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.