Sony STR-DG500 quiet on right channel until warmed up. - Page 1
Forum section: electronics / beginners

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: April 08, 2021, 10:20:17 am
content: Edit: found larger problem. Right channel is quiet. See reply #6 below.Dull vacuum display. Is it worth fixing?Amp is about 15 years old. I inherited it 2 years ago and in the last year, cos the telly has been on almost 24/7the display is now only readable if the room is dark. it doesn't really impede any use of the amp, however. It's just hard to know if it's off or on.An opportunity is coming up to remove it and everything else from the cabinet (room renovation) and I wonder if it's easy to fix with not much effort or just leave it.Anyone care to chime in?So far I have found this..https://www.eevblog.com/forum/chat/vacuum-fluorescent-display-rejuvenation/

author: Gyro
author_rank: Super Contributor
timestamp: April 08, 2021, 10:36:57 am
content: I think that thread pretty much covers all the bases.Check the VFD supply rail, relevant capacitors, filament voltage etc. Occasionally there's a series resistor in the filament supply that you may be able to lower or bridge out. Other than that, maybe look for a cheap (blown output) parts mule?Check the user manual for any display dimming function!

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: April 08, 2021, 10:52:07 am
content: Oh man, I wonder if someone 'found' a dimming function. All I do is turn it up loud or sometimes really loud.As for parts, I suspect the ones on ebay are for sale for this reason only. It's all light touch switches and relays so it seems pretty solid.Crouching down, I've just noticed a very faint, high pitch whine coming from the box, (not the speakers). Noise stops when unit is off. Wonder if that is related. Wonder if it's trying to tell me the PSU has a dry cap.

author: wizard69
author_rank: Super Contributor
timestamp: April 09, 2021, 02:08:57 am
content: Quote from: Ed.Kloonk on April 08, 2021, 10:52:07 amYou have no idea until you ask. Deoendign on what the part is and whom Sony purchased it from there is the possibility of a lot of NOS around. I wouldn't discount the possibility of good stuff being on E-Bay, I've made good scores off that place. Well i did before pricing went nuts there.QuoteCould be lots of things, possibly even normal. However you can't dismiss the possibility that the electrolytics (one or more) are bad. If that is the case the entire amp should be recapped. Knowing more about the buzz might helpNow to your original question of it being worth it, that is entirely up to you and what you value. A well designed amp should be well worth fixing. Frankly it prevents a lot of "electronic waste". If you are a beginner there is also a huge opportunity to learn, especially in this case the skills needed to track down noisy components and figure out why they are noisy.

author: james_s
author_rank: Super Contributor
timestamp: April 09, 2021, 02:21:40 am
content: I would check the voltages on the display, including the filament voltage, that should give you a pretty good idea of the state of the display. You could also mess with the various functions and get it to light up a rarely used display element, worn VFDs usually have quite a large variation between the brightness of heavily used and lightly used segments. If the whole thing is dim then it's probably low voltage, the whine suggests possibly a bad capacitor and it's about the right age for that to be an issue.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: April 09, 2021, 02:33:29 am
content: My good VFD amps have dimming functionsIt helps cut down on the noise.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: September 24, 2022, 04:54:38 am
content: Update.This amp.I've changed the thread subject title. One of the problems we thought was the speakers is the right channel on the amp taking 10 minutes to warm up.I've attached a schematic. Before I pull this boat anchor out of the cabinet and out from under everything else, I wonder what you think I should examine once I rip it open. I hope it's not the chip.It's the R channel, not SR. Yes, the schematic is a mess.

author: james_s
author_rank: Super Contributor
timestamp: September 24, 2022, 06:56:23 am
content: I wouldn't even bother with the schematic for chasing down a repeatable temperature related fault. Just let it warm up until the problem goes away then squirt parts with freeze spray from an upside down can of air duster until you find the part that makes the fault come back when it gets cold.

author: David_AVD
author_rank: Super Contributor
timestamp: September 25, 2022, 12:55:27 am
content: Does the bad channel come good slowly, or all of a sudden?Does it come good sooner if you give the volume a bit of a rev?Is the low channel clean sounding or distorted?

