BPL CB 154-D Electrolytic Capacitor Bridge, circa mid 1950s

[trobbins]

I'm just starting the restoration of a British Physical Laboratories Electrolytic Capacitor Bridge model CB 154-D, from I think the mid to late 1950's.

This is an imposing bit of lab grade kit due to its width and weight, and in the same enclosure style of a BPL RM 175-LZ MK II Megohmmeter that I have. Like other BPL gear, its production life continued with AVO as the Mk4 and 5 it seems. WW Apr 1957 has the only advert I can identify, but it is slightly different from the D model.

Just hoping someone has a manual for this equipment, or has a contact within British Vintage Wireless and Television Museum as it seems they may have it https://www.vintage-radio.net/forum/showthread.php?p=14031#post14031.

My unit is a little dusty on top, but visually only has one poor e-cap can, and one stressed resistor, so I'm hoping it will come up ok.

Some notable parts include many DoD Dubilier and TCC e-caps (rather than commercial grade), a BTH CG6E Germanium diode, a GEC GEX 5/1 diode, an X79 triode-hexode along with some EF86, some custom ww resistors for the 0.1uF to 1100uF span range switch in the bridge, and what looks like a large pot-core style transformer in a substantial metal enclosure. It has a 5R4GB for the 600V polarising voltage control, even though the current requirement must be pittance, with the CLC filter including two series connected 15W mains bulbs in series with a KM62 choke.

I can see an auction item for a latter AVO manual, and I may have to spruce up my swedish as the AEF museum has a manual.

Ciao, Tim

[trobbins]

The circuitry for the signal valves and polarising voltage was easy to discern so, in lieu of a manual/schematic, I've tested/replaced parts and confirmed ok dc idle operation. I replaced the higher voltage e-caps and the 0.1uF 500V foil Dubiliers, due mainly to highish leakage, and the TCC picopacks were bad. A few resistors had drifted, but in general the 1960'ish parts were still ok.

I'll continue to work through a few issues and functions, but it'll get difficult to go too much further without a schematic.

One issue I'll try and work through is that the max dialed polarising voltage is uncomfortably higher than the full-scale range switch selection levels (8V, 25V, 80V, 250V, 800V) for nominal mains voltage. Not too big a practical issue as there is a front panel voltmeter to set the applied level, but the 800V range can just reach the e-cap 450V x 2 limit I have used.

[trobbins]

Whoopy-do, AEF has just uploaded the instruction manual for their CB 154-D Mk2. Time to take this restoration to another level !

https://www.aef.se/Biblioteket/Tabeller/Dokumentarkiv_A-F.htm

[John_BS]

Lovely bit of kit: pity BPL's dcoumentation is a bit lacking in schematic annotation/service hints!
John

[trobbins]

Whoopy-do for the second time - it works.

The bridge appears to be accurate within its original spec, and for the resolution of the capacitance and TAN d scales, and the null detector works well and consistent with the scale resolutions.

I used a mid-1980's Sprague 735P metalized polypropylene capacitor that had been measured at 30.16uF and 0.004 D, but I don't recall the lab meter I used then, or the test frequency. My more modern MCP BR2822 meter appears to still be within cal, with compensated Kelvin leadset, and it measured 30.22uF 0.0008D 0.042R at 100Hz. This 154-D bridge uses mains 50Hz, and measures 30.1uF and <0.005D (ie. ESR<0.5ohm for |Xc|=106).

I then added 1R 1% and 10R 1% resistors in series with the PP cap, and measured D=0.01 and 0.095 (ie. ESR of 1.06 and 10.07), indicating PP capacitor ESR was ~0.06 ohm, and consistent with the MCP meter reading.

The 154-D uses precision WW resistors for a 5 decade capacitance measurement span covering 0.1uF to 11,000uF, so likely that the other decades are still within tolerance for capacitance measurement.

