If I were you, I would just pull that transistor and check the junctions with the diode test feature on my DMM. 99% of the time a bad transistor will have one or both junctions shorted or open.

-Matthew

Well, the IC has 7 leads to unsolder and the transistor only has three, so I guess I might as well check it first. I have an in-circuit transistor checker but have never had much luck with it.

Right now my dog is very sick, so I'm not going to have as much time as I'd like for this project for the next few days, but we'll get there.

Thanks for the suggestion.

Michele,

How is your dog?

Rich

Thanks very much for asking. She's not doing too well. We've been to three different vets now and no one has a clue. She started having diarrhea back in January. They tried three different alternative foods but that didn't help. Now she's losing protein and collecting fluid.

She had an abdominal ultrasound which was inconclusive. She's scheduled for an abdominal biopsy on Monday. Hopefully that will provide a definitive diagnosis. It looks like it will either be cancer or inflammatory bowel disease. Obviously we are hoping for the latter. Here she is, last summer:


Moving on to the VT-100, I'm afraid I'm not having much luck there either. I checked TR-6002 out of circuit using the component checker on my scope and it does not appear to be bad. I also pulled D-3272 IC-6005 and checked it. There was nothing oviously wrong with it either. Then I swapped in a different D-3272 but I still didn't get any output on pin 7.

However, I did notice two things. First, the rising and falling voltage I observed it apparently linked to the motor control trapezoid wave at TP4 and TP8. It is unstable and slowly precesses across the scope. Second, when I dial up the gain on the scope, I can see that there actually is a small sawtooth wave at pin 7 of IC-6005.

So I'm starting to think I may be barking up the wrong tree by suspecting IC-6005 and/or TR-6002. I'm thinking now that they're both trying to work, but something somewhere else is pulling down the signal that should appear at the R input to FF10 IC-6008.

I was thinking that the reason IC-6008 was not developing an output was because it had no R input. However, maybe some internal problem in IC-6008 is sucking down the R input. I know these FF-10's are not very reliable, and I have another one of Matthew's great FF-10 clones. I'm thinking of replacing IC-6008 and see what that gives.

Though I'll admit at this point I'm pretty much just shotgunning, and that's not a very efficient way to troubleshoot. I just wish I understood a little better how this board works.

I hope your dog is OK. One of my thoughts is that she may have been poisoned; is that a possibility? It happened to a neighbor's dog when I lived in Santa Barbara; it took a long time for the dog to get well again.

About the deck. Did you check any of the mylar caps in that circuit? They can get leaky and change value too, although this is not very common. Then again, that deck is not new and anything can happen with old components.

Also, you need to check the CTL pulse from the CTL head. This controls the speed of the tape.

She is a fine looking girl.

I am sorry to hear that and I know a bit what you are going through. My dog had something similar which came very close to killing her. Her official diagnoses was protein losing enderopathy. Her endoscopy and colonoscopy showed that her stomach and intestines were mostly bleeding sores. She had diarrhea and was puking for months. Was finally pooping red red blood. She was 4 and I thought we would lose her. Her creatin levels were so high her blood almost clotted in her arteries and her albumin was less that 1.0. Have your vet check those.

Turned out it was a food allergy (wheat)- plus she can't have any fatty foods at all. We feed her nothing but chicken and health food brown rice. Have to sprinkle fiber on it and add some supplements. Wheat was what almost killed her. And she was fine for her first 3 years.

Her vet bills ended up costing more than several new cars but she is 7 (middle aged now but they are always puppies in our minds) now and healthy and happy. Just have to warn our guests that if they sneak her food under the table we will kill them - and they don't quite believe us - but they don't. Try the diet change, it can't hurt and it really isn't that expensive. I will PM our recipe if you are interested.

Back to the VT100 as you said. It is absurdly ridiculously cool that you are restoring this weirdly rare thing that never worked great in the first place. And the MIT guy adding breakoff boards to his projects - that is over the top. I know you will get this working.

MIT guy - the tomb of the unknown tool has a mention in your honor for your help on this restoration. And if you don't mind me asking, where can you get 15 boards fabbed for $65?

