Idiot's Guide to breaking & Fixing the Sage 930A

The Sage 930A is a handy piece of test equipment, which can be had for a fairly reasonable price on eBay. Being an 80's vintage piece of electronics, it is also very repairable, so even units being sold as non-working can be quite easily brought back to life. From a design standpoint, it was quite leading edge, containing two break-through devices: the first DSP (the TI TMS320) and the first FPGA (the Xilinx XC2064).

Sage 930A Hardware Breakdown

CPU Board

The supervisor brains of the system; a 4 MHz Z80 CPU and supporting logic.

Daughter Board
Main Board
PCM ESF Board (two in system)

There are two identical PCM cards in the system, the only difference between them being a jumper setting and their position in the chassis. The ROMs on these cards are part of the system address bus, and the set will fail its boot-up ROM tests if either of the PCM cards are removed.

DSP Board

Handles signal processing and tone generation functions.

DC Board
AC Board

Not documented yet.

Display Board

Not documented yet. Largely dominated by VFD driver circuitry.

Backplane Board

Not documented yet. Very little active electronics.

Power Supply

In both of my Sage 930As, the Power-One SP584 power supply is a custom Power-One unit that appears to have been special ordered from Power-One. Later models carried a SP594, which is a very similar design. I'm guessing that the 930A power supply is based on the Power-One SPL53 design, an 80 watt power supply platform that was available in a number of output configurations, with up to 4 outputs. Here's an SPL53-1005:

SLP53-1005 PCB

Compared to a Sage SP594:


The Sage 930A power supply is based on a 4 output design, +5, +15, -15 and -52V, which is regulated down to -48V by the circuitry hanging off the left side of the board. Also, an opto-isolated 120 Hz powerline-derived timing reference signal is generated, the diodes constituting the bridge used for this are tucked in wherever there is free space on the PCB.

At first glance, this power supply has a number of what appear to be custom semiconductors. In one of my units, the main regulator IC carries a proprietary part number. Also, the main chopper transistor (marked SGS/ST) and the transistor connected to the feedback toroid (marked National) are undocumented. On the newer of my two supplies, the regulator IC is revealed to be a boring old LM723.

The Idiot's Guide...

Why this obsession over the power supply? My first unit worked when I first received it, then it refused to turn on when I tried it out a couple days later when I received my second 930A. They were supposed to play together. I checked the obvious things, and discovered that the power supply was not producing any output, but the high voltage DC supply side was working. The input filter capacitors weren't bleeding down, one 5 watt power resistor was missing, and a 1 watt power resistor was open-circuit. The 1 watt resistor connected the base of the chopper transistor to one of the high voltage rails. From the schematic, I'm struggling to make sense of the 5 watt resistor, but it ultimately functions as the bleed down resistor for the input filter caps. It appears the 5 watt resistor fell some time before the unit arrived at my door. With it missing, the 1 watt resistor was somehow doing double duty, and ultimately cooked itself and failed open, which is what killed the output.

The 1W resistor was a weird value, 510kΩ, that I didn't have in my stock of resistors. I also don't have many 1W resistors, so I went to the local electronics store and in the intervening time confused myself into thinking I wanted a 5.1kΩ 1W resistor, contrary to what I had written in the note to myself. They only had E series resistors too, so I came home with a 5.6kΩ 1W resistor that I happily soldered in place. I put on my flash protection gear, kevlar underpants and nomex toque, and flipped the switch.

The Plot Thickens

The chopper transistor didn't seem to appreciate suddenly being connected to 160 VDC at 100 times the current it expected, promptly turned into a 3 way wire, which popped the fuse.

It has already been established that the chopper transistor (Q5) is a mystery device, so I thought that I'd pull the supply from my second unit to see if maybe it would give me a clue. It didn't. What I did discover was that almost every single electrolytic capacitor on the power supply had managed to leak through to the underside of the PCB, and make a big mess out of everything. So off to Digikey I went, and hunted down mechanically and electrically compatible replacements for every single electrolytic capacitor. I also looked for possible replacements for Q5 and added a few of those to my order. Oh, and of course, 4 fuses. Given that mosfets are used elsewhere on the board, I assumed that a mosfet would have been used for the chopper transistor. This clouded my judgement. I started to trace out the PCB schematic:

Partially Reverse-engineered PCB

Based on some convincing documentation from a SGS databook, I convinced myself that Q5 was most likely a mosfet. I selected a few candidates, and ordered them. When everything arrived, I recapped both power supply boards. The electrolyte created a heat impervious lead oxide all over board #2, and it took a few days to finish that one. Board #1 took probably 20 minutes. Then I moved on to Q5. I plugged in my most likely mosfet candidate, powered the board up, and still nothing. No smoke, no fuse popping, just nothing. I hooked the working board up to my oscilloscope, and quickly realized that I didn't know what I was looking for.

