[John Tulett]

History
* I purchased this Spectral DMC-10 preamplifier, serial number 10491, new in 1983 from Straight Gain Electronics, Toronto ON.  I was very impressed with this preamplifier's sound quality; it performed extremely well with my Oracle Premier turntable originally fitted with a Kiseki Blue MC phono cartridge.

Figure 1. Spectral model DMC-10 Preamplifier - photo taken after repairs completed.
* I enjoyed listening to my audio equipment throughout 18 years of worldwide assignments in Canada, Japan, UAE and UK.  Although it was in full working condition, sadly I had to put my audio equipment into storage in 2001 due to shortage of living space.
* In 2022 I brought my preamplifier out of storage and powered it up for the first time in 20 years.  Although it was basically working and usable, there was something wrong with it.

Failure Issue
* Initially I noticed an increase in hum noise, later followed by the protection monitor LED illuminating Red.  At this stage the preamplifier's power supply had become hot and the unit inoperative.
* The heavy current load on the power supply was finally traced to two of the bypass capacitors leaking.  Although the leaking capacitor's electrolyte had spilled onto the printed circuit board, fortunately the damage was mostly on surface, and I was able to fully clean the affected area. 
* As a precaution considering the age of this unit, I felt it best to replace all electrolytic capacitors with new. 

Summary of Electrolytic Capacitors replacement

Table 1. Capacitor replacement summary.

Internal view of replaced components

Figure 2. New bypass capacitors (left side), new power supply capacitors (right side).


Figure 3. New power supply capacitors (center), protection circuit axial capacitors replaced by radials (right side).

Parts replaced

Figure 4. Original components replaced.
* Left bin: Power Supply capacitors qty 16.
* Center bin: Bypass capacitors qty 24 (two of these were leaking).
* Right bin: Protection circuit; Bipolar axial capacitors qty 2, axial capacitor qty 1.

Final notes
* The reason for this writing is to share my experience repairing a Spectral DMC-10 preamplifier.  It's not an endorsement of any component supplier, just to share what worked for me.
* Other than replacing all electrolytic capacitors and adjusting the internal trim-pots for DC offset, output bias current, and gain balance as per the DMC-10 Operation and Maintenance Manual, I did not make any modifications or changes to the circuitry.  With the excellent specifications of this unit (Fig. 5), I did not see the need for any changes.
* At time of this writing all repairs have been operating without issue, now enjoying listening to my repaired DMC-10 preamplifier with my rebuilt Oracle Premier turntable fitted with new Phasemation PP-2000 MC cartridge - this setup looks like new and sounds better than ever!

Appendix

Figure 5. Spectral DMC-10 Specifications - from brochure scan.

[FullRangeMan]

Congratulations for the successful repair. 
Nice pre-amp, hard to see a 1MHz preamp today.
Thanks for posting.

[toocool4]

Wow, great work John Tulett 

[HAL]

Love seeing great vintage gear brought back to life and making music again!  Congratulations! 

[Nick B]

What a nice piece to restore. Thanks for posting

[John Tulett]

Nice pre-amp, hard to see a 1MHz preamp today.
Thanks for posting.

Thank you for your kind comments and reply.
Indeed, the specifications of this preamp (Fig. 5 in the Appendix) were impressive back in 1983 and I believe still to this day.
To share one factor why I chose this DMC-10 preamp, was its phono stage directly accepted the low-voltage output from a moving coil cartridge, without the need for a separate step-up transformer or other device.
While most high-end preamps today readily accept either low output moving coil cartridges or higher output moving magnet cartridges, it was not common in the early 1980s.     

[FullRangeMan]

Prof.Keith O.Johnson knew what he was doing 
One of the greats in Audio.
https://referencerecordings.com/people/prof-keith-o-johnson/

[poseidonsvoice]

John,

Thanks for sharing. An excellent writeup 

You will agree, that this is more of a restoration project, which is important since capacitors age and in this case we are talking about 40 years. Replacement of caps is fairly easy, you just have to be careful about surface temperatures on the pcb so you don’t damage the pcb. Re-engineering however, is a very different ball of wax, especially when you are talking about a Spectral design which are high speed (>1MHZ) and the susceptibility of such to oscillations!

