Wednesday, May 6, 2009

The Onkyo 2500 MK II Receiver: a Tale of Two Dohs!

We have for your consideration the Onkyo TX 2500 MKII, the second of the 4 receiver Onkyo range from 1978-80. The 2500 listed for a not insignificant $360, the equivalent of about $450,000 in current dollars. For the sake of comparison the big dog, 100 watt, TX 6500 MK II listed at $650. Back in the day that was about 6 weeks take home pay for me, and was $250 more than my mortgage.



This unit was evidently last used in a barn or garage; it was filthy, but as often happens the thick pelt of dust and fur seemingly preserved it from the daily nicks and knocks. A thorough detailing left it close to pristine except for an extremely annoying bent tuning knob shaft. Not trying to straighten it is like not picking at a scab, but it worked and it's usually not worth the risk of making it worse. That last trip to the thrift or flea market is the primary source of hifi trauma.

Orion says the 2500 is capable of a tidy 40 watts RMS, no doubt a conservative rating. It has been my experience that the medium powered receivers of the '70s sound very good when just a bit of care is taken with the choice of speakers. The 30-50 watt-ers seems to occupy the sonic sweet spot, with a bit more slam than their lesser brethren, but without some of the complications that can creep into a bigger box. Simpler is often better. I also assume that much of what I hear is the result of price point design decisions unrelated to absolute wattage. It could be bigger main filters than the lesser units or outputs that hit a happier part of their thermal range. At any rate, that elusive mojo can show up almost anywhere, but the middle of the pack seems to be a good place to look.

This particular 2500 had one channel in the dumper and the usual noise in the other. I checked the amp transistors and all was well. I checked the output of the preamp and the channel that was down was extremely low, so I expected that a recapping would get us rocking. But... no. In an object lesson of 'look for the obvious, stupid' Mike pointed me towards the fuse block on the back of the unit. Yup, one was blown. Evidently there had been an event and a fuse had given its life to protect a speaker. Fuses on the back? Who knew? That was doh! #1.

The restoration replaced 54 caps with the usual suspects: Panasonic FMs and FCs with a few stray metallized stacked films. The 2500 has 4 boards: power supply, amp driver, preamp and the combo FM/AM/phono. The latter 3 were straight forward although the preamp was a bit cramped as usual due to proximity to the front panel controls. The power supply board is mounted upside down under the main filters. It had to be unscrewed, wire ties clipped, the main filters clamps unscrewed and some tugging before it succumbed to the iron. The 6800mfd @ 50 volt main filters were left as original. If I get ambitious I might double their values someday.

Here is the underside: very tidy. Note the power supply board at the lower right.


Here it is from topside:

Richard Avedon I am not. At least the photo is in focus. Sort of. Below is a close up of the tuner board in the vicinity of the leads to the lamps. Nice, eh? It is so well labeled that even a solder monkey like myself can follow it. No spatial relations brain squeeze when I try to figure out where the leg of a cap is. Many a resistor has been mistakenly desoldered from poorly marked board...



Onkyo has a reputation for quality tuners, and here is where the tale gets complicated. The fm tuner is a phase-locked loop (PLL) with nice meters for signal strength and center, and 3 lamps: locked, tuned and stereo. FM stereo worked but the other two lamps did not. The locked lamp was burned out and replaced successfully, but the tuned lamp was good, and would not light up. Drat.

Poking through the circuit revealed nothing that was obviously wrong; all the pertinent transistors seemed fine. So I took the easy way out. Sometimes it is easier to spend some money than to exercise your brain pan. I had an order pending to a new vendor, Acme Electronics, so I added a new FM chip, the BA1320 for $2.39 to my shopping cart of anvils and dehydrated boulders. Oh, and another FM chip and a big roll of solder wick. More on that later, perhaps.

Well, as you might expect, the new chip did not solve the problem. Hmm. So it was time for the 'A' team to track it down. Mike puttered for a few minutes and then determined that I must have missed testing the offending transistor: a C380 adjacent to the where the tuning knob connects to the board. Doh!



The impedance of your hand on the tuning knob alters the circuit and turns the lamp off. Below, note the white wire that connects the knob to the board. Just to the right is a pair of black transisitors. The upper was the offender. Note the light bulb on the board where the white wire connects: it's there to absorb a whack of static electricity when you grab the tuning knob.



Anyway, the 2500 is now in fine fettle. The tuner sounds very clean except for the digital side band hiss that pollutes KGSR (107.1) and KMFA (89.5). This leads inevitably to tech jokes, eg. 'this tuner is not digital ready', or 'that sure is an accurate rendering of digital hash'. Anyway, it sounds great on a stations that have not yet embraced the dark side of Ibiquity. I really need to track down tuner tweaks to avoid the hiss.

