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H724 power supply -15V instability (LAB-8/e)

T

thunter0512

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The LAB-8/e I am working on uses a H724 power supply (actually the H724A for 240V AC).

I am struggling with -15V instability. The nominal -15V drifts slowly between -13.2V and -14.5V and I cannot adjust it up to the -15V at all.
Interestingly the schematic for the -15V section of the power supply shown in the Maintenance Manual Volume 1 (Sept 73) does not match the circuit shown in the Engineering drawings (Revision AD - Dec 72) - the later shows 3 additional transistors Q4, Q5 and Q8 to do some further magic. The actual circuit seems to match the Engineering drawings.

The 5V supply is very stable and uses a very similar circuit.

The regulator used for both -15V and +5V is a standard LM723 but unfortunately in a metal TO5 case which is not readily available (there are some questionable ones on Ebay). I could try to butcher in a DIP version of the LM723 to see if it is causing the slow drifting, but would prefer a more systematic approach than just to start replacing random components.

I have not reformed the capacitors, but I don't think that this is the cause. Nothing gets warm at all.

Any thoughts or ideas on the stability issue?

Thanks and best regards
Tom Hunter
 
I have found the culprit. It was C19 on "Control Board A1" the -15V regulator.
C19 is a small 10 nF 100V 20% "Disk" capacitor - whatever DEC mean by "Disk".
It sits parallel to one of the 1k voltage divider resistors used in the voltage feedback circuit and reduced the 1k resistor to something around 800 ohm but varied which explains the bizarre behaviour I saw.
The supply now works very happily without the capacitor with stable power on all rails.
C19 was probably intended for some dampening effect when the load suddenly changes.
I want to find out first what "Disk" means and what properties it has before replacing it.
The cap C19 looks a lot like a diode with a glass body. I am not familiar with these.
I suspect a ceramic cap with the same rating will do fine.

If you know what these diode lookalike "Disk" capacitors are, please let me know.

Thanks and best regards
Tom "now stable -15V" Hunter
 
I think DEC mean ceramic when they say "disk". Those ones that look like diodes are glass encapsulated ceramic capacitors.
 
Hard to say without a picture. Disk capacitors are (were?) usually flat brown things in my experience, still available. What you’re describing sounds more like a polystyrene capacitor which are usually pretty reliable things.
 
Ceramic disc capacitors were green or brown (these are the two colours I have seen) 'discs' with two (2) wires sticking out of them (see for example https://www.mojotone.com/amp-parts/Ceramic-Disc-Capacitors/Ceramic-Disc-0-02uF-500V).

However, you can get ceramic capacitors in a glass envelope that do look like diodes (see for example https://forum.allaboutcircuits.com/threads/glass-ceramic-capacitor-use.159166/).

Our old Honeywell system has stacks of them as decoupling capacitors. When they go faulty (short circuit) in the test system - you know about it!

Dave
 
Thanks Dave,

The capacitor with glass envelope link you sent (https://forum.allaboutcircuits.com/threads/glass-ceramic-capacitor-use.159166/) is exactly the type of capacitor (C19 on Control Board A1) which went bad in the Lab-8/e supply.

In my case it wasn't a short circuit but it behaved more like a variable resistor which drifted slowly with time.

I will replace the offending cap with a standard ceramic type with the same ratings, even though the supply is very stable without it. While the soldering station is hot I will replace all caps of this type on both Control Board A1 and A2.

Thanks
Tom Hunter
 
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