Yamaha G100 B212 dc offset

[vintagekiki]

2. What's not good:
a) Amp puts out a horrid low volume distorted sound, all inputs.
b) The CO+ terminal voltage referred to on page 3 of the service manual reads 50.3V. Should be at 31V +/- 1V. In an unloaded/no input condition adjusting trimpot VR2 brings this down to ~37V minimum.
c) Idling current reading is -0.05mV (should be +5mV +/-1mV).

Distorted sound occurs due to small idling current (227uA = 0.05mV/0.22 Ohm)

To avoid unnecessary financial costs that may arise during repairs, install a protection in the + power supply, which consists of a resistor limiter 220-470 ohms / 50W and more and a 1 ohm resistor sensor for monitoring all current changes.

Remove the 3ADC fuse from the fuse holder
Connect a resistor limiter to the ends of the fuse holder.

Set switch to Repair Mode
- Monitor any current changes on the sensor resistor 1 ohm. (1mV = 1mA)

1 - How much change the voltage on the sensor resistor 1 ohm with change B220 (3 – idling current adjustment)¹⁾
If the voltage on the sensor resistor 1 ohm changes by changing B220, set the voltage on the sensor resistor 1 ohm to 25mV (25mv = 25mA)

2 - How much change the voltage on TP2 with change B470k (2 - center voltage adjustment)¹⁾
If the voltage on TP2 changes by changing B470k, set the voltage on TP2 to half the value of the voltage measured on TP1.

Tip
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/50421-acoustic-370-repair?p=769431#post769431

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/50421-acoustic-370-repair/page2#post769631

https://www.ssguitar.com/index.php?topic=4500.0
You must log in or sign up

[Rhod]

Thanks, vintagekiki. Is the advantage of doing this that it allows measurement and adjustment in a way that is not possible with the lamp limiter connected?

[vintagekiki]

Yes.
Resistor limiter (220 .. 470 Ohm / min 50W) limit the DC current of the faulty power amp, so you can perform the necessary measurements.
In the worst case for UTP1 = 100 V the current does not exceed the value of 0.2 - 0.5 A. When the amp is brought to the correct state, the current will be within idling current.

The lamp limiter protects the primary transformer. The resistor limiter precisely limits the current from the power suplly.

[Rhod]

Thanks, vintagekiki. There will now be a bit of a hiatus while I continue to source a spare transistor and high wattage resistor etc... I will report back when I get a delivery!
Thanks, everyone....

[vintagekiki]

You can make a high wattage resistor with several less power resistors connected in parallel.
Example 5 resistor of 1 kOhm / 10W connected in parallel gives 200 Ohm / 50W or 5 resistor of 2.2 kOhm / 10W connected in parallel gives 440 Ohm / 50W.

[Rhod]

Good point, vintagekiki! I did do a trawl through some old PCBs to see if I could accumulate enough cement resistors - but no, sadly! I have ordered a direct replacement TO33 transistor from the place I linked to above. Bit nervous about this and wondering if I should just have ordered the TO220 equivalent from a known supplier. Any way to check that the part is genuine using just a DMM (or two?).
Many thanks.

[vintagekiki]

From the 30 year practice, the most common failures in power amp are the output transistors (TR23/TR24), the driver transistors (TR21/TR22), the emitters resistance of the output transistors (0.22/ 2W).

When the power amp repair, replace the defective output transistor(s), the emitter resistor(s) of the defective output transistor(s), the both resistors (150 Ohm) of the base/emitter of the output transistors.
If output transistors (TR23/TR24) or drivers (TR21/TR22) is defects they are changed in pairs. Matching the transistors is performs for current gain (hfe) and for collector-emitter breakdown voltage (V_CEO).

After removing the defective part, check and adjust the idle current (25-30 mA per pair of output transistors).
Check the DC offset (adjust the voltage on TP2 to half the value of the voltage measured on TP1) and that's it.
All measurements and adjustments are made with a resistor limiter in the power supply (limiter switch to repair mode), without an input signal.

When the power amp is repaired and adjustment as the service manual says, set the resistor limiter switch to idle mode and check idling current adjustment and center voltage adjustment.
If fine-tuning is required, do so carefully.

1)
Testing a transistors
https://www.ssguitar.com/index.php?topic=4500.msg36078#msg36078

Mounting a transistor to heatsink
https://www.ssguitar.com/index.php?topic=4500.msg36029#msg36029

[Rhod]

Right - back after a short while. Got some components delivered, and have made the resistor limiter. I've got a couple of questions...
How do you source matched pairs of different transistors? I'm familiar with the concept of matching valves/tubes - get a bunch, test them all, pick the closest....I guess when matching the same transistor if you get it from the same production run, it's likely to match pretty closely...but what about a complementary pair which will be two different transistors from different runs - do you insert additional components into the circuit in order match them? Or rely on your supplier to do this for you?
How crucial is this (and indeed replacing the working complementary transistor at the same time as the blown one?) Where are we on the scale that runs from 1:slightly better sound to 10:you will destroy your amp?!

