HRO MX update 4

The HRO powered up and made noises on the first attempt, but I couldn't actually tune in any stations with the 180-400 KHz coil pack installed. However with the 0.9-2.0 MHz coil pack installed (the only other one I have at present) it was a different story! 

It's currently sat on the bench playing Smooth Radio on 1332kHz, and a quick tune around found a few European stations as well. I'm well pleased! 

I hadn't planned an external restoration, but it will get a thorough clean and tidy. The worst of the chips and scratches will probably get filled with black paint.

My next task will be to get some coil packs for other frequency bands. AM broadcast stations are ok to prove the basic operation, but I want to see how it copes with CW and SSB signals on the amateur bands.

Keep listening!

HRO MX update 3

Whilst checking components above the chassis, I noticed that the connections to the Local Oscillator coil have been modified.

One of the coil contacts was originally connected to a capacitor mounted on a ceramic insulator on the chassis. I believe that this provides temperature compensation.

This connection has been broken, and in its place are an additional inductor and variable capacitor below the chassis. These form a series tuned circuit between the coil connection and ground.

I've no idea what the purpose of this modification might be, and a quick search on the web didn't turn up anything relevant. I somehow doubt that it's an 'official' modification though.

I'm tempted to remove it and restore the the original circuit, but I think I'll do a bit more research first.

Update: Having failed to find any info on the modification, I decided to remove it and restore the original wiring. 

Keep listening!

Then there were three!

I'm lucky that my wife understands and to some degree shares my interest in vintage radio. Many of my friends' partners would object to the number of old sets that have taken up residence in our house over the last few years. 

However, I noticed a less than enthusiastic response when I arrived home with another HRO-MX receiver last night! 

The HRO is a classic, but it was never intended to be nice to look at; unlike some of the domestic sets in my collection, with their wonderful wooden veneer finish and stylish Art Deco lines.

This latest HRO actually belongs to a good friend of mine, who was happy to lend it to me so that I can compare a couple of things with the one currently on my bench.

I'd better not tell her that I've also agreed to get it working for him, which means it will be staying in our house for a while yet...

Keep listening!

HRO MX update 2

As standard, HRO receivers have terminals on the rear which carry high voltages and should be considered to be hazardous. As this set is intended for use in working public demonstrations it is necessary to make these safe.

The original HRO loudspeaker unit had an internal transformer, and presented an impedance of around 7000 Ohms to the receiver.  The loudspeaker sockets on the rear of the HRO are wired directly to HT+ and the anode (plate) of the output valve, which means that it would be possible to get a nasty shock from them.

The speaker sockets were supposed to be shorted together if the HRO was ever used without a loudspeaker attached (i.e with headphones only). If this wasn't done it could result in excessive screen grid current and damage to the audio output valve.

My first modification was to fit a loudspeaker output transformer to the inside rear face of the chassis, with the safe low-voltage speaker output via the re-wired terminals. The transformer I used was a 'universal' type with several tappings, but fortunately it came with a data sheet showing the different ratios. One of these gave a primary impedance of 6500 Ohms with a 4 Ohm secondary load which is close enough for this application.

Plugs of the correct size to fit these old sockets are difficult to find today, so I decided to swap the sockets for screw terminals. I used an old screw terminal strip which looks the same as other terminals fitted to the HRO.

Further along the chassis, two more screw terminals were provided for muting the receiver during transmission. As this was done by interrupting the HT supply to the receiver, these terminals form part of ther HT wiring and carry full HT voltage.

The two wires to the terminals were simply disconnected and transferred to a new tagstrip, installed inside the chassis near to the original connections. This method ensures that the wiring can easily be restored in the future if required.

I'm getting close to the point when I can apply power and see if it works. With a set of this age I suspect that there may be a few problems ahead, but perhaps I may be pleasantly surprised.

More to follow... so keep listening!

HRO MX update 1

The electronic restoration is going well, with all the wax paper capacitors replaced with metal film polyester types. As I blogged previously, the set wasn't in original condition, so I'm not going to to hide modern components used under the chassis.

It isn't uncommon for high value vintage resistors to go well out of tolerance, but I was surprised to find that several were more than twice their original resistance value. Some of the smaller value ones were also way off, so I decided to change them all. For this I used 2 Watt 5% carbon types; the larger size of these makes them easier to work with, and they don't look as odd as modern 0.25W ones would.

As expected, the two 10uF electrolytic cathode bypass capacitors were both very leaky.  I removed these completely by drilling out the rivets securing them, and replacing them with modern axial types.

