Dell Pentium Pro Restoration – The Plan

After getting the system out of the closet, I took photos to document the current condition. Overall, it’s in nice shape given its age. I didn’t see anything structural that was damaged, e.g. no missing or broken plastic.

A Dell Dimension XPS Pro200n
Front of the Dell Dimension XPS Pro200n
Rear view of the Dell Dimension XPS Pro200n
Designed for Microsoft Windows 95
The original “Designed for Windows 95” sticker was even intact!

The plastic on the case as well as on the drives has yellowed over time. This is from exposure to UV light, and it can be reversed using a process called Retr0bright. There are some smudges and other marks that will need to be cleaned up as well.

906FX or 90GFX
Is that a 6 or a G?

A sticker on the back panel has a barcode with the text “906FX” or “90GFX,” but I think it’s the former. Perhaps this is a precursor to the Dell service tag? Searching for this string, in either form, yields nothing relevant.

Moving on, it looks like both a video card and a NIC are installed. I’m fairly certain I added this NIC at some point after I took de facto ownership of the system. We must have had a modem installed, I think originally, because this was right around the time I was heavily into BBSing, and my dad would have been using Compuserve or Nando.

Iomega Zip drive
Long live the Zip drive!

I do distinctly remember my dad opting for the Iomega Zip drive when we configured and ordered the system. At the time this was seen as a convenient and inexpensive way of storing a large amount of data on removable and rewritable media– all 100MB of it.

Inside view of the Dell
Decades-old dust, mmm…

Removing the side panel was a cinch. The panel uses a single thumbscrew, and then two clips: one at the top, and one at the bottom that you depress and then slide the panel off. There was a decent amount of dust that I immediately vacuumed out, but pretty much everything is coated in it.

I hadn’t seen SIMM memory modules in quite awhile, and it was cool to see a vertical mounting scheme for the HDD as I didn’t think that was a popular method back then.

Matrox video card
Matrox PCI video card

The video card is made by Matrox. The large QFP IC reads “Powered by MGA 64-bit graphics” and has a part number of “IS-MGA-2064W-R3.” I did a quick search on this, and found an entry at the VGA Museum. There are many versions available, but I do see what look like 4 DRAM modules on the card. They are SEC KM4232W259A modules at 1 MB each, so this video card has 4 MB of RAM. There’s also a code next to the Matrox silkscreen at the top of the PCB that reads “590-05,” and this corresponds with the “Matrox MGA Millennium 4MB IBM” listed at the VGA Museum. We’ll dive into the video card some more later on, but we now know that it is a Matrox MGA Millennium 4MB video card.

3COM network interface card

The only other card is a 3COM PCI NIC. I’m fairly certain this did not come with the system, so I’ll need to verify that later on.

Front panel removed
Front panel removed

The front panel was removed by pressing three levers along the right side of the panel. With that removed, I could take out the floppy drive.

Sony floppy drive
Sony MPF920-F 3.5″ floppy drive

The floppy drive: a Sony MPF920-F 3.5″. It looks pristine in the above photo, but it sustained some accidental damage the next day. More on this later as well.

IBM 4.3GB hard drive
IBM DCAA-34330 4.3GB hard drive

The hard drive is an IBM DCAA-34330 4.3GB IDE.

Drive caddy
5.25″ and 3.5″ bay caddy

The optical drive and the Zip drive were housed in this removable caddy. Removing this made the structure of the case a little flimsy, so I’m not sure why this was meant to be removable. Maybe removing the other side panel is more of a pain than I’m anticipating…

NEC CDR-1600A CD-ROM drive

The CD-ROM drive is a NEC CDR-1600A. At the bottom, the manufacture date is listed as April 1997, so we know the system cannot be older than that assuming this drive was never upgraded or changed.

Iomega Zip drive
The venerable Zip drive

And finally, the Iomega Zip drive that was touched on earlier.

We’ll get to the motherboard and other interesting bits later on.

