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Old 11th Jan 2021, 12:42 am   #1
factory
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Default HP 5325A & 5325B universal counters

Here is what I've been working on recently, a HP 5325A (12.5MHz) and 5325B (20MHz) universal counter, both are from the late 1960's, I've had the HP 5325A for a few years and acquired the broken HP 5325B locally last year, however I noticed something wasn't right with the 5325A either, the overflow lamp was on all the time.
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This was traced to transistor Q1 which is supposed to ground the supply from R2 & extinguish the lamp when not required. The transistor still measured as two diode junctions with the DMM, but wasn't working in circuit, it was part no. 1854-0232 which is apparently a 2N3440, one of those was fitted & the overflow lamp problem was fixed.
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Onto the 5325B, I had previously cleaned the push button switch contacts (as recommended by another collector on the eevblog forum), but it had made no difference and it still refused to count anything, not even the self check.

The first problem was traced to the A4 function selector board, it has a few minor differences to the earlier counter (for the 20MHz spec rather than 12.5MHz, plus some added resistors from the logic gate outputs going to the BNC's on the back), I verified the A4 board was faulty using the older counter. And using the 548A logic clip I traced the fault to IC6 1820-0380 (marked SN74H53 on the underside) which had no output, this was temporarily replaced with a standard 7453 as I haven't got any of the 'H' version.
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That fixed the lack of any count appearing on the display, but also revealed a second problem, it would only count using the first five display tubes and one of those would only display a few numbers, mainly 4,5,6 & 7 then blanked again before repeating.
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This took quite a bit longer to diagnose the failed IC's with the 548A & 546A, I removed the board & powered the 5V rail from a bench PSU while fault finding, due to the amount of wires that would have needed disconnecting/reconnecting in the counter each time.
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David
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Old 11th Jan 2021, 12:52 am   #2
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Default Re: HP 5325A & 5325B universal counters

Part two...

Two of the decade counter IC's & one of the buffer/storage IC's were faulty, with either missing outputs or were overloading the outputs of the preceding IC. The buffer/storage IC had been attacked by a previous owner, with a pin cut off & a random diode attached, which had also been cut through (the IC still didn't work when reconnected).
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All the IC's for the counting & display section are custom parts, thankfully I had a parts board that donated some replacements, I fitted some IC sockets to the board in case of future problems.
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Here are the two counters after repair, measuring the 1MHz output from my Racal 9839 and the faulty parts.
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I still have one problem with the 5325B, that is the crystal oven not getting warm.

David
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Old 11th Jan 2021, 3:11 am   #3
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Default Re: HP 5325A & 5325B universal counters

usually, a crystal to be used in an oven is not a normal 'AT' cut orientation with the rotated S-shaped temperature curve. They are usually a cut with a parabolic shaped curve, centred on the oven temperature, at the required frequency.

These crystals are therefore quite a long way off at room temperature.

This gives the interesting discrepancy that if you turn on two frequency counters, the one with the expensive oven reference option gives a far worse error than the one with the simple crystal reference, and then over a period of many minutes, the ovened unit gets better and then should wind up the more accurate of the two.

So fixing the oven is quite important.

David
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Old 11th Jan 2021, 10:45 am   #4
barrymagrec
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Default Re: HP 5325A & 5325B universal counters

Does the 5325B have the same feature as the earlier 5245L of the oven heater being permanently on while the unit is connected to the mains? This means that the oscillator comes up to stable frequency almost immediately the unit is switched on by the front panel switch.
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Old 11th Jan 2021, 1:43 pm   #5
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Default Re: HP 5325A & 5325B universal counters

If the 5325B's oven is anything like the 5245L's, it's likely that the internal wiring in the oven has burnt itself to a crisp and subsequently burnt out its mains transformer. I've had to do quite an extensive rebuild on my 5245L (documented here somewhere) to get it working reliably, which I'm happy to report it still does.

