HRO help
 

I have been trying to figure out a problem with my HRO-5 (was restored by another but not able to obtain assistance) but not getting very far.

The HRO-5 appears to have 'lost the secondary grid voltage' on the 6K7 valves which is my simple description but I cannot seem to locate the fault. This is obtained by division of the input voltage B+ but the R all seem to be fine so scratching head ... (I could wholesale replace them all just in case but seems a bit like overkill)

I have managed to check the voltages on the pins on both my faulty HRO and a working one (at RAF Duxford) and pin Pin4 ( Screen Grid?) on many of mine are at 0V (2nd IF V6 (6K7), 1st IF V5 (6K7), 2nd RF V2 (6K7), 1st RF V1(6K7)) when on the working HRO-5 are at about 138V DC. And Pin 5 & 8 are at 0V on V5 1st IF (6K7) and V2 2nd RF (6K7) whereas on working are at 4.33V DC. Anode voltage on these valves are all c230V so the B+ is working.

R10 is fine and is 15K
R32 appears to be fine 1K (168V at R30 end, 158V at other feeding R10)
R30 is a 2K2 1/4 watt and is fine.
R11 is 3K (a 2K7 is fitted and was there when I got the set)
R15 is 33K

One end of R10 is at 158V DC the other connected to pin 4 V6 is at zero volts.

I tried with all the 6K7 valves removed V1, V2, V5, V6 but no change in the voltage values.

Pin 6 of V7 which comes from the same B+ source initially is at 168V through R18 which is a bit high at 53K not 47K.

The AF stage V9 is working fine as I can inject a signal into it and get plenty of amplification.

Anyone got any idea what might be the problem or suggestions?

72

Dom
M1KTA

Re: HRO help
 

Check if there is a screen decoupling capacitor gone leaky do you have a circuit diagram. Mick.


Check if there is a screen decoupling capacitor gone leaky. Do you have a circuit diagram?

Mick.

Re: HRO help
 

Hi Dom, sounds like a short from G2s to earth, maybe C23, but R10 would run rather hot!

Peter

Re: HRO help
 

With the HRO-5 turned over, dial towards you... Is C23 the second 0,1uF 400V from the right hand side between V2 and V3, connected to pin 4 of V2?

I might go and grab the digital camera if not clear.

I don't have any 400V 0.1uF but I have a couple modern 0.1uF 600V versions... right now I want to get it working, will sort the proper replacement later.

72

Dom
M1KTA

Re: HRO help
 

Only a very poor one.

I have a very bad photo copy of a location image that sort of helps to ID the components.

72

Dom
M1KTA

Re: HRO help
 

Hi Dom,

http://www.helen.radio.btinternet.co...ircuit_hro.htm

HRO MX good copy

Peter

Re: HRO help
 

Thanks.

This is the HRO-M version but I 'think' the circuit is the same for the HRO-5 which I have. You also suspect C23 on far left of the diagram the grid voltage bypass?

72

Dom
M1KTA

Re: HRO help
 

Thanks... C23 replaced with a modern Sprague 400V 0.1uF and the radio springs to life ;-)

I think I might take the time to make up some look-a-like 0.1uF capacitors as I have a number of vintage caps that I could replace the insides with modern versions.

Thanks to all that read the post, responded and the pm's.

72

Dom
M1KTA

Re: HRO help
 

This might help someone else...

The heater indicator bulb had also blown, small bayonet type just to the right of the tuning dial and suffice to say a pig to get to it and replace. Anyway I replaced it with something I had to hand a simple car side and tail bulb. RU233 12V 4W BA9 cost 60p. The fun bit was getting the old one out and replacing the new.

1/4" bore rubber hose (like the sort used in school science labs with a Bunsen burner), snug fit over the new bulb allowing it to be fitted, once locked into position the rubber tube can be rotated off.

72

Dom
M1KTA