The original configuration

Before going any further, it might be helpful to have something to compare the current hardware setup to, so we can determine if this hardware is actually original to the system or not.

The problem, of course, is that we’re dealing with hardware from the mid-to-late 90s. There’s no mention of this system on the Dell website in any sort of archive, so instead we’ll have to rely on archived websites (if they exist) and any print media that might give us clues.

I know this system was purchased between 1995 and 1997. How? Because that’s about roughly when the Pentium Pro was manufactured. We can further narrow that down from the CD-ROM drive manufacture date of April 1997, so it’s likely on or after that date. With those dates in mind, we can look at archived versions of screenshot from 1996
Microsoft BackOffice? Haven’t thought about that in decades.

The earliest archived version of the site is from Dec 21, 1996 on the Wayback Machine. Clicking on Dimension Desktops, they have configurations listed for both Home Office and Small Business. Let’s try Home Office:

Dimension XPS Pro200n

    200MHz Pentium® Pro Processor
    Mini Tower Model
    32MB EDO Memory
    256KB Internal L2 Cache
    3.2GB Hard Drive [9.5ms]
    20TD Trinitron Monitor (19.0" v.i.s., .26dp, 1600 X 1200 max. res.)
    Matrox Millennium 4MB WRAM Video Card
    NEW 12X EIDE CD-ROM Drive
    AWE32 Wave Table Sound Card
    Altec ACS-490 Full Dolby Surround Sound Speakers w/ Subwoofer
    33.6 U.S. Robotics Telephony Modem
    MS Office Professional with Bookshelf for Windows 95®
    MS Office 97®, Small Business Edition Upgrade Coupon
    MS Windows 95/MS Plus!® CD/30 Days Free Support
    MS Mouse
    3 Year Limited Warranty with 1 Year On-Site Service 

Price Configure Buy Product Code #501119

What a powerhouse. How about the Small Business configuration?

Dimension XPS Pro200n

    Mini Tower Model
    64MB EDO Memory with ECC
    256KB Internal L2 Cache
    3.2GB Hard Drive [9.5ms]
    20TD Trinitron Monitor (19.0" v.i.s., .26dp, 1600 X 1200 max. res.)
    Imagine 128 Series 2 Graphics Accelerator with 4MB VRAM
    NEW 12X EIDE CD-ROM Drive
    MS Office Professional with Bookshelf for Windows 95
    Microsoft Windows NT Workstation 4.0/30 Days Free Support/
    MS Mouse
    3 Year Limited Warranty with 1 Year On-Site Service 

Price Configure Buy Product Code #501110

$4,299 is equivalent to $8,000 in today’s money. That’s insane. These are the highest configurations I pulled from the archived site. They did actually have a configurator linked, but the server-side dependencies are long gone. On a side note, the other, cheaper configuration listed for the Small Business category used a SCSI HDD and CD-ROM drive. Both were smaller in capacity and slower in speed, respectively, to the higher priced config.

Moving forward in time, let’s see when the Pro200n was no longer listed on their site. Well, the very next archive from June 5, 1997 no longer lists the Pro200n or any Pentium Pro desktops as available configurations. This makes sense, because processor speeds and technology in general were rapidly advancing at the time.

To track this down further, let’s look for advertisements. I assumed they would be advertising the system right up until the point it was no longer available. I hopped over to and searched for “pro200n.”

I found a ton of results, and thankfully, a bunch of scanned and archived PC Mag issues. The latest advertisement I could find with system specs was from the June 24, 1997 edition:

June 24, 1997 advertisement in PC Mag
June 24, 1997 advertisement in PC Mag

Some of the specs match up, but this system does not have two USB ports, so it must be an earlier configuration.