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: September 25, 2022, 01:00:59 am
content: Quote from: David_AVD on September 25, 2022, 12:55:27 amSlowly. 10 minutes.QuoteCannot seem to make it make any difference.QuoteSounds weak when displaying symptoms, seems fine, when it finally warms up.

author: David_AVD
author_rank: Super Contributor
timestamp: September 25, 2022, 01:13:42 am
content: The slowly comes good is a little unusual, otherwise I was thinking bad solder joint or cruddy contacts on the speaker relay.So many amps that run warm suffer from cracked joints, mainly in the higher powered areas.I'd be comparing L/R at the input to the main amp section to see whether the fault is in the pre-amp or power amp and go from there.Did you sort the VFD yet? If not, check the AC voltage across the filament connections. There should also be a large negative bias when measured from either pin to 0V.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 08, 2022, 12:28:58 pm
content: Update.Amp on bench. Lid removed. 20 mins only spent looking around before bed.Right channel now seems to be weak (compared to L) when warmed up now. So that's good or bad depending on how you look at it.Tried different input sources(analog/digital), swapping L/R input (just in case).Probing around as-is with no input or speakers connected.It has a push button to sequence though 4 different speaker modes, A, B, A+B, OFF. Relays click on and off accordingly.Voltage rails are +/- ~54Vdc around the preamp chips are as they should be, but here's the weird bit, drop down to 38Vdc when selecting A+B. The AC-in connector near the bridge rectifier reads from 84Vac down to 57Vac again, with A+B.That seems to drop a lot of voltage only when in bridge mode and with no input.Yet the power meter on the plug shows 30-40 watts between modes. No real consistency.

author: David_AVD
author_rank: Super Contributor
timestamp: October 08, 2022, 11:28:47 pm
content: Some amplifiers have a DPDT switch (or relay) that selects between lower or higher voltage windings on the transformer secondary. This is to select 4 Ohms or 8 Ohm usage. I've seen the switch (but not the relay) go intermittent and cause havoc with the rails. I always spray clean them as a matter of course. That won't any L/R channel differences of course.I have seen the odd power relay (switches the main transformer primary) go bad, but from memory the armature was sticking so the contacts never closed.Cracked joints and corroded wire links (especially ones that are under glue beside filter caps) are also pretty common and can cause all sorts of weirdness. This is especially true when they are associated with the 0V line.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 09, 2022, 03:36:55 am
content: Quote from: David_AVD on October 08, 2022, 11:28:47 pmYou were right. Block diagram shows relay near a pair of secondary windings, a relay driver and a control line labelled bridgeable. Thanks. That makes be feel a lot better. I can justify dismantling the thing so I can attack the back side of the board.Next week. Maybe.

author: james_s
author_rank: Super Contributor
timestamp: October 11, 2022, 06:18:49 pm
content: Rather than relying on ears to determine a channel is a bit quiet, feed all channels with a tone from a signal generator and measure the output amplitude with a scope. That will make it very easy to see any mismatch or drift and may offer other clues to what is going on. Just be careful if you try to look at multiple channels at once, some amplifiers have all the speaker negative terminals tied to the same point, some do not.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 11, 2022, 11:48:31 pm
content: I trust my ears before I'd trust any of those new fangled scopes.Yeah. When I clear the bench of the current mischief and mayhem, I'll do just that. My rigol has a pretty terrible inbuilt function gen that is surprisingly good enough for most of my purposes. I think I do have a proper sig gen around here somewhere if not.At the same time or nearer, I've got to fiddle with a big EQ with either dodgy sliders or dead caps or crap solder on a few bands. Need to get the bench set up, the amp disassembled, a chunk of spare time and all the planets to align.