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Old 27th Apr 2017, 6:16 pm   #1
Radio Tech
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Default GEC BRT402D Receiver.

Hi folks

Got this set off Hammonds (Dave) and got it on the bench to start work on it, to say the least it's had a hard time, mains transformer scorched, and all the caps are corroded and blown out to the point of splitting. It's going to take some time to get it in working order, to say the least a challenge of a lifetime.

Best wishes

Ken
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Old 27th Apr 2017, 6:43 pm   #2
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Default Re: GEC BRT402D Receiver.

All I can say is "Good luck!"

And I'll be following your path-towards-insanity out of interest.
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Old 27th Apr 2017, 7:15 pm   #3
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Default Re: GEC BRT402D Receiver.

As my son works in the area I had thought about going for it myself. Sound like it would have been more work than I would have wanted to take on.

Before spending any time and money on the set there is one test that needs to be carried out. The final IF transformer has a habit of going S/C between primary and secondary. From what I have read on here no-one has managed to repair one or find a suitable substitute. Fingers crossed that yours is OK.

Good luck and keep posting your progress.

Al
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Old 27th Apr 2017, 7:20 pm   #4
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Default Re: GEC BRT402D Receiver.

Good luck from me, too. It's a fine, even a deservedly classic receiver- unfortunately, it uses about three-quarters the chassis area of an AR88 to accommodate a similar number of valves with similar topology but about twice the number of passives and nearly twice the HT consumption.... so it runs H-O-T. That also means it's very packed in there, it's best to take time and care over each bit and don't set hard deadlines per stage or you'll be on the road to demoralisation. It rewards with excellent audio quality, good coverage and one of the nicest tuning drives around, though.
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Old 27th Apr 2017, 7:50 pm   #5
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Default Re: GEC BRT402D Receiver.

I have one of these. I gave up on it many years ago due to the aforementioned final IFT going intermittent and causing massive crackles. The problem is they are variable selectivity jobs with taps on them so not easy to replace. I think it was all discussed on here quite a while back now. Good luck with the restoration. I found mine very difficult to work on due to close packed components. Mine is now languishing under a bench in my garage. Shame really, as I think they would be a good receiver in their day.
Alan.
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Old 28th Apr 2017, 12:04 am   #6
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Default Re: GEC BRT402D Receiver.

I wonder why GEC entered the receiver market, which was rather crowded in the 1940s/50s? Did GEC develop receivers before the BRT series, or was this a new venture? Did they continue to make receivers after this?
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Old 28th Apr 2017, 1:27 pm   #7
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Default Re: GEC BRT402D Receiver.

Presumably they wanted to demonstrate the availability of a full range of equipment, as befits a major player in the Electronics world. A fit would have had a contract price rather than an individual price list. Cost is no barrier to sales at these levels.
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Old 28th Apr 2017, 5:25 pm   #8
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Default Re: GEC BRT402D Receiver.

Hi folks

Found my thread again, Sorry Mods, I put it in wrong place. That said, I have started work on this receiver and yes quite a lot of the caps are tucked in awkward places to say the least. The one thing that puzzles me is why did GEC not identify all the components on the overall layouts, it would have made life easy and also on the circuit diagram the capacitors are not clearly identified as to whether they are electrolytic or not and neither are the caps themselves even where there is a capacitor on a cathode of a valve its not marked, most of the caps I am finding have a highly discoloured sleeve on and the markings have long gone so I am having to guess half the time, the 3 x 2uf smoothing caps of course have given up doing their job so therefore killing the HT voltage from the rectifier, I think if push comes to shove I should be able to replace these with at least a couple of 16uf and an 8uF provided these are at least 450v and can handle the ripple.

We will see

Ken
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Old 28th Apr 2017, 5:30 pm   #9
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Default Re: GEC BRT402D Receiver.

I think you can safely assume that any capacitor with a value in excess of 1uF is an electrolytic.
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Old 28th Apr 2017, 6:48 pm   #10
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Default Re: GEC BRT402D Receiver.

Ken, there are no electrolytics in the set- as a (rather expensive!) professional receiver of its time, they were avoided as suspect components. Just as well really, as they would quickly have been broiled to extinction. If it's the 3x 2uF paper blocks under the chassis with 22k composition resistors strung between them that you mean, these are for the c. -100V AGC amplifier supply- only around 2mA is drawn from this rail but it does need to be a smooth 2mA. It uses 2 rather bulky metal rectifiers and these are unlikely to be as good as they were nearly 70 years ago... The HT smoothing capacitors are 4uF and 2uF paper blocks above chassis next to the mains transformer- as the off-load HT is only around 400V, you could get away with electrolytics here, but it goes without saying that 105 degree types would be wise!,

Colin
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Old 29th Apr 2017, 12:35 am   #11
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Default Re: GEC BRT402D Receiver.

