Let's fix this beautiful old mixer [Satt sam 82] - Page 1
Forum section: electronics / repair

author: aabbcc
author_rank: Regular Contributor
timestamp: June 25, 2018, 06:44:57 pm
content: So I scored a real nice old Swedish made mixer a while back for cheap, usually I spend most of my time building synths (raaaarely troubleshooting) so I thought this might be a good chance to learn somethingThe mixer is an old Satt sam 82 filled with nice old Lundahl transformers.Stolen photoThe problem is that it wont power up all. Visually everything looks fine so I started checking all the traces and swapped the fuse which had blown on the psu but it still wont power on.I decided to remove the psu section from the synth in order to more easily try and troubleshoot the thing. All the test points reads around 0.5v when plugged in. The only place where I'm actually able to get a reading is before the current enters the big transformer where I get the big scarry 240v, everything after that I get basically 0v, which leads me to believe the transformer has died.Now I have no idea what to do next though, can I somehow test the transformer? I checked all the leads and tracks going to the transformer and at least they seem to be connected. Furthermore I have a rigol ds1054z and a brymen 867s at my disposal.Click the link for more photos of the psuhttps://photos.app.goo.gl/LKJHqjevXyVSZYw19Service manual:https://data2.manualslib.com/pdf5/101/10098/1009764-satt_electronics/sam_82.pdf?373d3cacdef1450e71f59cd434f92921Psu overlay: Page 23Psu information: Page 24Psu parts list: Page 35-36Schematics: Page 43-44

author: Andy Watson
author_rank: Super Contributor
timestamp: June 25, 2018, 07:02:03 pm
content: Measaure the resistance of the transformer windings. You might be able to do this without disconnecting it - the low resistance of the of the secondareis should dominate the other components in the circuit. Eitherway you can compare the two secondary resistance values - it is unlikely that both secondareis will lhave become open. You should be able to measure the primaries without a problem. If the windings are ok, check the resistance through all parts of the primary circuit.

author: abraxa
author_rank: Frequent Contributor
timestamp: June 25, 2018, 07:17:36 pm
content: My assumption? The voltage switcher (14) has oxidized contacts. Disconnect from mains before testing it using the ohmmeter. If that's not it, use the ohmmeter on the primary windings of the transformer. The resistance should be very low.

author: aabbcc
author_rank: Regular Contributor
timestamp: June 25, 2018, 07:25:57 pm
content: Quote from: Andy Watson on June 25, 2018, 07:02:03 pmThanks! Measured the resistance of the wingdings (Didnt understand the part about the secondaries). When measuring resistance on the rest of the psu what exactly am I looking for?Transformer resistanceEdit: Ah so primary=input, secondary=output. All primary then measures 0Quote from: abraxa on June 25, 2018, 07:17:36 pmChecked the contacts with a loupe, can't see any oxidation. Primaries all measure 0.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 25, 2018, 07:56:56 pm
content: Quote from: aabbcc on June 25, 2018, 07:25:57 pm

author: aabbcc
author_rank: Regular Contributor
timestamp: June 25, 2018, 08:11:32 pm
content: Quote from: Andy Watson on June 25, 2018, 07:56:56 pm

author: abraxa
author_rank: Frequent Contributor
timestamp: June 26, 2018, 07:49:48 pm
content: When you say e.g. "Pin 1", what other pin do you measure against? I would've understood if you had written "0<->1: 0 ohms" but giving only one point of the connection makes it unclear what you actually mean.

author: aabbcc
author_rank: Regular Contributor
timestamp: June 26, 2018, 08:17:46 pm
content: Quote from: abraxa on June 26, 2018, 07:49:48 pmMy bad, I did all the measurements against a ground point on the pcb.I emailed Lundhal if they had a spare of this old transformer and they had not. They did however tell me that the the transformer has 4 secondaries which each should measure 27 vdc. I measured the secondaries and they were only around 1-4 vdc.Also the windings have this weird yellow resin-ish substance on them, is this normal?

author: abraxa
author_rank: Frequent Contributor
timestamp: June 26, 2018, 09:09:01 pm
content: I wouldn't worry about the resin, it doesn't affect the performance of the transformer except for maybe some louder humming.QuoteThe same ground point for both primary and secondary windings? They don't share a common ground, so you'd need to use different reference points for each side.How many ohms do you measure between L and N of the line power connector? For 240V, both windings are connected in series, so I'd expect you to measure 0.12 or 0.24 ohms, depending on how you measured the primary side before.QuoteTo test the transformer, you really need to measure its output voltages without any connected circuitry. It's a high-impedance transformer, so the voltage will break down when it's overloaded. If - for whatever reason - there is a fault in the voltage regulation circuitry, the transformer will show low output voltages, just as you're seeing. So I wouldn't blame the transformer for it just yet.If you have a lab power supply that can limit the current, you could for example apply 25V to one of the transformer secondary windings while limiting the current to, say, 300 mA. If the power supply is still in current limiting after a few seconds (it will initially limit the current as the uncharged capacitors charge up), you know that the voltage regulator circuitry connected to that winding is faulty.