We did consider that. I can't imagine anyone doing that deliberately - we're on good terms with our neighbors and we don't let our dog annoy anyone.

But there is wildlilfe in the area and we've seen everything from deer to rabbits and wild turkeys passing through the yard. She might have picked up something from one of them. I guess we'll have to see what the biopsy reveals.

I did check the control pulses and they look good. I know the tape is running at the correct speed because the audio sounds right. It's just the video head drum that's rotating at double speed.

This evening I checked C6021 and C6024, the capacitors under TP6 and TP7. They were OK.

Very interesting - that's just what our vet said. And Foofie also has a wheat allergy apparently (although she did fine with wheat for years - she's almost 8 now). At least she's not vomiting, not showing any blood in the stool, and still has a good appetite.

I'm glad your dog pulled through. Hopefully the Foof will be OK too. She isn't "like" a member of our family - she is a member of our family.

And thanks for the encouragement on the VT-100. It's frustrating because most of the servo board is working now and I feel like I'm real close, but there's still just one more bug hiding in there somewhere. I guess I'll do some more component checking tomorrow.

This is kinda long, but it's interesting. Please bear with me. Last night I was thinking about why this servo board isn't working and I suddenly had a rare flash of brilliance. I looked at the schematic again and noticed UJT (unijunction transistor)-6002. Originally I had kind of glossed over this figuring it was just some sort of amplifier.

Turns out, it's not. It's not really a unijunction transistor either. It's actually a PUT - programmable UJT. And this whole little section around it is a classic relaxation oscillator! Check it out:


R6030 is the charging resistor that charges up C6024. When it's charged, it goes over the hump of the characteristic curve and discharges through the UJT. Resistors R6031 and R6032 are the programming resistors (hence the name PUT) that allow you to set nu, the standoff ratio. The whole process repeats and voila, oscillator.

If you follow the R input of FF10 IC 6008 back, you end up right at the gate of the PUT. So this must be where the R input to the FF10 comes from! And not TR-6002 as I had previously thought.

So I measured the voltages around it. The gate should be 4.7 v., I was getting 4.2 v. OK, that's a bit low. The anode should be 1.95 v., I was getting 1.35 v. Hmm, now that doesn't look good. Also, the Q output of the FF10 (pin 2) should be 4.5 v. and I was only geting 1.49 v. That doesn't look good at all. And there was still no signal at the R input and no signal at the UJT. I was positive I had found the problem.

I pulled UJT6002 and tested it. Sure enough, the anode-cathode junction was completely open. The UJT wasn't oscillating. I replaced it with another one (that did check good on my curve tracer) and powered up, convinced that would do the trick.

It didn't The incorrect voltages were all still there. I checked the oscillator charging resistor R6030 and it was OK. The 1S214 diode hanging off the gate was good. The oscillator capacitor C6024 was OK. About all that was left was the FF10 IC 6008 itself. I had another one of Matthew's FF10 clones so I put that in.

Well that changed things alright, but now everything is wrong in a new way. Check out the new voltages in red in the image above. Of particular concern is that the A and G voltages of the UJT are now identical, and they're way too high. I'll bet I just fried this one too . I still have no signal at the FF10 R input and still no oscillation at UJT6002.

Now I'm really stuck. How did replacing this FF10 cause 8.5 volts appear at its Q output (but still no signal) where it should only show 4.5? And why is this UJT still not oscillating?

I am at a real dead end here now folks. I could sure use some help. Thanks.

A potentially dumb question, but did you replace the UJT with one which has similar specs? If the new part has a very different IV curve than the old one it won't oscillate.

-Matthew

Good question. As it happens I actually scored a VT-100 on Ebay last week, so the UJT I used was the identical part off the same board.

I had this fantasy of restoring this Ebay machine, but that was dashed when I found that the video heads were gone. And by "gone" I mean missing, not just worn out. Not only that, but the slip rings were missing, the video drum pickup coil had its wires snipped, and all of the FF10's on the servo board had been removed. Apparently, this machine had been vandalized by the previous owner for parts. Fortunately for me, they left the UJT on PX506.

And while this machine was advertised as "non functioning", I was less than pleased to discover it missing a few minor parts like the video heads and slip rings.

