Yamaha G100 B212 dc offset

[glebert]

5c. since the collector of TR18 is lower than the collector of TR24 the output is not forward biased. Usually there will be about 0.5 to 0.7V across the base-emitter junction of each of the transistors, so the base of TR23 (which is also the collector of TR21) should be about 0.6V higher than the emitter (which should be about equal to the collector of TR24), the base of TR21 (which is the collector of TR20) should be 0.6V higher than that, the base of TR20 (which is the collector ot TR18) should be 0.6V higher than that. So the collector of TR18 should be about 1.5-2.1V higher than the collector of TR24/

Can you check the voltage across D1? That is not actually a single diode but a diode assembly that is about equal to 3 diodes. Beyond that I would measure voltages at all terminals of TR17, 18, 19 since that TR18 collector voltage is quite a bit lower than the service manual gives.

[vintagekiki]

Please let me know if it would be worth me starting a new thread as we've moved away from the original query in the subject line! Thank you!

No need to open a new thread. The topic thus stretches and loses content.
The title of the existing tread is "Yamaha G100 B212 dc offset" and it covers the problems of Yamaha G100 B212.
Go ahead, keep explaining Yamaha problem, and with the help of forum tips, you will fix your Yamaha.

[Rhod]

Thank you, glebert - when I finish work today I'll check out those values. That diode array component is interesting - I've read another thread about how to fabricate an alternative to that hard to find item. I've got a couple of similar ones (possibly the same) in an old Yamaha mixing desk that was also the subject of a thread and fixed with the support of the folks here (and is now fully hooked up to my son's synthesisers/audio interface) resplendent with its mock vintage amber leds lighting up the VU meters! But I digress.

vintagekiki - that's a really kind and encouraging post. Appreciated. It's that sort of spirit that some (including me) have noticed more keenly during lockdown: communities being supportive, valuing relationships above 'success' and therefore going the extra mile for others. Am I extrapolating a bit much? Maybe, but hey!

[g1]

Are you still on the limiter lamp? If so, it is for idle conditions only. You can't make any conclusions about audio signal when using the lamp.
If it is staying dim, then you don't need it anymore.

[Rhod]

Good thinking, g1 - but no, audio testing done plugged straight into the socket, and readings below also taken without the lamp limiter.

Voltage drop across D1 = 1.6V (53.1V/51.5V)

B C E
TR17 88.9 90.7 91.8
TR18 89.7 50.7 90.6
TR19 89.8 34.3 90.8
TR20 50.7 94.4 53.9
TR21 53.9 94.4 53.9
TR22 53.3 0.01 54.8
TR23 53.7 94.5 53.7
TR24 0.0 54.2 0.0

Comments:
Well, some of those don't look right! Like the 0V on TRs 22 and 24...
Base of TR23 is connected to emitter of TR21 (not collector)
Some of the B-E readings are close-ish to the value glebert mentions (0.5-0.7V) but others some way out e.g. same voltage at B and E of TR23, and the expected voltage rises not apparent or reversed e.g. TR24 C to TR 18 C shows a 3.5V drop rather than a 1.5-21.V rise.

[glebert]

If you pull out TR23 and TR24 does the collector voltage of TR22 go up to something above zero?

[Rhod]

No, remains stubbornly at zero with those transistors pulled...

[Dave H]

No, remains stubbornly at zero with those transistors pulled...

Then TR22 is suspect. Its base is 1.5V lower than its emitter but it has no collector current.

[Rhod]

Looks like you are on to something, Dave. TR22 measures open circuit across EB (both directions). It's a 2SA483 in a TO-66 package. Not available (nor any of the equivalents) at any of the usual trusted names. I could take a punt on something like this:
https://www.ebay.co.uk/itm/2SA483-TR...AAAOSwUIle6RFD
or this:
https://www.ebay.co.uk/itm/2SA483-Tr...sAAOSwR-9eFI54 who is a UK seller

or is there an alternative you would recommend?

Thanks!

[The Dude]

I wouldn't have problems buying the part from an eBay seller, as long as 1) it's a reputable seller, and 2) it's a NOS original part and not a Chinese fake.

[Enzo]

I have to go to storage anyway, I will see what I have. I know I have 100 2N6212

But you can probably use a TO220 in its place.

[Rhod]

Enzo - that's really kind of you. I know you know this, but just in case - I am in the UK, quite a long way from Lansing, Michigan!

The Dude - I'm with you. In fact, I haven't got any problem with buying from eBay sellers whose reputation I don't know: I know I am taking a calculated risk. But with things like electronic components it's very difficult to tell whether the products are fake, and the only thing to go on with the sellers is their eBay feedback, which may not be 100% reliable. Having said that, one of the links does have an address/contact details and a separate website. Anyone in the UK had experience of www.littlediode.com ?

Thanks, all.

[teemuk]

I would be hesitant to trust the service manual figure for +CO. It's probably a mistake.

If you look at it, to ensure equal output voltage swing towards positive and negative directions +CO should be approximately half of +B voltage. About 50 volts is about half of 98 volts. 31 volts IS NOT.

Note, for example, that service manual for series II (same power amp) suggests +CO of 45V for G100 (with +B of circa. 90V) and +CO of 32V for G50 with much lower +B (circa 60 volts).

[Dave H]

If the schematic is correct then 31V for +CO has to be a mistake. The schematic has TR18 collector at 56.6V so +CO can't be 31V as it's only 3 x 0.7V below TR18 collector.

[Rhod]

That all makes sense...and is quite reassuring, thank you. It didn’t make sense to me that the +CO was meant to be 30 odd V yet everyone on this forum seemed quite content to discuss the 50+V figures that I was finding...