author: aabbcc
author_rank: Regular Contributor
timestamp: June 27, 2018, 04:19:06 pm
content: Quote from: abraxa on June 26, 2018, 09:09:01 pmMu multimeter measures 0 when I try to measure between L and N.Unfourtunately I dont have a bench psu. Any other tips on how to proceed?

author: janoc
author_rank: Super Contributor
timestamp: June 27, 2018, 07:37:50 pm
content: Quote from: aabbcc on June 27, 2018, 04:19:06 pmA much easier way to test is to simply desolder/disconnect the diode bridges. Then you should be able to measure some AC voltage (make sure you don't have your meter set to DC!) at each of the secondaries when the transformer is powered up. If you get the 27V you should be getting, then the trafo is likely OK and the problem is downstream - diode bridges, voltage regulators, capacitors on the power rails, etc. If you don't get any meaningful voltage there and the primary is powered (i.e. has those 240V on it - careful there!), then the trafo is likely shot and you will need to replace it. Sourcing a mains transformer with 2x24V or so output shouldn't be that difficult these days - the exact voltage likely isn't critical as the power supply produces +-15V out of it.If the trafo is good then disconnect everything from the power supply module so if there is a fault you don't blow it (and also if the mixer circuitry is actually faulty it will allow you to fight only one battle at a time). Then start testing each of the power rails - is there an output voltage? If not, you need to track down why from the schematic. Try to divide the circuit in in parts and measure voltage - e.g. if you have DC voltage after the diode bridge and before the regulator but nothing at the output, then the problem is likely in the regulator. Etc.

author: aabbcc
author_rank: Regular Contributor
timestamp: June 27, 2018, 08:32:08 pm
content: Quote from: janoc on June 27, 2018, 07:37:50 pmArgh crap, while waiting for more help I actually desoldered all the diodes to check if they were working, they were, so I soldered them back in hehe...In the meantime I got some very good help from a geerslutz member, I will quote him here for reference it sounds like the below information is enough to conclude that the transformer is indeed kaput? If not then I will continue with your very helpful suggestions!QuoteWhich resulted in:Resistance measurements points:1-2: 171 ohm1-3: 195 ohm1-0: open circuit, no-connection1-4: open circuit, no-connection1-5: open circuit, no-connection1-6: open circuit, no-connection1-0 (second 0 in photo): open circuit, no-connection4-2: no-connection4-3: open circuit, no-connection4-0: open circuit, no-connection4-5: open circuit, no-connection4-6: open circuit, no-connection4-7: open circuit, no-connection4-0: open circuit, no-connectionVoltages:N-L: 237 AC volt1-3: 0 volt4-6: 237 AC voltSo from above he concluded that the first winding 1-3 looks ok but that the second winding 4-6 is "open" so that the transformer is dead :deadbanana:Apparently my measurements indicate that the first winding (1-3) looks OK.But the second winding (4-6) is "open", so the transformer is dead.

author: glarsson
author_rank: Frequent Contributor
timestamp: June 27, 2018, 08:51:15 pm
content: Note that the D4 bridge rectifier in the first post schematic is drawn wrong. The diodes are facing the wrong direction.QuoteI was a bit confused when you previously wrote that the primaries measured 0 ohms. Open circuit makes more sense, but you still mix the unmixables (0 ohm and open circuit).

author: abraxa
author_rank: Frequent Contributor
timestamp: June 27, 2018, 09:16:00 pm
content: I think it's time you explain to us how exactly you're measuring. As glarsson points out, "0 ohm" and "open circuit" (infinite ohm) are mutually exclusive, so it's unclear what you mean and how you come to that conclusion. Ideally, you'd write "0 ohms" instead of just "0" to avoid misunderstandings. They could stem from the assumption that maybe you're measuring "0 V".You saidQuote...which, if taken as ohms, suggests that the transformer windings are conducting as they should.Also, you saidQuote...which would be 0V if one of the primary windings was broken since for 240V, one windinding is connected using 110V and the other in series using 127V configuration. Meaning that both need to be conductive for any current to flow and the transformer to do any work. Come to think of it, though, you write "VDC". A transformer outputs an AC voltage, however. Please repeat the measurement using AC range.Either way, I still think the transformer is fine and the circuitry is at fault. I'm looking at the two ICs, Q1/Q3 and C1/C2/C4/C5 in particular, since a fault in one of those would short the transformer outputs directly.By the way, one rather direct way to test the transformer is to power it up and listen. Does it hum at all? If not then it's most likely broken - if it does, the primary side is working at least.