And for those following the Foof saga, she was supposed to have surgery this morning but went into ventricular fibrillation upon receiving the anesthetic. They were able to bring her back. We are fortunate to live within driving distance of perhaps the best veterinary college and hospital in the country. So that's where she is tonight, in ICU. They're supposed to call us tomorrow with an update. We were told that "no news is good news". I'm dreading the ring of the phone right now.

I'd gladly throw my VT-100 and all of my old radios into the crusher if it would bring my dog back to health.

Michele -
Any update on the Foof? I really feel for you in your struggle with this. We lost our beloved Samoyed Meshka a couple of years ago last month to cancer and I know what it is like having a sick pet. Best and blessings in all of this.

-Matthew

Thanks very much for asking. She was supposed to have an abdominal biospy on Monday but ended up going into ventricular tachycardia when they gave he the anesthetic, so they had to stop the surgery.

Fortunately, we live within driving distance of a nationally known veterinary college and hospital. Our vet arranged to have her admitted there. We drove down there and she spent the night in ICU. They brought in a cardiologist who managed to stabilize her yesterday, and they were able to get the biospy done today. We should have the results of that by Monday.

They say she's alert and active and might even be able to come home tomorrow. However, the underlying disease, whatever it is, is still there. When we get the word on that, we can come up with a treatment plan. So I guess for now that's good news but the ultimate outcome is still uncertain. Suffice it to say we have our fingers crossed.

Sorry to hear about your loss. Samoyeds are awesome dogs too.

Any more news? What were the results of the biopsy? Is she doing better?

-Matthew

I was wondering myself. Hope she's OK.

Thanks for asking - I really appreciate the support. We just got the results from the biopsy yesterday. It isn't cancer and it isn't a lymphangectia (or however you say that), so that was a huge relief. The final diagnosis is irritable bowel disease.

The bad news is that she's still having nothing but diarrhea and drinking more than double her usual amount of water. The vet at the hospital said she had to eat nothing but Purina HA which is fine, except she wouldn't eat it. Then he said to try Eukanuba Salmon and Rice.

She loved that for about two days but now isn't too keen on that either. The vet says we have to stick with this for two weeks to try and resolve the inflammation. If it doesn't get better, we'll have to try medication like steroids.

She's low on energy but she's been through a lot. At least she's back with us and there's hope for an eventual resolution to this. It could have been a lot worse.

And just to keep this a bit on topic, I'm also at a complete impasse with the VT-100. I had a bad UJT but when I replaced it, nothing changed. Then I replaced that FF-10 on a hunch and somehow that seems to have cooked the new UJT.

So now I'm afraid to put even more new parts in there for fear of ruining them too. It's clear that the UJT oscillator isn't running, and I haven't a clue why.

Thanks for the update on your dog.

You may have a leaky diode at the UJT, or a leaky capacitor (if you haven't replaced them already). I don't see how the new FF10 could have fried the UJT, but I suppose it's possible.

Are the power supply voltages still OK in that circuit?

Check very carefully for a broken trace on the R input of the FF10. Something just does not add up with the voltages in your diagram. The R input of the FF10 appears to be connected to an 8.48V node and a 6.87V node (through VR6003 and R6030 respectively both of which on the same order as the input impedance of R) but R is still at 0V?? That does not compute for me unless there is a broken connection or unless the R input of the FF10 is somehow shorted to ground.

I really wish that you were closer so we could get together and do a bit of troubleshooting. Two minds tend to be better than one and this thing had me quite intrigued.

-Matthew

Thanks Matthew and Johnnysan for looking at it. You're right - it does not compute for me either. And since I only have one more official Akai UJT left to sub in, I need to be extra careful at this point.

I will look at both of your suggestions and report back. And I also wish there was a way to have someone else actually look at it. I readily admit that my solid-state troubleshooting skills are nowhere as good as my tube knowledge (whixh isn't that great itself) and that's really hampering me at this poimt.

It is important to know for sure that the circuit did damage the replacement UJT. If nothing else, it is an important clue. You should remove it from the circuit and test it on the curve tracer if you have not already done so.

-Matthew