I'm the new owner of this project:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260406195193&viewitem=&salenotsupported

The seller has a lot of positive feedback from selling receivers and turntables, so hopefully he knows how to pack. It has no tubes, and it's dirty, but I couldn't see any other major problems. The seller even said the power transformer tested ok.

At first glance, it looked like it had some mismatched knobs, but another SX-802 picture I found has the same ugly knobs:

http://www.unzen.com/hobby/audio/sx802.jpg

Can anyone help me with a schematic, or other info? I probably have most of the tubes in my stash, but I'd like to start getting them together while I wait for it. I also expect to be replacing dozens of leaky Japanese caps. I wouldn't say no to a top cover, or knob bright if anyone has spares.

Those are the original knobs. I have an SM-83 (bitchen unit!) with the same knobs. I replaced them for aesthetics but kept them, just in case.

Based on the tuning dial, it appears to be a Japanese market model. The US version appears to be th SX-82. I would like to modify the tuner for US frequencies, and comparing the schematics would help me decide if it's practical.

Can anyone help me with a schematic of either the SX-802, or SX-82?

I found a full schematic if anyone else needs it.

I found a full schematic if anyone else needs it.
You might want to consider putting the schematic into the Pioneer database for future AKer reference, if it's not already there:
http://www.audiokarma.org/forums/showthread.php?t=91705

Nice receiver there; good luck with it. I have the similar looking SM-Q305, which runs 6BQ5 in the OPs.

It arrived without damage, and the only nasty surprise is that lots of resistors are out of tolerance. It looks like the tuner has already been converted to 88-108MHz, but I'll have to go through it and check their work.

I just discovered one more problem. The input select switch has a carbon track between the B+ switch for the tuner, and the right channel line level audio switch. I'm surprised they put 300v on the same switch wafer as line level audio.

I'm attempting to take apart the switch and repair it, but if it can't be repaired, are there any options for replacement? Does anyone still make replacement input selector switches? I would hate to have to scrap it for such a simple problem.

I managed to fix the input switch, and it seems to be holding. I had to go in twice because there were two wafers that were arcing.

I can't believe how many bad resistors there are in this receiver. I'm having to replace almost all of the carbon composition resistors. Most of them are between 10 and 25% off, but some are as much as double, or half their correct value. After going through the phase splitter and output circuit, I only had six original resistors left (and one of those looks like it's breaking down under full voltage).

I managed to fix the input switch, and it seems to be holding. I had to go in twice because there were two wafers that were arcing.

I can't believe how many bad resistors there are in this receiver. I'm having to replace almost all of the carbon composition resistors. Most of them are between 10 and 25% off, but some are as much as double, or half their correct value. After going through the phase splitter and output circuit, I only had six original resistors left (and one of those looks like it's breaking down under full voltage).

At that point, why not replace all of the resistors with new parts known to be in spec? I can't see much reason to keep a couple of resistors while replacing most of them.

I have rebuilt Pioneer SM-83 tube amplifier some time ago. You can see my complete restoration walkthrough (link below), I think at may be helpful because all Pioneer tube units seem to have very similar problems.

Walkthrough
http://www.vacuum-tube.eu/audio_pioneer_sm83_restoration_en.html

and photo gallery at

http://www.vacuum-tube.eu/photogallery_pioneer_sm83/index.html

I did some measurements of SM-83, and will post on my web site upon availability of free time.

I just discovered one more problem. The input select switch has a carbon track between the B+ switch for the tuner, and the right channel line level audio switch. I'm surprised they put 300v on the same switch wafer as line level audio.

I'm attempting to take apart the switch and repair it, but if it can't be repaired, are there any options for replacement? Does anyone still make replacement input selector switches? I would hate to have to scrap it for such a simple problem.

I have the similar problem with input selector switch, it simply falls in pieces. Even worse, internal composition of the amplifier is SO dense, that I'm was unable to install any existing rotary switch inside (which would match pos/poles of old ones). So I simply removed all circuits I do not need (microphone, crystal pickup, tape head). Additionally, in SM-83 input selector switch changes negative feedback value, so if your rotary switch will be like main, this will result in very loud clicks.

