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Hi folks,

I recently acquired a HP 5315A Universal Counter in mint condition. It works perfectly too, but for two issues that annoy me.
First, it hums noticably loud. Its definitely a 100 Hz mains hum. The unit works nicely, the casing is not getting warm after an hour of usage.
I have the service manual for it, but before taking it apart I'd like to hear if other 5315A owners have the same problem.
Second: my 5315A does not have the battery option fitted, but still the unit hums as soon as the power plug is in, even if the unit is turned off. I'm thinking of making the power switch a "real" power switch, since I do not use the counter very often. Has anyone done a mod like this on a 5315?

Cheers,
Wolfgang

The HP5315A/B is a very interesting counter to work with. I’m not sure of the cause of your humming, but when plugged in, there is power to the PCB always. That keeps the oscillator stable.
This counter and the HP5316 are well documented in manuals and HP journals. Everything is controlled by 3 chips. A custom counter chip, a Mostek MK3870 clone Microcontroller and an Intersil ICM7218 display controller. There are several generations of PCBs over the +15 years of production. The simplest mod is to replace the XO or TXO with an OCXO (oven controlled) daughter board. Google for this mod.

The HP5315A/B was a low cost design, so there was no HPIB interface. The HP5316 was essentially the identical design but included an HPIB interface. It is 2 times the depth of the HP5315A and much heavier.

I am in the process of developing a USB interface to the HP5315 to log the display to a PC. I’ll provide more info as the project proceeds. Basically, what ever mode is selected on the front panel will be interpreted into 8-9 digit mantissa, 1 digit exponent, units and associated flags. Logging will be selectable from 1-600 seconds.
The design is based on a daughter board which uses the MK3870 and a Microchip USB Microcontroller. The daughter board plugs back into the MK3870 socket.
Keep tuned.

Hi Wolfgang,

I don't have an HP 5315A, but in my HP 5328A the power switch is wired this way because they want to keep the oscillator running always that the counter is plugged into the mains.

This is good for oscillator stability and if your unit is equipped with an OCXO, it insures that you will have zero warm up time. I would keep the power switch wired as it is now unless you use an external reference, or the calibration will suffer.

The hum is certainly caused by the transformer laminations "singing" and it can be corrected tightening the screws, if any, or even with some varnish.

Regards,

Ignacio, EB4APL


El 27/03/2019 a las 9:43, Wolfgang Mahringer escribió:

Hi folks,

I recently acquired a HP 5315A Universal Counter in mint condition. It works perfectly too, but for two issues that annoy me.
First, it hums noticably loud. Its definitely a 100 Hz mains hum. The unit works nicely, the casing is not getting warm after an hour of usage.
I have the service manual for it, but before taking it apart I'd like to hear if other 5315A owners have the same problem.
Second: my 5315A does not have the battery option fitted, but still the unit hums as soon as the power plug is in, even if the unit is turned off. I'm thinking of making the power switch a "real" power switch, since I do not use the counter very often. Has anyone done a mod like this on a 5315?

Cheers,
Wolfgang

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Hi folks,

Thanks for all your tips. I have just been in that counter. My main concern was that there was something bad going on causing that humming, like shorted diodes or caps. But there isn't, all voltages are perfectly in spec. It's really just the transformer being noisy.
Modding the existing power switch would need some serious rework on that nice gold plated PCB. I don't want to do that, since the unit is in perfect shape outside and inside and I want to keep it that way.
So an external mains power switch is the way to go here....

Cheers,
Wolfgang