McIntosh MC2500 Rehab

Mcintosh MC2500 Power Amplifier 

Today were showcasing a Black/Black plated McIntosh MC2500 Power Amplifier.

Albeit very large and heavy the MC2500 is a true work horse, capable of driving 1OHM loads at rated power via their innovative AutoFormers, and thermostatically controlled fan options adds to the versatility of the amp. Integrated with McIntosh’s renown Power Guard feature. Mcintosh’s Power Guard feature has evolved over the years and is still considered one of the best detection circuits out there, all in a fraction of a second the circuit analyzes the input/output waveforms for premature distortion( D+N), when D+N does occurring the proceeding input signal is dropped until a clean waveform is present and gain then compensated.  This particular rebuild, retained the original 33,000MFD capacitors after verifying at voltage levels ESR/ratings. 

Driver Assemblies PCBID: 046783

The MC2500 uses a dual symmetric NPN/PNP driver topography and Autoformers.  On these boards the original electrolytic were replaced with a audio grade Nichicon KG/KA and UVP Nichicon BP capacitors.  The tracking potentiometers were updated as well as the initial differential pair transistors with matched ZTX low signal transistors matched within 1% along with current monitor Q4.  Final stage drivers Q10/13 were updated with matched TO-220 cased 1220/2690 Bi-Polar transistors.  Remaining 4148 general diodes were updated with new 4148 type. Remaining film capacitors were replaced with high grade WIMA polypropylene type including the .47MFD electrolytic to a WIMA film. 

At this point the TO-3’s were removed and re-greased along with crimp thermostat/molex connector re-crimped and cleaned. At this time I like to pull the 33,000MFD assemblies and replaced the original .22/100 film with a MPK polypropylene .22/630 film capacitor.

Power Supply Assembly PCBID: 045743

The Power supply and Meter Control Assemblies both lie in the front of the chassis parallel to the main meter displays.  Both PCB’s are vertical molex oriented assemblies.  On the power supply assembly we replaced the electrolytic with all high temp (105C) long life Panasonic FR capacitors along with updating the two Y2 rated ceramic capacitors with Vishay X1Y2 type .01 capacitors.  Both regulating BJT’s Q3/Q2 which are TIP devices were updated to MJE15033/32 TO-220 transistors with new thermal compound applied. I want to note this assembly uses a MOTO MPSU57 transistor which has a fairly sketchy life, we replaced this with a MJE15030 type TO-220 BJT.

Meter Display Assembly PCBID: 045619

Located next to the power supply assembly the power guard detect and meter simply and driven via this assembly.  For this stage we used all high temp (105C) low impedance Nichicon PW capacitors along with UVP Nichicon Bipolar type. Updated the potentiometer network with Bourn precision devices and addressed Q1/2 which is a BC239 to a 5088 BJT device, we noted a leaking BC239 under load and notated a low gain (HFE-11) which resolves the intermittent power guard indication.  Both 1.5MFD films were updated to 1MFD Solen MPK polypropylene film capacitors.

The Input/RCA assembly was pulled and the electrolytic replaced with a high temp (105C) low impedance Nichicon PW, the RCA/Thermo/MONO switches were all deeply cleaned, many customer have noted issues with their thermostatic switches.    I highly suggest using LED's in this assembly, the original incandescents literally bake the lamp clips, we can tell from a prior shop had replaced with the lamps with mismatched voltage type that the clips are brittle and have broken away, LED's have almost no radiant heat to combat. 

Audio Notizen

Sansui 8080 Receiver

Sansui 8080 Receiver

I wanted to take a posts and highlight a restoration of a Sansui 8080 Receiver. The 8080 is up there with the 9090/8080DB variants. Rated at a modest 80W @ .2%THD VS the 9090, the 8080 features some distinct differences in design apart from the 8080DB.  The 8080 features a quasi complimentary output stage where as the 8080/9090DB features a NPN/PNP push pull topography, the 8080 also only features a single RIAA input and formats the outputs to parallel rather than in series like the DB versions.  The 9090 and 8080 share an identical MOS FET FM front end unique to those two.

Power Supply F2546 Protection F2547 FM PS F2431

We started off with the regulated power supplies and protection networks. The main supply assembly F2546 also houses a insert card type connector for the driver assembly F2436.  All of the electrolytic includes the four 4.7/100 Bipolar capacitors were replaced with Nichicon UVP Bipolar and Nichicon low impedance PW and Panasonic FC high temp (105C) capacitors.  A common cathode blocking rectifier diode was replaced with a TO-220 type MUR1620 device with a common cathode configuration, along with the 2SD313 NPN transistor replaced with a 2073 type TO-220 and thermally coupled to the mounting plate. The FM Front End PS had the electrolytic replaced with the Panasonic FC type and remaining 2SD313 with a 2073 NPN BJT.  Along with the 6800MFD Filter capacitors updated.