Quote:
Originally Posted by CarbonMike View Post
I wonder why GEC entered the receiver market, which was rather crowded in the 1940s/50s? Did GEC develop receivers before the BRT series, or was this a new venture? Did they continue to make receivers after this?
I don’t know whether GEC offered a similar-specification professional communications receiver before WWII. But post-WWII it probably saw that there was a significant market for new such receivers amongst government and professional users, in contradistinction to the consumer and amateur markets, for which much military/government surplus equipment was available.

The GEC BRT400 was mentioned briefly in Wireless World (WW) 1949 January (page 14) and was the subject of a full test in WW 1949 May (page 171ff). Here are some advertisements from the 1950s:

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My understanding is that the various suffix letter versions and the BRT402 differed from the original only in detail. All had the same basic circuit, of the standard-for-its-time single-conversion 2 x RF, 2 x IF form. Perhaps a standout feature was its amplified AGC system, which effectively used DC amplification (in a combined rectifier/amplifier stage) rather than say a side-chain extra IF stage as used by Marconi on some of its receivers.

The BRT400 series had a long production run of around 20 years, and was replaced by the RC410/R solid-state model, announced in WW 1968 February. So one assumes that it was a worthwhile exercise from GEC’s viewpoint. The RC410/R evidently had a long gestation period. It was mentioned in “Technical Topics" (TT) for 1966 July, then again 1968 January as only just having entered production, with a late change to cascode jfets in the RF stages. It was definitely of the transitional period, combining full synthesis with a valve-era topology, tracking-tuned RF section and double conversion with 1.6 MHz and 100 kHz IFs. It had two tuned RF stages, so effectively had a four-gang front end. Whilst this was common, perhaps even de rigueur for higher performance valved communications receivers, it was apparently rare in solid-state practice, where, if an additional RF “gang” was desired, it was done as a bandpass, either input or interstage, around a single RF stage. (The Eddystone EC958, EC964 and Marconi Hydrus provided contemporary examples of the bandpass approach.) The turbulence created by paradigm shift produces some interesting side effects.

The RC410/R was evidently caught up in the events of the time. In later 1968, following its 1967 acquisition of AEI, GEC took over English Electric (EE), with which came Marconi, a subsidiary of EE since the later 1940s, and Eddystone, a subsidiary of Marconi since 1965. So GEC had three lines of professional HF communications receivers, obvious targets for rationalization which was of course the Weinstock mantra. At the time, the Marconi and Eddystone lines had not been fully integrated, although Eddystone had been a supplier to Marconi since the early 1950s. But there were signs of co-ordination. The Eddystone EC958 had been designed with marine main receiver applications in mind, and the marine version, in the guise of the Marconi Nebula, was announced at the same time as Marconi’s own-design Apollo marine main receiver (see WW 1971 December, page 609). And the Marconi H2310 Argo and H2311 were EC958 variants.

The RC410/R (and the RC411/R with coverage extended to VLF) were listed under the Marconi name in the WW 1970 June survey of communications receivers. But their subsequent fate is I think depicted by this advertisement from WW 1971 July:

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Its purely speculation, but I surmise that the Eddystone EC958 rather than the RC410/R was seen as the best contender in that part of the professional HF general coverage receiver market that was a distinct performance and price level below the lofty level inhabited by the sophisticated upconversion receivers from Racal, Redifon, Plessey, etc. The EC958 production run was quite long at about 15 years.

Incidentally, the GEC BRT402 was used at Quartz Hill for HF broadcast relay purposes, until superseded by the Marconi HR21; see: http://www.zl6qh.com/000468.html. I vaguely recall that there was a step-change quality improvement in the BBC news rebroadcasts on the National programme, so that was probably the cause, although I did not know that at the time.


Cheers,
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Old 29th Apr 2017, 6:47 am   #12
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Default Re: GEC BRT402D Receiver.

The BRT400 series are usable as search receivers, for trawling across a band to see what's there. The tuning gearbox is smooth and feels good.

The RC410 series are quite different. I had one of the early ones. The tuning is by decade switches and the switches are geared together with Geneva-Wheel mechanisms. You turn the right-hand tuning knob and it goes clunk-clunk-clunk in 1kHz steps (100Hz if you have it pulled out for 'low gear') one in every 10 clunks is a CLUNK when the next decade switch increments. You feel the added load! The small grey knobs to the left of the big blue kHz knob are the 10kHz and 100kHz switches. You can disengage these and turn them to do a quicker QSY.

This arrangement is substantially more convenient than a bank of thumb-wheel switches, but it's not free tuning bu a long chalk.

Inside the set is on large SRBP PCBs which warp.

I think the market had it right, the Eddystone was the better radio. The RC410 was better as a fixed link machine.

Nowadays the RC410 is a historic artefact, not a daily user for anyone who has one. I had no interest in keeping the one I had.

David
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Old 29th Apr 2017, 2:59 pm   #13
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Default Re: GEC BRT402D Receiver.

I have an RC410R here, awaiting restoration, along with the full service manual down to component level.

It looks a bit of a nightmare (which is why it's still awaiting) and has one badly warped Paxolin PCB in the synthesiser. It was working 17 years ago...

