Hi.

I have a couple of these old broadcast limiting amps, made here in Melbourne in the 1970's by 'Design 2000' (yes, awesome name!)

One of the units was not working when I bought it, so I took them both in to the 'Design 2000' office (which still exists in Upper Ferntree Gully!) along with the operational notes, circuit etc, and they said they'd get one of their old guys in at some stage to have a look over it and see if they could get the second unit up and running for me.   They were pretty excited to have a look as they are pretty rare units, well regarded, and they don't even own one themselves anymore.

Well, unfortunately nobody there seemed to get very far as they were not successful at making any progress on the non-working unit, and when I got them home again the 'working one' doesn't seem to do any limiting anymore, either!

hmm.. anti-climactic.

The amplifier design is based around the opto-coupler approach to limiting, with the original opto cell being a smallish black block  (might be called MTD 533-6?) but the manual also includes a service notice, with possible / recommended upgrade to PL5533-2H type cell, which is red in colour.  From what I can tell, the later units came from the factory with the red cell.

I have one of each type. 
Interestingly, one of the control module boards is installed the other way around (ie. 180 degrees rotated in the case).. not sure what that's about.

Anyway, with neither of my units working (one passes audio and amplifies, but no gain reduction) .. I think I need to get replacement gain cells for these .. Can anyone recommend how I even begin to work out what type to get, if they are still available at all, or should i try to rebuild the existing ones with new LED &/or LDR components?

Couple pic's of the units in question:

black cell unit


red cell unit


Circuit

Michael Tibes

Just some thoughts: before you dive too deep into assuming the LDR cell being the culprit I'd do a serious problem analysis - or replace all the electrolytics and tantals first and analyze then. Also checking the LDR should not be too difficult, I assume that the two connections before the input amp are probably the LDRs and that the third pair connected to the sidechain amp should be the LED. My gut tells me to leave the LDR unit alone in terms of soldering as long as possible because if it was still ok it might break in the process - and it might be the one thing in that unit you can't replace with original specs because you don't know them. LDRs differ a lot in terms of resistance and timing.

In my opinion it should be possible to test the LDRs as well as to basically fix the unit even if the LDRs don't work (obviously it would just amplify and not compress). So hunting for a replacement LDR might be the last step you need to do.

Michael

Just some thoughts: before you dive too deep into assuming the LDR cell being the culprit I'd do a serious problem analysis - or replace all the electrolytics and tantals first and analyze then. Also checking the LDR should not be too difficult, I assume that the two connections before the input amp are probably the LDRs and that the third pair connected to the sidechain amp should be the LED. My gut tells me to leave the LDR unit alone in terms of soldering as long as possible because if it was still ok it might break in the process - and it might be the one thing in that unit you can't replace with original specs because you don't know them. LDRs differ a lot in terms of resistance and timing.

In my opinion it should be possible to test the LDRs as well as to basically fix the unit even if the LDRs don't work (obviously it would just amplify and not compress). So hunting for a replacement LDR might be the last step you need to do.

Michael

Thanks Michael.  All good points.
One of my units does pass audio (and amplifies), but just doesn't limit.  This one I more so suspect the opto cell.
The other unit does not pass audio and I will follow your suggestions as to getting it to pass audio and amplify first, then look at the cell. 

In either case, I would love to learn more about how to ascertain what spec's the cell should have, and how to replicate the characteristics (but I can imagine there's just no easy way to do this!)

Cheers.
Ev

Michael Tibes

You could still first investigate whether the sidechain works and you might be able to check the opto cell still being built in. Check whether you can measure a resistance across the LDRs and how the LED behaves (open / shortcut / diode test). It might only be possible to determine the parameters of the LDRs with extensive tests - unless you're able to open a case and see a type printed on the actual LDR. There is a lot of information about the T4B on the net and you might find useful insights on how determine the characteristics from that.

Michael

PRR

It is a feedback limiter. The cell specs will not be TOO strict.

It is a question, for 1973, if the light is incandescent (slow) or LED (fast, but very new for the day). (Neon and green-glow are ruled out by 20V supply.) Clearly if they could get LEDs, they would use them. I've sketched how I understand the guts of the PL5533.

But of course the other responses are correct. You don't need the cell to pass audio; basic audio troubleshooting gets you there. You do not need the cell to diagnose the no-limit. If you put an LED in the LED-side of where the cell goes, does it blink audio peaks?

gyraf

R53 + RV1 (1K +10K) across -20V indicates max. current adjustable between ca. 1.8 and 20mA. Which supports the theory of LED's, as it's too little to reliably light an incandescent bulb..

Simply verify all working voltages, and then trace signal through the unit..

Jakob E.

 
..note to self: don't let Harman run your company..

jayRC

Thanks for posting your experiences with the limiter.

7HT in Hobart used one of these Design 2000 transmission limiters in the early 1970s.  From memory, it worked well enough, if basic (no phase rotation or clipping). 

I'd agree with the advice so far, i.e. it's unlikely to be a faulty optocoupler, more likely to be capacitors or perhaps an IC. 

Note that the circuit actually shows the opto module as PL 5S33 , not '-5333'.  Even so, the only tech details I could find online are listed on a UK component supplier's site, littlediode.com

PL5S33

Product specifications
Decay Time     100mS
V(isol) Max.  3.0kV
R(on)     250 ohms
R(off)    10Mohms
Outp Brkdwn Vlt Min  80
PD Max.  400mW
Min On Current (Cntrl)    25mA
Min On Volt (Cntrl)    2.0V
t(r) (S)    100uS
Type of Inp    LED Lamp
Number of Inp Chan.    1
Number of Outp Chan.    1

https://www.littlediode.com/components/PL5S33.html

Similar opto-coupler units:

Vactrols
http://denethor.wlu.ca/pc300/optoisolators/analogoptoisolatorintroduction.pdf

or these:

Fairchild H11F1M, H11F2M, H11F3M
Photo FET Optocouplers
https://www.fairchildsemi.com/datasheets/H1/H11F3M.pdf

Silonex used to make a large range of opto-couplers and photo devices.  The parent company changed hands a few times but I think it still exists (just like Design 2000).  An Australian company, Crest Components, lists Silonex parts: www.crestcomponents.com.au

Here's a useful Silonex app note via Crest's site:
http://www.cresttech.com.au/pdf/Silonex/levelcontrol.pdf

Good luck with your fault finding.
« Last Edit: October 26, 2017, 12:58:42 AM by jayRC »

Thanks for posting your experiences with the limiter.

7HT in Hobart used one of these Design 2000 transmission limiters in the early 1970s.  From memory, it worked well enough, if basic (no phase rotation or clipping).  ...

Thanks so much for your helpful replies, guys!

I think I had somehow managed to miss the most recent couple, but a conversation with friends broguht me back here to find more answers.. 
Legends!

Ricardus

Is everyone seeing the pics? I'm not.
Audio mastering for hire..