Thanks for all the help so far. I have a cheap oscilloscope coming with a signal generator built in. Now I need to find out exactly how to place the leads to trace the signal. I know I need to compare the right channel (good one) to the problem left channel to check for differences.
You need to understand the signal path in the schematics - are you comfortable with this?
The collectors of TR9 and TR11 are the first places to look. Each of those 'blocks' is a gain stage. You won't see the signal direct from the head (to small) on many scopes. You use the scope like a DVM, remember the scope is a voltmeter that traces voltage against time.
I used the DVM to test all the voltage on the transistors with the power on but no signal going through. The voltage on left ch tr 10 and tr 11 are all over the place. It happens after the power has been on for a few minutes. When first turned on they are more constant. The right ch tr10 and tr11 stay steady. The left channel tr8 and tr9 have consistent voltage after warm up. So it looks like the problem is in that left ch "block". Can you suggest where I should look in that "block" for the possible problem?
Like I said I am a newbie at this but I am watching some youtube basic electronics classes and hope to learn enough to fix some of this old equipment. Thanks again for the help.
Like I said I am a newbie at this but I am watching some youtube basic electronics classes and hope to learn enough to fix some of this old equipment. Thanks again for the help.
So just to be clear you are seeing the voltages here move all over the place:
The first thing to check is that the voltage on R58 and R63 is constant (this is the supply) and it looks as though it should be around 23 volts. Typically 😉 the diagram seems to have an error and the connection of the supply this point is missing. Either a 'blob' has been missed or a direct link, either would do the same.
Anyhow, check the supply is constant on those two resistors. You have already replaced the transistors as I recall.
Assuming that is OK can you check the voltage on the junction of R58/R59 and C46. This should be fairly constant even under fault conditions and be within a couple of volts of the 23 volt supply. If the voltage is dropping a lot at that point either the cap C46 or resistor R58 is suspect.
My instincts tell me C51 might be a suspect, particularly if it is a small 'compressed disc' ceramic type. To try and prove that one way or another just lift (to isolate) one end of the cap. Hopefully the stage remains stable without that (not oscillating) and you can see if the voltages are now stable or not.
Lets get to that point 🙂
The first thing to check is that the voltage on R58 and R63 is constant (this is the supply) and it looks as though it should be around 23 volts. Typically 😉 the diagram seems to have an error and the connection of the supply this point is missing. Either a 'blob' has been missed or a direct link, either would do the same.
Anyhow, check the supply is constant on those two resistors. You have already replaced the transistors as I recall.
Assuming that is OK can you check the voltage on the junction of R58/R59 and C46. This should be fairly constant even under fault conditions and be within a couple of volts of the 23 volt supply. If the voltage is dropping a lot at that point either the cap C46 or resistor R58 is suspect.
My instincts tell me C51 might be a suspect, particularly if it is a small 'compressed disc' ceramic type. To try and prove that one way or another just lift (to isolate) one end of the cap. Hopefully the stage remains stable without that (not oscillating) and you can see if the voltages are now stable or not.
Lets get to that point 🙂
Have you replaced ALL the 2SC458? The corrosion on the legs will affect the DC levels. As Mooly says above start by measuring the power rails and move down to the transistors. See where in the circuit becomes unstable. Also with older circuits you often need push the probes of the multi-meter quite hard to ensure good contact.
Here's what I found so far:
r57 v jumps both sides
r58 OK
r59 22.3 - v jumps
r63 22.1 - v jumps
c51 v jumps both sides (c51 is a resistor marked red, red, green, gold, white?)
vr4 one lead v jumps other .1mv
I couldn't find junction of r58/r59 and c46
tr10 b=1, c=.3 and e= v jumps
Could it be vr4 that's the problem?
I bought this Akai and a Dokorder from a church that wanted to make space. Both may have been stored in damp environment. the Dokorder is an early 70' machine and after fiddling with the switches a bit it sounds pretty good. go figure.
r57 v jumps both sides
r58 OK
r59 22.3 - v jumps
r63 22.1 - v jumps
c51 v jumps both sides (c51 is a resistor marked red, red, green, gold, white?)
vr4 one lead v jumps other .1mv
I couldn't find junction of r58/r59 and c46
tr10 b=1, c=.3 and e= v jumps
Could it be vr4 that's the problem?
I bought this Akai and a Dokorder from a church that wanted to make space. Both may have been stored in damp environment. the Dokorder is an early 70' machine and after fiddling with the switches a bit it sounds pretty good. go figure.
That needs investigating. One easy option might be just to short out C45 and then see if the voltages on those two transistors are stable. They should be steady. You could also try lifting (to isolate it) one end of R57.
Is the voltage on TR9 collector stable? Any change or jumping around here would transfer to the next stage.
That sounds like a tubular ceramic cap. Usually a light-ish yellowy green background and they look highly glazed. Very common in equipment like this. Usually very reliable unlike the compressed disc ceramics.
Red/red is 22 but the green band doesn't compute. 22pF looks the expected value in that part of the circuit. Lets not worry over that one at this point. Red red and black would be 22 and no multiplier and so 22pF. No matter at this stage.
I shorted C45 and the voltage jumps seemed to go away. I had the signal generator hooked up and got good left channel signal on the VU meter. So I spooled on a tape and was going to listen to see if the sound quality was there and the control buttons were not functioning and when I looked the VU meter had gone dark. I found a blown fuse and am not sure which fuse it is on the schematics. The fuses are not marked on the PC board. It is the right most fuse if you are looking from the back. Went to the store and bought some more fuses and put one in and it blew again. Undid the jumpers that shorted C45 and put in another fuse and it blew again. The VU meters and the control switch servos are one the circuit but I not sure what else is.