Good luck with your project!

http://www.vacuum-tube.eu/images/pioneer_sm83/pioneer_sm83_photo_1_lowres.jpg

http://www.vacuum-tube.eu/images/pioneer_sm83/pioneer_sm83_top_before_restoration_lowres.jpg

http://www.vacuum-tube.eu/images/pioneer_sm83/pioneer_sm83_restoration_kit_lowres.jpg

http://www.vacuum-tube.eu/images/pioneer_sm83/pioneer_sm83_finished_circuit_lowres.jpg

As cramped as yours is, it looks empty compared to my SX802. I'm starting to appreciate the quality of the better US brands like Scott and Fisher.

Things are going well so far. The input switch seems to be holding so far. I gave it a good coat of silicone dielectric grease to try to prevent arcing, and lubricate it. I'm almost done with the resistors. For now, I'm leaving the ones that are within tolerance in place, but I don't mind going back in if something acts up. My pots are also out of tolerance, but they track each other fairly well, so I'm going to leave them alone.

The caps weren't as bad as I had expected. The large electrolytics, and most of the other caps test as good as new (including leakage current at full voltage). All the .047 "oil caps" were leaky off the scale, but the smaller oil caps are a different design, and have no measurable leakage current at 400v. About half the small electrolytics were open, so I'm replacing all of them. Again, I like to leave things as original as possible, and I don't mind going back in if I have to.

Apart from that, I had to fix a few messy repairs, and replace some open power resistors. I still need to look at the tuner which was modified for the US FM band. I think they only modified the oscillator, but left the RF amps on the Japanese FM band. Needless to say, the FM sensitivity sucks. I also need to figure out why the tuning eye is totally dark, even though it has heater and B+.

There are a few design flaws with this receiver. First, FM mono, and AM only send sound to the left channel unless you switch the receiver to mono mode. I thought it was broken, but it's wired this way on purpose. I would have to replace the input select switch to change it. The other problem is that the aux input level control severely rolls off the high end unless it's set to maximum. Clearly, a 500k pot is much too high for this application.

For whatever reason Pioneer tried to miniaturize their gears, and as the result, all internals are extremely dense and crammed. Anyway, I'm currently listening Sarah Brightman melody on my SM-83, its fantastic !

The tuner modification is coming along well. As I suspcted, the oscillator had been modified, but not the RF amp. After swapping two 20pF caps for 10pF caps as shown on the US schematic, it has good sensitivity and covers the full FM band.

One thing that has me confused is the tuning eye. The tube socket was wired completely wrong. It had B+ on the cathode, and everything else except the heaters was wrong. Several pins that should have been connected had clearly never seen solder, and the soldering looked like a factory job to me. The only thing I can think of is that the Japanese SX-802 was meant to take a different eye tube than the SX-82. It had the correct tube for the SX-82, and it works well after rewiring it as shown on the SX-82 schematic.

I'm just about ready to call this one finished. The tuner and amp are both working well, and all the controls work as they should. I would definitely discourage a beginner from getting a Pioneer. The poor quality resistors, switches, and pots made it a challenging restoration.

I decided that the volume pot had to go because the tracking was just too uneven in the lower part of its range. I bought some audio taper pots from Mouser, and as luck would have it, I was able to make the phenolic wafers fit the original pots. It took a little work, but the new wafers were a good fit. I even managed to add a loudness tap by using some conductive paint to make the connection.

Several of the large tubular ceramic power resistors were open. Mounting modern power resistors can be challenging, but Mouser had some power resistors that were an exact match for the originals.

The good points on this receiver are the power transformer, and tube sockets. The transformer is large, and runs nice and cool. The tube sockets have very tight contacts, and I didn't have to clean them. Normally, I have to deoxit the sockets, and clean the tube pins. It also gets an A+ having a complete lack of hum.

