Trying to identify power/pilot lamp of Technics SU-8080 - Page 1
Forum section: electronics / repair

author: klausman
author_rank: Regular Contributor
timestamp: September 29, 2025, 10:13:01 am
content: This thing has blown:Itseemsto read "XAMR21S 350". The S might be a 5, but I don't think so.I am trying to replace it. At first glance into the service manual, I was sure I could replace it with an LED, since the SM says this is a 6.3V/40mA bulb. Knowing not to trust service manuals, I measured the voltage this is run at, and sure enough it's ~35VAC (between A and B in the below schematic). Now I don't know if that means the power supply situation of this thing is broken (and hence it blew), or if the SM is just wrong. I suspect the latter.Here's where it sits in the schematic:About the only way I can see 35VAC not being the intended voltage is R505 or R501 being shorted/way-too-low, and I haven't tested them yet, but I doubt that is the problem here. I will also check the what the T2 delivers on the ORG-WHT pair. What I am sure of is that it's AC voltage, so just dropping in an LED and a resistor is not ideal. I'll also check voltages at C-B and D-A.I would have attached the whole SM but it's too big. I can be found on Elektrotanya, though.edited: better schematic

author: tunk
author_rank: Super Contributor
timestamp: September 29, 2025, 11:11:31 am
content: Wouldn't really know.Is it an on/off indicator light?If so, then you could connect it to a DC rail.And you're measuring an open circuit, i.e. 35VAC should be correct.If the bulb was working, it may be around 6VAC over the bulb.

author: klausman
author_rank: Regular Contributor
timestamp: September 29, 2025, 11:24:30 am
content: Quote from: tunk on September 29, 2025, 11:11:31 amYeah, it's the front-panel power-on indicatorQuote from: tunk on September 29, 2025, 11:11:31 amI'll have to see if I can find one within reach of the place. The amp is a rats nest of wires already. Also, I am kinda torn between "repair it as close to original design as possible" and "replace outdated incadescent lights with LEDs." The latter wouldn't betoomuch of a style mess: the two other indicator lights the amp has already are LEDs.Quote from: tunk on September 29, 2025, 11:11:31 amAh,you're right of course. Might see if I can get something that's spec and if it goes up in smoke, at least it was only pennies.

author: themadhippy
author_rank: Super Contributor
timestamp: September 29, 2025, 01:25:07 pm
content: chuck a 180 ohm resistor were the lamp should go,that should give ya a ball park idea of whats wot

author: klausman
author_rank: Regular Contributor
timestamp: September 29, 2025, 01:53:35 pm
content: Quote from: themadhippy on September 29, 2025, 01:25:07 pmAt both 200Ohm and 150Ohm (I don't have a180 on hand), the voltage completely collapses to well <1V.I could of course try smaller ans smaller resistors, but that seems like a gamble ending in a short and some smoke (or a blown fuse).<- stupid idea. I also tried 1K and there I finally get something that doesn't seem to be noise: about 150mV.

author: tunk
author_rank: Super Contributor
timestamp: September 29, 2025, 03:22:33 pm
content: I guess the reason for themadhippy's suggestion is: 6.3V/0.04A= 158 OhmNot sure if I read the schematics correctly, but if R505 and R501 is inseries with the bulb, then 150mV with a 1k resistor doesn't make sense.Have you measured the values of the two resistors, and maybe the voltagesacross them?

author: klausman
author_rank: Regular Contributor
timestamp: September 30, 2025, 08:34:27 am
content: Quote from: tunk on September 29, 2025, 03:22:33 pmBoth R505 (68 ohm) and R501 (5.6 Ohm) measure ok (501 is slightly high at ~6 ohm). With a 150 Ohm load connected, both have very little voltage across them, ~10mV for 505 and 0 across 501 (I think my 34461A -should be measuring something here, but it's all 0s, so now I am trying to find if there is a short somewhere). Across the 150 ohm load, I measured ~23mV.

author: Swake
author_rank: Frequent Contributor
timestamp: October 02, 2025, 04:48:10 pm
content: I thought for a moment there was something voodoo going on here as the lamp is connected to two different windings on TR2.The BLU wire of TR2 is connected to a ground.The GRN wire of TR2 is also connected to ground. Not certain if it is the same ground as above but lets assume.The potential difference between BLU and GRN is 44.3 - 34.7 = 9.6 V That is 3.3 V more than what the lamp would like to see.For the lamp to get a current of 40mA the shunt is -> 3.3V / 0.04A = 82.5 Ohm.Given there is a 68 Ohm and a 5.6 Ohm resistor and also a fuse (which does have some resistance too) in line with the lamp this seems to match somewhat.Have you checked the fuse F8? If you are 100% sure that fuse is ok, then check the entire path of wires to the bulb because it is loose somwhere.