April 22, 1997 advertisement in PC Mag
April 22, 1997 advertisement in PC Mag

An advertisement in the April 22, 1997 edition looks like it matches up except for the CD-ROM drive. Let’s look a bit later…

May 6, 1997 advertisement in PC Mag
May 6, 1997 advertisement in PC Mag

This matches my system except for the hard drive capacity. So it’s likely the system was purchased somewhere in April or May 1997, but no later since Dell added USB ports as standard some time in June. I know Dell offered options when ordering a PC, so it’s entirely possible that my dad opted for a smaller hard drive, or upgraded to the faster CD-ROM at the time.

No NIC is listed in any configuration, so that confirms I installed this later on. We’ll disregard it for now.

The Plan

I want to retain as much of the original system as possible, but also keep it true to Dell’s spec. That means we’ll work off of the above advertisements to get the system up to its most likely original configuration. We know, for instance, that there is no modem or sound card. The sound card was standard, so that’s definitely missing. The modem was an option, and I know the system had one at some point, so we’ll make the assumption it had the modem listed in the advert.

So here’s a run-through of what I have in mind:

  1. Evaluate the condition of all components
  2. Dust removal and clean-up
  3. Repair and refurbish all components as needed
  4. Replace missing or non-original components
  5. Test and first power-on
  6. Restore plastic
  7. Software installation
  8. Bask in 90s glory

Much of what I plan to do isn’t necessary for a running system. Hell, it probably runs just fine right now. I’m sure I could power it up and expect a working system. The point of this whole exercise though is the journey. We’ll be getting down to the most minute details in this restoration, and hopefully learning lots of history, technical and engineering knowledge, and maybe even discover a few surprises along the way.


Dell Pentium Pro Restoration – Intro


By the time I was forming memories as a young child, we always had a computer in the house. My dad was a software and database engineer; he worked in several industries, including in the aerospace field where he wrote software for Pratt & Whitney rocket engines. The first computer I remember was a Tandy TRS-80 Model II with a Motorola 68k CISC and two 8″ floppy drives. I have memories of seeing 8″ floppy disc mailers arrive in the mail, and I was fascinated by the idea that information could be conveyed by something that had no discernible features on it. Most of my early computer exposure was through observation: I would watch my dad and my older brother type on a keyboard and interact with the computer.

Our first real family computer was an Intel i386DX 33 MHz PC that we got for Christmas one year. It was built by a local computer store called One Step Computers located in MacGregor Village in Cary, NC. I have a distinct memory of walking down the stairs that Christmas morning, and seeing the brilliant, vivid colors on the CRT monitor as it sat on the floor waiting for my siblings and I to discover it. A fish screensaver on Windows 3.1 was on, and my sister and I stared in amazement. That was my first real “wow” moment at what computers could do– as a kid, you’re not impressed by rocket engine software, but by a crude representation of fish.

I was pretty much hooked at that point. I lived and breathed as much as I could about computers from that point forward. My dad was still involved in software engineering, and a few years later he needed an upgrade, and then another after that. Eventually, one came in the form of a top-of-the-line Dell Dimension XPS Pro200n. This was the absolute pinnacle of home PCs at the time, and it sported a 200 MHz Intel Pentium Pro CPU. For awhile, this was strictly his computer, and we weren’t allowed to use it.

A Dell Dimension XPS Pro200n
The Pro200n as it sits today

Those were exciting, accelerating times though. It wasn’t long before he needed *another* upgrade, and I finally got my hands on the Pentium Pro I had been longing for. Even for me though, this new found speed didn’t last long, and I eventually moved onto newer and faster hardware.

I never remember actually learning to type, and it seemed to come completely naturally to me. I think maybe this was because I observed them so closely — seeing what fingers pressed what keys. Maybe I simply emulated that when I first started typing on a typewriter in early elementary school. By the time I hit typing class in 6th grade, I would finish my assignments in 5 minutes and then walk around fixing problems the teacher had with other PCs in the lab.

I don’t know why, but we ended up hanging onto the Dell and for whatever reason, never quite decided to part ways with it. It persisted through so many moves, rearrangements, and so on. Even during attempts to reduce clutter and throw the old things out, the Dell remained. There’s so much from the 90s I wish I had kept: *so* much computer hardware and software, music CDs, other electronics, the list is huge. Fast forward decades later, and in my sixth or seventh move, the Dell is still here.

