TEAC A3440 /REVOX A (HS) 77 - troubleshooting help please

[Top Cap]

Hi Thebes1, just establish that you have a 24V supply as mentioned earlier at the emitter of Q732. You should be able to check for this on the remote socket at the back of the machine. Measure between pin 7 (0V) and pin 8 (+24V). The voltages on the logic steering diodes seem to vary on my machine which gives me cause for concern . I would expect 12V and in fact I actually measured this on D705 cathode and D709 cathode, again these can be checked at the remote socket on pins 2 and 3 respectively. It is the Stop line that I find rather strange as this should, in theory, also be at 12V on the remote socket pin 4 but on my machine is only 0.7V even with the take up arm off the micro switch point. This is a bit worrying and I will have to wait till my workbench is clear before I can investigate. One theory I have is that Q712, with no base return resistor, may not be fully turning off after doing a reset. But anyway, measure your voltages and let me know what you get. BTW all the 12V signals should go to 0V when their respective buttons are pressed on the machine.
Les

[Thebes1]

Hey Les, took some measurements. Cool idea using the remote connections to make these tests.

I did measurements directly at the emitter on the Q732 at got a reading of 24.6v.

I also measured at Pin1 and got a reading of 24.0v. (Pin 7 is unused in my model and Pin 1 corresponds with Play on my schematic)

Pins 2 and 3 measured in at 11.7v each, and all the above readings went to zero when their proper button was pushed.

The only real oddity was Pin 4 read nothing and then jumped to 11.7v when the Play button was pushed, but only with the automatic shutoff lever in the "on" position. It dropped to zero when the Stop button was pushed, again with the lever "on"/engaged. With the lever off, no readings.

So, what next?

[Top Cap]

Fine Thebes1, Pin 4 is the STOP line and your readings prove the microswitch is working ok by releasing it from 0V when the R/H arm is moved from its rest position. As we have already established that the pinch wheel solenoid and its supply is correct, the fault surely must be down to the drive components after the Play flip flop. Check that when Play is selected the collector voltage of Q702 goes low, the Vsat for this transistor is 0.3V so you should get a nice low figure. This should result in Q720 being turned on and its collector should rise to +24V. This in turn should be turning Q721 on which will provide a ground return for the pinch wheel solenoid. That's how it should work in theory but there are a couple of interlocking circuits involved as well but at least check out the above and if a fault cannot be found here then you may have to check out the other interlocks that appear to be linked to the Record and Pause circuits. After all, there is a possibility that the machine is stuck in Pause mode! Good fun innit!

[Thebes1]

Originally Posted by Top Cap

After all, there is a possibility that the machine is stuck in Pause mode! Good fun innit!

Well now you are just being cruel.

Ok, not sure how to interpret this but I'll lay it out for you.

One of my problems is I'm not sure which is the base, emitter and collector legs on some of these transistors. However,

On the right leg of Q702 (with the flat part of the transistor facing towards me) the reading is .694mv with Play on and this rises to 42.3mv when the Stop button is hit.

On Q720 I get a voltage reading of 24.7v on BOTH right and left legs legs and this does not change with either Play or Stop pressed , nor with the auto shutoff lever engaged or off. If I short the middle leg to either side the solenoid engages.

Now I've read the datasheet on Q721 and if I have it right, the left leg is the Base, the Middle one is the Collector and the right one is the Emitter.

My readings are left leg, -.6mv or basically nil, the middle leg, 14.1v and the right leg -.8 or basically nil.

So, how am I doing?

Marty

[Top Cap]

Hi Marty, well most of the transistors have the same pin layout, viewed from the top with the flat facing you, of emitter on left, collector in centre and base on the right.
For the time being just concentrate on the centre collector lead of the transistors.
Pressing Play should turn Q701 off and you should see its centre lead rise to quite a significant voltage. This should cause Q702 to turn on and the Base voltage you measured as .694mV is what you would expect across its Base/Emitter junction. The fact that it stays there until Stop is pressed gives me confidence in saying the the flip flop is probably working OK.
The centre leg of Q702 should be quite low when Play is pressed but rise when Stop is pressed. As the flip flop requires this low voltage to keep the flip flop set (via D718) then in all probability the voltages here on Q702 are also correct but check them anyway.