I greatly appreciate your help on this.
I greatly appreciate your help on this.
Shorting C45 simply removes any possibility of fluctuations form the preceding stage entering the TR10 and TR11 stage. It just shorts the signal to ground.
The fuse issue is unrelated to anything in the signal stages you are working on. Given it is a weird coincidence first check that you really have linked this cap out and not some other one.
F3 supplies the bulbs and also the 5 volt regulator. Make sure there is no short on the 5 volt rail coming from TR12 emitter and also check no short across C19 and that no shorts across any of the four diodes in the bridge.
The fuse issue is unrelated to anything in the signal stages you are working on. Given it is a weird coincidence first check that you really have linked this cap out and not some other one.
F3 supplies the bulbs and also the 5 volt regulator. Make sure there is no short on the 5 volt rail coming from TR12 emitter and also check no short across C19 and that no shorts across any of the four diodes in the bridge.
Mooli, I tested what you suggested and here is what I found:
on tr12 DMM set to ohms: B to grd = 12K, E to grd = 5M, E to grd = 5.6M
DMM set to diode: (first letter pos probe second letter neg probe) BC ran up to 1V and then OL, BE .928, EB .530, CB .550, EC .442, CE .444
Does this indicate a bad TR12? There is no short across C19. The bridge is good.
on tr12 DMM set to ohms: B to grd = 12K, E to grd = 5M, E to grd = 5.6M
DMM set to diode: (first letter pos probe second letter neg probe) BC ran up to 1V and then OL, BE .928, EB .530, CB .550, EC .442, CE .444
Does this indicate a bad TR12? There is no short across C19. The bridge is good.
Its not possible to check the transistor in circuit if that is what you are doing 🙂 You would have to remove it. There should be no continuity (use the diode range though) between C and E with the red lead on the collector.
If you do remove it then you can power the unit up and check that you have around 5.8 volts across the Zener diode that goes to the base of the transistor. Its marked 6.2 volt so you may see higher than 5.8 volts. The meter lights should of course illuminate as well. If that is the case then it would be wise to just replace the transistor but there is then the possibility that if the 5 volt rail has gone high that there could be other collateral damage elsewhere.
Depending on the package outline of the transistor there should be many common types that would be suitable such as TIP31 or TIP41 or BD131.
If you do remove it then you can power the unit up and check that you have around 5.8 volts across the Zener diode that goes to the base of the transistor. Its marked 6.2 volt so you may see higher than 5.8 volts. The meter lights should of course illuminate as well. If that is the case then it would be wise to just replace the transistor but there is then the possibility that if the 5 volt rail has gone high that there could be other collateral damage elsewhere.
Depending on the package outline of the transistor there should be many common types that would be suitable such as TIP31 or TIP41 or BD131.
I took TR12 out and checked it: BC .577, BE .580
With TR12 out turned power on and VU meters light up but control buttons are still locked.
The D8 Zener Diode, with power on, I get -OL on DVM
With TR12 out turned power on and VU meters light up but control buttons are still locked.
The D8 Zener Diode, with power on, I get -OL on DVM
That sounds like you are using the diode or ohms range on the meter rather than using a voltage range? The fact it now power up and meters light suggests there is a short on the 5 volt rail supplied by TR12. At this point it is not worth risking refitting TR12 without replacement.
You need to measure the actual voltage across the diode with the transistor removed.
TR12 is used as a 5 volt regulator and the worry is that an overvolts condition may have damaged the circuitry fed off that 5 volt supply.
If you have around 6 volts across the Zener then I would replace the transistor anyway. Beyond that... you could be looking at further collateral damage. Also there is the real risk any replacement will fail if there is a short or overcurrent condition.
The 2SD794 seems to be a T0126 outline device so a BD131 should be fine as a replacement. If it is not on a heatsink then you could use a TIP31 or TIP41 but you would have to fit those turned 180 degree as B and E are reversed.
I found the short that was causing the blown fuses. The 2sd794 was not insulated properly from the heat sink. When I put it back in without attaching it to the heat sink it worked. I was watching a youtube of some tech working on an Akai similar to mine and he found, after a great amount of effort, that a transistor power regulator was not insulated. He did not have a lot of good things to say about Akai.
If I provide a tone or signal to the RCA inputs where should I be able to find that with my scope probe? The signal goes through a 100k resistor from the input and I loose it at that point.
I checked the voltages on all the transistors and they seem to be on or pretty close to what's on the schematic. I still have the weak channel with lots of pops and cracks.
If I provide a tone or signal to the RCA inputs where should I be able to find that with my scope probe? The signal goes through a 100k resistor from the input and I loose it at that point.
I checked the voltages on all the transistors and they seem to be on or pretty close to what's on the schematic. I still have the weak channel with lots of pops and cracks.
👍 excellent.
You need to clarify something first 🙂
The thread started with us (me anyway) assuming you meant the issue was during playback. If the issue is occurring with the signal just looped through from line in to line out then we are looking at a different area of the circuit.
If you want to trace the signal from the line input first set the mic record level control to minimum and the 'line record level' to maximum. Apply the same signal to both left and right line inputs (use 440Hz or 1kHz sine tone) and adjust the generator level so you see something like 100mv peak to peak on capacitor C9. Check you see the same level on both channels.
Now move to the negative end of C11 on both channels and check you see the same levels between channels. If that is OK then move to the negative end of C20 on both channels.
If the levels are the same at C20 then you issue is after that point (assuming it is not just a playback issue)
I would say Akai build was par for the time. When it comes to the sound they are still one of the best at consumer level despite their age.
Have you cleaned the input level pots? Common issue with equipment of that age.
age.