The tuning is mechanical, with a servomotor controlled by a phase locked loop system and a varicap for fine tuning.

The 4-gang tuning box has the ranges selected by a Ledex turning a set of wafer switches. Plenty of noisy clunking going on there as of course it only turns in one direction!

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Old 29th Apr 2017, 4:58 pm   #14
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Default Re: GEC BRT402D Receiver.

Thank you every one for your contribution and as Collin has mentioned the fact that they did not use electrolytic caps that takes the confusion out of the equation. I found out the problem with the HT dying, the 4uf smoothing cap positive had a wire linked to the headphone socket this terminal going direct to chassis and as luck has it the 2uf and 4uf smoothing were ok and had no sign of leakage. but still plenty of caps to change yet, I keep finding them everywhere.

More to follow soon.

Ken
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Old 29th Apr 2017, 6:37 pm   #15
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Default Re: GEC BRT402D Receiver.

I'm reading your thread with interest Ken. The sets you have passed on to myself and other Forum members have been nothing like this communications RX and I have often wondered about the GEC range, so I am intrigued to see you take on what has already been described as a considerable challenge! Perhaps you will be able to overcome some of it's apparent design problems in the process of renovation/restoration?
Thanks for posting your progress so far

Dave W
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Old 29th Apr 2017, 7:42 pm   #16
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Default Re: GEC BRT402D Receiver.

The most serious shortcomings are at the foundations of the design.

The box is too small. Having components built over each other to this extent is a nightmare for repair. You could say it's OK when it's going and when it was new, but the poor airflow leads to bad cooling and hence bad reliability.

They chose to avoid electrolytic large value capacitors and to compensate for the poor smoothing, added an active 'capacitance multiplier' valve and a huge resistor. Perhaps this could be undone. Without the active shunt smoother, there will be signifacantly less HT power consumed, and one fewer heater to feed. This should reduce the heat stress somewhat. The ripple can be reduced by more capacitance - meaning electrolytic ones.

More heat reduction could be got by going for silicon HT rectifiers. They save the heater powe of the original rectifier, though their voltqge drop may need to bey simulated with some series R, alternatively a cooler (higher voltage) tap could be selected on the mains transformer and a little less heater voltage tolerated.

Every watt you can save would help.

David
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Old 29th Apr 2017, 8:51 pm   #17
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Default Re: GEC BRT402D Receiver.

Quote:
Originally Posted by Radio Tech View Post
I found out the problem with the HT dying, the 4uf smoothing cap positive had a wire linked to the headphone socket
Sometimes, I've scratched my head at just what folk have done to old kit, but that has to be one of the most "exciting" mods I've heard about- was it a GCHQ punishment for loose-tongued operatives?
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Old 29th Apr 2017, 10:46 pm   #18
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Default Re: GEC BRT402D Receiver.

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Originally Posted by Radio Wrangler View Post
Every watt you can save would help.
The shunt stabiliser (S130) and it's 3k feed resistor are also very power hungry. Of the 40mA the resistor is passing only 10mA is supplying the LO and BFO.

With today's cheap high voltage FETs it would be very easy to replace the active hum reduction and shunt stabilisers with series pass regulators saving a good few watts of HT and heater power.

Al
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Old 30th Apr 2017, 9:13 pm   #19
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Default Re: GEC BRT402D Receiver.

Yes, the S130 looks more like something from the 1900s, not the 1940s! I wouldn't be surprised if it was a pre-war GEC (?) item. I'm not sure when gas-filled shunt stabilisers became widespread and commonplace- the AR88 was notably a mass early adopter but there were probably earlier mass production examples. Blumlein seems to have published the classic reference/comparator/series-pass full electronic regulator as early as 1933.

The hum-canceller circuit, as well as adding a dollop of extra heat, also gives us the mother of all "that" capacitors in the shape of a 100nF waxy between raw HT and the grid of the shunt power tetrode. In their keenness to avoid electrolytics, GEC did seemingly get themselves into a bit of a losing spiral- the smoothing valve adds something like 35mA extra HT drain and another amp of heater current, so the transformer got even bigger and the heat just kept building. A smaller transformer and no smoothing valve might even have meant available space for more paper smoothing block in the first place, and surely electrolytics were good enough by war's end to at least get a look in- some, notably US kit, had IO-base electrolytics for quick replacement.

As well as the Achilles' heel capacitor mentioned above, there's an anti-hunting capacitor between the amplified AGC line and the 1st IF amp screen grid- this is also a candidate for early replacement as any leakage could make several W81s rather ill. For some reason (probably physical size!) it's only rated at 250V when even the cathode decouplers are rated at 350V- it's in a particularly tight spot at the front of the chassis, next to the oscillator box and under the selectivity switch....
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Old 1st May 2017, 8:12 am   #20
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Default Re: GEC BRT402D Receiver.

Hi folks

Here are some pictures of the areas that have had the capacitors changed, to say the least, yes it is a difficult set to repair and I will not know for certain all is going to work afterward, and even some of the valves have very little emission, we live in hope.

Ken
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