Need some help getting a DEC VT100 working

[MattisLind]

BTW. When testing a germanium transistor one has to understnad that the readings will be quite different because of the lower forward drop. The forward drop is around 0.2-0.3 V rather than 0.6-0.7V.

 

[rjarratt]

Thanks Mattis. I did find some of those parts on eBay (should have said), but I was hoping to find a modern equivalent rather than something that might fail again due to age. Interesting what you say about PNP vs NPN, the 2SB411 is PNP as you say, but the Feb82 schematic shows an NPN. However my board does have an additional resistor across C443 (see p58 of the schematic https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.pdf), so perhaps that is the difference?

 

[Radix]

Top tip from fixing these back in the 1980s... If you have the monitor board out, remove the solder from the choke next to the two electrolytics which has a piece of ferrite strapped to it.. make sure the wires are bent flat to the PCB for a good connection and re-solder it...

It carries a lot of peak current and the connections go bad, resulting in the resistor in parallel with it going up in smoke... It's a simple thing but saves some drama later..

Robin

 

[rjarratt]

I have had a separate suggestion that the transistor marked B411 can't possibly be a germanium transistor, so that now makes two people telling me this. It is very clearly marked as B411, but if it isn't a germanium transistor what could it possibly be?

Bear in mind it is Q414 on p58 of the schematic https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.pdf. My board does not fully match this schematic despite having the same DEC part number. However, the part of the circuit showing Q414 is the same, except for a resistor in parallel with C443 and for CR406, which is marked as optional, and is also fitted.

 

[MauriceH]

In that schematic its a BU407D (with diode) , so any can do the job who suited in this TV type of fly-back circuit.
Used in many TVs back in the late 1970's, early 1980's.
So use the more common BU508D (8A amps) , they can be found in TO3 housing , or TO-247.

And! Beware of those old "Sparque" capacitors, they suspect of very leaky. Like C439

 

[MattisLind]

I have had a separate suggestion that the transistor marked B411 can't possibly be a germanium transistor, so that now makes two people telling me this. It is very clearly marked as B411, but if it isn't a germanium transistor what could it possibly be?

You can easily test if it is germanium or not. Remove them and mesure the PN-junctions. If there are around 0.7V voltage drop on BE/BC it is silicon. If around 0.3V it is germanium.

But the marking on the chip shows that it is a Sanyo 2SB411. Not many alternatives.

 

[MattisLind]

BTW. I think you should trace out the difference between your circuit and the schematic. Then it might be possible to understand why they use two germanium PNP-transistors to replace one silicon NPN-transistor.

 

[rjarratt]

In that schematic its a BU407D (with diode) , so any can do the job who suited in this TV type of fly-back circuit.
Used in many TVs back in the late 1970's, early 1980's.
So use the more common BU508D (8A amps) , they can be found in TO3 housing , or TO-247.

And! Beware of those old "Sparque" capacitors, they suspect of very leaky. Like C439

I have already replaced C439 and all the other electrolytics. I haven't been able to find a BU508D in a TO3 case so far, but it still remains to be seen whether my board really uses a BU407D as shown in the schematic, so not really sure it would be right to replace it with a BU508D anyway.

 

[rjarratt]

You can easily test if it is germanium or not. Remove them and mesure the PN-junctions. If there are around 0.7V voltage drop on BE/BC it is silicon. If around 0.3V it is germanium.

But the marking on the chip shows that it is a Sanyo 2SB411. Not many alternatives.

This is what I have found with the part removed using my diode tester. I also tested CR406 which has the same markings:

Joint	Q414	CR406
B-C	2.7V	Open
C-B	0.1V (beep for short circuit)	0.1V (no beep)
B-E	1.4V	1.8V
E-B	0.15V	0.15V
E-C	0.16V	0.16V
C-E	1.45V	1.8V

So it would seem to indeed be a germanium transistor, but the differences make me think that at least one of them is faulty. The apparent short circuit is the bit that is a bit concerning, not sure if the DMM is being fooled by the fact that it is not a silicon part and that it is testing fine as a germanium transistor.

 

[rjarratt]

BTW. I think you should trace out the difference between your circuit and the schematic. Then it might be possible to understand why they use two germanium PNP-transistors to replace one silicon NPN-transistor.

Yes, my next task is to draw the schematic. I have already done the Q414 side of T403, I now need to do the Q413 side which is where the main differences between my board and the schematic in the print set

 

[jmdhuse]

Hello All and Happy Holidays to those who are celebrating...

As I am recovering from a bout with COVID and am not supposed to be out and about, I decided to tackle my own VT101 restoration project. Would it be appropriate to ask some questions of my own in this thread, or would that be considered hijacking? I want to be respectful to the original poster who started this thread...

Thanks, Jon.

 

[MattisLind]

This is what I have found with the part removed using my diode tester. I also tested CR406 which has the same markings:

Joint	Q414	CR406
B-C	2.7V	Open
C-B	0.1V (beep for short circuit)	0.1V (no beep)
B-E	1.4V	1.8V
E-B	0.15V	0.15V
E-C	0.16V	0.16V
C-E	1.45V	1.8V

So it would seem to indeed be a germanium transistor, but the differences make me think that at least one of them is faulty. The apparent short circuit is the bit that is a bit concerning, not sure if the DMM is being fooled by the fact that it is not a silicon part and that it is testing fine as a germanium transistor.

I think these measurements looks strange. On the other hand I have not much experience working withe ge transistors. I have read that ge transistors are leakier than si transistors. Perhaps what we see in the measurements.

Maybe you could hook up a couple of lab supplies with current limiting and pull a milliamp or so out of the base and see how the current through the collector is reacting.

 

[rjarratt]

Hello All and Happy Holidays to those who are celebrating...

As I am recovering from a bout with COVID and am not supposed to be out and about, I decided to tackle my own VT101 restoration project. Would it be appropriate to ask some questions of my own in this thread, or would that be considered hijacking? I want to be respectful to the original poster who started this thread...

Thanks, Jon.

You raise a good point. When I resurrected this thread I thought it would be OK given that it was about the same board. However, I have since wondered if I should have started a new thread too. Any thoughts and guidance for next time would be useful.

 

[jmdhuse]

You raise a good point. When I resurrected this thread I thought it would be OK given that it was about the same board. However, I have since wondered if I should have started a new thread too. Any thoughts and guidance for next time would be useful.

As I think about it, it made sense for you to restart this thread, because the previous conversation had concluded successfully... otherwise there would be confusion about issues and answers. Since you are still in the middle of debugging your VT100, I think I will start a separate thread. Anyway, good luck with it!