radios and the different models, if anyone knows what service manual might work I bet he does.
Welcome to the forum.

73
Jeff

 

[Tech5]

Thank you for the reply and welcome Jeff! =)

There are two markings on the main board: 2994HB and EPT295013Z. I have only found one service manual but it does not match the driver transistors for the dimable back ground illumination, which is done – I think – in three steps, of the display. It would be much appreciated if someone could shed some light on this matter. The leds for the illumination flicker like a amusement park right now! =)

73
Bjorn

 

[nomadradio]

The dimmer circuit places one or both of two half-Watt resistors in series with the display's bulbs. A surface mount transistor on each of the two resistors selects one, the other or both resistors in parallel to get three brightness levels.

Just one problem. Those two resistors overheat. And if someone replaces the original bulbs with a higher-current type, those resistors will get cooked, along with the circuit board under them. Damaged resistors can make the display backlight flicker randomly.

I have just enough fear of damaging the surface-mount circuit board that we simply wire new LEDs to illuminate the display directly to the radio's DC power. You can no longer select brightness, but they won't flicker any more.

Consider also that those two resistors depend on the high current draw of the original bulbs to obtain the dimmer settings. LEDs don't draw enough current to get dimmed by the stock resistors. Haven't felt brave enough to try changing them to a higher resistance value correct for LEDs.

73

 

[Tech5]

Thanks nomadradio for allo the valuable information!

Aha! On the Hercule schematic I have they use 4 transistors with different values for the collector resistors so with other words that is far from the two transistors you wrote about. I had a quick look at the display board and these two resistors get really hot and partly cooked.

Well … feed the leds fom the dc power 13,8 volts would certainly solve the problem, but if I do not find an easy solution with these leds I may use small light bulbs instead and also replace the two resistors. By the way, it seems that the switching for the instrumentation light uses anoter circuit to control the background illumination.

However, I measured the voltage after the last led, IE cathode side which goes to the resistors, and the voltage counting from step 1 to 3 was: 9,5v, 8,2v and 7,9v. What I noticed when I checked the voltage with my scope, when the leds where flickering, it first looked like it was pulse width modulated but it did not look like that, it was more as random pulses.

Nomadradio, do you by chance have schematic on this type of display?

73

 

[nomadradio]

The display circuit board should have a "EPT"-prefixed number on it. That would serve to narrow down which board your radio has. The manufacturer has changed production versions more than once in the last 20-plus years that type radio has been produced.

73

 

[Tech5]

Thank you nomadradio!

I took out the display board with the the DPT prefix which can be seen in the attached photo. I found the two transistors Q2, Q4 and the resistors R4 (82R) and R6 (56R).

 

[nomadradio]

Here's the diagram I have for that board.

73

 

[Tech5]

Thank you so much nomadradio!

Yepp, that is the right schematic for this display. They wrongly placed PNP transistors in the schematic for the dim switching, which are NPN, but everything else is perfect as far as I can see. I will hook up the scope, just for fun, on the base on both transistors in order to see the base drive coming from the shift register.
Thanks again nomadradio!

73s

Bjorn

 

[Tech5]

Hi again!

This is what I did to get the LED illumination for the display to work and will do just about the same with the meter LEDs as soon as I got time to do it. I may change values on the resistors R4 and R6 in order to get a bit better dim steps, but it would have been nice to have the LEDs PWM controlled. Thanks again nomadradio for the help with the display/CPU schematic!

73s