GEC BRT400 Receiver

[Alf Fisher]

Hello folks,

One of these radios followed me home recently and I note there has been much discussion on here about them although nothing in the last couple of years.

I've found two versions of scanned manuals online and the rest are copies of the same files where the scan has been very poor for the photographs or the circuits break up in pixels when you zoom in.
Does anyone have an original manual that I could scan and make a decent job of it?

Has anyone got details of the components on the tag strips? In many equipments of military and professional origins, component idents are marked with their circuit references in Indian ink. I was somewhat disappointed to see that the components in this radio aren't marked at all. The first thing I wanted to do was change 'THAT' capacitor but its all so crammed in that its difficult to trace wires to find the beastie. Of course an alternative approach would be replace all the capacitors

The next thing I'd like to know is how to remove the case from the chassis. Please can anyone enlighten me?

Thanks in advance,

Alf

[ms660]

The audio output valve g1 coupling capacitors are C103 & C104 in the schematic, 0.1uF & 0.005uF respectively, they are marked on the tag strips in Fig. 24 in the manual in the link below:

https://www.rigpix.com/miscrx/gec_brt400-402_manual.pdf

Lawrence.

[Silicon]

I have one of these but it may have been modified. It will be a long slow job when I do tackle it.
I will probably create an alternative to the shunt regulator power supply.

These radios have a reputation for running hot and destroying mains transformers.
IF transformers can go faulty and are difficult to replace.

Before spending time and money on it, I recommend that you thoroughly check the expensive and irreplaceable parts.

The VMARS website may have a manual.

[Alf Fisher]

Thanks Lawrence,
I missed that one - guess I should have gone to specsavers!

Cheers,
Alf

[Alf Fisher]

Hi Silicon,
Yes I spent a few hours yesterday reading through all the posts.

It has more than a reputation for over heating - it does as is evidenced by the toasted panel on top of the mains transformer and the black marks on the underside of the lid.

I will be doing away with that electronic smoothing valve and fitting a suitable capacitor in its place. May need to increase the value of R100 if the HT is too high.
I also thought about changing the output valve for something else. I'm sure I can find one that takes less heater current and lower anode current. I don't know what the normal HT current demand is supposed to be but I measured it at 180 mA and the mains input current was a bit under 0.6A. Its mostly OK and I did receive Radio 5 live - albeit badly. The AGC doesnt work as the the receiver was overloading. On backing off the RF gain about a third, the signal came through quite well.
I don't think the smoothing valve could have ever worked because I discovered a dry joint on the connection of the hum pot slider that is supposed to be grounded. With the leaky capacitor feeding the grid of the smoothing valve and the bottom end of R106 and the pot wiper and the cathode all floating, there's no way it could pull any current.
Its on hold at the moment as I've got other things that need my attention.

Cheers,
Alf

[Alf Fisher]

So that's the answer to the first question sorted.

The question now is:- "How does the case come off".

I have been told by one person that they have seen it off but not how it was done.

Advice welcome.

Alf

[Station X]

Full manual can be downloaded here if you don't already have it:-

https://bama.edebris.com/manuals/ge/brt400/

[G4XWDJim]

Originally Posted by Alf Fisher

The question now is:- "How does the case come off".


Alf

If I remember correctly it is not a case as in most receivers but comes off in sections.
Not an easy set to work on.

Jim

[Alf Fisher]

Thanks Jim,

To me I think you're right. I had another look this morning and I think it might be just the left and right sides that unbolt from the chassis but the there is a plate across the back that has what look like 4BA bolts but they're impossible to get at until the sides are removed - a chicken and egg situation if ever there was one.

I've had other things to deal with recently so haven't had a serious look at it for a week or more.

Cheers,
Alf

[G4XWDJim]

About twenty years ago I had two of these sets. I spent ages working on one, a 402 model perhaps but didn’t find it enjoyable everything being excessively awkward, particularly working on the chassis end of the tag boards.

It worked quite nicely and looked impressive with good sound and was reasonably stable but as every little thing turned into a big job I disposed of the second untouched.

I had a picture in an RSGB handbook 1960 maybe, of a room with many shelves full of them, must have been a hot place to be listening.

Jim

[Alf Fisher]

Quite a lot of progress has been made since my last posting on this heavy receiver.

I succeeded in taking off the case and the front panel and have made a number of repairs to get it working more safely. As you will see from the photo, I've now fitted an IEC mains connector instead of having wires hanging out with a choc block on them and a proper fuse holder in place of the Slydlok ones. I've also taken the advice of David, (Radio Wrangler) and got rid of the smoothing valve and fitted a nice new electrolytic.

I found the HT voltage was excessively high and I have no idea why they specified a transformer having 350-0-350 HT winding. The voltage on the 4uF reservoir capacitor was 380 volts which is crazy. I fitted a 390 ohm aluminium clad resistor at the cathode of the rectifier and increased the value of the so-called smoothing resistor from 220 ohms to 390 ohms which fits the clips perfectly and was what was available in the junk box. The HT rail voltage is now 245v which is what it says in the manual. The total HT current is now about 120mA instead of 190mA. When I get the case back on, I expect it to run much less hot and take less power from the mains.

Cheers,
Alf

[Ed_Dinning]

Hi Alf, these sets have a reputation for running hot and damaging mains transformers; it could be due to the high HT and you may have solved a long standing problem with your resistors

Ed

[Alf Fisher]

Hi Ed,
I find it difficult to understand how these sets can burn out the mains transformer. The power consumption quoted in the manual is 135 watts yet the transformer is a massive lump that looks to be capable of 400-500 watts. The output transformer is also a huge lump.