The Protection assembly is located in the front end of the chassis under the FM PS assembly.  All of the electrolytics again were replaced with low impedance Nichicon PW’s and high temp (105C) Panasonic FC  and two Nichicon UVP Bipolar capacitors. The relay input mylar was replaced with a high quality Panasonic ECW polypropylene film along with the four remaining .1/50MFD mylar to ECW film capacitors.  The remaining 10D1 type diode were replaced with an Ultra-Fast UF4005 type diode and the MY4 Omron replaced.

Power Amplifier Assembly F2463

Like the 9090 versions the 8080 features a socketed single symmetric PCB design, fearing a differential front end and quasi complimentary output.  The electrolytics on this assembly were replaced with high temp (105C) Nichicon PW’s and Nichicon KL along with two UVP Nichicon Bipolar 100MFD capacitors.   The differential pair TR01-4 were replaced with 1% matched 1845 NPN transistors. remaining 2nd-3rd driver BJT’s were replaced with matched KSC2690/1220 compliments and 2073/940 BJT compliments with new thermal compound applied at the driver pairs TR11-14.  TR07/08 is thermally coupled to the collector of the driver pairs via a heatsink tab. The remaining2473 diodes were replaced with general 4148 type and the two VD1212 glass pack diodes to two 4148 in series. Going back and updated the 4.7K/1K to bourns potentiometers and thermal compound to the outputs once resistance points are obtained.  The bias breakdown is using an active mA measurement of 30mA or 15 (3mv<) at each .33 emitter resistor. (V=R(2)/I)

Tone Control & Assemblies F2544 F2543 & RIAA F2541

Mounted under the front end of the chassis next to the protection circuit the AF assemblies are fairly easy to get to after breaking the unit down.  As we typically, the AF electrolytic were replaced with a audio grade Nichicon KA and Nichicon fine gold (100MFD) capacitors.  Updating 1MFD> with high quality WIMA polypropylene film capacitors and Nichicon UPV Bipolar. 

Each PCB contain a low signal NPN BJT which is replaced with a quite, more reliable KSC1845 transistor, this consists the later network of the front end BA312 IC network, which is basically a

 NPN SET gain driver.

Similar to the above F2544, the RIAA F2541 uses a BA312 IC as a basic gain driver.  The electrolytic within this stage were replaced with audio grade Nichicon KA and low impedance Nichicon PW (470MFD) capacitors, two high grade WIMA polypropylene films were installed replacing an electrolytic (1MFD>).

New 8V SMD custom Fuse Lamps installed and function indicators and its ready for the verification tests...

Audio Notizen

Frequency Response

15hz-30Khz @ +/- .79dB

Continous Rated Output @ 8OHM

25.62Vrms  (82.04W/CH) @ .086% dB THD+N

@ 1.00W/CH @ 0.014%dB THD+N

Intermodulation Distortion

60Hz/7Khz 4:1 @ 0.031%

AV Gain @ 28.17dB

Zoomed in look at subsonic-reference-high frequency responses;

10Hz Sub-Sonic

1Khz Reference Mid Point

20Khz High Frequency Point

Pioneer SX-550 Receiver Highlight

Pioneer SX-550 Receiver

We have a little Pioneer SX-550 Receiver in today, thought we'd show some highlights. Produced for a limited run from 1977 to 77" its features a 3 gang tuning capacitor and modest 20W rated output. The 550 was the entry class for many SX users because of its price point.

Switch/Repair & Modification
One of the common issues with the lower series SX Pioneer's is the integrated Power/Speaker Selector switch. Both sections of the phenolic based speaker switch and integrated power switch are prone to excessive carbon build up and then AC arching. often presenting as fluctuating AC/ flicker lamps. If you place you ear near (*caution...) you can actually hear the arching.

One fix is to tear down the switch and phenolic assembly by desolder and removing the unit. As photo'd below its important to scour and clean the contact points and clean and treat with a high quality non residue cleaner.

In some cases, such as this.... no matter how much cleaning occurs the switch is done for.  In that case I recommend salvaging and cleaning the speaker selector and we did and adding a new power switch to the rear of the chassis for discrete mounting. A 5A DPDT single throw was chosen for this unit with a Vishay X1Y2 safety capacitor installed parallel the switch.

