AR88D PSU question.

[TubeType]

Hello,
Yes, ruminating. That is what I was doing in Post 17.
Al, your description of how and why bleeders are used was very good.
Kindest regards,

[pwdrive]

Hello Al, what's the verdict then, will you fit a lower value or leave as is.

Lawrence.

[Michaelr]

Remember to take into account that the HT will rise quite a bit in "Trans".

I wonder if the design considers the situation when the AR88 is powered by its vibrator power supply instead of mains.

Mike

[Skywave]

Lawrence: I've fitted a 100k-ohm resistor as an additional bleeder: +HT1 to chassis.

Mike: Yes, the HT will rise in TRANS - and by quite a lot too, since in that mode, the PSU is effectively on no-load. But I'm not too concerned about that in this receiver. I consider the chances of myself switching this set off at the a.c. supply whilst it is in TRANS to be remote. And even in TRANS, the existing 1 megohm resistor bleeder discharges the capacitors to 100-v in about 25 seconds. Moreover, I suspect that that is why the original bleeder was fitted to the input side of the L/C filter, as opposed to the filter's O/P - as is usually the case. So, you might ask, why not simply replace the 1 megohm bleeder with the 100k-ohm one? And the answer is: two bleeders are better than one!

Al.

[Michaelr]

Lawrence: I've fitted a 100k-ohm resistor as an additional bleeder: +HT1 to chassis.

Mike: Yes, the HT will rise in TRANS - and by quite a lot too, since in that mode, the PSU is effectively on no-load. But I'm not too concerned about that in this receiver. I consider the chances of myself switching this set off at the a.c. supply whilst it is in TRANS to be remote. And even in TRANS, the existing 1 megohm resistor bleeder discharges the capacitors to 100-v in about 25 seconds. Moreover, I suspect that that is why the original bleeder was fitted to the input side of the L/C filter, as opposed to the filter's O/P - as is usually the case. So, you might ask, why not simply replace the 1 megohm bleeder with the 100k-ohm one? And the answer is: two bleeders are better than one!

Al.

Hi Al,

Having 2 "little bleeders" is better than one is good practice.

I think Lawrence was really wanting to know why they had fitted the 1 Meghm instead of a lower value like 100k. Having done a bit of surfing on t'internet it seems that the vibrator unit companion to the AR88 used an OZ4 valve for rectification. According to its spec the advised max current is 75ma, this will I guess be running at pretty near maximum rating. I wonder if the higher value bleeder is to sympathise with this.Of course battery duration would be improved by about 4 percent as well.

Mike

[Skywave]

Mike - Why two 560k-ohms resistors and why those particular values? Terry has supplied a possible answer in Post #17; for myself, I simply don't know.
However, your comment about the OZ4 does 'fit in' with those chosen resistor values. With regard to this particular AR88, whilst it is my ownership, it is extremely unlikely that it will ever be powered other than from a.c. mains. In the event of my selling it to a new owner, (a) I think it is highly unlikely (but not totally out of the question) that it will be powered from a source other that a.c. mains; (b) whenever I do make any modifications to a receiver, I always thoroughly document any such changes - and pass that documentation on and try to arrange that any such changes can be easily reversed by that new owner, should he/she so desire.

Al.

[Michaelr]

Hi Al,

I am not suggesting that you do not do what you are doing on the contrary I would do it If I had an Ar88.

Purely in answer to Lawrence.

Mike

[pwdrive]

It is interesting to note that they are not shown in the AR88 manual but they are on the AR88D so I am guessing a modification by the manufactures, also from what I have read they changed the filter caps from mono bloc to seperates at some point (not sure which way round chronologically that would have been) I wonder if the max wkg DC voltage were the same beween these two different types of arrangement.
I think I too will lower the original value when mine is next on the bench.

Lawrence.

[Skywave]

Lawrence: Do you know if when the caps. were changed from mono-blocks to separates, was there a change of capacitance values accompanying that change?

Al.

[pwdrive]

Hi Al, I can not say with any certainty,my source for these capacitor changes was:

http://www.radioblvd.com/ar88.htm

Part way down page...RH column headed power supply....toward the bottom of that column.

The owner of that site, Mr Henry Rogers might answer a query if you E Mail him, he has helped me in the past as regarding AR88 and HRO serial numbers and he seems very knowledgable.

Lawrence.