I love nostalgia, and especially during these times of climate change, a pandemic, massive inequality, and so on– the experience of being an ignorant, naive kid during the 90s is something I mentally visit from time to time as a respite. I would’ve LOLed at the thought of that had you told me during the time, of course.

When I saw the Dell sitting in my office closet a few weeks ago though, I realized it was probably the greatest link I still have to that era. A crazy idea popped into my head: what if I restored the system back to its original glory? I’m sure the system would probably run just fine again, but what if I restored the system to absolutely new condition as an homage to that era of computing? I decided I would keep the Dell most likely forever, and I want it to be in working condition as long as possible. As an adult you often realize the importance of history, and of preserving that history. When we lose or forget history, no matter how insignificant it may seem, we lose a piece of our collective human journey through this absurd universe.

Dimension XPS Pro200n

So my plan is to document what I learn about the system, the how and why of restoration, and maybe relive some of the 90s through that process. I have no real end goal other than to end up with the computer as it would’ve been when it was new. Some parts of the restoration may be ridiculously in-depth, and maybe even seem pointless to some, but ultimately it’s not really about the end goal. It’s about the process, and what I (or we) learn along the way.


Brother HL-L2360DW repair

The problem

My Brother HL-L2360DW laser printer went into a reboot loop after a power outage. The LCD initializes and displays solid blocks, followed by a blank screen, and the Wifi button LED blinks once, and then the cycle repeats. Holding the “GO” button either during startup or while it’s looping puts the printer into “Users Mode” which displayed on the LCD. The service manual for this particular model does not mention this mode, but I found another service manual that did. I tried every possible option, but always ended up in the same place.

I removed the access panels on the printer and quickly checked voltages from the low-voltage power supply. I didn’t suspect a problem here, and I didn’t find one… all the voltages measured OK, but that’s always the first step. I then turned to the main PCB (p/n B512386-4) as the issue was almost definitely there. I tried disconnecting all external connections: the HV power supply, laser unit, motor, etc but this had no effect on the issue.

The service manual has a simplified block diagram showing the main PCB components and their connections. There is a Renasas ARM MPU in a 144-pin QFP package that serves as the main controller. This is powered by a 3.3V rail that is generated by a DC-DC converter. This voltage checked OK, as expected. The MPU uses I2C to communicate with an EEPROM as well as an IC which is labeled “HYPNOS” in the diagram. Among other things, the HYPNOS appears to be responsible for sending the AC zero crossing signal to the MPU which is probably for controlling a triac or SCR. I checked the SCL and SDA lines on my scope, and confirmed that the MPU is sending clock as well as data. Entering into “Users Mode” immediately ceases I2C communication, and it doesn’t begin again until the machine cycles.

The MPU also uses SPI to communicate with a serial flash IC (a cFeon QH64) labeled U7 on the board, which is likely used for the firmware. I was able to see SPI clock and data on the scope, so it seemed like the main MPU was intact.

I don’t think the EEPROM (a ST M24C32-W) stores anything absolutely necessary for startup and some of the parameters can actually be set in a maintenance mode according to the service manual. There are also a number of error codes related to EEPROM, DRAM, and flash ROM failure. I haven’t received any error codes, though. At this point I was thinking that the firmware was most likely corrupted given the behavior of the system.

I knew I would need a replacement board due to the current state of repair parts and software availability. I was unable to find an exact replacement board, but I was able to find a board for a Dell E310DW which is essentially a re-badged HL-L2360DW. The part number for the board differs by one digit (B512386-5), and it visually looks identical.

