Yamaha CA600 advice

[dave cox]

Possibly, there is an extra button to switch between ohms / diode and an enuciator on the display to indicate which is selected.

dc

[greenstar]

Just for the sake of clarity. Digitec M3800. New battery fitted. I've got a better one (and test leads) but it;s hiding at the moment. I have about half an hour a day to spend on this at present. Will do my homework next.
Tony

['LIVEWIRE?']

Apart from the colour, your M3800 is virtually identical to mine, which is yellow. The only difference is that on my M3800 the high current terminal is marked 20A not 10A. The ohms/diode test switch is the furthest to the left in the Ohms Range

[greenstar]

Well, for what it's worth, 2SB531 C/E no beep or reading either way, C/B no beep either way, 471 one way.
2SB371 C/E no beep or reading either way, C/B no beep, 463 one way.
With Andy's experience, doesn't look good.

[Station X]

If that's on the diode range, I'd say the transistors are good with forward volt drops of 472mV and 463mV respectively.

['LIVEWIRE?']

With a good transistor, there should be a lowish forward reading between C & B and between B &E, and high or o/c readings in any other direction, so a beep should be heard one way round between the points mentioned. Transistors can be checked on the normal resistance ranges as well as the 'diode test' setting, when, again, low forward readings between C&B and between B&E should be expected. Low readings in either direction between C&E indicate with 99% certainty that the transistor under test is faulty. Over the may years I've ben involved with electronics I have undoubtedly found from time to time that a transistor will check OK on an AVO or other multimeter, but break down in circuit, where higher voltages &/or currents may be present. Edit; Crossed with Graham - I'll second his comments re; the forward voltage drops.

[Station X]

I don't disagree, but none of my DMM's have a beep function on the diode test range. One has a continuous tone to denote low resistance on the continuity range.

It's a case of RTFM for the meter you're using.

[greenstar]

Finally ... looked at the link Graham gave, to a very clear description of transistor testing ...
Wish I had looked earlier, but guess I am guilty of doing a job whilst frazzled and rushing.
2SB531 Black to base B/C 460 B/E 473 Red to base B/C high B/E high C/E high C/E high
2SD371 Black to base B/C high B/E high Red to base B/C 473 B/E 457 C/E high C/E high
I think this looks ok?

[Station X]

Those are good results.

[julie_m]

Very occasionally, a short on the speaker lines can just blow the power fuses and leave the output transistors alone. (Actually, that's what's supposed to happen; but it doesn't always go that way, in an endurance competition between a fuse and a semiconductor junction.) You could try replacing the fuses with resistors or even tungsten filament lamps, and powering the unit up (no speaker). Check the voltages across the resistors, which should be minimal with only the quiescent current flowing (remember volts = amps * ohms) and evenly matched. If the output DC offset voltage is below 0.1V then consider applying full power and testing with a load.

[greenstar]

Julie, that's great, now, what sort of value resistance? A few hundred? What is the output DC offset voltage and where is it measured? Sorry about this, but fixing transistor amps is not my usual thing!

[julie_m]

We need to select the resistors to limit the current to a safe value. If we use 0.5W metal film resistors and arrange for them to be dissipating 0.5W when passing the maximum amount of current we expect, then they will fail if the current greatly exceeds that amount.

I'll leave all my workings for those playing along at home

According to the servicing manual, the quiescent current is measured across two 0.47Ω resistors in series, for 0.94Ω; and should be 0.047V. Using Ohm's Law, this gives a current I = V / R
= 0.047 / 0.94
= 0.05A, or 50mA.
Multiply that by 3 for a margin of safety (the preceding stages will consume some current too, and this is a maximum to stay within as opposed to a target to aim for) and we get 150mA. Now we want to choose a resistor value such that it will be dissipating 0.5W at 150mA; if the current exceeds that value, it will overheat and fail open-circuit just like a fuse.

Now P = V * I; so V = P / I
= 0.5 / 0.15
= 3.33333 V
and we can work out the resistor value using R = V / I
= 3.33333 / 0.15
= 22.2222Ω. The nearest preferred value would be 22Ω, and they need to be rated 0.5W.

So if we replace each fuse with a 22Ω resistor, this will not burn up at 150mA (which will be enough current to know for sure whether or not the amplifier is working) or less. In fact, since the supply rails are ±32V and the fuses are rated 2A, the short-circuit current will be limited by the 22Ω resistors to about 1.5A -- within the fuse rating -- and only for as long as they last trying to get rid of the best part of 50W.

Rehearse your route around the test points on the board for measuring the required voltages before you apply power. When you do, remember, you are half-expecting trouble. There is a possibility of the resistors going pop, if something besides the output transistors is faulty (or if they are faulty, but just not in a way that shows up with a simple low-power test). Fit lengths of insulated wire (16/0.2 will be fine) to extend them away from the amplifier if needs be. Don't let them touch anything heat-sensitive (especially not you).

The DC offset is simply the voltage at the speaker output with no signal at the input. The service manual actually says 10mV; but don't worry too much if it's as much as 100mV, as the biasing may be affected by the resistors. Measure this first; and if the resistors keep the smoke in, then measure the other voltages at the test points indicated on the wiring diagram in the manual. If the resistors start overheating, switch off the power at once.

If the resistors hold up and all voltages measure as expected, then you can try fitting the correct 2A fuses (or even some lower value ones temporarily, if you don't plan to turn the volume up too high; they will just blow sooner in case of a fault.)

[greenstar]

Thanks Julie. WiIl be a day or two before I have a sane space to check this out, but will report back.

[greenstar]

I am very pleased to report a successful outcome. No heating at all of the resistors, I was a little concerned as I expected a little, voltages read about right, so i took the plunge, fitted fuses and tested with 'phones and a CD deck. Worked!
So back in the house and set it up with my other separates, and sounds wonderful. I did put a Pioneer SA520 amp in there for the duration, but didn't like the sound so much although it works well. That one will go to my computer for Youtube etc. I have checked the LS cables very carefully for potential shorts.
Photo below of the somewhat ramshackle system. I am no audiophile and bumbling my way as usual, but find I have definite preferences. I very much like the Yamaha Natural Sound system, and have the tuner to match, also a nice bit of kit. Technics SB3430 speakers which seem to sound good, and a Lenco deck newly added from the last RWB, Wonderful to hear my LP's properly again.
The thing on top is a timer.
Possibly this is another thread ... it's a bit of a dog;s breakfast but for the moment, sorted!
Many thanks for your input and patience, I have learned a little and am not quite so ignorant of transistors. Especially thanks to Julie for the patient explanations.
Best, Tony

[Station X]

Nothing wrong apart from blown fuses then?

[greenstar]

If there is I am not aware of it - works perfectly. However, there may have been, so I am sure all the checks were the best idea. Including checking the speaker cables. I was moving things about a bit so there may have been a stray wire. Otherwise the blown fuses are mystery.

[julie_m]

Result! It sounds as though the fuses must have done exactly what they were supposed to, this time.

I have to say, I did get a strange feeling this was going to be an easy fix ..... Glad to have been of help, and thanks for posting the good news.

Anyway, time!