VR14 "meltdown"

[thunter0512]

After running my newly built VC8E in my PDP-8/e beautifully displaying on a VR14 sadly the fun suddenly came to an end.
The 3A line fuse of the VR14 had blown and of course the replacement fuse blew immediately too.

After some probing with a multimeter I found that the +/-22V power supply's "bridge rectifier" made up of two chassis mounted DM-15 diode modules had a shorted diode.
I replaced the two DM-15 modules with a single modern bridge rectifier and the 3A line fuse was happy again.

Unfortunately the +/-22V rails were now around +/-34V.
I turns out that the 2N4399 & 2N5302 power transistors in the power supply are all shorting. I am sure that there is an underlying cause for all this trouble. At this point everything is suspect.

The 2N4399 and 2N5302 transistors are no longer readily available, but staff at the Living Computer Museum (LCM) in Seattle have written a blog where they recommend to replace these transistors with higher spec-ed 2N5684G and 2N5686G.
These newer more powerful transistors are still available but rather expensive. I need at least 4 of each to replace all the power transistors in both the power supply and X/Y amplifiers.
The prices I found are:

• 2N5684G: US$15.89
• 2N5686G: US$21.20

So that is about US$150 for all the transistors.

It is very likely that due to another yet unidentified fault these die immediately too, so the repair cost quickly escalates. I think these expensive transistors are acting as fuses for something else.

Any suggestions on how to proceed?

Also is there a cheap source for either the original (2N4399 and 2N5302) transistors or the replacements suggested by the LCM (2N5684G and 2N5686G)?

Thanks
Tom

 

[NeXT]

I'm afraid those prices are about right for power transistors. I had to replace four in a set in a 12v 41A power supply at $12 each. D:
Usually once I see a power supply take out the MOSFETs or power transistor I beep out the rest of the diodes (pulling them if I have to) and do a complete recap. a shorted MOSFET will regularly avalanche a bunch of other failures and if you don't verify the rest isn't at the lest shorted it will pop them again.
It's also possible your transistors shorted and the brief load before the fuse popped roached the bridge rectifier, so you are seeing an effect and not a cause.

 

[Chuck(G)]

Too bad you're not in the States; I can find NOS stock on both devices for about USD$4.00 each--but shipping to Oz would likely wipe out any savings.

 

[thunter0512]

Too bad you're not in the States; I can find NOS stock on both devices for about USD$4.00 each--but shipping to Oz would likely wipe out any savings.

Thanks Chuck. Could you please send me a PM with a link to the supplier.
At that price I would buy at least 12 each so shipping would be not as significant.

I have two VR14 displays. One came with the LAB-8/e (and works fine) and the other appears to historically have been part of a GT40.
It is the GT40 version which popped the fuse, bridge rectifier diodes, +/-22V supply regulator transistors and who knows what else.

I would like to have enough spares of the power transistors to keep both VR14 displays going.

 

[pbirkel@gmail.com]

Thanks Chuck. Could you please send me a PM with a link to the supplier.

I'd also link to see those link(s), as might others. Power transistor replacement is an ongoing problem for many ...

 

[m_thompson]

The VR14_and_VR20_Troubleshooting_Procedures_Jun72.pdf manual describes several ECOs to the VR14 that need to be done to improve reliability. You should read this manual and make sure that your VR14 has all of the improvements implemented.

 

[thunter0512]

The VR14_and_VR20_Troubleshooting_Procedures_Jun72.pdf manual describes several ECOs to the VR14 that need to be done to improve reliability. You should read this manual and make sure that your VR14 has all of the improvements implemented.

Thanks Mike. I have read that manual.
The LCM also had a nice blog about their VR14 repair including the transistors they substituted for the original (and they referred to the manual you described):

The SAGA of the DEC VR-14 – ENGBLG

For now I still have to discover the root cause of the current set of VR14 failures otherwise I likely just keep using the transistors as fuses.

Actually it would make sense to wire in fuses before the power transistors to protect them. Fuses are cheap, the transistors are expensive.

 

[Chuck(G)]

The joke, as told to me by a DEC CE, many years ago, was that DEC used "transistor-protected fuses". In other words, the problem wasn't uncommon when these things were mainstream.

 

[thunter0512]

Chuck forgot to send me a link to the US$4.00 power transistors, but eventually I found some on the US version of Ebay and bought a lifetime supply. These will arrive end of January or early February.

I also got one set of 8 of the recommended replacement transistors from Digikey and hope my wife won't check the next credit card statement. These should arrive by the end of this week. I need 4 for the deflection amplifier and 4 for the +/-22V supply.

While waiting for the transistors to arrive in Australia I am exploring further with my second VR14.

 

[Chuck(G)]

Sorry, it was on my list. Just saying that stateside, they're not ridiculously expensive. Electronic Surplus has a few, as do the NTE sellers.

 

[thunter0512]

I have now rebuilt the VR14 by replacing all power transistors with the ones recommended in LCM's The SAGA of the DEC VR-14 - ENGBLG.

There was also a faulty MC1709 op-amp on the G836 regulator board. The gold coated steel pins of the op-amp where badly corroded and 2 fell off when I unsoldered it. For good measure I also replaced the other MC1709 op-amp and the 2N2904, 2N2219, MJE2955 and MJE3055 transistors. I substituted the readily available and cheap MJE2955T/MJE3055T in in TO220 packages for the ridiculously priced TO127 packaged MJE2955/MJE3055 transistors. The world stock of the TO127 versions appear to be held by a single Ebay seller called "semitronus".

I have also checked that all VR14 and G836 ECO have been implemented (see VR14 ECOs specifically in http://bitsavers.org/pdf/dec/graphics/VR14/VR14_and_VR20_Troubleshooting_Procedures_Jun72.pdf). The only ECOs missing are the "voltage transient related" ones, but these are poorly documented and would require a different CRT socket to fit the spark gaps.

Unfortunately DEC manufacturing bent all components pins on the G836 regulator board before soldering. This makes replacing those parts with thicker leads or pins very difficult. The MJE2955 and MJE3055 have very substantial leads which you cannot un-bend while trying to de-solder them. Furthermore the PCB pads were too small for these pins meaning that the transistors have been press-fitted into those pads. I cut the transistor leads and then tried to pull them out from the opposite side but the pads were destroyed/damaged in the process. I had to fit new PCB eyelets to replace the damaged pads. I made sure that the eyelets were larger than the originals to make future repairs much easier.

Here are some nice photos showing the VR14 (actually it is a VR14L - i..e. the display part of a GT40), the PDP-8/e running VC8E diagnostics and the LAB-8/e in the background:

 

[m_thompson]

Very nice. It will probably work forever now.