National HRO MX5.

[a223bfg]

Hi my first write up in the success stories section, I always get carried away and forget to take before or during photos !

This particular HRO came from one of the BVWS Harpenden auctions along with another as the coils for the 2 were mixed between the 2 lot's (any excuse for another receiver). When viewed it appeared fairly original with just the addition of a voltage stablizer valve on top of the chassis and an additional switch next to the audio gain on the front panel.
Once home however and out of it's case it became apparent that at some point in it's life it had been "improved". An ECC83 based product detector had been added below the chassis and the rest of the circuit had been considerably altered with the crystal filter disconnected and the cw osc transformer disconected !
I am usually a plug it in and see what happens person but in this case I decided discesion was the best option, I started by cutting out all the later wiring associated with the modifications and started again. The original loom had fortunately only been cut in a couple of places so this was repaired and reconnected in the original circuit. The majority of the resistors had been replaced but 90% of these were way out of tollerance so were replaced along with all the capacitors as the replacements fitted were of varying ratings and condition.
The worst part was rewiring inside the crystal filter box where the wiring had just been pulled out of and there a million screws and nuts to undo to access anything.
Once powered up it worked straight away and realignment went without a hitch. I was concerned that the c/w osc transformer may have been disconnected to to failure but it was ok. The case and front panel were given a good scrubbing and a black gromet fitted to fill the hole left by the removed switch.
I am very impressed by it's perormance compared to much more complicated receivers such as the SX28 & AR88 with a low noise floor and the National bandspread dial making listening to SSB easy.

I still have a small problem with the "s" meter to sort but it sits in my workshop playing gold most of the time I am in there.

Phil

[MichaelR]

Nice one Phil,

I too am a big fan of the HRO receiver, it is amazing how well they perform even on the modern bands considering they are an 80 year old design.

What is your problem with the S meter

Mike

[a223bfg]

Hi Mike,

Thanks for replying, the "s" meter pegs over to the stop when switched on, I have checked and rechecked the circuit and components and now suspect the meter it's self so I have to bring myself to drag my other HRO out of the attic to swap the meters to prove of disprove this.

Phil

[MichaelR]

Phil,

You must have the RF gain control set to about 9.5. If you reduce the RF gain then the S meter does fling over to the right. Of course the meter should not move at all if it is switched out of circuit

The procedure to set it up is to set RF gain to 9.5 , turn off the BFO and have no Aerial attached. Adjust the chassis pot for zero ( be careful the original pot is open frame and easy to get full HT with a prodding finger).

The S meter is only used on AM stations ( no BFO ).

Mike

[a223bfg]

Hi Mike,

Adjusting the pot has no effect, the needle stays firmly on the r/h stop. I have checked the pot and that is ok so this is what leads me to think it could be the meter it's self.

Phil

[MichaelR]

I would check you balance bridge circuit for wiring correctly and also the values of the resistors. It is unlikely to be your meter. If you cannot switch the meter out of circuit then there is a wirng fault.

Mike

[a223bfg]

Hi Mike,

I can switch the meter out of circuit ok it's when I switch it in it immediatley goes to full deflection.

Phil

[MichaelR]

OK Phil,

The bridge will be sensitive to the HT voltage which should be 250V. There is a resistor connected to the wiper of the S meter balance adjuster (R11) and B+ it has a value from 500ohms to 2k5ohms usually set at the factory. You may need to change this to a value that allows you to get a balance under the conditions I stated in my previous post. First Check the other resistors in the bridge circuit are at the values in the circuit diagram and your wiring is correct

Mike

[a223bfg]

Hi Mike,

I will give that a go when I get this RA17 off the bench.

Thanks again.

Phil

[johnn]

Phil, but oh what a goodie. Lot of work in a small write up!
I haven't got one yet...John.

[neil29]

Its always good to know the set has been repaired properly when you do it yourself . cheers neil.

[ms660]

Hello Phil...good job done on the HRO, nice write up too. I have never had a meter go faulty on them yet though..I do not know off hand if the HRO meters have an internal shunt or not, if they do it might have gone O/C or come adrift but from what you say it looks like the actual movement is OK.
If the center zero pot is of the LIVE shaft type it might be worth checking the insulating washer under the pot nut, if the pot touches chassis then the meter will kick hard over when it is switched on. (I can't remember which way!)
Don't forget Mikes resistor tip if you get it going but not balancing correctly.
It must of taken you quite a while to do the job, I have a similar one to do, someone in the past has replaced the RF, Mixer and Osc. valves with B7G's I think that they must have encountered drift problems after the modification as afterwards they seem to have planted an octal stabalizer valve into it as well...it still drifts quite a lot even after the normall warm up time, who ever did the mod. had done a very tidy job though so the rest of the set is
A1. I have been trying to source four of the original National ceramic octal bases so I can put it back to how it was but no luck yet...there is a bloke in the States that has four but he wants a lot of money for them.
I too am a big HRO fan, such a simple design and low componet count for what they can achieve...radio engineering I guess.
Good luck with the meter.

Lawrence.

[MichaelR]

Lawrence,

Spot on with the fibre washer , this is probably the cause of the fault.

Mike

[ms660]

It might be Mike.. the reason I mentioned it was that it caught me out once along with a couple of belts! before I got to know those receivers. On mine the WW pot has a wear spot just as the meter is approaching zero, I removed that resistor you mentioned and replaced it with the nearest preffered value to get the slider off the wear spot.

Lawrence.

[peter_scott]

Hi Phil,

It's nice to hear of others enjoying the HRO. I've had one for a good few years now and like you am very impressed by its low noise floor and good sensitivity.

Peter

[a223bfg]

Thanks to all for the kind comments, yes the HRO is a very impressive receiver considering it was conceived in the 30's.
Further to the s meter saga I have checked all the components and the wiring even changing RV32 the balancing pot and the only way I can get the meter to balance is to replace R11 with an 8K or higher but this means that the meter is totally insensitive any ideas ?

One side product of the hunt for a replacement RV32 is that I have discovered 2 ceramic octal valve holders that are identical to those fitted in my set, I you are interested Lawrance i'll take some photo's of them later.

Phil

[MichaelR]

Have you made sure the BFO is off and the RF gain pot is set at 9.5 before you try and balance.

When you say insensitive do you mean that the needle does not move?. Have you made sure that the meter is wired correctly from the polarity point of view.

When you reduce the RF gain does the needle move up the scale, if not check out the RF gain circuitry/wiring.

regards
Mike

[a223bfg]

Hi Mike,
Yes BFO off, RF gain 9.5. The needle does not move with a signal applied but does move up the scale when the RF gain is reduced, I did try reversing the polarity on the meter but this did not improve things.

Phil

[ms660]

Hello Phil...yes a photo would be good, if they are the right ones I'll have them if thats OK..if it gets to that stage I will PM you.

Many thanks.

Lawrence.

[MichaelR]

Originally Posted by a223bfg

I have checked all the components and the wiring even changing RV32 the balancing pot and the only way I can get the meter to balance is to replace R11 with an 8K or higher but this means that the meter is totally insensitive any ideas ?


Phil

Phil

I have just noted that you have replaced R11 ( which I have misquoted to you in a previous post) . It is R30 that is set at the factory and should be connected to the wiper of the RV32.I would make sure that R10 and R11 are set at what they shoud be .

R10 ....15Kohm
R11 ,,,,, 2k5 ohm

R30 upto 2Kohm set to allow zeroing of S meter with the 1Kohm preset RV32.

If this does not work can you measure B+ this should be 250V, we will have to look all around the bridge circuit and check the valves.

Apologies
Mike