author: aabbcc
author_rank: Regular Contributor
timestamp: June 27, 2018, 09:30:02 pm
content: Quote from: glarsson on June 27, 2018, 08:51:15 pmMy bad, my multimeter just said 0 so I interpreted it as 0 ohm : [Quote from: abraxa on June 27, 2018, 09:16:00 pmThe voltage measurement, again I mixed up dc and ac when writing the post, so for the secondaries I was getting 1-4 AC voltage depending on which of the 8 output points of the transformer I was measuring.Apologize, got it mixed up, measuring right now and getting 240 v AC between N and L.Some more AC voltage measurments:L-point 1: 240L-point 2: 240L-point 3: 240L-point 0: 0L-point 4: 0L-point 5: 240L-point 6: 240L-point 7: 240L-point 0 (second one in photo): 0N-point 1: 0N-point 2: 0N-point 3: 0N-point 0: 240N-point 4: 240N-point 5: 0N-point 6: 0N-point 7: 0N-point 0 (second one in photo): 240I also as suggested in a previous post desoldered D4 (bridge rectifier) and am still getting only around 1 volt ac at the secondaries of the transformer that connect to the unsoldered bridge rectifier.And also there is no audible hum at all, the transformer is dead quite when powering the psu.

author: janoc
author_rank: Super Contributor
timestamp: June 27, 2018, 10:06:14 pm
content: Quote from: aabbcc on June 27, 2018, 09:30:02 pm0 ohm = dead short. That's why your are confusing the heck out of everyone. What does your meter show in ohms range/continuity mode when you touch the probes together?Most meters will show something like "0L" for "overload" when seeing open circuit. Zero ohms (a short) would be indicated as an actual 0.0, with no "L".

author: aabbcc
author_rank: Regular Contributor
timestamp: June 27, 2018, 10:16:49 pm
content: Quote from: janoc on June 27, 2018, 10:06:14 pmIn resistance/continuity range when touching the probes togheter I get000.04 ohmwhen they are not touching each other the meter reads.0L

author: janoc
author_rank: Super Contributor
timestamp: June 27, 2018, 10:47:41 pm
content: Quote from: aabbcc on June 27, 2018, 10:16:49 pmYep. So the first one is what is "zero ohm"/dead short and the later one an open circuit condition. You should clarify where did you get which measurement.

author: aabbcc
author_rank: Regular Contributor
timestamp: June 27, 2018, 11:11:29 pm
content: Quote from: janoc on June 27, 2018, 10:47:41 pmThanks, clarified the previous post where I wrote 0 ohm, it should have been no connection/open circuit

author: Anks
author_rank: Frequent Contributor
timestamp: June 28, 2018, 06:37:18 am
content: The simplest way forward here and to prove out the transformer is to remove D1 and D4 diode bridges and measure for ac at the transformer leads. If no voltage on the primary check that you have continuity through the voltage selector. If the voltage on the primary but not on the secondary check resistance from the two points that AC reaches the transformer. If that is open you have a open primary. If this measures good then you should see output on the secondary if the secondary is good. Remember to be careful on the primary side and do this without the diode bridges D1 D4 in circuit.

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 07, 2018, 02:45:28 am
content: Hi!There seems to be a lot of confusion worse confounded going on here!The simplest test I recommend is to remove the bridge-rectifier diode assemblies trom the power supply PCB and connect a 24V 2.2W m.e.s. bulb (about 100mA rating) across each winding and power the mixer's transformer up - the bulbs should light full brilliance without any signs of distress from the transformer such as buzzing, running hot, smells, etc.The d.c. resistance of each primary should be about 10-50 ohms total and about 1-3 ohms between the 110V and 127V tappings, and about 2.5-5 ohms for each secondary winding - each supply section has two sections of identical secondary in parallel.Service Manual:-https://www.dropbox.com/s/0uhrngb2ha1qj99/sam_82.pdf?dl=0Make sure the mains transformer is OK by trying two bulbs on it as per my suggestion then we can look at what to do next!According to the Service manual, the internal power supply provides ±15V at 150mA so a small 24V 30W R-Core Transformer such as:-https://www.ebay.co.uk/itm/115V-230V-30W-R-Core-Transformer-for-Audio-AMP-Amplifier-Preamps-DAC-24V-24V/232182678223?hash=item360f2a82cf:g:TNwAAOSwJ7RYTtdZ. . .can be used to repair your Mixer, altho' you would have to drill mounting-holes for it on the bottom metalwork and probably extend the leads to connect to the voltage selector and the PCB.This suggested transformer provides 0.62A per winding, so allowing for the 0.5 x de-rating factor needed for bridge rectifiers, it is capable of providing 320mA D.C., which meets the psu spectfication of 250mA max. given in the S.M.Chris Williams