The picture shows that the BBC used lots them in racks at Tatsfield and how the temperature must have risen in that room with them being on 24/7. I wonder what MTBF they got in that environment. Surely they would have had air conditioning.

Another mod I've been thinking of doing is to change the output valve to a 7B5. Its the same base but the cathode resistor will need to be changed but it would reduce the heater load and the HT current compared with the KT81.

Regards,
Alf

[G6Tanuki]

The whole interior of a BRT400 got hot. Bad enough to carbonise the Paxolin tagstrips on which some high power resistors were mounted.

I guess that if the insides of the cabinet are running that hot it must have fatigued the insulation of the transformer to the point of breakdown, rather than this being due to the transformer generating heat itself.

Fitting a less-sweaty output valve makes sense too, and upping the cathode resistance to reduce the standing current as well.

I regularly use a 470 Ohm cathode resistor with a 6V6 in communication receivers where you are more interested in cool running to prevent drift on the HF bands rather than hi-fi or loud audio.

[Ed_Dinning]

Hi Alf, a G6 has said, not poor rating of each item, just poor total thermal design is rubbish

Ed

[Andrew Sinclair]

I had a BRT400K which was the last version in production. Mine didn't run excessively hot once I had fixed all the faults!
The set has paper smoothing capacitors, as electrolytics were considered unreliable at the time. This gave an unacceptable level of hum, so this was fixed by using a hum canceling valve (EL84 in mine). A high power resistor was fitted in the HT line between the power supply and the rest of the set. The hum cancelling valve anode goes to the output of this resistor. The valve grid is AC coupled to the HT line and an un-bypased variable resistor is used in the cathode for bias. By adjusting the resistor, the valve gain can be set to almost completely cancel the hum.
The problem is that the capacitor in the grid behaves exactly like THAT capacitor in a broadcast receiver output stage. If it leaks DC, the grid goes positive and the valve tries to short out the HT rail.
On my example, the output valve, (a second EL84) had a leaky grid capacitor in addition to the "hum eating valve" so the set had got hot enough to burn out it's wiring harness!
Once I had fitted modern capacitors to the EL84 grids and repaired the damage, the set ran reliably for the 14 years I had it. It ran a little warmer than say an RA17, but not excessively so. The hum pot never needed adjusting.
I believe the BBC used them for a number of duties, including receiving signals for re-broadcast. The detector is very linear. The AGC circuit has a DC amplifier valve and works extremely well too. The audio output transformer is massive to give high primary inductance and therefore good frequency response. The set is an excellent receiver for listening to broadcast signals.
I imagine the poor maintenance man at the BBC station in in the picture spent his life running his hand over the cabinets to see which sets were getting hotter than usual and about to catch fire!
Andrew

[Alf Fisher]

Yes Ed, poor thermal design.
Mounting two big wirewound resistors right next to each other on top of the transformer and then put a paxolin panel on top to impede the cooling. Its no wonder the panel got burnt.

I fitted a 7B5 output valve with a 620 ohm cathode resistor and fitted a heatsink on the front side of the mains transformer. On this are two 330 ohm resistors in the AC feed to the rectifier and what was the smoothing resistor which is now 400 ohms. I've left the dropper for the stabiliser on its own on top of the transformer.
With these my mods, the current drawn from the mains is reduced from 520mA to 410mA and it runs a lot cooler.

Alf

[Ian - G4JQT]

Not off topic I hope, but when I first arrived at the BBC Receiving Station at Crowsley Park in 1979, we still had about 20 RA17s and 10 or so BRT400s - I forget the exact version. The rest were RA1772s - fantastic sets even by today's standards.

All the sets were on 24/7 for years and years, but the only 'smoker' I can remember was a BRT400!

Ian

[Radio Wrangler]

Further heat reduction measures would be to replace the HT rectifier with silicon diodes and a dropper resistor. The dropper could be an aluminium cased one on a finned heatsink outside the cabinet - it's a good opportunity to move heat outside.

With some of these things done the thought is to reduce the HT voltage by tap changing, voltage bucking or some such trick rather than resistive drop. But you don't want o scale the heater supply down, so it's the HT only that needs treatment. A transformer rewind would be the best approach. Transformer bucking could be done on the HT secondary, but for a full wave rectifier, a bucking transformer with two secondaries would be needed, and it would need HT grade insulation between them.

The BTR400 is nice-sounding and a reasonable performer for AM broadcast, but would I put this much effort into fixing design choices which cause the unreliability?.... they are heritage bloopers after all.

I think I'd just stick to my AR88 or maybe succumb to the thought of finding an RA1772 and doing a bit of power supply cooling work on that. I was given a GEC RC410 which was the next generation after BRT400. No heat problems but huge paxolin boards bending like bananas with moisture absorption, an early frequency synthesiser with iffy performance, tuned by a monster array of switches mechanically interlinked with Geneva-wheel mechanisms. Horrible to tune around with. But, it had a servo-tuned preselector. Imagine if Racal had stuck one of those in the RA17 or RA1772 families.

The difficulty with improving a set is deciding when to stop. If there is something better available, then maybe the old thing should be kept pretty close to original to show what things were like 'back then'? Maybe some heat reduction to make a BRT400 into a more reliable demonstrator could be justified. As it is, the planet ought to thank Alf

David

[turretslug]

That arrangement of big, hot resistors in a thick Paxolin sandwich on top of a hot transformer strikes me as about as bad as it could have been. I'm stumped as to why GEC couldn't have taken advantage of their tubular construction by mounting them vertically on a cheap steel (or even aluminium, plenty of the latter swilling around at the time) plate mounted vertically in front of the mains transformer over the block capacitors. The whole PSU arrangement seems to have been chasing one problem but providing several others to replace it in the process!