Panasonic SU-2800 Audio Amp. No Sound (STK2028) - Page 1
Forum section: electronics / repair

author: Dannyx
author_rank: Regular Contributor
timestamp: July 11, 2025, 08:58:37 am
content: Good day folks. Someone gave me this old Panasonic (National) SU2800 amp to look at. It outputs no sound at all - nothing, not even a buzz or a hiss, NADA ! It is stone dead. I can't find any useful info on this thing and even some of the parts are rather elusive.The difficulty starts with the main audio amp itself: it's an STK2028 and there is no clear datasheet for this. The only "thing" that comes close is like a scrapbook someone put together in a PDF on Scribd: some snippets someone took from some service manual and a photo of a page where STK2028 is hand-written next to a pinout of an IC which looks the same, but doesn't explicitly say "2028" anywhere....maybe it's an equivalent, but I'm really not certain. I've attached it if you're curious. The pinout does seem to match to a degree, but the middle two pins which are NC in the capture, are connected to GND in my case.Taking some basic measurements to the best of my ability, without having a scope on hand, shows that there is +/-35v going to both VCC+ and VCC- pins, but that's about all I can measure with a DMM.One thing which stood out and doesn't look right to me is the input pins on either end: one reads 1.3v, but the other one reads 27v, close to the full 35v rail ! I'm pretty sure this shouldn't happen. Tracking down where the two pins connect leads me to a pair of A1123 transistors. They both check out fine, as far as I was able to test in diode mode. Going further back, the transistors' bases seem to be driven by a pair of theseAN7060. Thankfully, these are less elusive than the 2028, though you can tell these are very old parts, because the datasheet is once again a scan of its physical form and it's in Japanese...whatever. At least it shows a pinout I can follow. This opens up another can of worms: pin 6, the output pin, connects to those bases and they both output 35v - the same as the supply rail, which I find odd....but that's reading it in DC mode with my DMM. I was also expecting pin 5, VCC-, to read -35v, but it's close to 0v, so that's another thing which is not entirely clear.There's also the matter of this exploded resistor near the balance pot on the front panel. It might be irrelevant for now, because all it does is connect the metal shell of the balance pot itself to chassis GND. There was probably some high leakage that occurred between whatever audio source was connected to the amp and the chassis and that caused the resistor to burn, but I didn't bother with it for now.Any feedback on this thing would be greatly appreciated. Cheers !

author: BILLPOD
author_rank: Frequent Contributor
timestamp: July 11, 2025, 12:53:30 pm
content: Good Morning Dannyx, Usually the first step in diagnosing a 'DEAD' electronic device is to see if you havepower anywhere in the device, and that result will lead to where to go next. As always, look over the circuits to see where mains voltage should be present. And always practice 'Safe Probing'

author: themadhippy
author_rank: Super Contributor
timestamp: July 11, 2025, 01:01:28 pm
content: QuoteDid you bother reading the post before jumping for the AI generated response?QuoteIf your certain its an input id try lifting that pin and seeing if the voltage is coming from the chip or the support circuit.

author: Dannyx
author_rank: Regular Contributor
timestamp: July 11, 2025, 01:09:44 pm
content: I was thinking the same: I need to figure out where that voltage is coming from, but I was afraid the STK might not like its input pin floating, so I postponed doing that.During my searches, I came acrossTHIS: it's a schematic for a Technics amp which seems to share most of the circuit with mine - it even uses the same STK and AN ICs and is kind enough to tell me what each pin on those ICs should read, so I was right about one thing: the AN chips are supposed to have -6v on their 5th leg, but I get nothing there (about 1v), so I need to investigate further.Unfortunately, the zener regulator which is responsible for that -6v rail based around Q401 and Q404 is slightly different in my case - close, but not identical, so I'll have to do some more digging there....

