AR88D PSU question.

[pwdrive]

I realise this is Al's post but I don't get the equipotential bit, can someone talk me through it, if the resistors were connected to the other side of the heater I can maybe see something in it.

Lawrence.

[Andrewausfa]

I have the exact same resistors on my LF so I assume they are original. They are R57 and 58 on the circuit.

[attachment=3812]


Andrew

[Skywave]

Thank you for your reply, Terry.
I have spent many years maintaining & designing high-voltage power supplies, with or without valves, and I am fully aware of the dangers and the necessary precautions that need be taken during the design and maintenance of such power units to reduce the chances of accidental electric shock.

When I produced my opening Post, it was late last night: too late to go outside and take some measurements: that I have now done. With the receiver's mode switch in the "TRANS" position (therefore, no load on the PSU by the receiver itself), I measured the HT at the point where those Rs are connected to the HT line. Then whilst monitoring that voltage, I switched off the incoming a.c. mains, and timed the rate of fall of that HT. Next, I repeated all of that, but with those resistors removed. Comparing the two results has now thoroughly convinced me that those resistors are indeed bleeder resistors. With them fitted, when the a.c. was switched off, the HT fell to 100v in 25 seconds. With those Rs disconnected, the time required for that HT to fall to 100v was 5 minutes: therefore, case proven.

In retrospect, there are 2 factors which were confusing me:
1. the ohmic value of those resistors: I'm more accustomed to expecting values round about 100 k-ohm;
2. their position in the circuit. It is far more usual to see a bleeder at the output of an L/C filter, rather than its input. Of course, since those 2 points are connected by the low resistance values of the smoothing chokes, it makes little difference.

Finally, on the grounds of safety, since with the passage of time carbon composition resistors of high ohmic value tend to rise in value, or even go O/C, I shall replace those two Rs. with one wirewound resistor of about 470 ~ 680 k-ohms.

Thank you everyone who contributed to this Thread.

Al.

[Paula A]

thanks for the circuit.. I was under the impression they were from each end of the filament with a centre tapped 5v winding. As shown it's immediately obvious they are to discharge the HT rail.

[pwdrive]

As well as being bleed resistors in the discharge sense what about trickle current to reduce cathode poisoning if in TX position for long periods or is that a bit fancifull.

Lawrence.

[Paula A]

Direct heated?... thats doubtful. It would be more likely to use a relay in the HT line for that sort of purpose. Only time I have come across a relay used in a protection circuit was to protect large output valves in the event of a grid bias fault.

[TubeType]

Hello,

After chewing on this for a while, I decided to search for the AR88D manual online. Lo and behold, I came across the schematics for the original AR88 plus its descendants. The original AR88 did not use the bleeder resistors. The descendants did use them. I will bet the field technicians complained until they were added to the later versions. The resistor's combined value was chosen to provide a fairly long RC time constant. This would gently discharge the three electrolytic filters (smoothers). If you replace them, keep their total ohmic value at approimately 1M Ohm.

Question: Why are there two 560K Ohm resistors in series rather than one 1.1M Ohm?
Answer #1: RCA had a ton of them in stock.
Answer #2: The brother-in-law effect. David Sarnoff's brother-in-law made 560K Ohm resistors.

AR88 Full Wave Rectifier Circuit w/o Bleeder

[pwdrive]

Thanks Terry, the manual I have and was refering to is now clearly for the AR88 and not a later variant.

[Michaelr]

Yes Terry,

Having seen the circuit now , hum reduction does not come into it. You are right on all accounts, I guess there is also protection from possible power surge ( in the "Trans position" ) in stormy weather etc if the HT momentarily pushes very high and beyond the rating of the caps, it will reduce the overvoltage time.

Mike

[pwdrive]

Just to say that I agree with Al in his earlier post insofar that I would have expected a lower value discharge resistor if they were fitted with quick discharge/shock hazzard in mind, I would say 100k or a lot less maybe. Clearly they are supposed to be there though which the main thing. I have since discovered that the manual I was reading from had AR88 (R's not fitted) written on the front and not AR88D (R's fitted)I must try harder.

Lawrence.