Am I going Barmy?

[Linnovice]

I'm back with my Revox A77 end of tape sensor problem. I've cleaned and lubed the relays (spay cleaner and gentle paper towel). I've checked all resistors, diodes and caps. All are within spec. I've checked the resistance on the light sensor itself, again, well within spec. Done all this a couple of times. I found one resistor with a dead short and changed it.

I've now reassembled the machine and the ******* thing still doesn't work. With machine standing and no tape, play, ff and rwd are inoperative. If I cover the sensor with my thumb all functions run normally, which you'd expect. Take my thumb away and illuminate sensor. ****** thing keeps running. I'm missing something but for the life of me I don't know what it is. There are no covers on the transport but that shouldn't make a difference, should it?

Anyone have any idea?

Mike

[Linnovice]

The controls are only inoperative when I turn the machine on from cold. When it's been on a while and warmed up the end of tape is not working at all.

[dseymo1]

Sounds as though hairdryer / freeze spray diagnosis is called for.

[Linnovice]

Be really handy if I knew what that was. Being as bald as a coot it's been some since I've used a hairdryer . . .

[TonyDuell]

Freeze spray (or freezer spray) is an aerosol can that when sprayed on something gets it cold (down to -20C possibly). You can buy it from electronic shops like Maplin. A hair dryer, of course is a source of a blast of warm air.

The idea is that you use these to cool or warm various components and see if that causes the fault to appear or disappear, indicating that said component is temperature-sensitive (when presumably it shouldn't be, and that therefore that component is faulty).

[vidjoman]

Originally Posted by Linnovice

I found one resistor with a dead short and changed it.

Unlikely that a resistor will go "short circuit " - is there a cap across that resistor? Or even a switch contact shorted when you tested the resistor.

[Linnovice]

Thanks Tony for the explanation, obviously food for thought. Not being an expert on the electronics front I'm sort of learning on the fly.

vidjoman. I changed the suspect resistor (0.47) and out of the board it measures as a dead short on an AVO meter so I can only assume it's faulty.

[Michael Maurice]

Unless you've got a low ohms meter, you are unlikely to get an accurate reading of a 0.47R resistor

Edit:

There are no 0.47 ohm resistors in the drive control circuit there are 3 0.47µF capacitors perhaps you could tell us which resistor you replaced.

[Grubhead]

Just to be clear on how the freezer spray works. Wait for the fault to act up, then spray one suspect component a time. If it pops back to life then it could be faulty. Then you heat it up deliberate. To see if it plays up again. I tend to use a soldering iron on the metal parts of a component (don't short it out) as it is quicker and more precise than a hair-dryer. Only target with the spray one part at a time, as you can't narrow it down otherwise.

[Linnovice]

Quite right Michael, a slip of the decimal point (or short term memory) it was R123, 4.7.

[Diabolical Artificer]

I'd check Q111 or Q112 surely these switch the relay's? I havn't figured out what goes where as the Revox A77 schematic is hard to interprit, but the LDR - IE end of tape detector I think changes the bias on Q111 (B107) switches it on, thus turning on the meatier B112 B140 which enables play.

So, does the resistance change on the LDR when light/dark? I can't see anything else in the circuit which could go wrong. I'd check the LDR, find it's dark/on resistance and light/off resistance and try swapping it for fixed resistors. First one, then the other.

I've found low value resistors do go SC BTW. That or high.

Andy.

[Linnovice]

Hi Andy, nice to hear from you again. I've checked out Q111 & Q112 using my AVO and they seem to be good. That was in circuit though. I've got a sneaky feeling it is the LDR as the lower reading is a bit high (very Monty Python) ie.

Right: from cold, light 10kOhm, dark 39kOhms
After 20 minutes light 15kOhms, dark 30kOhms.
That's measured on an AVO Meter on the 10kOhm scale. Looking at my service manual that is out of spec. Should be less than 2k light and more than 20k dark so presumably the reading of 10/15kO is way over spec.
I've bought a spare one from a chap in Germany so set that this afternoon and see what happens.
Mike

[Linnovice]

Hi All, cracked it! It was the LDR that was faulty. Changed it out this afternoon and now have a happy A77.
Thanks for all the advice and suggestions. All part of a very valuable learning curve that is greatly appreciated. So, cheers to all.
Must remember where to put the decimal point though. Remember? Hmmmm, possibly in another life ��

[Diabolical Artificer]

Nice one Mike. If the LDR is faulty then it won't turn on/off the tranny because the LDR sets bias current to the tranny.

Glad you got it sorted, sanity restored : ) Andy.