author: coromonadalix
author_rank: Super Contributor
timestamp: July 11, 2025, 02:07:58 pm
content: https://elektrotanya.com/technics_sa-103.pdf/download.htmlthis one has similaritiesfor stk based amps like this, yes you check the stk2028 datasheets for needed voltages, if all seems fine, you follow the sound with a signal tracer / scope to the stk inputs, if the sound is thereand the muting circuits are ok not killing the sound, you may have an failed stk ampi dont think this one su-2700 or Technics Sa-103 or the SU-2800 had speakers relays in themas you may know STK monolithics packages where prone to many failures ...pins 13 and 4 are the negative psupins 15 and 2 are the positive onepins 9 and 8 are groundspins 1 and 16 the audio inputs7 and 10 needs to be checked if you have some speaker problems / shorts, or the stk has failed etc ...stk 2028 schematic attached here

author: Dannyx
author_rank: Regular Contributor
timestamp: July 11, 2025, 04:51:26 pm
content: Thanks, but I'm not sure that is the correct diagram of the 2028, as it does not match the pins in the service manual, namely the IN and OUT pins.There's also the matter of THIS thing I found near the -6v regulator: D401 is a resistor placed where the schematic (and marking on the board) indicates a zener diode should go. The solder looks untouched, so it's not a hack someone did - it most likely came like this from the factory, so I'm not sure what to make of this. It may have something to do with the missing -6v. I'll have to pull it off to measure it, since it reads some BS values in-circuit.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 11, 2025, 06:59:47 pm
content: Quote from: Dannyx on July 11, 2025, 04:51:26 pmIt is a zener. We have seen this before. The colour code represents the type and voltage of the diode. There is a thread on here but it is difficult to find amongst all the other requests for resistor identification. I think the manufacturer might have beenVisheyPanasonic - MAZ4000 series .Edit: Found the thread:https://www.eevblog.com/forum/beginners/old-zener-identification-460028/msg5901116/#msg5901116

author: Dannyx
author_rank: Regular Contributor
timestamp: July 12, 2025, 08:52:43 am
content: Assuming the schematic matches, the part number of said zener should be the correct one.

author: Dannyx
author_rank: Regular Contributor
timestamp: July 14, 2025, 08:44:49 am
content: Ok, back to work on this amp, so here's some more findings:D401 is indeed a 6v zener and checked out fine. I actually removed it, put it in series with a 200ohm resistor and hit it with 12v from my bench power supply. It clamped fine to 6v, so we can move on from that.It just so happens the original paper is punched right where I need it the most, namely towards the bottom where the rest of the circuit is....@-holesAnyway....I figured I should measure "inwards" from the Collector of Q401 (which is essentially V-) and compare what I get to the values on the little labels shown in the schematic.1) I get nothing (0.6v) on the Cathode side of D402 (and Emitter of Q401). The label says there should be -28.5v there...so that's N.G.2) Ironically, I get -1.3v on the Anode side of D402 (before R401), BUT that just so happens to be the value that's expected on pins 5/12 of the STK amp. Pin 5 of the STK itself reads fine, but pin 12 reads 25v, so that's also N.G. and could point to a dead STK.3) AFTER R401 (Anode of D401) I get nothing (0.6v). That's also where pin 5 of the 2 AN ICs connect. Those expect -6v, which is also not happening. I even tried disconnecting the two pins, thinking perhaps one of them is dragging the rail down, but no: still nothing after R401...4) Pin 16 of the STK IC reads 27v. This should be the signal input, so it shouldn't be this high. Pin 1 on the other hand, the other channel, reads fine (1.3v).5) Collector of Q404 (and base of Q403) read -35v, while the schematic shows it as 0v...also N.G.I checked the PNP transistors as much as I could with my DMM and they all checked out fine, with diode drops of around 0.7v. I even tried swapping Q301 and Q302 to see if those 27v show up on the other pin (pin 1), but no: it's still pin 16 which remains "high" all the time.Another thing I tried which yielded unexpected results was to desolder pin 15 to see if cutting VCC+ to the "affected" channel also causes pin 16 to go down...which it DID, but now the other "channel" had nothing on it either, namely pin 1 and 12 which previously read 1.3v were now dead, which doesn't make sense to me, because I assumed the two channels of the STK should be independent, so I don't see how un-powering one side affects the other, but apparently there is some connection between the two...Not sure whether I should go ahead and replace the STK and hope for the best.