Thursday, July 23, 2015

Pioneer SX-550 Receiver

Pioneer SX-550 Receiver

Here we've got a nice Pioneer SX-550 in for a restoration. The unit came in quite dirty from typical wear and tear and the faceplate, quite the build up on the plate and knobs. I was extremely happy with how clean the knobs the plate came out, night and day from when the unit was dropped off. 

Power Supply
The single assemblage PCB here plants the driver, tone and power supply all in one location. The power supply utilizes two 6800 microfarad capacitors in the rectification/driver filtering. This does use an interesting secondary in-line fuse array for protection as well. All capacitors here were upgraded with high temp long life panasonic FC/FR capacitors, general use diodes were replaced 
with modern UF4XXX series. The filter capacitors were replaced with 6800 microfarad Cornells.

The RIAA consists of dual 7136 pick-up IC's, all electrolytics including small value tantalums were replaced with audio grade Nichicons and low impedance high reliability Nichicon PW's. Whats interesting in this topography is the attention in design to hi/low gain loss typical here and how they compensated in pre-amplication. 

Driver Assembly
The amplifier circuit consists of a pair of dual FET differential transistors in the 1St stage. In the final stage I want to highlight a upgrade that may be of some benefit for continued years of use, updating the output devices to a more reliable TIP41/42 TO-220 transistor package (2SB596/2SD526). The biasing procedure is effortless with easy to access (cept #36) "test points". For all electrolytics here Nichicon KA and KW audio grades and WIMA PP's were utilized.

RF (AM/FM) stages
Per my usual I think the Nichicon PW's really work great here, a nice reliable low impedance capacitor. Several tantalum capacitors in odd ball values are present and should be updated with electrolytics as well. 

New 8V 300mA lamps and a deep cleaning wraps it up, a very nice little side setup receiver.

Tuesday, July 21, 2015

Sansui 1000A Repair & Modification

Sansui 1000A Valve Receiver 

Here we've got a Sansui 1000A Valve Receiver. Their are a few variants of the 1000A/T topographies plus different bias configurations to note. 

This particular unit came in for extreme hum and red plating issues. A few modifications can/should be integrated as well. For  a more consistent reliable cathode draw or bias current a 10 ohm 1/4-1/2W MOX should be used at the 7591 power valve cathode pins to ground, remove the original jumper and replace. One can also tie the dependent bias potentiometers to a single control if desired. 

Another issue brought to my attention was the 117VAC original rating. If we look at the driver section schematic we see posted Vdc of 470Vdc+ for a plate voltage. Most modern homes here stateside operate in  120-125VAC range. Looking at initial Vdc rating we see peaks of 482/480Vdc. so increasing come capacitor voltages is a good here. 

7591 Pin-Out Configuration

For the red plating, after ruling out mismatched 7591 valves we look at the bias circuit. The grid resistors 1K and bias 200K resistors are typically out of spec on these units, another common fail point is the oil-type .3uf coupling capacitors. It is highly recommended to replace these with a 500Vdc+ electrolytic or polypropylene in the given voltage. I also recommend replacing electrolytic axials here as the axials in the time period in my opinion are dodgy at best.  Monitor again for about 30mA draw at the cathode resistor and that solves that. A thing to keep in mind, because of the auto-biasing 7591 topography, matched quads are of some benefit here. The 1st stage ax7's were fine but the consistent left channel hum was the result of a bad 6AN8 valve. 


New coupling capacitors (.33 630Vdc PP, new .47 & .22uf PP capacitors, new 30uf radial panasonic FC high temp long life electrolytic)


Keep an eye out for our Sansui 1000A Restoration posts coming soon!

Tuesday, July 14, 2015

Madison Area Summer Concert Highlights

Just wanted to take a moment and talk about some local venues in the Madison area. 

This time of the year were fortunate enough to have a plethora of outdoor  music venues to explore. One thing Madison does well every summer is music, over the last three or fours years the available shows has been outstanding and comparable to much larger metropolitan areas. 

A few spots are highlighted below including paid and free events. Id also like to notate Maximum Ink,  the only source for "homegrown" music expose, has an excellent section in the back outlining local/tri county and festival music events. 

Maximum Ink "A Home Grown Music Magazine Since 1996"

WUD Music Presents (Wisconsin Union Directorate Music Committee) Free
Starting Every Thursday into Saturday evenings typically around 5PM they host a series of free concerts at the Rathskeller/Memorial Union Terrace and a few at The Sett. The Rathskeller also host easy going sessions Fridays 5-7PM. Check their Facebook and WUD links for updated listings.

I recommend showing up a little early if you want a table and chairs, otherwise their is plenty of side and lake front seating available. Local and afar Beers and icecream/food options are available in and out of the Memorial Union. This is a personal favorite, I caught the Isthmus Jazz Fest here with Susan Hoffer, the Sangria kicks ass too.

Live on King Street (Majestic and Local Sponsors) FREE
Live on King Street is a series of concerts hosted on King Street in downtown Madison starting June 26th and running through September 18th in the cast of the Capitol building. This is its 5th year hosting and growing bigger and better each year.  I am incredibly excited to catch Benjamin Booker on September 18th. Remember their are at least 3 parking garages down there and the concerts are still FREE!  I recommend a Beer at Pauls or Coffee at Michelangelo's prior :)

Sunset Music Series (East Side Club) $
The East Side has hosted an impressive line up already this summer and continues the tradition with an affordable lake side outdoor mashup. Starting at 5PM most events with music at 6PM, for most children and teens shows are free with nominal 6$ tickets for adults. Ample parking for the most part and they welcome bringing your own chairs or paddle up to the dock! Plenty of food and beverage options available for cart vendors as well. 

Olbrich Gardens Summer Concert Series $
This is a relatively new series. Hosted at the world famous Olbrich Gardens on the West side. The grounds are a beautiful spot for concerts, arrive early to walk the paths and grab onsite parking. The offering is wide swatch from Jazz to Honky-Tonk at 7pm July 14th, 21st and 28th remaining. $1 minimum suggested donations and don't forget, bring a picnic and chairs or a blanket!