I am rolling this article out the door without significant critical listening, stuff is piling up on the bench. So, with luck, I will tack on an update on how it sounds in a week or so.

14 comments:

Anonymous said...

just got 1 for 4.99 at goodwill plays good one channel not blown but seems weak ??i know nothing but will learn?

katty said...

I usually listen loud music in my house, that´s why I bought a big stereo, I like to spend my time at home I think it was the costa rica investment opportunities because I acquired my house and now I can do everything I want.

Anonymous said...

Anon, I just picked one up at a Goodwill as well. There is a fuse block on the back under a plastic cover, check it. I got a Sansui recently with the same problem, and replacing the fuses did the trick (channel down but not out).

Does anyone know about the servo lock feature? I replaced all the dial lamps with 7.3V fuse-lamp types, it lights and looks great, but the LOCKED and TUNE lamps will not light. The STEREO lamp does. I visually inspected the non-functioning lamps and they look OK. Is there something in the circuitry I can check on the tuner for this?

I used to sell these in retail years ago, and I seem to remember that when you get close to a station the servo lock takes over and drags the tuner into center and there is a small click when that happens. I am not getting that with this unit; however the tuning indicator does swing to center when I am close to a station and stays there until I move the dial. I think it actually is locking, but the SERVO and TUNED indicators aren't functioning for some reason.

Any help is appreciated. Thanks.

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swampducks said...

So cool, just found this blog. Makes me want to open up my Onkyo and see if I can fix the little "locked" light that's been out for years. Not being an electronics weenie never occurred to me to try. My receiver is still chugging along wonderfully all these years after I bought it new back in.... um... '78 I think?

Anonymous said...

I just got one of these from my father up in Georgia. He found it at a appliance recycling center in north Georgia. It had been rained on for two days and was just sitting there at the dump. He picked it up, took it back to his garage and dried it out. Took 4 days to get it to fire up. He found a Harmon Karmen a couple days ago at the same place so says "you want this one". I never seen it so said yeah whatever. I got it and it was dirty, just a dirty garage/shop stereo. I took it apart, cleaned it hooked speakers up to it, hooked the computer up to it, and man is it cherry. This thing cleaned up to brand new, cherry condition. Very warm feel and sounds great. The only problem with this unit is the Locked/Stereo/Tuned lights dont light up and the auto lock radio station fuction dosn't work either, if its supposed to take over by itself and go to a station. Evey thing else is cherry. Not a scratch on it like brand new. I can't imagine the receiver he found to give this one up. I'm into vintage receivers now for sure. Grew up around them and they are so sweet!!

Onkyo TX-NR609 said...

What do you think about the new Onkyo TX-NR609?

Classic Receivers said...

Nice Onkyo. I have a TX-4500 that I love. Someday I'll pony up for the 6500 Mark II. Glad to see that you got yours working properly. They are difficult to find in fully functioning condition.

dualref said...

I bought one of these back in 1977 and used them with a pair of JBL 4311 speakers. Very, very clear sound. These receivers rocked! I think these receivers were underrated, they had a lot more power than 25WPC.
Unfortunately around 1983 it walked out the door of my condo while I was at work.....

vermont said...

Glad to find this blog! A friend pulled one of these out of dumpster and gave it to me knowing that I was a bit of a gearhead. It looks stunning bot doesn't work... so far, so good. So off I go to look for fuses...

gquinton said...

I have had one since 1977 or 78. It was put away for a while, but now back in service. a few problems...The Servo lock lights don't light, and the tape monitor switches affect whether or not the speakers work. If i wiggle them I will get R or L or both. I'd like to get this working 100%. Do you think using an electronics spray cleaner will help?

purtain said...

Greetings, a while ago got a TX-2500MKII in very good shape. I am really impressed by the sound. I've replaced the tuner/locked/stereo lamps. The stereo lamp is not lighting, however, (the other two are), and I am not getting FM stereo sound, only mono. Any suggestions. Thanks, Richard

YEMEN K.D said...

Hello. love the blog and lucky to find such a source. I just scored one of these from a friend who had it sitting for years and I'm trying to get it up and running, but bit of a novice with these units. My receiver fires up and lights as usual but there is no sound. I've tried all the speaker troubleshooting antics to no avail. Wondering if you have any recommendations or anything I should try looking at first?
Thanks in advance...

Anonymous said...

I laid eyes on a TX-2500 at second hand store. I plugged it in and it powered on. Next I McGuivered some wire and a speaker, sound emitted from one channel. Oh, what's this....a fuse box with two fuses and one is diffused. Cross my fingers....half price day too but no returns. Gamble.... I bought it. Came home and cleaned it and inspected. Went to the store, came back with new fuses, fired it up and viola. Sweet Jesus. This thing sounds good, really good. Better than I thought and better than my TX-860. Score. The power button is a bit sticky. Careful is the key word.