New multi-facet LED's were installed along with a LED function indicators. to give off a new clean/ white glow!

Audio Notiz
The resulting verification report was as expected with a slight deviation in the low frequency to sub-sonic frequency response at 10Hz to 35Hz with a noted deviation of +1dB.

Kenwood Model 600 Supreme Series

Kenwood Model 600 Supreme 

Here is another Kenwood Model 600 Supreme we have in for a restoration. The Kenwood 600 is a TOTL variant quietly produced and exported by kenwood-trio. part of the infamous Supreme series, manufactured from 1973 to 1979’ this features the popular dual power supply topography designed by kenwood and later adopted with Kentronics which later produced the Accuphase series. From research it would indicate that their are potentially less than the 1500 or so 600’s out there produced and who knows how many left in good repair.

With an impressive 18,000MFD of filter a side for a total of 72,000MFD of reserve!, the 600 also touts a limited ALPS 44 stepped attenuator and a ALPS coaxial balance fader! The 600 used excessive heatsink platforms and specific shielding channels for input/output leads along with bottom chassis well.

Power Supply X00-1720-10 & Protection X00-1700-20

The dual mono power supply assembly is a slide connected PCB into a molex base at the chassis, supplied with independent 18,000MFD caps.  The electrolytic were all replaced with a high temp (105C) long life Nichicon PW and Panasonic FR/FC type with VP 100MFD bi polar capacitors.

Through several variations I was able to correct the 40/28 and 14V output voltages by updating the 2sc1419/2sa755 BJT’s to heavy duty On Semi MJ15/32/33 type TO-220 transistors along with their TO-92 pairs of KSA992/KSC1845 type. 

BEFORE

Noted dZ5-9 were updated to general type 1N4148 diodes and the V06B type diode at Dz10 updated to a ultra-fast 4005 type diode. I want to also note that the glue used for the large original 100MFD caps had severally corroded the diode at Dz10 and associated resistors which had to be replaced.

AFTER

The ProtectionPCB is a dual facet over current detection and excessive offset detection.  Protection transistors Q9-11 were all updated to 733/1845 BJT’s. I want to note that pinout for Qz9/10 (733/1845) are different than the PCB topography. Qz11 was updated to a KSC6909AYS class (2SD415) and a new Omron MY2-24V was installed.

The original four 18,000MFD filter capacitors were replaced with a high quality Nippon Chem-Con 18K 75V filter capacitors with adapted screw terminations.

Originals...

New Nippon 18Kmfd

Amplifier Stages X07-1440-00

The driver stage consists of an initial 1st order dual FET package IC differential 2SK58, which is enclosed in its own shielding!, the proceeding second stage differential is comprised of a pair of NPN transistors (front end gain bump). The 2nd order differential were swapped with 1% matched ZTX694 bjt’s (All BJT’s watch pinout*) and 3rd order differentiate KSA1381 type. Major upgrades to the transistor packages should be done as we did here due to their high failure rate at Q6 (2sc1452) to a KSC3503 for the constant current BJT.

All the original bias and especially the offset course/fine post should be replaced, we choose a high quality Bourns precisions multi turn trimmer. All the electrolytic capacitors were replaced with a audio grade  Nichicon KA (10MFD) the remaining films were replaced with a high grade WIMA polypropylene .1MFD and a Panasonic ECW high grade polypropylene .01MFD capacitors. One of the prized components in the driver stage is the TO-3s were Sanken 1164/747A output devices capable of a 40A drive(PVP)!, which were removed and MICA/compound applied.

Pre-Amplifier (RIAA) X08-1560-10, Tone Control X11-1310-00 & Filter Assembly X12-1130-10

Unique to the technilogical advances prevalent within the 600, the RIAA stage is comprised entirely of  JFET devices. In a 3rd order array the initial FET’s comprise of a Class A differential + drive stage with a constant current regulation.  All the original electrolytic were replaced with audio grade Nichicon KA and Fine-Gold capacitors, while 1MFD< were replaced with high grade WIMA polypropylene film capacitors. 

The tone stage is designed again around a class front end, using a Shunt-Regulated-Push-Pull (SRPP) configuration using  FET’s. (2SK Japanese devices) The Tone and filter assemblies were treated as above, with all electrolytics replaced with audio grade Nichicon KA and high grade WIMA polypropylene film capacitors. The input stage of the filter assembly had the original mylar/poly films upgrade to high grade Panasonic ECW polypropylene type. 

