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Old 8th Mar 2017, 10:42 pm   #1
Gridiron
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Default Coutant Bench Power Supply Identification

Can anyone identify the Coutant supply shown in the photo, it looks similar to the LA400-2 but the circuit, from a previous post, is different. A couple of the electrolytics are dated 1966, which is earlier than I thought a supply using 2N3055's would be. Could it be a LA400-1?
Thanks for any help, Mike.
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Old 8th Mar 2017, 11:08 pm   #2
kalee20
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Default Re: Coutant Bench Power Supply Identification

It does look like the LA series.

I think Coutant set up their Ilfracombe factory about 1970 - this one says Reading which was where they were at beforehand (in fact they still had a sales office at Reading till the early 1980's).
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Old 9th Mar 2017, 7:20 pm   #3
WME_bill
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Default Re: Coutant Bench Power Supply Identification

Coutant Bench Power Supply.
Looking hard at your photograph, it appears the maximum output is 15V, 4A = 60W.
That would suggest a LA 400 series. Whether series 1 or series 2 remains the question. I have an LB500 series 2 dated 1968, so your 1966 date may not be too helpful.

Can you tell us what is inside. The LA400-2 has 3 of 2N3055 pass transistors and a fourth one as driver. The control board uses BC108 or similar.
The LA400 series 1 uses ME1402 or similar, but still the 4 of 2N3055.
Other versions use a 723 on the control board, and high current ones use 2 of 2N3055 with a couple of SCR.
Bench Power Supply units from all makers seemed to keep varying slightly during a production run, with the same basic control board and different transformer and pass transistors depending on the model output voltage and current. Meter panels can change depending upon availability.
I am curious why you want to know the exact model number and series. Do please enlighten us.
wme_bill.

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Old 9th Mar 2017, 8:19 pm   #4
Gridiron
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Default Re: Coutant Bench Power Supply Identification

I wanted to know what model it is so I could try and obtain service information for it as regulation of the output voltage was poor at lower voltages, fluctuating randomly about 0.25-0.5 volts, the wirewound resistor used to set the limiting current was also burnt out and the value was not legible. Cleaning the multiturn pot seems to have cured the former problem and I replaced the burnt out resistor with a 2.2 ohm 7 watt one.
There are 3 2N3055 output transistor and another on a separate small heatsink. The circuit board has several 2N3053's and several Motorola metal cased transistors with SM prefixes and also a dual transistor which look original, presumably the dual transistor is for temperature stability. No BC108's.
I would have to open it up to read the actual numbers, it was working satisfactorily when I tested it this morning.
Regards Mike.
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Old 9th Mar 2017, 10:13 pm   #5
kalee20
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Default Re: Coutant Bench Power Supply Identification

Is this any use? Extract from the instruction book for the LA and LB series.
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Old 10th Mar 2017, 3:43 pm   #6
WME_bill
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Default Re: Coutant Bench Power Supply Identification

Coutant LA series bench power supply.
Now we know that you want the manual, perhaps the attached will help.
You will be lucky to get the exact manual for a power supply, as there are always so many variants, and the availability of particular components means that you will usually find un-documented alternatives used.
I attach a circuit which I think will cover what you have. The actual manual offers little more.
The two comparator long tailed pairs, which balance the voltage and the current sensing are sometimes dual transistors (it sounds as though that is what you have), and sometimes two identical ones clipped together to match temperature effects.
Your trouble with voltage drifting suggest the pair Vt1 and Vt2. Try replacing them with two similar general purpose transistors. Match the Vbe or Hfe at say 1ma current, or just choose two from the same manufacturing batch and measure the Vbe in circuit, ideally to <5mv.
I have bought five, and found that I can get good match by selection for one pair or even two pairs. Type not particularly critical = BC107/108, BC547, 2N3904, 2N930, MPS6514. Then clip them together or put in a common heat sink. Mullard used to use a little S shaped bit of brass or phosphor bronze on their matched FET eg BFS21.
Your current sensing resistor replacement sounds a bit high. The manual I have suggests 0.5 ohm for the L200-2. If the resistor was burnt out, then check the current sensing pair Vt3 & Vt4 also, as it sounds as though they have failed.
wme_Bill
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Old 10th Mar 2017, 11:13 pm   #7
Gridiron
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Default Re: Coutant Bench Power Supply Identification

Thanks WME_Bill and Kalee20 for the information you supplied. I will check tomorrow if the circuit is similar to the supply I have and let you know the results.
Mike.
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Old 13th Mar 2017, 12:00 am   #8
Gridiron
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Default Re: Coutant Bench Power Supply Identification

The PCB in the supply does not appear to be similar to the one in the information posted by WME_Bill , see photo. The board reference number (obscured by the flash) is PCB174-04 Iss. 2
Mike.
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