Since a replacement was on the way, I decided to go a bit further to see if I could figure out exactly what was at fault. I desoldered the EEPROM (U1) and there was no change in the behavior. I desoldered the QH64 flash IC (U7), and bam– no output on the LCD. Does this mean that the firmware is actually intact and that the issue is with the EEPROM? I soldered the QH64 back in, and was back to where I started. Observing the I2C and SPI lines, it is clear that the system is simply looping. It attempts to boot, something fails, it restarts, repeat.

Interestingly, the previously mentioned HYPNOS has an output pin labeled “CPURST” as in “CPU reset.” There were several ICs on the board that I was unable to find any datasheets for, so at first I wasn’t sure exactly which one was the HYPNOS. The block diagram showed 7 pinouts, one of which was tied to the AC signal for the zero crossing detection. I was able to trace the pin header for the AC signal from the power supply, and eventually found a 20-pin SMD that looked like it might be the one. I probed around with the scope, and found I2C communication on one of the pins, so I’m fairly confident it was the right one. The IC itself is labeled “510A” with “DN5” on the second line, but otherwise no other identification. I desoldered it, and the system would not boot, so nothing gained there.

The repair

I received and installed the replacement board, and the printer is back up and running, albeit as a Dell now:

This at least confirmed what I already knew. The printer was intact other than something on the main PCB. On closer inspection, the Dell board is not quite identical to the Brother, as the flash IC is different. It is a GigaDevice 25Q64CSIG and there is a 10k pull-up resistor on the CS pin which is absent on the original board. The EEPROM is different as well. This new board uses a slower SPI clock at 25MHz, compared to 50MHz for the QH64. I’m thinking that the Dell board might be a slightly newer revision given the part number. I wonder if Brother had issues with the older design given they left CS floating.

I de-soldered the flash IC and whacked it onto a breakout board. I hooked it up to a Bus Pirate:

I used spiflash to extract a .bin of the firmware. binwalk showed that the firmware consists of ARMEB instructions, which is ARM’s “old” application binary interface (ABI) that uses big-endian:

strings output showed an entry for “Users Mode” so it appears that this is indeed part of the firmware. Hmm… does this mean the firmware was actually intact all along? For grins and giggles, I also found entries for “Dell Printer E310dw.” I suppose I could change the name of the printer back to the original Brother model if I wanted… or something entirely custom!

I went ahead and wrote the .bin from the Q64 from the Dell board to the QH64 from the Brother board.

That completed successfully, so I re-soldered the QH64 back onto the original Brother board. I reinstalled the board and turned it on.

Success! Sort of…

The printer booted into maintenance mode. I checked the service manual for the different commands, and printed a test page using command “09”. It worked! With that success, I chose command “01” which automatically set the EEPROM parameters. I then printed a “maintenance information” report using command “77” which confirmed that this Dell firmware is indeed at least newer than the version shown in the service manual:

HL-L2360 version information from service manual
Version information after flashing firmware to Brother board

The “SW CheckSum” shows “NG” which I assume means “not good,” but I’m not sure what this really indicates or what it is calculating a checksum for. After exiting maintenance mode, everything seemed okay. Unfortunately, I then noticed there was no network option in the main menu, and the Wifi button did not respond.

The board uses a Realtek PHY, and after checking the IO line to the MCU, it was clear that something was going awry and the MCU was choosing to shut down the PHY. I went back into maintenance mode and selected option “80” which displays machine log information including the MAC address. No MAC address was listed, so that was almost definitely the issue.

The service manual indicated that the MAC address is stored on the EEPROM, but it is not an initialized parameter. In other words, it is pre-defined on the EEPROM and there’s no way to update or change it from the printer itself:

So, I de-soldered the EEPROM again in hopes of seeing if I could either manually edit the MAC address, or perhaps dump the contents of the Dell board EEPROM onto this one. During this process, the original EEPROM died. It developed an internal short which killed the chip when I was trying to read it. I probably exposed it to too much handling and heat.

I slapped the Dell board back in and the printer (and networking) works fine of course. At some point I may order another EEPROM IC and try the above read & write sequence. For now though, I’m done with this project and mostly accomplished what I set out to do: repair the printer, and determine what went wrong with the original. I think both the firmware and EEPROM were corrupted during the power loss / surge event.