Q720 is a PNP transistor and the logic low (if I may call it that) on Q702 during play should turn this transistor on so its centre lead rises to +24V.
The same thing should be happening to the centre lead of Q719.
From your description this appears not to be happening so it could be two open circuit transistors? (unlikely) or the Base voltages are not getting taken down by Q702.

"Quote - On Q720 I get a voltage reading of 24.7v on BOTH right and left legs legs and this does not change with either Play or Stop pressed , nor with the auto shutoff lever engaged or off. If I short the middle leg to either side the solenoid engages."
What you are doing here is simulating Q720 being turned On and this is operating Q721 so this latter transistor appears OK.

So it looks like Q720 and maybe Q719 are not being turned On but as Q702 appears to be working then attention should perhaps be directed to the junction of R744 and R745. If this point is being forced high by Q718 then the Play command from the Q701/Q702 flip flop will be ignored.
Check Q718 is not short circuit.
I have yet to unravel this part of the circuit as it gets not complicated but very confusing. What is the purpose of this piece of circuit? Is it to give an over ride of some sort or perhaps used to give a controlled sequence of events for the timing of the reel motors and pinch wheel solenoid?
Something I shall have to look into next time I get my machine onto the work bench. It is very frustrating sometimes trying to reverse engineer from just a circuit diagram, even more so when there are so many interactive elements.
Hope I have given you some useful checking points.
I have a full set of pdf's for this machine, if you do not have them please PM me. They are useful for tracing out the circuit boards and identifying components.
Les

[Michael Maurice]

Originally Posted by Top Cap

So it looks like Q720 and maybe Q719 are not being turned On but as Q702 appears to be working then attention should perhaps be directed to the junction of R744 and R745. If this point is being forced high by Q718 then the Play command from the Q701/Q702 flip flop will be ignored.
Check Q718 is not short circuit.

I have yet to unravel this part of the circuit as it gets not complicated but very confusing. What is the purpose of this piece of circuit? Is it to give an over ride of some sort or perhaps used to give a controlled sequence of events for the timing of the reel motors and pinch wheel solenoid?

Les

I'm not that familiar with this deck, but the controlled sequence of events may be that there is a delay to ensure the reels have stopped before play is actually activated.

Michael

[Thebes1]

Well Les, I think we are finally getting somewhere.

Q701 rises from .44mv to 23.7 with Play on, and Q702 does exactly the opposite so the flip/flop seems to be working at that point.

Both Q720 and Q 719 register nil on the center leg and 24 volt on the base and emitter legs.

Q718 is strangest of all registering 24v on all THREE legs in either Play or Stop.

However, when I check it in circuit with the diode tester function on my DMM it registers exactly the same as all the other 2SA733 transistors I've tested. In other words, according to the tester it's fine.

The collector leg on Q718 is soldered on the bottom of the board to one of your famous two sided connector pins. However, when I use the short tester on my DMM it shows a completed circuit from that point to the next two sided collector on the topside of the board. so I'm thinking it isn't he pin.

R747 and R745 are 82K resistors and measure respectably 18K and 24K but I have not desoldered them to get an actual out of circuit reading.

I'm not exactly sure where we are going but I do think we are getting closer.

[Top Cap]

