Vintage audio (Sanyo) - one channel audio - Page 1
Forum section: electronics / repair

author: michalism
author_rank: Regular Contributor
timestamp: November 17, 2018, 04:03:59 pm
content: HelloI am trying to restore a Sanyo G-2601KCA music centre. From what I know it's from the 70's.My problem is that only the right channel is working. Left channel is almost dead.I do not have service manual, so I'm working in blind mode, and being a newbie, this is a real challenge!Anyhow, I think that the output stage is working, because none of the output transistors seems shorted and if I apply a signal to the base of the transistors I hear a buzz on both channels.I checked where the audio signal is fed to the amplifier board, and the signal on the left channel is too weak! I tried two sources - radio and tape.that is why I am suspecting a problem with the small board holding the volume and balance controls in the front.What I am looking for is below:1. any potential sources for a service manual? I tried all the ones described in the sticky note, but I was not able to find anything2. how can I eliminate the possibility of a bad power transistor? the unit is using germanium transistors and finding replacement parts is not easy, and I do not want to buy replacement parts unless it's absolutely necessary.3. Can these old green polyester caps go bad? is it possible that they might be my problem.I forgot to mention that I already replaced some leaky electrolytic capacitors.Any advice would be much appreciated!!!Thank you very much in advance!Michalis

author: madires
author_rank: Super Contributor
timestamp: November 17, 2018, 05:01:38 pm
content: You can check the BJTs with the diode test of your DMM, i.e. one diode between B and C, and a second one between B and E. The polarity depends on the type (NPN/PNP). There are several videos on YouTube about this simple check. If you have a signal tracer or a scope then follow the signal path starting at the source. This way it's easy to find the stage causing the problem. The green polyester caps can go bad sometimes (cracked or white tarnish).

author: michalism
author_rank: Regular Contributor
timestamp: November 17, 2018, 06:18:57 pm
content: Quote from: madires on November 17, 2018, 05:01:38 pmThat's what I did to check the transistors, and they read fine. Also all transistors measure the same, so that is one reason why I believe the problem is elsewhere.I am using a scope to trace the signal. I am going backwards now, but all indications point towards the small board with the balance controls...Thanks for taking the time to reply

author: commongrounder
author_rank: Frequent Contributor
timestamp: November 17, 2018, 07:00:42 pm
content: Check or replace every electrolytic capacitor you can find. They are cheap, especially the small values. Caps that are in the signal path that have dried out, gained ESR, or otherwise lost value, will cause a drop in signal. Good luck with the restoration!

author: Twoflower
author_rank: Frequent Contributor
timestamp: November 17, 2018, 07:18:18 pm
content: Check also the mechanics. Including switches and variable resistors like volume balance, ... I've also a Sanyo amplifier having some trouble with them.

author: michalism
author_rank: Regular Contributor
timestamp: November 17, 2018, 07:25:22 pm
content: I am a little hesitant to swap out all electrolytics. I did this once in an old HMV radio receiver and it created more problems than solved.I am checking the capacitors and replacing all those that are out of range or have large ESR.From a mechanics point of view I can't find something being completely off, can you please be more specific?I checked for cold joints, I found none.All potentiometers are cleaned, the sound is really smooth and there's no intermittent break, so I tend to believe it's a broken cable/ track or a complete failure or a part.

author: Twoflower
author_rank: Frequent Contributor
timestamp: November 17, 2018, 07:44:58 pm
content: I haven't looked which one is the definitive culprit on my amp (JA-220). But I have also one channel failing; wild switching usually helped. If I get it running, it works as it should. That's the reason why I never fixed it, it was just never annoying enough. The amp is currently not in use anymore.But I think that's the source-selector that not switches one channel reliable. But also the balance adjust has some trouble. I have the feeling that all the switches and variable resistors build-up some oxidation layers or other kind of defects.

author: floobydust
author_rank: Super Contributor
timestamp: November 17, 2018, 08:00:31 pm
content: One post of a fuzzy portion of the schematic, enough to see signal flowfrom post #25.The amplifier is basic, only a few transistors and not all germanium. You can take voltage measurements and just compare the two channels to find the trouble. There are only a handful of electrolytic capacitors, if you take careful pictures then replacing them can go smoothly.

author: michalism
author_rank: Regular Contributor
timestamp: November 18, 2018, 07:06:11 am
content: Thanks for the schematic! It should help a lot!