Verification Report 4/8 OHM Reports

Marantz 2230 Receiver Restoration

Marantz 2230 Receiver 

Today were showcasing another Marantz 2230 Receiver restoration! One of my absolute favorite Marantz receivers with looks to go, the Bleu LED’s we install in these turn out phenomenal. 

"Best in its class. With impressive specifications and Marantz-exclusive features, the Model 2230 offers more value and performance than any other receiver in its class. We've known that for years. But just recently, one of the nation's most respected independent consumer testing organizations verified it. In not one, but two separate categories -- featuresand performance. It delivers 60 Watts continuous power, both channels driven, from 20 Hz to 20 kHz with total harmonic and inter-modulation distortion well under 0.5%. Stepped, three-zone tone controls for BASS, MID, TREBLE. Provisions for tape decks, phones, record changers and 4-channel adaptors. Ultra low-distortion FET, RF and IF circuitry. Massive heat sinks; automatic protection for internal circuitry and associated speakers, plus the famous Marantz gold-anodized front panel. Looking for a moderately priced receiver? The Marantz 2230 out-does them all."

- excerpted from a 1973 Marantz stereo equipment catalog

Power Supply P800

The regulated 30V power supply is a simple design supplying 66V rails for the drivers and 14/35.6V for the auxiliary assemblies.  All of the electrolytic capacitors were replaced with a high temp (105C) long life Panasonic FR/FC capacitor, The original 2SC1213 NPN transistor was replaced with a modern TO-92L KSC2383 BJT along with the 14V /500mW zener at H806. The mounting glue is etched off as well, the noted 30V TP is at J808. The original filter capacitor was upgraded to a low ESR long life 10,000MFD capacitor and both coupling capacitors upgraded to two low ESR, long life 6800MFD capacitors. A new 7A DPDT power switch and Vishay X1Y2 safety capacitor was installed as well.

P400 Pre-Amplifier

I decided to try some very nice Solen polypropylenes on this stage and the phono to replace with original polypropylene films. I think the the MPK Solen is an incredibly film, cost appropriate yet ever bit as good as the EVO Oil’s we’ve tried extensively.   The remaining electrolytic were replaced with audio grade Nichicon KA/ Fine-Gold series. Per our protocol all 1MFD (AF) stage caps were replaced with a high grade WIMA polypropylene film capacitor. The P400 is designed around a small signal BJT complimentary gain driver stage with variable bass/mid/high attenuations. This same topography is seen in MANY of the 22XX series such as the 2245/70.

BEFORE

AFTER

P700 RIAA/PHONO

The phono stage is in the 2230 is one of the best, bar none. With a nominal Av of 40dB its more than enough drive. Their are some major updates this assembly needs though. The notorious 2SC458 transistors are well known to break down under load with pre-emptive failure and potential noise floor issues, these were replaced with 1% matched Hfe 1845 BJT’s. The original tantalums were replaced with a audio grade Nichicon KA capacitor and Fine Gold 100MFD Nichicon. The original films were again replaced with a high grade Solen MPK polypropylene film capacitors. The original VD glass pack diodes were also replaced with tH709 constructed of two 4148 diodes in series and 708/707H single 1N4148 diodes. 

P750 Driver Assembly

I’ve always been a huge fan of the 2230 topography, very simply push-pull design thats tried and true, very stabile and reliable unlike the 2270. Using a single H751 as a gain drivers to the H752 down to the 3rd order devices. It employs a integrated current limiting protection of the output devices through a current deviation circuit.

All the electorlytics above 1MFD were replaced with a audio grade Nichicon KA and Fine Gold, the 1MFD pre-driver coupling capacitor was replaced with a high grade WIMA polypropylene film capacitors along with a 100PF  WIMA polypropylene at the input capacitor C761. The output coupling C760 .1 MFD film was replaced with a high grade MKS PP WIMA film capacitor as well.  Both Bias and clipping potentiometers were replaced with precision tolerance Bourns pots. new thermal compound was applied to the outputs and thermal tracking diode H005 SV-3A. Its very important to update the glass pack at H757 S3016R with a general 4148 diode for reliability. 

I want to take a special focus on the before and after verification reports, the results are tremendous! Lowered total THD+N response, more linear frequency response at the peak sub sonic and ultra high frequencies, about 3 watts gained with an even better intermodulation response (4:1 60hz/7Khz)

BEFORE Restoration

AFTER Restoration