author: klausman
author_rank: Regular Contributor
timestamp: October 03, 2025, 09:29:30 am
content: Quote from: Swake on October 02, 2025, 04:48:10 pmI am not sure I'd agree on the wiring colors. To me, the diagram says that the bulb is connected to ORG on T2 (marked as 34.7VAC) directly, and via R505 (1W, 68 Ohm), R501 (1/2W, 5.6 Ohm) and F8 to WHT on T2 (marked as 0V). This would fit with the 35V I see on the open circuit. I have not verified this on my unit yet, but will do so on the weekend, as well as chekcing F8 again (I checked it before, along with all the others, but I want to be sure), as well as the cabling. The latter is going to be a major undertaking thanks to the routing of said cabling being buried as deep as can be.

author: Swake
author_rank: Frequent Contributor
timestamp: October 03, 2025, 10:40:38 am
content: The top and bottom windings on TR2 are galvanically isolated. Looking only at the WHT and ORG wires it is possible that your 34401 registers 35 V as its input impedance is very high, however current of useful size cannot flow unless there is another path.I think that path is going over the BLU tap of the top winding back to the middle GRN tap of the bottom winding. The GRN is connected to ground at point J18. Following the BLU wire it connects to point 415 and from there on it is a 'thicker' line, which is common to indicate ground. Therefore I believe these 2 taps are connected somewhere not visible in this part of the schematic.

author: klausman
author_rank: Regular Contributor
timestamp: October 03, 2025, 11:21:27 am
content: Quote from: Swake on October 03, 2025, 10:40:38 amAh, gotcha. I looked around the full schematic, and one of the two (GRN) goes to the negative speaker terminals (T1's GRN to the main set, T2's to remote), and BLU is just tied to chassis ground at some point. I didn't trace GRN further since I know that speaker negative is chassis ground on this amp by design.So time to figure out what is going on with that lamp wiring. I am also getting a box of replacement bulbs (by spec...) today, and will give one a try, see what happens. At worst I am blowing a few pennies worth of light bulb.

author: nfb
author_rank: Contributor
timestamp: October 03, 2025, 12:12:42 pm
content: Check voltages at points 508, 509 relative to ground without the bulb. There should be about 35V AC at 509 to ground and about 45V AC at 508 to ground.

author: klausman
author_rank: Regular Contributor
timestamp: October 04, 2025, 02:15:54 pm
content: Quote from: nfb on October 03, 2025, 12:12:42 pm508: 0.85V509: 35V420/520: 0.85VAlso checked F8 (ok) and the voltage on either side of it, again 0.85V.So I attached one of the spare bulbs I got, And measured voltage across it, <1mV. The bulb didn't even glow briefly (and it didn't blow). I'm a bit out of ideas now.

author: nfb
author_rank: Contributor
timestamp: October 04, 2025, 03:29:19 pm
content: The problem is in the 45v branch. Check voltage between points 410 - 415. There should be 45V AC there, these points are connected to secondary winding of the T2 transformer (WHT BLU) through the F8 fuse.If there is 45V AC on 410 - 415 check if 415 point has connection to ground.

author: klausman
author_rank: Regular Contributor
timestamp: October 04, 2025, 04:28:46 pm
content: Quote from: nfb on October 04, 2025, 03:29:19 pm410-415 is only ~1.53VAC. 415 is connected to chassis ground.I had a hunch about the voltage selector (S15), so I turned the amp off, unplugged it and worked the switch back and forth a few times (not contact cleaner or anything). That resulted in 410-415 now being 1.8VAC. Do you think a faulty voltage selector might be the problem here?

author: Swake
author_rank: Frequent Contributor
timestamp: October 04, 2025, 06:28:06 pm
content: No, the voltage selector is not at cause here. If it would be bad al other windings and voltages would show an issue as well.You're closing in to the issue: Something is wrong between the transformer T2 top winding and the points 410 and 415.Measure the 45 VAC as close as possible to the transformer, preferably directly on the end of its wires. Are these wires terminated in a connector or soldered on the PCB? Then work your way up to the points 410 and 415 one step at a time. As soon as the 45 VAC disappears you know the fault is before that point.Please remain careful, depending on the conditions 45 VAC is already at the level that it might tickle.