author: michalism
author_rank: Regular Contributor
timestamp: November 18, 2018, 03:54:11 pm
content: ok, I may be getting a little closer to a faulty part.The amplifier part has 4 transistors, let's name them Q1-Q4, Q1 being the closest to the input signal.I rechecked the input stage of the amp, and at least for the radio signal, both channels were at the same level. But the transistor on the right channel seemed dead. After poking around, I noticed that touching Q3 produced a buzz on the right channel!Before that, I measured some voltage (I think 1-2V) on the base of left channel's Q1, but there was no voltage on the base of right channel's Q1.All passives on the amp board are ok, also all transistors give the correct readings on my DMM.Is it still possible that the transistor is at fault?The transistors are 2SC537-D.What would be a suitable replacement?Should I change all transistors in one go, or go slow?

author: floobydust
author_rank: Super Contributor
timestamp: November 18, 2018, 05:42:57 pm
content: 2SC537 is an general purpose part nowadays with no special requirements here.NPN silicon planar 20V 0.1A, Group D is hFE unknown, I could not find a datasheet. Japanese ECB pinout.I would considerKSC1845(2SC1845).If you are willing to bend the leads, transistors having EBC or CBE pinout could also work.Example BC546,547,548,549,550 in group B or C for hFE. I would go with medium gain and low noiseBC549B or BC550B.It all depends on what transistors are easy to get in your locale.Just replace the bad transistors as they fail. Sometimes they go noisy or open-circuit.I think the electrolytic capacitors are pretty old here, at 45 years there are only a handful of 1,10,47uF signal-coupling parts, so I would just change them.

author: michalism
author_rank: Regular Contributor
timestamp: November 18, 2018, 07:07:02 pm
content: Quote from: floobydust on November 18, 2018, 05:42:57 pmGreat, thank you very much! I will use this info to pick the proper replacement.Is it possible for a transistor to measure ok on the DMM, but still be noisy?

author: michalism
author_rank: Regular Contributor
timestamp: November 19, 2018, 08:52:56 am
content: Hello!I ordered a bunch of BC546C's to test. I was not able to find any KSC1845 or ZTX1051A that got suggested from another source. I will go to another store when I get a chance, but I'll first check the ones I ordered.I checked in my parts bin, and found several transistors that seem to be a match. For example, C1815, C945 and S9014 all have specs that seem within limits and have hfe>500.I'm tempted to give them a try, but then I'm afraid that I may cause more issues.What do you think?Also, what worries me is that the transistors in my parts bin are from ebay, I just got them to play with, and not really use them in mission critical situations...

author: floobydust
author_rank: Super Contributor
timestamp: November 19, 2018, 06:34:43 pm
content: You gotta have fun fixing this old gear and be open to learning. Just keep the soldering iron away from melting the dial-cord lol. Take PCB pictures so you don't forget pinout/polarity.If you have transistors that test good, try drop one in. eBay and Aliexpress are known for fakes so sometimes a bad part can confuse a person when doing a repair.The Sanyo pre-amp stages are two transistors working together, so any one being bad can upset the other.The output stage power transistors are the high failure-rate parts and the ones that can make smoke.The pre-amp parts really have no danger if thety get messed up.

author: michalism
author_rank: Regular Contributor
timestamp: November 20, 2018, 04:23:39 pm
content: For sure I'm having fun! :pBut now I am getting to a point that need some more directions...I swapped out the suspicious transistor, but the problem is still there, and I do get the same behaviour when I poke it with an insulated tool.In the attached picture, this is Q4.I tried to trace the incoming signal on both channels. The difference I found is that the right channel leaves the emitter of the transistor (Q4 in the pic) with a DC level added to it, while this is not the case for the left channel. I mean the signal has no dc component after it leaves Q2.Also, I took some measurements with respect to ground on Q1 which is the transistor that receives the power for the board.Right channel: Vc= 25.4, Vb = 6.6, Ve = 6Left channel: Vc = 25.4, Vb = 3.2, Ve = 0,69As you can understand, I am confused, and i'm not sure what to look for.All capacitors are measuring fine, and all resistors also are within limits.I have the schematic of the system that I have found in another forum, but it is too blurry to understand and I think it is a little cropped. I am attaching it in case any of you can share some advice!Thank you

author: floobydust
author_rank: Super Contributor
timestamp: November 20, 2018, 07:30:26 pm
content: The signal flow is in your picture is from right to left.Signal comes in, goes through Q4, Q3, Q2, Q1 and then to the output transistors.Q4, Q3 are part of the pre-amp section, which then goes to drive the volume/tone/balance controls and that signal then comes back into Q2, Q1. I can figure it out if you post a picture of the other side of the pcb.The left channel Q1 measurements indicate Q1 is bad. Base voltage 3.2, emitter 0.69V is wrong, the E-B voltage should be at most 0.7V difference if the transistor is biased on. So I think Q1 is open E-B junction but I would check Q2 as well. The two parts are connected together.If you give voltage measurements for all 4 parts, it's easier to take a guess.There's two ways to troubleshoot - take a multimeter on diode-test and check the transistors. Or measure voltages with power on.