The HRO has balanced valve heater wiring to reduce hum. A tapped wirewound resistor is wired across the 6.3V AC  heater supply, with the tap at the middle of the resistor connected to chassis. Unfortunately the paxolin former of this resistor was broken. According to the manual, the balancing resistor is 64 Ohms with a rating of 3 watts, so I replaced it with a pair of 33 Ohm wirewound resistors mounted on a tagstrip fitted to the side of the chassis.

Whoever had done the previous work on this set obviously didn't share my obsession with tidyness, so the  component replacement also provided a useful opportunity to tidy up the wiring. Many of the solder joints were also of questionable quality and were reworked. All new component leads were sleeved, and anchored mechanically by wrapping round the tags before soldering.

This audio amplifier valve in this HRO-MX should have been a type 42 pentode, but it had been replaced by a 6V6GT  as used in the later HRO-5. Comparing the circuit diagrams of both models showed that the cathode resistor used with the 6V6 should be 300 Ohms, and not the original 500 Ohms which had been kept when the valve was changed.

In the next update I'll describe the modifications to the rear connector wiring and the addition of the loudspeaker output transformer.

Keep listening!

National HRO MX receiver

I started the restoration of this classic receiver by removing the cabinet to allow a close visual inspection of the chassis. 

My first impressions were that the set was complete, although unfortunately not in original condition.

Many of the capacitors and some of the resistors have been changed, and there is an octal based 6V6GT valve in place of the original amplifier valve.

Whoever fitted this didn't fit an audio output transformer, which was a popular modification for amateur use. (The external HRO speaker had a built in transformer, and presented an impedance of around 7000 ohms to the radio.)

Somebody had cut wires to the meter backlight, to the lamp on the front panel, and also two wires between the IF stages and the crystal filter unit on top of the chassis!

Whilst I was a bit disappointed that the radio isn't in original condition (and therefore somewhat rare) it does give me more flexibility with the restoration. For example, I don't see the need to re-stuff the capacitors as I did with my Wartime Civilian Receiver, and I can use modern resistors where necessary.

I think I'll keep the exterior and above chassis areas looking as original as I can. Under the chassis I'll make whatever changes I need to get the best performance, whilst keeping the circuit as standard as possible.

The first tasks will be to replace the damaged wiring, and then replace all the wax paper and electrolytic capacitors.

It's going to be a fairly big job, but I'll post more updates as I go. 

Keep listening!

Making progress

I've had quite a few DAC90A problems recently, with one set in particular needing no less than three faults fixing, in addition to the usual rewiring and component replacement. 

The worst of these was yet another broken wave change switch. This looked like it might be a problem, as fitting a modern replacement would have required hacking of the chassis to make it fit. Fortunately I managed to find a secondhand switch amongst an Internet auction lot for a few quid.

Fortunately my own ivory coloured DAC90A didn't throw any unexpected problems at me, and even the 'no expense spared' restoration was the quickest I've done yet. I guess I've picked up a few tips and tricks since I did the first one!

I've been trying to get all the domestic sets done so that I can move on to something that I've been looking forward to for a while.

Coming soon... A 1940's National HRO receiver. 

Keep listening!

Capacitor Reforming

Here's a picture of the DAC90A currently on my workbench undergoing reforming of the smoothing capacitors.

 

Reforming is an important process that should always be done before applying power to a set that has been unused for a long time; without it there is a serious risk that damage may result.

The reformer on the left applies a current limited HT voltage to the capacitor, with the applied voltage and leakage current shown on the meters. If all is well, the current will drop to almost nothing after a few minutes, as the capacitor reforms.  

With this set the leakage currents of both sections dropped to around 1mA after about ten minutes, which is an acceptable leakage for capacitors of these values (16uF and 32uF)

There will be more radio recycling blogs coming along soon...

Keep listening!

Four Bush DAC90As

Happy New Year!

I had two DAC90As in the workshop over Christmas. One came in to have a vintage style braided cable fitted, which is a quick and easy job. The other was a complete restoration for friends, and it's currently waiting for a replacement wavechange switch. Somebody had completely wrecked the original by forcing it past the end stops!

It's been a while since I had so many sets of one type in the house, I decided to take a photo of them all together. The Ivory one and the one on the right are mine, the other two are the customer's.

The green document is a DAC90A sales leaflet produced by Selfridges in Oxford Street, London, circa 1955. Back then a brown DAC90A would have cost £16 and 15 shillings, and a cream coloured one would have cost a pound more.

I'm hoping to spend lots of dark January evenings in the workshop working through the 'to do'pile. Expect more radio recycling tales soon.

Keep listening!