Gee Marty, things are getting a bit tougher now. If the base of Q718 is at 24V then I would expect it to be turned off? It could be that you are getting 24V on its collector via another route. I notice that the Stop Sensor board controls Q718 and boy does it have a journey. The info I have suggests that the stop sensor is located on the L/H reel motor but if I deliberately stop the motor on my machine there is no effect. Maybe my stop sensor is defective?
Personally I would have thought the capstan motor would be the place for it so that if the capstan stopped for any reason then a Stop command would be initiated. First of all though, I am beginning to think that Q716 and Q717 form another flip flop for Q718. When the machine is powered up, Q715 places a short duration low going power-on-reset signal to the base of Q716 which being a PNP transistor is forced on. Its collector goes high and thus turns on Q717. This would indeed turn on Q718 but a signal from the STOP sensor board must force this flip flop to reset. The signal comes from Q805 on the stop sensor board but goes through a shorting link (P601) on the power board before going to the control board. It does not however go through the main connectors but appears to be connected to pad 14?
Crazy I know but I did say that this part of the logic workings was very hard to understand, so many diodes feeding LED's and other circuit elements it gets quite mad.
I can't understand the 24V on all three legs of Q718 though, if Q717 collector is high then there should not be any 24V at Q718 collector unless you are seeing the 24V via R753,R752 and the diode D732.
Do you see a low voltage at the junction of R744 and R745 when Play is selected?
Les

[Top Cap]

Hi Marty, I have sussed out the purpose of the Stop Sensor. It is used when you switch the machine to Play from either FF or RWD. Q718 temporarily over-rides the output of the Play flip flop which is trying to engage the Pinch wheel solenoid. Make sense really as you don't want this engaging whilst your fast winding . The Stop sensor waits until the left hand reel has stopped then Q718 turns off, allowing the Play flip flop signal to engage the pinchwheel solenoid and the slow speed relay for the right hand reel.
Hope this helps
Les

[Thebes1]

Ok, Les. I've tested the diodes and caps on the stop sensor board. Q805 tests fine. I then used my DMM to test for continuity between that board, the power board through the shorting link, and onto Pad 14 and the path seems fine, no shorts or breaks. I think they just ran the wire this way primarily for engineering neatness so it would not have to be run in front of the motor.

I then tested Q717 and on the center pin I get 2.4v on the right pin 6.7mv and on the left I get nil. No change on any of these pins on stop or play or with auto arm engaged or not.

I retested Q818 and I'm still getting 24.v on all 3 pins.

D732 tests fine but I am not sure how I would test the junction of R744 and R745. Would this be via one of the connectors pins? If it's the trace on the bottom of the board it would be impossible to do this with the board installed.

Do you think I should take these various resistors out of circuit and test them?

Have I mentioned I really appreciate the time your taking to solve this mystery.

[Top Cap]

Hi Marty, well Q718 def has to be off or else all Play commands are ignored.
It might be worth physically removing it for checking, also Q717 in case it is leaky. Incidentally, does the machine operate in its FF or Rewind settings?
Pretty well running out of ideas here so maybe a review is in order:-
You have proved that the collector of Q702 goes low when Play is selected thus proving the flip flop is working and also that the STOP line is not holding anything back.
Junction of R744/R745 should go low to engage slow speed and operate the pinch wheel solenoid but it does not, presumably because Q718 is ON and putting 24V on this point.
Q718 should be off however because with no reels operating at this time, the base of Q717 will be at 0V (which it is) thus the base of Q718 should be high and thus turned OFF.
Hmmm. is it worth trying the machine with Q718 removed, it should be ok as long as you do not select play when the machine is laced up and running in FF or Rewind. The deck MUST work with Q718 removed else 24V is getting onto the output of the play flip flop from another source.

Quite a puzzle!
Les

[Top Cap]

BTW, it may be worth checking C711 and Q715 as a leaky component here would over-ride Q717 and force Q718 to be permanently ON.
Les

[Thebes1]

Les, if I ever have to have heart surgery I'm gonna want you to do the analysis before they operate.

We have liftoff!!!!!!!!!!!

First, I re-tested Q717 and it tested ok. Then I removed Q718, fired it up and hit Play.

Eureka she works!

Q718 itself tests ok out of the circuit so I'm thinking maybe just a cold solder joint. I reinstall and hit Play. No joy. So I head over to Radio Shack which just happens to have a 2.2 elecrolytic cap which is the value called for on C711.

Solder the new cap in, hit Play, and wallah, she runs!

Pinch roller is not making full contact and have to adjust that, cue up a tape etc. but I suspect this vintage player is on it's way to providing me hours of enjoyment.

Can't thank you enough. This was like an itch that wouldn't go away, and without your assistance it would be in my pile of "I'll get to it later when I actually know what I'm doing" stuff.