author: michalism
author_rank: Regular Contributor
timestamp: November 21, 2018, 06:08:26 am
content: Thank you! This is what I needed, what path to start with, because my measurements are incorrect both at the signal entry, as well as the power input.The transistors measure fine, but I can change them anyway and see if it makes a difference.Q2 is new from my parts bin, but it is from ebay, so I will put another one in just to make sure.I will post more measurements and pictures later today.

author: michalism
author_rank: Regular Contributor
timestamp: November 25, 2018, 08:15:59 am
content: UPDATE:I am almost sure that the problem is not with the transistors, but in any case I will replace all transistors of that channel to see what difference it makes.Unfortunatelly, I've been battling with a nasty cold for the most part of last week, and I made no significant progress. I'm almost back to normal today, but we are expecting some visitors at home so there's no way I can convince the BOSS to lock myself in the "lab" and get busy with my "work"...Next week does not look promising either, due to the actual work being piled up in the office.so, please hang in there, I will do my best to provide an update as soon as possible!

author: michalism
author_rank: Regular Contributor
timestamp: December 07, 2018, 04:42:28 pm
content: Hi again...back after a long break...Eventually, I swapped out all old transistors from the right channel, and replaced them with brand new C1815.Nothing changed...and all old transistors seem to be fine by all multimeter checks. I do not have a component tester to be absolutely sure.Anyhow, I did some voltage measurements on every transistor and put them side by side in the attached file.Measurements with a single letter are with respect to ground, and with two letters are the voltage between the respective pins (BC, BE and CE).Since all measurements are completely off, I am confused what to check for. Initially, I was suspecting that the biasing of Q4 might be the problem, but I did not find a shorted or open component around that area.I do not want to randomly start checking every single component to find the fault. I am trying to understand the logic and method around this problem...so any kind of help is more than welcome!!!Thank you!

author: michalism
author_rank: Regular Contributor
timestamp: December 23, 2018, 09:28:28 pm
content: Hi again, and very warm season greetings!!I have been measuring the resistor values on the amplifier board, and I am noticing a strange behaviour from a particular resistor.There's a 100k resistor that measures correctly out of circuit, but in circuit it's resistance rises quickly.What may be causing this behaviour?The corresponding resistance on the properly working channel measures 10k in circuit.I am trying to work-out a draft schematic, but in the mean time, I would appreciate some help!Thank you!

author: floobydust
author_rank: Super Contributor
timestamp: December 23, 2018, 10:36:50 pm
content: It's kind of a difficult problem, if the transistors are OK.Q4 and Q3 work together, Q4 is off instead of being biased on, and has low collector voltage. Q3 is in the same boat. I would check Q3 for C-E short. Otherwise, a problem with collector power for Q3. That has a 6.8k resistor feed that go to the middle R-C filter network for power to both channels. Then a 1uF capacitor on the collector which then goes to the volume control "A" if I am reading the fuzzy schematic right. That cap I would check too.Normal for the 100k resistor in-circuit reading to change as the nearby caps charge up due to the multimeter injecting some current for ohms.A pic of the PCB backside will make the schematic easier to draw.

author: michalism
author_rank: Regular Contributor
timestamp: December 23, 2018, 11:50:10 pm
content: Your reading of the schematic is correct, this is what I am tracing with the DMM. I am uploading the pcb backside as you're requesting. Please note that all transistors are replaced on the faulty channel, so are the electrolytics. I have double-checked the transistors, but I will do it again!I see some pads that are ready to be ripped off the pcb, but all connections are fine, so I do not think this is my problem right now.The 6.8k resistor is checked ok...Also, you mention: "Normal for the 100k resistor in-circuit reading to change as the nearby caps charge up due to the multimeter injecting some current for ohms."My question is why the same resistor on the other channel does not change? I understand the effect of the capacitor, but I am getting this behaviour only on the faulty channel...Thanks for your time!