Most of the information online about switch-mode power supplies covers more modern designs with PWM controllers and opto-isolated feedback. This supply had used a linear regulator combined with a discrete oscillator to drive the chopper, and magnetically coupled feedback. I learned a lot from Ken Shiriff's excellent article, "Apple didn't revolutionize power supplies; new transistors did". I also ordered a couple 80's vintage books on switchmode power supplies and set to learning in my spare time. In the mean time, summer kicked in and I had to take a break from this project.


4 months having passed since starting this project, it became apparent that I needed to get everything put back together, or I would never be able to remember what went where. So I thought, why not just ask Power-One what this transistor is? I contacted their technical support, and while they were very willing to help me with my quest, they reported they had nothing on file for the SP584 or SP594.

So close, yet so far!

But then I remembered that while searching eBay for Power-One power supplies, I came across units that looked very similar to the SP584/SP594, save for the goofy outboard circuitry. I went back on eBay, refined my searches, and found out that the SLP53 series were not only almost identical as far as the input and switching circuitry was concerned, but that they were also quite common. So I went back to Power-One and asked them if they had data for those units on file. In short order, I received a screen-shot of the line from a BOM that looked like this:

TSTR NPN 850V 10A TO-220 AZ EA 1.0000 Q5

Finally! I checked this data against a 1988 vintage SGS databook, and they definitely sold a transistor that matched this description in the 80's. This had to be it! That model was long since discontinued, so my next stop was the Digikey parametric search tool, where I found a close match with the ST BUL510. While it's also obsolete, Digikey still has a stock of them. I ordered 4. They arrived the next day, I installed one, and it worked.

While I had the ear of a friendly engineer, I figured I'd ask about the other mystery device, the transistor connected to the feedback transformer. I received this:

BF1 TRSTR;PNP;60V;800mA;hfe 300;TO92;;;1.000 EACH 1 Q4

The closest modern match for this one is the BC32740TA.

Replacement Parts

For those that are following along at home, here are all of the part numbers for the replacement electrolytic capacitors, the fuse and the two mystery transistors:

Manufacturer Part NumberManufacturerDigi-Key Part NumberCustomer ReferenceDescription
10ZLH1000MEFC10X12.5Rubycon1189-1072-NDCAP ALUM 1000UF 20% 10V RADIAL
35ZLH330MEFC10X12.5Rubycon1189-1891-NDCAP ALUM 330UF 20% 35V RADIAL
UPW2A330MPDNichicon493-1971-NDCAP ALUM 33UF 20% 100V RADIAL
UPW2A221MHDNichicon493-1981-NDCAP ALUM 220UF 20% 100V RADIAL
ULD1H220MDD1TDNichicon493-16251-1-NDCAP ALUM 22UF 20% 50V THRU HOLE
LGX2D471MELA25Nichicon493-7840-NDCAP ALUM 470UF 20% 200V SNAP
ULD1H010MDD1TDNichicon493-14724-1-NDCAP ALUM 1UF 20% 50V THRU HOLE
URZ1V220MDD1TDNichicon493-11563-1-NDCAP ALUM 22UF 20% 35V RADIAL
UVP1J220MPD1TDNichicon493-12711-1-NDCAP ALUM 22UF 20% 63V RADIAL
0318003.HXPLittelfuse Inc.F2610-NDFuseFUSE GLASS 3A 250VAC 3AB 3AG
BUL510STMicroelectronics497-7204-5-NDChopper Transistor (Q5)TRANS NPN 450V 10A TO-220
BC32740TAON SemiconductorBC32740TACT-NDFeedback Transistor (Q4)TRANS PNP 45V 0.8A TO-92