The second picture is a close up of one of the IF cans. I was surprised to see a Pioneer logo that's very close to the new Pioneer logo on a 45 year old receiver. You can certainly see where they got the inspiration for the new logo.

thats interesting that those small gray caps did not leak under voltage, I had a 100% failure rate on both an allied and kenwood of the same era.

did you check with a cap tester, or just apply voltage and look for current flow? I used a heathkit cap tester with the tuning eye tube.

Trio/Kenwood integrated amps of tube era look suspiciously similar to Pioneer units. I guess who really manufactured them?

As for poor qiality resistors and potentiometers I have to partially disagree - these units have not been designed to run for 40 - 45 years. Carbon composition resistors seem to "decomposed" after so long usage because of heat, aging or whatever.

thats interesting that those small gray caps did not leak under voltage, I had a 100% failure rate on both an allied and kenwood of the same era.

did you check with a cap tester, or just apply voltage and look for current flow? I used a heathkit cap tester with the tuning eye tube.


There were two distinctly different types of gray caps. One type did have 100% failure, but the other type were all good. I figure that they must be good quality caps if they've held up well for this long. I used a Sencore LC-101 which can apply up to 1000v and display down to 0.1uA leakage.

Trio/Kenwood integrated amps of tube era look suspiciously similar to Pioneer units. I guess who really manufactured them?

As for poor qiality resistors and potentiometers I have to partially disagree - these units have not been designed to run for 40 - 45 years. Carbon composition resistors seem to "decomposed" after so long usage because of heat, aging or whatever.

I'm sure other Japanese companies shared a lot of parts suppliers and design philosophy.

I agree that it was never designed to last this long, but compared to other items of this age, the resistors and pots didn't hold up well. I've never had to replace such a large percentage of the resistors. Normally, I find only a small number of bad resistors, even in TVs that run much hotter than a receiver. It's clearly a low mileage receiver, but maybe it was stored under poor conditions.

Normally, I find only a small number of bad resistors, even in TVs that run much hotter than a receiver. It's clearly a low mileage receiver, but maybe it was stored under poor conditions.

Carbon composition resistors is a common problem for all Pioneer owners I know, it is not result poor storage condition. I'm suspect thermal condition - Pioneer models share very very small chassis, and heat from the output tubes have been partially accumulated in circuit chamber.

I did a few more minor things to it over the weekend. First, there was a lout popping noise when switching to the tuner, even with the volume at minimum. I traced it to the switching of the B+ to the oscillator tubes. I'm still not sure exactly what was happening, but some how it was generating noise that found its way into the phase inverter. I played around with various methods of bypassing, or decoupling the tuner B+ from the rest of the power supply, but nothing helped. Finally, I had good results by adding .0015uF caps from the high side of the AM and FM osc plate resistors to ground. I'm open to suggestions if anyone else has any ideas.

Next, I replaced both the 300v 20uF caps in the MPX circuit with higher voltage replacements. I found that the voltage on those caps was getting close to 400v during warm up.

Finally, I turned the tape head input into a line input. It only required a few minor changes to the wiring of the input selector, and would be easy to reverse. I don't think I've even seen a tape head input that showed any signs that it had ever had anything plugged into it.

I've run into an interesting problem with biasing the output tubes. If you look at the schematic, you'll see that the 450v supply goes to the output transformers, and is dropped by a 2.5k resistor to 310v for the screens. From there, it's dropped to 260v by a 1.5k resistor which feeds the tuner osc tubes (point A).

The problem is that the 8-10 mA used by the tuner drops the screen voltage by a good 20v. This is resulting in a large change in output tube bias when it's switched to the tuner. In aux mode, the screen voltage is 367v with 15mA cathode current per tube. When I switch to tuner, the screen voltage drops to 247v, and the cathode current drops to 10mA. Is it normal for the screen voltage to affect the cathode current so much? The only solution I can think of is to add a separate screen supply, or keep the tuner powered all the time.

Well, rather than redesigning the screen supply, I found that it was easier to power the tuner from somewhere else. There's 225v between L10 and R116, and the tuner doesn't seem to mind the slightly lower B+ with slightly higher ripple. Dropping the value of the FM osc plate resistor compensated for the voltage difference. Later Pioneer receivers use a very similar change in the power supply.