What I'll do in a day or two is write up a short A,B,C for folks acquiring one of these so they'll have a starting point , and I suspect your troubleshooting on this will help many others over the years.

OH, have I mentioned I have a Tandberg 64 with wow and flutter problems, and then there's the Roberts 770.....

[Station X]

Well done Top Cap aka Les. Remote fault finding can be exceedingly difficult, but you've come up trumps here. Award yourself a .

[Top Cap]

That's Brilliant Marty, well I do all of my fault finding by circuit diagram though when I was a young lad it was completely different . I wonder if the same capacitor could be behind the phantom "Suddenly STOPS but goes again after 2 seconds" fault, it normally shows up within the first few minutes of doing a recording but once it has been and gone, never shows up for the rest of the night! . I know it is slightly OT but I did a modification to my machine and converted the mono headphone output to stereo, it makes so much difference. The strip-board amplifier can be mounted with the original and wired to the phone socket which incidentally is a stereo one but wired as Mono at present.
The single volume control can be replaced with a stereo one of the same size, I found a local company "Cricklewood Electronics" here in the UK stocked a dead ringer complete with splined shaft.
They also stock all of the original transistor type numbers so I was able to make an exact copy apart from the circuit board of course. If the pictures are not too big, I shall try to attach them so you can have a look.
All the Best
Les

[Michael Maurice]

It would be interesting to know if that capacitor had gone open or short circuit, my guess would be to have gone open.

If you have a diagram Les, just of that part of the circuit, we could p-erhaps see how it interacts with the other components and how it caused the problem.

Michael

[Top Cap]

OK Chaps, I have put together something on the attachment, just hope the resolution is all right. It's a crazy circuit throughout, I reckon nowadays you could easily replace the control logic board with just one PIC, it's working out the algorithm. 40 years ago probably no problem but I have too many turns on the proverbial coil now

Just hope I have sussed out the circuit correctly but if you find another twist to it let me know.
Regards
Les

[Thebes1]

Cool board you built there Les.

Well I tested the cap I replaced and not surprisingly in appears to be Ok, but using a DMM without a cap testing function is crude at best.

Did do a boo,boo, however and left the machine powered on for several hours and now the capstan motor won't spin, even though the solenoid is tripping like it should and the take up reels are trying to turn. Probably not surprising given the age of it. Pretty likely there are still some dodgey (as you Brits say) parts still in there. It did trip a fuse so somethings not right electronically. First thing I'll do tomorrow is work back from the motor and see if anything obvious presents itself.

Did get a chance to listen to if for a short while and it sounds pretty bad, lots of wow and flutter, so I'll have to see if I can get the tension closer on spindle and clean the heads again.


Still and all I'm pretty happy and very appreciative of your efforts.

[Michael Maurice]

Leaving a machine on for several hours should not in itself cause it to fail. I have left my Revox on for a couple of weeks continuously, (probrably because all the bulbs have gone).

Going back to the circuit, If the two lines that go off at the LHS of the drawing are play activators ie, the remote control or the play switch itself then by when these lines are switched low, the junction of R737, D726, C710 and C711 will be low. when they are released, that junction goes high, and C711 delivers a pulse to the flip flop. I assume that C711 went open circuit so the pulse wasn't being delivered to the flip flop.

Well thats my theory anyway. It would be interesting to see if Les is in agreement.

[Top Cap]

Yep, the left hand lines go to the FF and REW flip flops so that when these are operated, they discharge C710 via D803. The resultant -ve going pulse from C711 is suppressed by D730. When the STOP button is pressed, whichever flip flop was operated will now cause a pulse similar to when powering up so once again Q718 will come into play and turn off the slow speed reel motor relay and disengage the pinch wheel. These of course should be off anyway because the PLAY flip flop has not been set, I think it could be a belt and braces interlock?
Marty, have you checked the capstan drive belt? A machine left for this long would invite the gooey belt syndrome and apart from giving severe wow and flutter, would of course give non operation of the capstan if it failed completely.
Les