Phono preamp

A friend of mine designed a simple phono preamp for me to use with my newly purchased turntable. Once he was done with the design, I used EAGLE to design PCB layouts for both the power supply and amplifier boards. They were fabricated by OSH Park.

The power supply outputs 27V and uses a LM317LZ regulator. The amp section uses 2N5088 transistors.

The build was relatively unremarkable. I used an aluminum Hammond chassis, and point-to-point wired everything before, between, and after the PCBs.

It sounds great and has been trouble-free for years!


Fisher CA-272 rechassis

I bought a Fisher CA-272 100w/ch amplifier that had very noisy controls. Specifically, the amplifier had a built-in EQ and the sliders were deteriorated to the point of being unrepairable. Instead of ditching the amp, which sounded relatively good, I decided to transplant it into a new chassis.

The cool thing about this amplifier was the beautiful pair of heatsinks for the output transistors. I decided to show those off a bit in the new chassis. Since I was ditching the front panel, I had some power to play with as well.


I replaced any capacitors in the audio path with Nichicon KL series as these are known in audiophile circles to have the best performance in these applications.

For the volume potentiometer, I used an ALPS “Blue Velvet” RK27112A00CC.


For the chassis I decided on a steel Hammond 1441. The power transformer of this amp was pretty hefty, so I needed a relatively stout panel to mount it. Also, working with steel is a bit easier than aluminum when it comes to machining.


After drilling holes for the board mounts, transformer, RCA jacks, and power cord, I mounted everything in place.

To show off the heatsinks, I cut rectangular slots in the top removable panel using a dremel.


The front panel of the original amplifier was fed using +17.2VDC, so I used this source for LED lighting. I decided to illuminate the heatsinks that would be protruding through the top panel. Eight amber LEDs were connected in series along with a 470 Ω current-limiting resistor. The array draws about 30mA.

I used a combination of Bivar LED holders and Panduit adhesive cord clips to secure the LEDs around the perimeter of the heatsink cutouts.

To position the board and heatsinks at the correct level, I used Keystone standoffs to offset the board from the bottom of the chassis.

Putting it all together

I was pleased with the end result, but I wish the lighting was a bit more diffuse. At some point I plan on addressing this. I’d also like to clean up the edges of the heatsink cutouts using grommet edging.


Compaq Portable PC

The Compaq Portable was one of the world’s first 100% IBM PC compatible computers, and definitely one of the first real “portable” computers. It’s not really portable by today’s standards as it is nearly 30lbs, but in 1983, it was groundbreaking. A co-worker posted one for sale on a company bulletin board as non-working. He bought it new when it was first released, and hadn’t powered it on until just recently. When he did, there was apparently a loud noise and smoke followed.

The problems

Working on old irreplaceable equipment is always a little daunting. Parts availability can be questionable or non-existent, and it’s easy to get in over your head on how far a restoration should go. In this case, I decided that I would restore the system back to working order, but would not go as far as a complete cosmetic restoration. I love vintage hardware, but there are others much, much more dedicated and knowledgeable to preserving these historical artifacts than I am.

I disassembled the entirety of the unit including the CRT. The issue that the previous owner ran into was easy to spot: a tantalum capacitor had shorted and exploded on the power supply board.

After cleaning the surrounding area, it appeared that no other components were damaged, at least from what I could tell visually. Given tantalum capacitors’ propensity to fail in this manner, and the age of the system, I decided it would be justifiable to replace all of them.

Additionally, the keyboard used capacitive foam & foil discs that had long since disintegrated. Luckily, many other vintage systems used similar keyboards (made by Keytronic) so replacements are available.

The repairs

Replacing the tantalum capacitors was straightforward. Before replacing the keyboard foil discs, I slowly powered the system on using a variac and dim bulb tester. After ensuring voltages were at the expected values, I slowly increased the variac to maximum voltage. The system booted!