author: nfb
author_rank: Contributor
timestamp: October 04, 2025, 06:33:53 pm
content: I'm afraid that it looks like a broken transformer winding (BLU WHT). I would double check the blue and white cables, solder joints near the fuse and the fuse, if you're lucky there is a small chance that a problem is there. As you have the 35VAC voltage the voltage selector is probably good.

author: klausman
author_rank: Regular Contributor
timestamp: October 05, 2025, 07:46:26 am
content: Quote from: nfb on October 04, 2025, 06:33:53 pmYep. With the amp off, here is no continuity between points 410 and 415, which would normally should be low-ohms. As it is, I get megohms. I think I checked all the cable leading to those points as thorughly as I can. where there's access to bare copper, I tested continuity and the rest of the cable runs have no damage I could see. Well, that means the transformer is toast, and is probably unobtainium. What a shame. I'll give this one more pass and then I'll have to decide what to do with this unit. It's probably e-waste/sell-for-parts now :'(ETA: Well, would you look at that. I gave the cabling another more thorough check:Looks like some part of the metal framing sliced into the wires and shorted the winding, blowing it open. testing continuity on those two exposed wire spots still gives me megohms, confirming the transformer is busted.

author: Swake
author_rank: Frequent Contributor
timestamp: October 05, 2025, 08:00:19 am
content: You have not convinced me yet... of course it is possible that transformer failed but you have not complained about anything else than the bulb not working while that top winding on TR2 is powering other things too. You would have noticed something else is wrong.Pierce the blue and white wiresas close as possibleto TR2 transformer and measure AC voltage at that place. Use needles to pierce the isolation. If you have nothing that can pierce the isolation cut some away with a box cutter. Also make sure you do this on the correct transformer, the other one is having the same colors of wires, eventually do this on both transformers then you have something to compare.

author: klausman
author_rank: Regular Contributor
timestamp: October 05, 2025, 08:05:12 am
content: Quote from: Swake on October 05, 2025, 08:00:19 amRe: piercing: see above edited post. As for does everything else work: I think so, I have only looked at the outputs on an oscilloscope and they looked fine for both channels (I tested both sine and square waves, and into 100W 8Ohm resistors). I didn't note down what voltage I get before clipping, but will do that next, after I put some tape on those damaged wires.

author: klausman
author_rank: Regular Contributor
timestamp: October 05, 2025, 08:19:09 am
content: Quote from: Swake on October 05, 2025, 08:00:19 amI did measure the voltage between those wires, at the damaged spots: it was <1VAC.After makeshift taping over the damage, I hooked up everything again: 250mV signal into the tuner input, 100W/8ohm resistors on the speaker terminals and checking output voltage with the oscilloscope.The amp is specified at 72W, so for 8 ohms, that would mean: sqrt(72 watt * 8 ohm) = 24V. However, the right channel starts clipping at about 16V, which means we only get (16V)² / 8 ohm = 32 watts. I am not gonna lie, I am amazed the amp still works at all, and even puts out nontrivial power. I'll hook up some cheap speakers I don't care about much to see, I meanhearwhat's coming out of it with music fed in.

author: Swake
author_rank: Frequent Contributor
timestamp: October 05, 2025, 08:33:59 am
content: Quickly checked the rest of the schematic. The top windings of both transformers are feed into the same rectifier diodes essentially complementing each other. One less winding indeed means less power and maybe some more distortion at higher power output. That is exactly what you noticed now.Don't push that amplifier all too hard and it should still work. Don't put more than one pair of 8 Ohm speakers. More speakers or lower impedance speakers will overwork (=overheat) the remaining transformer very quickly.Found some pictures on the net, there seem to be 2 versions of this amplifier, one having 4 larger fuses and one with 8 fuses of 20mm. Also a Japanese version named A80, having slightly different internals.The design dates back end of the 70's, impressive actually.

author: klausman
author_rank: Regular Contributor
timestamp: October 05, 2025, 08:37:52 am
content: I just did a little test with some cheap satellite speakers, and the music sounds fine, in that there is no distortion, even at somewhat elevated levels.Dangit, if this thing was entirely toast, I'd chuck it or tear it apart for parts. But now I have a sickly amp giving me puppy dog eyes. I guess I'll hang on to it and keep a saved search for a spare transformer or broken SU-8080 on ebay etc. Maybe one day, I can replace this blown T2.Thanks everybody for all the help on this thread! What started as the hunt for a mystery bulb turned out quite the puzzle (for me, at least, I am sure some here would've sussed this out in ten minutes)