Also - having sourced a replacement transistor, well, it smells funny. What do you guys think? The printing is much simpler than the original, the casting seems slightly worse. On the other hand, they weigh exactly the same.

[Rhod]

OK - to work.

With resistor limiter in Repair Mode
- Monitor any current changes on the sensor resistor 1 ohm. (1mV = 1mA)

1 - How much change the voltage on the sensor resistor 1 ohm with change B220 (3 – idling current adjustment)¹⁾
4.1mV drop across the 1 ohm resistor. Adjusting B220 makes no difference to the reading.

I note, vintagekiki, that you said "if" B220 changes the voltage drop across the 1 ohm resistor. What do I do if it doesn't?!

Stage 2 completed succesfully:
2 - How much change the voltage on TP2 with change B470k (2 - center voltage adjustment)¹⁾
If the voltage on TP2 changes by changing B470k, set the voltage on TP2 to half the value of the voltage measured on TP1.
TP1 voltage = 89.8V
B470k adjusted till TP2 = 44.9V

Thank you!

[vintagekiki]

1 - How much change the voltage on the sensor resistor 1 ohm with change B220 (3 – idling current adjustment)¹⁾
4.1mV drop across the 1 ohm resistor. Adjusting B220 makes no difference to the reading.

I note, vintagekiki, that you said "if" B220 changes the voltage drop across the 1 ohm resistor. What do I do if it doesn't?!

Setting and measuring the idling current is done with the output transistors TR23 / TR24 in the circuit.
Voltage drop 4.1mV across the 1 ohm resistor is low idling current 4.1 mA
If no difference to the reading when adjust B220, check B220, D1, TR20.

1)

... ...

If I am not able to find the original replacement transistor, when the repair I install transistors that match with power dissipation (P_D) and collector-emitter breakdown voltage (V_CEO) values of the original transistors.
Match the transistors for current gain (hfe) is in the range +/ - 10%

[Rhod]

Thanks, vintagekiki. I've put in the replacement transistor and set the centre voltage as described before - voltage at TP2 1/2 that at TP1 (rather than the figure in the service manual which as previous posters noted seems to be incorrect) and the idle current so that the voltage at the testing point as per the service manual was 4.9mV - this was at the extent of the adjustment pot and lies just within the +/-1mV allowance.

I know I haven't finished the job - I've got a bag of new caps to replace the 40 year old ones on the boards and I haven't yet replaced the other driver transistor as is recommended, BUT I couldn't resist just having a bit of a listen to see if it was working.

Well, it is, and WHAT A SOUND! I know these have a reputation as sleepers and some of the series have been used by heavyweight jazz guitarists - it's the reason I picked this one up as my valve amp, nice though it is, doesn't have enough headroom and I haven't got the funds for a more powerful one. Well, now I don't need one. This things sounds beautiful. Having played a valve amp for about 10 years after a variety of SS amps before that I was really surprised by just how good the tone is, especially with a p90 pickup btw. If this thing keeps going (and after a recap why shouldn't it?!) I don't think I will ever need to buy another guitar amp!

So, after all that excitement...I'm just wondering how occasional repairers like me, who has plenty of amplifiers (guitar and hi-fi) but all with different transistors manage to get a matched transistor when they order replacements. Which suppliers offer a matching service? It's not an option I've ever seen!

[Enzo]

MAtched to what? The only time I worry about matching is when there are multiple parallel output transistors in high power amps. If you are thinking matching the NPN to the PNP, forget it.

SOlid state amps are heavily fed back and correct their own distortions for a large part.

[vintagekiki]

... ...

... voltage at the testing point as per the service manual was 4.9mV

The 4.9mV voltage drop (TP4 / TP1) at the 0.22 / 5W resistor corresponds to the idling current 23mA. It's OK.

I know I haven't finished the job ... BUT I couldn't resist just having a bit of a listen to see if it was working.
Well, it is, and WHAT A SOUND! ...This things sounds beautiful ... I was really surprised by just how good the tone ...

Congratulations.
If the amplifier works well as you described, if you are satisfied with the sound and if the amplifier adjustments within the limits as described in the service manual, nothing more needs to be changed.

... how occasional repairers ... manage to get a matched transistor when they order replacements.

When replacing transistors, the market dictates what you will install (which are currently available), because you must do the procedure 2 - center voltage adjustment and 3 - idling current adjustment anyway.
To replace a transistor the most important thing is to match by power dissipation (PD); collector-emitter breakdown voltage (VCEO) and layout case to original transistors.
For output transistors I mainly use for NPN MJ15003 and for PNP MJ15004 (250W/ 140V)

[Rhod]

Thank you, vintagekiki and Enzo. Appreciate your help - and that from others on the forum. This group has brought two bits of vintage kit back to life (both Yamaha by coincidence). The other, a powered mixing desk, was used for the first time in a socially distanced outdoor band practice last week. It worked perfectly and was much admired, particularly the illuminated Vu meters as the evening light faded! This guitar amp too will give a lot of pleasure. So thanks, folks!