The polarising voltage and leakage section is functional but still needs some work, as the panel meter reads a few % high, and the leakage metering is fine for 50uAFS, but NQR for 0.5 and 5mAFS.

The on-line CB 154-D Mk2 manual shows some subtle differences to this 154-D, but certainly equivalent for purposes of fault-finding and restoration. And yes John, the manual is woeful with respect to description of operation.

This is a de Sauty - Wien style bridge, with a fixed 50Hz AC voltage, transformer coupled with the DC polarizing voltage, as the bridge excitation voltage. The detector is a transformer winding coupled to the EF86 based amplifiers. The X79 likely has a tuned load for 50Hz, although I haven't done a frequency sweep on its response, and then the BTH CG6E Germanium diode detector for meter rectification.

It certainly is an easy bridge to use, and although not metrology grade for the time, it was used in ASTOR brand's radio and TV manufacturing facility in Melbourne, so appears to have had an easy life for a few decades, and then taken home by one of the employees to live out its retirement in a cupboard, until now

[trobbins]

It appears to be fully functional now.

Resistor R14 in the leakage current measurement circuit had blown open - perhaps due to a faulty cap shorting through the 0.5W rated R14, which was replaced with a more robust resistor.

The voltmeter required slight tweaks to the divider resistors, and is now accurate across the ranges.

I dumbed down the max polarising voltage to be about the range limit with no load (ie. no leakage current), by padding resistance in series with the control rheostat R45. During reforming, any significant leakage current will sag the max level to below its range limit, which is perhaps a good thing. For modern e-caps that would almost never have more than a 500-600V max working level, some caution is needed to read the voltmeter scale correctly as 450-500V could be easily exceeded by accident. Perhaps hobbling the 800V range to 600V would be a good thing.

My interest was drawn to the manual's general information statement:
'It is helpful to note that both capacitance and power factor as well as leakage current vary markedly with applied polarising voltage.'

So I put a sample new Rubycon BXW 100uF 450V 105C e-cap through its paces to get:

Polarising Vdc ; C(uF) ; TAN Dx100 ; Leakage (uA)
60 ; 90.8 ; 2.2 ; <1
150 ; 91.2 ; 2.4 ; 2-5
300 ; 92.1 ; 2.6 ; 2-8
400 ; 93 ; 2.7 ; 10-15
440 ; 93.2 ; 2.8 ; 18-20

As expected, leakage current takes time to subside, and although I didn't wait any mandatory time like 1 or 5 or 10 mins, I did wait some time. The Cap and TAN D weren't changing as leakage current subsided, so a simple parallel R model for leakage wasn't influencing bridge balance.

[Bear2Biker]

Hi Tim, Whoopy-do over here aswell.

I got one just like the photos (with righthand meter) in a semi working state some time ago, as it's quite a long unit to work on I just let it sit there, the 2 ballast tubes at the rear had blown and absolutely no markings on them so was unsure what Barretta model they were, seems like they are just 15W bulbs, simples.

So now with your detective work & restoration notes and the manual from Biblioteket I'm good to go (well maybe later in the year/New year), so thanks Tim for posting on the Forum.

[trobbins]

Xclent. I hope you have a relatively painless restoration.

It seems reasonable that any equipment fault would likely be due to electrolytic caps - whether internal to the unit, or as a part being tested ! My 15W (aka oven style) bulbs were both ok, so hadn't endured any shorting DUT or internal e-caps.

I was able to button up my unit, and finish off my restoration doc: https://www.dalmura.com.au/static/BPL%20Electrolytic%20Condenser%20Bridge%20CB%20154-D.pdf

[Bear2Biker]

Tim,

What an interesting breakdown and description of your efforts to bring it back to life, loads of pertinent and useful information in there, excellent pictures too, very clean and tidy inside.

As LED bulbs seem to be taking over even these small ones I'll get a few "oven/fridge " bulbs as spares, the ones in mine are E27 but I'll change the holders to E14.