author: michalism
author_rank: Regular Contributor
timestamp: December 26, 2018, 11:24:04 am
content: Hi!I forgot to upload the backside of the pcb...sorry for this

author: floobydust
author_rank: Super Contributor
timestamp: December 28, 2018, 12:01:20 am
content: I flipped your PCB pic so it's a top-view and I drew in some stuff.The blk? wire return from the left channel volume control/tone controls I do not see in the coax cable.red- L out to volume/tone controlblk- L in from volume/tone controlgrn- R out to volume/tone controlwht- R in from volume/tone controlshield- GNDThe resistor you changed does not seem to go from Q2-C, it goes to Q2-E which I think is a problem. It should go to the pad/hole just before.{ it's OK just the pic at a bad angle}It looks like a 27k ohm part on the right channel, but not what you used. Can you check.It's 22k on other picturesedit: fixed mistakes

author: michalism
author_rank: Regular Contributor
timestamp: December 28, 2018, 02:26:07 pm
content: wow!!Much obliged and really amazed by what you did!The black wire is "hiding" under the red wire, but it is there.I am not really sure that I correctly understand what to check. I have replaced a 22k resistor that goes to Q2-C. I have circled it with a blue mark in the attachment. It's also the one with a slightly lifted pad. I suppose this is the one you're asking to be checked (?). I am confused as I do not see an alternative way to connect it, and besides I have marked the spot before I removed the old resistor...checked with the DMM too and it is indeed connected to Q2-C.I have also replaced the "drifting" 100k resistor (in red circle), but this is related to Q3 & Q4 so I assume that's not the one you wanted to be checked.EDIT: I am including a backlit picture of the top side of the board in case it helps. I have again marked the replaced resistors. Sorry it's a little shaky, I will try to get a better shot later tonight.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: December 28, 2018, 04:52:44 pm
content: Quote from: madires on November 17, 2018, 05:01:38 pmWith power transistors it is quite common for a bad one to pass both those "diode" checks but to be shorted C to E.Sent from my SM-G900V using Tapatalk

author: michalism
author_rank: Regular Contributor
timestamp: December 28, 2018, 05:25:46 pm
content: Does not seem to be a problem with the power/ output transistors. If I feed the signal from the right channel to the left one, I get sound. So the output stage is working.

author: Andrewausfa
author_rank: Contributor
timestamp: December 29, 2018, 11:27:58 am
content: Just thinking, have you thought about using a signal tracer to find where your signal is disappearing? Feed an audio tone in at your input and work backwards.

author: michalism
author_rank: Regular Contributor
timestamp: December 30, 2018, 02:58:38 pm
content: Yes. I traced the incoming signal and it is lost, or not amplified right after it enters the pre-amp section.we are trying to figure out why Q3 and/or Q4 do not turn on.

author: floobydust
author_rank: Super Contributor
timestamp: December 31, 2018, 10:25:12 pm
content: Old Japan consumer electronics have clever ways to get the most out of each transistor. I guess it's from the transistor radio days post-war.I drew a partial schematic to make this all easier. The pictures are helpful.I think the 6k8 resistor on Q3-C is bad for the L channel. It's a bit strange, the 6k8 looks like 680R on the other channel? - red and brown and orange sometimes look the same on these resistors. My point is the Left and Right channels should have the same part values.Voltage measurements at the power RC filters could also help.I couldn't see the part leads on the new 22k, so I got that wrong - disregard all that, I'll fix the post.There might be troubles further down the line, as Q1 still has low voltages I think.

author: michalism
author_rank: Regular Contributor
timestamp: January 02, 2019, 06:53:05 am
content: This partial schematic is really key!It took me a few minutes to find the fault after checking your schematic!!!There's a broken pad interrupting the path to the 6k8 resistor of the left channel!The schematic helped me to understand the way the circuit is powered, and I suppose a more experienced troubleshooter would have fopund this much sooner and without the need of a schematic.My mistake was that I was considering the 25V were coming from the power supply, and paid no real attention to the common resistors (5k6 and 820 Ohms).Anyhow, I patched the track and now I have audio on both channels!!!That proves that the old transistors are working fine, but I will leave the new ones ion unless I notice any difference to the audio on each channel.There are still other issues I need to address, but I think this is a huge leap!!!I really appreciate the feedback from all, but extra credits to floodydust for the schematics and the analysis!!! Can't really express my gratitude!