Yamaha Electronic Organ Main Amp Circuit. Fault Somewhere. - Page 1
Forum section: electronics / repair

author: abrogard
author_rank: Newbie
timestamp: December 20, 2018, 05:26:08 am
content: Can anyone help me fix my Yamaha organ? It emits a loud 'white' noise when switched on. And I find fuse 3 on the main amp board was burned out and burns out again when I replace.I don't know enough electronics (virtually none) to be able to figure out what to look at next.

author: commongrounder
author_rank: Frequent Contributor
timestamp: December 20, 2018, 10:42:26 pm
content: Do you own a multi-meter? It’s pretty likely that at least one or both of the output transistors (tr6 and tr7) in the amplifier circuit have failed. You are going to need a meter to look for shorted devices. Don’t replace the fuse again until you’ve corrected the problem, to avoid stressing the power supply.

author: abrogard
author_rank: Newbie
timestamp: December 20, 2018, 10:50:03 pm
content: Yes, I have a multimeter. Not sure how to use it to test the transistors though.And I'll have to pull the chassis out to get at the board and would it work then if plugged into the mains but the 'input' and 'output' plugs not plugged in?I've got pics of the machine and you can see the location of the board down there at bottom left of the cabinet.https://imgur.com/a/DZGxouzp.s. I see these very cheap DSO's on ebay and such. Would I need one of those?

author: bsfeechannel
author_rank: Super Contributor
timestamp: December 20, 2018, 11:30:38 pm
content: What is the model of the organ?

author: DaJMasta
author_rank: Super Contributor
timestamp: December 21, 2018, 12:00:13 am
content: With a multimeter you can check using resistance or diode modes to see if transistors are shorted - which is a symptom of most total failures. Just check each pin with both of the others, looking for shorts.

author: abrogard
author_rank: Newbie
timestamp: December 21, 2018, 12:56:49 am
content: Quote from: bsfeechannel on December 20, 2018, 11:30:38 pmBK7iQuote from: bsfeechannel on December 20, 2018, 11:30:38 pmthanks for that. will do as soon as I've got time free of all these xmassy things...

author: bsfeechannel
author_rank: Super Contributor
timestamp: December 21, 2018, 01:54:10 am
content: Quote from: abrogard on December 21, 2018, 12:56:49 amThanks. I saw your posts on other forums. It would be a good idea if you could publish the whole service manual with the complete schematics, not only the circuit you posted above. Although in good probability the problem, I think, is related to the output transistors, as mentioned by the other members, there must be stuff connected to connectors US1-S and US2-S that we need to rule out as a source of problems.

author: abrogard
author_rank: Newbie
timestamp: December 21, 2018, 03:21:24 am
content: Happy to.I've put it here. It is a pdf file:http://abrogard.com/If you just click on the link on that page it should start downloading. I tested and it worked for me.

author: bsfeechannel
author_rank: Super Contributor
timestamp: December 21, 2018, 04:24:15 pm
content: Cool. With a bit of luck, it won't be necessary to pull the chassis out, at least for the time being.Before we proceed, let me ask you a question. The loud white noise you heard was continuous or just a burst?Meanwhile, show us, please, a close up picture of your multimeter so that we can see its range switch, its binding posts and the display in detail, together with its leads (but not connected to the binding posts).

author: abrogard
author_rank: Newbie
timestamp: December 21, 2018, 09:25:03 pm
content: That's pretty easy. There's pics on the web. It is a DT-9205AAnd the noise? It is continuous. We left the thing on all night by accident and the next morning it suddenly burst into life with this loud continuous noise. And then it would do it again whenever I switched it on, with only a few seconds before it began during which I found I could press a key and get a note just as though it was okay.https://www.petervis.com/meters/Excel_DT9205A/Excel_DT9205A.htmlMine doesn't have the 'auto power-off', unfortunately. And that push button on/off means tossed in jumbled work bench drawer it can all too easily get switched and drain the battery away..... I found out.... the hard way....

author: bsfeechannel
author_rank: Super Contributor
timestamp: December 22, 2018, 02:37:50 am
content: Quote from: abrogard on December 21, 2018, 09:25:03 pmBrilliant. I have exactly the same meter.QuoteThat piece of information is important and will help us understand what is going on later.QuoteAlright. Let's get your meter to work.First off, for your safety, please, turn off the organ, if it is not already, anddisconnect the power plug from the wall socket. There are hazardous voltages around that area of the circuit, even with the organ turned off.Now turn the range switch of your meter to the 20K position (next to the Ω sign). Connect the black lead to the COM binding post, and the red one to the VΩ binding post. Turn on your meter on and short the probes out to make sure it is indicating zero or close to zero ohms.If that is ok, disconnect both the "IN PUT" and "OUT PUT" plugs from their sockets on the board. That way we will prepare it for probing.Now, let's concentrate on the "OUT PUT" socket. What you probably have is an octal socket. Locate a notch in the center and count the number of pins like in the picture below.Connect the meter probes to the pins and report what you read:Black probe 4, red probe 3Black probe 4, red probe 2Black probe 4, red probe 8Red probe 4, black probe 3Red probe 4, black probe 2Red probe 4, black probe 8We say pin, but of course the socket has holes. If you find it difficult to insert the probes due to the small diameter of the holes, and/or the constraints of space, we will have no choice but to pull out the chassis.In that case it will be better to probe the socket from the underside, in which case the numbering of the pins will follow what you have in the schematics (just flip the picture above and count 1, 2, etc. clockwise from the notch).If you do that you have to take care not to bend any components or change the trimpots (those with a red blob of paint).We're probing for short-circuits. So good electric contact may be established between the probes and the pins.

author: abrogard
author_rank: Newbie
timestamp: November 23, 2019, 11:51:56 pm
content: Hi.......... I'm back again. 11 months later. Sorry. Don't know how I lost touch in the first place but I do know I've had a hell of year this last year.So I'm about ready to have a go at this organ again.Just wondering if this 'connection' is still good or I should start another thread?