I don’t do much restorations, my last one was a restoration of a Sansui 9090 integrated amplifier (from the 70’s). Construction quality back then was pretty amazing. A lot you can learn with a scope and sine wave generator!

I currently enjoy doing de novo builds, for no other reason than boredom to be honest 

Here are three I completed this year:

Poseidon’s Voice MOFO
Poseidon’s Voice FH9HVX
Poseidon’s Voice Neurochrome Modulus 686

Many of us advanced hobbyists share the fruits of our labor over on the diyaudio forum, so I encourage you to post over there as well 

One frequent section is ‘Rawson’s Repair Reflections’ where we all collectively end up repairing the horribly constructed builds by Rawson of Nelson Pass’ designs (giving him no credit and only profiting by the way). One can learn a lot from someone else’s mistakes, especially as a hobbyist. Please take a look, it’s fun to see how bad some constructions are and then rectify them to be safe, and operable.

In one month’s time, Burning Amp Festival will be held which I go to every year. It’s fun hanging out with dignitaries such as Nelson Pass, Tom Christiansen, Mark Johnson, etc…and this year, it is in Petaluma, CA (wine country). Please join us if you can!

Best,
Anand.

[Murf99]

Hi John
I literally just chanced upon your fascinating and instructive article whilst researching for my own Oracle rebuild.
You may not believe this but I was there ( at SGE when you auditioned / collected your kit !!! )
I was good friends with Jean - the Manager and worked / helped out  on Saturdays  !!!!
I bought my own similar kit there in Jan 83 ( so so long ago )  :-
Oracle Delphi MK1 B / FR 64 Black ( Ltd Ed ) - just refurbed her, sounding lovely 42 years on ! - Kiseki Blue, Spectral Pre Amp, Krell KSA 50, Vandersteen 2B - it sounded rather nice in my dedicated listening room, ah the good old days.
Great article
Regards
John
Essex UK

[John Tulett]

You may not believe this but I was there ( at SGE when you auditioned / collected your kit !!! )

Hello John in UK,

Thank you for sharing your story, a trip down memory lane!

Indeed, we must have met at Straight Gain Electronics.
Almost by accident I found SGE visiting Toronto in February 1979, while walking along Yonge St. spotted a sign 'Audio' in the side street, 15 Elm St. 
Here's a picture that my brother (he also purchased audio equipment from SGE) found posted on Facebook showing a winter view of Elm St. in the 1980s, it's low resolution but if you zoom in on the left-hand side you can see the wording "Audio" in gold lettering on black sign that was Straight Gain Electronics.　



Displayed in the SGE store window was a Dayton Wright pre-amp which intrigued me, so I went into the store, met Peter Chu for the first time and bought the pre-amp.
In September 1980 I re-visited SGE and Peter Chu convinced me to trade in the Dayton Wright pre-amp for an AGI 511 pre-amp, which I still use. Refurbishment story posted here.
"AGI Model 511 Preamplifier Restoration" https://www.audiocircle.com/index.php?topic=186321.0
I also purchased from Peter Chu at SGE the Acoustat Model 2 electrostatic speakers, which I still have and enjoy daily. A couple of refurbishment articles posted here.
"Acoustat Magne-Kinetic Interface MK-121-2 - A Successful Restoration"
https://www.diyaudio.com/community/threads/acoustat-magne-kinetic-interface-mk-121-2-a-successful-restoration.404764/
"Acoustat Stator Wire Repair: Hints and Lessons Learned"
https://www.diyaudio.com/community/threads/acoustat-stator-wire-repair-hints-and-lessons-learned.404881/
As already mentioned, the Oracle Premiere turntable and the Spectral DMC-10 pre-amp were also purchased from SGE.
"Oracle Premiere Turntable: A Successful Restoration" https://www.audiocircle.com/index.php?topic=186862.0
"Spectral Model DMC-10 Preamplifier - A Successful Repair" https://www.audiocircle.com/index.php?topic=186474.0
Everything SGE sold was top-of-the-line in audio fidelity and beat many other hi-end products costing much more.
And a quality that I only appreciated in later years, all this vintage audio equipment is repairable/restorable, something you don't see enough of these days.

Again, thank you for sharing your story.
John

[Murf99]

Hi John

Here it is
Regards
John