Since the system was now working, I went to work on replacing the keyboard discs. This wasn’t a fun or interesting job, but about an hour later, I had a working, usable system on hand.

The 5.25″ drives seem to have a little trouble reading the included MS-DOS (version 1.11!) diskettes. I did not attempt any further repair, as I had mostly accomplished what I set out to do, and I did not want to risk damaging these diskettes.


Pioneer SX-300

The Pioneer SX-300 was originally released in 1973 and stayed in production until 1976. It was a small, simple receiver that output 7W per channel. I found one unexpectedly at a nearby estate sale. It was sitting in a basement covered with dust, and the back panel was falling off.

Since the condition was unknown, and who knows how long it had been since it was powered up, a variac was used combined with a dim bulb tester to bring it back to life. This was done slowly to help reform any old electrolytic capacitors, and to ensure there were no potentially damaging short circuits. Luckily, it powered up successfully.

The problems

Any receiver this old will have several areas that need addressing. The volume pot, tone controls, and switches all needed cleaning with contact cleaner. The right channel was noisy as well.

The amplifier section primarily uses 2SC1344s and 2SC1345s, both of which are long out of production. Over time, they are susceptible to becoming noisy and failing. These in particular suffered from “black leg” syndrome, most likely oxidation, but it’s unclear whether this is indicative of an issue.

I used cold spray to see if I could isolate the noisy transistor(s) while the radio was playing. I was able to identify the culprit: a 2SC1344.

The repairs

Since these radios are worth a decent amount of money, and just from the historical value of them alone, I decided to re-vamp the entire radio to ensure it will play trouble-free for years. That meant replacing all the transistors and electrolytic capacitors, as well as a thorough cleaning of all the pots.

I substituted 2N5088 and 2N5210 transistors in place of the 2SC1344 and 2SC1345 respectively. These were compatible with the circuit topology and required no other changes. The footprint of these transistors was different, so I had to carefully bend them into the correct orientation before soldering.

All the electrolytic capacitors were replaced, mostly with Nichicon KL series. These are low leakage capacitors that are excellent for audio applications.

The tuner section worked beautifully and required no alignment.


RCA Dimensia MPA-120

RCA’s Dimensia line, aside from being pronounced like the neurodegenerative condition, was their top-of-the-line component system produced in the 80s. I purchased the MPA-120 amplifier that was part of this line from a Craigslist post. It was said to power on, but would eventually go into and would stay in protect mode.

It’s a cool chassis design with a separate VU meter for each channel on the front panel display. The amplifier topology utilizes STK modules as is relatively typical for many amplifiers from this era. It is rated at 120 W per channel into 8 Ω.

The problem

A visual inspection revealed a charred resistor that actually measured correctly with a DMM. Since the amplifier was portrayed as at least partially working, I powered it on and verified the behavior. The protection mode would kick in after being powered for a few minutes.

After checking the components surrounding the STK modules, and verifying no other visible damage, I began to suspect heat may be an issue. As mentioned above, the protection mode would engage after a few minutes. If the unit was powered off and on immediately, it would almost instantly go back into protection mode.

I used a heat gun to apply heat to various parts of the main amplifier board to see if I could get the protection mode to engage faster, rather than simply waiting for the entire unit to heat up. Eventually I found that heating the protection components themselves would trigger the protection mode.

The Unisonic UPC1237 was used as the IC for controlling the protection mode. On pin 7, a capacitor is used to control the muting period when Vcc is applied. In the MPA-120, a 100uF electrolytic is used.

The repair

Since no other issues could be found, either the protection IC or the timing capacitor could have been at fault. I removed the timing capacitor and checked the ESR, which returned with a normal value. I replaced the capacitor regardless to eliminate it as a possibility, and this turned out to be the culprit. The amplifier no longer went into protection mode even after being on for several hours. The capacitor must have had a high leakage current causing the IC to go back into protection mode.