Some questions about an old school Yamaha Amp - Page 1
Forum section: electronics / repair

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 27, 2020, 06:56:03 am
content: This is a very old and rusty amp, model is AVC-30. I Was told that it had no sound at output posts but it turned out giving sound though very coarse and unstable. I gutted everything off from the chassis for convenience. It's still very inconvenient because too many cables connecting all the different boards here and there and, what's worse, they are mostly fixed connection so no way to disconnect and redo to make it neater. It's a total mess on the desk.With some fiddling, it now stopped giving sound (possibly poor contact somewhere or short circuit inadvertently at one point). I'll try to figure it out. But I had a question when I first started to diagnose it.This is a model with a mains voltage selector (for the main transformer) suitable for 110/120/220/240V mains. This is all good. What's strange to me is that it's standby transformer does not have a voltage selector (I don't think the one for the main one can work with it as well). So when the unit is connected to mains of 240V it will 240V as well. According to the manual, its secondary side should give 12.3V AC. What I have is about 30V. Also I observed that, when the unit is on, this standby transformer gets extremely hot. It must have been like this for very long. if not its entire life so far when on, because the part of the case above it can be seen a sign of prolonged exposed to hot air). Since I don't need other mains voltages any more, I removed the voltage selector, shorted the relay switching the main transformer. Then I'm able to reconnected the primary of the standby transformer to one of the now-redundant wires giving roughly 110V (using the main transformer as an auto-transformer). This way I got a close to 12.3V voltage (15V or so) at the secondary side. Now it does not get very hot any more (the reason is apparent) but the boards downstream of it seem working as they used to.So my questions now are -- is the standby transformer intended to be directly connect to 240V AC if this is the mains voltage? Is the standby transformer likely to get very hot under the normal working mode?(Its service manual ishere)

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 27, 2020, 06:56:37 am
content: More photos

author: orbanp
author_rank: Regular Contributor
timestamp: September 27, 2020, 03:34:12 pm
content: Chances are that the standby transformer is sized for 240V.When only fed by 110V or so, it still provides enough input voltage for the linear regulator that follows it.When fed by 240V the linear regulator dissipates more power (close to twice) but that is probably well within its capabilities.Regards, Peter

author: SeanB
author_rank: Super Contributor
timestamp: September 27, 2020, 06:33:46 pm
content: Had the same issue with a really fancy lightbulb, which has a 12VDC power supply, generated by using a small 30VAC 1W transformer to still enable the linear regulator to deliver 12V even feeding 100VAC into the primary. Transformer well cooked, so I replaced it with a slightly used 18VAC one, and a new filter capacitor, also well cooked, for some reason, living between the hot transformer and a suffering Zetex darlington that did the linear regulation. Powered right up, and now I have a fully working fancy light, with absolutely no use for something that is both obscenely expensive and quackery. Now sits in the cupboard, bought cheap on auction as "not working, bulb faulty", but still has the original lamp in it, 12V 100W halogen, used to make polarised light at an obscenely poor efficiency.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 27, 2020, 10:07:11 pm
content: Quote from: orbanp on September 27, 2020, 03:34:12 pmThanks. That's probably the case. I have isolated this part and will do a bit more test and check with this module (not fore repair really but for learning purposes). I would not imagine that Yamaha engineers cocked up with this simple thing. I was VERY curious, though, why it gets that hot to a point it keeps burning the case above it? It's not too hard or too much cost to use the specs of one notch higher for this standby transformer.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 27, 2020, 11:15:31 pm
content: Quote from: SeanB on September 27, 2020, 06:33:46 pmI don't understand why Yamaha would have chosen to have the standby transformer so cooked when the unit is on. The model seems to be of good solid construction (with solid aluminium facia) which is not compatible with an obscenely austere design.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 29, 2020, 09:14:55 am
content: Now I understand that the standby circuitry is ok with a quite wide range of voltage from the secondary side of the standby transformer, which is rectified (half-wave) by D121. Because of the Zener diode Z123 which fixes the base voltage of Q118 to 11.5V or so, and because of the relative fixed Vbe of Q118 (~0.7V), the emitter voltage of Q118 can be 'regulated' to be around 12.2V with a wide range of its collector voltage as long as it is not cut off. In other words, the range of 110V-240V AC mains for the standby transformer is totally fine.This might be very straightforward to experienced eyes but it took me some time to figure out.Q118 is D1090. Use this trick, maybe I can make a very simple adjust power supply by replacing D123 with a variable resistor (voltage divider)? If using the main transformer of this unit which gives a secondary voltage of 82V or so, it seems the DC output can be roughly between 1V-80V DC?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: September 29, 2020, 09:38:20 am
content: The same trick is used here to create the symmetrical +/-15V DC rails.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: October 02, 2020, 11:27:38 pm
content: One more question, what does the specs of the main capacitor “65WV” mean?

author: oPossum
author_rank: Super Contributor
timestamp: October 03, 2020, 01:19:40 am
content: WV is Working VoltageThe voltage must not exceed that.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: October 03, 2020, 03:57:03 am
content: Quote from: oPossum on October 03, 2020, 01:19:40 amThanks!