Musings and comments about our common interest
I spent some quality time during this weekend to get the oscilloscope working. As I said in previous installments of the blog, the most likely culprit of the problem was the VRAM chip. These were built with a battery inside, and were used to keep the calibration variables. If it is not working, it not work properly until calibrated - but the calibration would then last only until powered off - and you'd have to recalibrate each time! Mine was even worse - it wouldn't show a trace. Running the test routines it failed on the VRAM (as expected) but also in the AD and DA tests.
I disassembled the machine, which was extrmely clean inside. The disassembly procedure was well explained in the service manual, and I managed to get the beautiful main board, equipped with a complete computer of the time - starting with the Morotola 68000 microprocessor (way more powerful than the 80X88 that equipped the primitive IBM PCs. The 68000 processor was also used in the Apple Macintosh)
The problem was that the integrated circuits were directly soldered to the board - no sockets in sight. Then it was risky to apply too much heat to solder or unsolder the chips.
After all the effort, we switched on the machine. It worked. Then we proceed to the recalibration as per the user manual (the machine is self-calibrating, which saves a lot of time getting the proper equipment to calibrate it. You only need a cinch-to-cinch connector to run from one of the calibrating outputs in the back of the oscilloscope to one of the four channels)
After the recalibration, we powered off and on again, and run the self-tests. This is what we got:
This meant that we had succeeded. This unit is a little bit battered on the outside, but excellent inside - I hadn't seen so clean a machine since a long time ago. Built like a tank, too.
I then proceeded to see the time response of the filters of the new Meridian Explorer 2 DAC, with MQA. I could then see the ringing of the filters, which is supposed to be all after the pulse (as opposed to Linear phase filters, which "ring" before and after the event.). These filters are supposed to be nicer on the ear. Lets see them applied to a sqare wave:
I already commented in a previous blog that I was repairing an 54501a oscilloscope. I was impressed by the quality of its circuit and general build; but also its logic was miles ahead of what I was used with analog oscilloscopes. Moreover, you could print out a screen through HP-IB to a suitable printer (that we also have).
The 100 MHz bandwidth may sound limited nowadays, with a lot of equipment running in the gigahertz range, from calculators to computers; on the other hand, it is plenty enough for any digital or analog audio applications, and this is what I am planning to use it for. Nowadays, with the advent of the MQA audio codecs, it will be priceless to perform all kind of analysis on how different waveforms are encoded.
As I have let the unit in the mountains' laboratory, I am not able to post any pictures - but i will during this weekend. The problem this unit seems to have is that it has lost the capacity to store a calibration. It used a vram memory chip that included its own battery, and it has run dry after all these years. The solution is to unsolder the old chip and solder in a replacement. This is a 5 euro chip, so it is a quite safe investment. Apparently, once this problem is sorted, it can be self-calibrated and the new values stored. Then -fingers crossed - all self tests will pass again.
I'll keep you posted. Please also email me if you have a similar machine to repair, or are looking for similar equipment.
During this week end I have been beginning to repair our unit of HP54501a oscilloscope from the early nineties.
What a design! The main circuit and all the other circuits (keyboard, power supply, CRT driving circuits) are of exceptional design. The main circuit is built around a Motorola 68000 microprocessor, and the layout and the PCB itself are incredibly beautiful and well done. This is the quality of HP of that era, second to none.
Also the clarity of the manuals and the accompanying documents is exceptional. You get a course on oscilloscope basics for free when you read them!
I will post updates as the repair go. Suffice to say that the self test information was extremely clear and conducive to an easy diagnostic.
More on next issues...
There are two items of the measuring area pending testing and cleaning:
A HP 54501A Oscilloscope. One of the first Digital Oscilloscopes by HP, with the quality of the past HP;
A HP 8591E Spectrum Analyzer. This is a very interesting machine, which many amateurs don’t have due to its high price. We’ll try to make a special price since we got a decent deal ourselves.
Also, there are many other items coming our way - including boxed, near mint calculators - stay tuned!