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Old 11th Jan 2021, 3:31 pm   #6
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Default Re: HP 5325A & 5325B universal counters

Both the 5325A/5325B use a smaller oven assembly than the 5243L/5245L*, it's part no 05325-60012, according to the diagram it only contains the crystal & a heater, the heater is supposed to measure between 60 & 80 ohms at room temperature. The supply for the heater is from the +/-12V supply that is shared with other parts of the counter, no separate transformer in this one.
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There is a later manual for the 5325B on the web, that shows they removed the oven assembly from later 5325B's, instead they used a different A7 board with a discrete crystal, oddly the notes on the diagram still refer to the heater measurement.

Quote:
Originally Posted by cmjones01 View Post
If the 5325B's oven is anything like the 5245L's, it's likely that the internal wiring in the oven has burnt itself to a crisp and subsequently burnt out its mains transformer. I've had to do quite an extensive rebuild on my 5245L (documented here somewhere) to get it working reliably, which I'm happy to report it still does.

Chris
*My 5245L is a bit later than the one you repaired, it has the 10544A oven assembly. I acquired an earlier 5243L (only rated for 20MHz) last year with the same oven oscillator assembly as your 5245L, the 5243L has problems with the oven controller, which doesn't switch the heater on (I found out after fitting an extra fuse-holder for it), the heater element is OK. But that will be something for a new thread when I revisit it.

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Old 11th Jan 2021, 3:57 pm   #7
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Default Re: HP 5325A & 5325B universal counters

That grey can is used for various crystal arrangements, not all have heaters inside. The internal foam houses a crystal in the glass envelope you'd associate with a 6V6GT, minus the base. Gold metallised electrodes on the quartz and en evacuated glass housing... rather classy!

I've got one here in pieces just as a demo object for lectures on crystal oscillators.

David
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Old 11th Jan 2021, 4:19 pm   #8
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Default Re: HP 5325A & 5325B universal counters

That maybe from a 5232A-69A crystal assembly, which is used in many other HP counters and is in a similar sized grey can. I think you've just explained why that one has fewer connections, just before I dug a manual out.

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Old 12th Jan 2021, 4:04 pm   #9
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Default Re: HP 5325A & 5325B universal counters

Well the oven heater in the 5235B isn't open-circuit, but is a bit lower than the spec given at 49.6Ω (should be 60Ω to 80Ω at room temperature), I also checked the current draw at start up & what it stabilised at after around 10 minutes. Start up current was about 0.5A dropping to 16mA after a while.
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The 5325A oven assembly has a heater resistance of 80.6Ω, current at start up was about 375mA, dropping to 45mA after a while.
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Ambient temperature was approx 18°C, I haven't opened the oven assembly yet* but it could have some shorted turns on the heater element. The manual actually gives a part number for the heater (RT1) 0410-0161.
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*I've been a little distracted after noticing some burnt components on the input amplifier boards in the older 5325A, how I missed that before I don't know.
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David
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Old 15th Jan 2021, 12:27 am   #10
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Default Re: HP 5325A & 5325B universal counters

Well I'm not sure who was responsible for the burnt resistors being lower than 100kΩ, as I discovered the very first batch of HP 5325A's (prefix 820) came with a 82Ω resistor for R3, but this counter is a later 844 prefix and the input amplifier boards have no space for the capacitor C20 that is in parallel with the 100kΩ. I wouldn't normally do this but I decided to modify both boards to match the 844 prefix manual, i.e. how it should have left the factory.
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First I had to strip all the components from the burnt areas & clean the carbonised remains away, some Araldite was needed to fill in where one board was damaged.
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The components were all tested, one Zener diode had failed (0.3V one way & 0.9V the other way), testing the remaining ones I have decided a 3.3V Zener closely matches the spec of the rest of them (approx 3.8V at 10mA), one of those has been temporarily fitted & I have ordered a couple of the original 3.83V part as they weren't too expensive (part no. 1902-3059).

I replaced a couple of the resistors that had faded markings, fitted the 100kΩ for R3 and a 1000pF capacitor on the rear of the boards, I also added a link on the back to properly repair a damaged track that previously had a bit of IC leg bodged on.
Both boards tested OK on all attenuator settings, guess I'm lucky the protection circuit did it's job & didn't EOL the dual FET.
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David

Last edited by factory; 15th Jan 2021 at 12:34 am.
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