letterbeacon

Problem with Peerless 4651A - Altec/ RS124 input TX
« on: April 23, 2020, 06:53:16 PM »
I'm having trouble with an RS124 build and as I've narrowed the problem down to the input transformer, I thought I'd ask in here.

I'm getting a hi pass effect which I think is coming from the input TX - a Peerless 4651A transformer. I've taken it out of circuit, connected it to my interface and used REW to run some sweeps on it, and I'm getting a huge bass roll off on it - see image attached.

I'm measuring 609r between the primaries and 2.13k between the secondaries.

I'm afraid I don't know very much about transformers, so this might be a silly question, but is my transformer irreparably broken?


EmRR

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #1 on: April 23, 2020, 07:14:17 PM »
Assuming it's the steep continuous slope, it looks like a winding is open so you are just getting leakage.  Odd you still measure resistance on the windings, those numbers sound vaguely correct.  Is there any cabling, clip leads, anything else in your testing that might instead be the problem? 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #2 on: April 23, 2020, 07:36:49 PM »
Assuming it's the steep continuous slope, it looks like a winding is open so you are just getting leakage.  Odd you still measure resistance on the windings, those numbers sound vaguely correct.  Is there any cabling, clip leads, anything else in your testing that might instead be the problem?

Sorry yes the steep slope is the transformer. I have this problem when it's in circuit, and also when I've taken it out and used croc clips, so I don't think it's any cabling.

I've just double checked the resistance on the wirings and yes I am still measuring the same results.

This might be a dumb thing to ask, but I have the primaries twisted quite tightly - could that be an issue?

EmRR

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #3 on: April 23, 2020, 07:53:21 PM »
No real need to do that, but shouldn't be a problem. 

CJ took this one apart, there's a thread.  I see he had DC resistances of 594 : 2K3, so you are in zone with that. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #4 on: April 23, 2020, 07:58:50 PM »
Is this big hi pass effect the symptom of a broken transformer, something can’t be fixed?

Is this a common occurance?

mjrippe

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #5 on: April 23, 2020, 08:16:08 PM »
If the windings measure ok DCR, perhaps you have magnetized it?  You can hit it with a degausser (tape head demagnetizer) or there is a method with low frequency sine waves that has been discussed here.

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #6 on: April 23, 2020, 08:48:43 PM »
Ok will give that a try. Just been searching on here and it seems like sending it low frequencies sine waves might do the trick. If not I assume degaussers are much the same? there's a few on eBay that seem pretty reasonable.

EmRR

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #7 on: April 23, 2020, 09:31:15 PM »
Is this big hi pass effect the symptom of a broken transformer, something can’t be fixed?

Is this a common occurance?

It's a symptom of an open winding, usually.    I've never seen it anywhere else.    Perhaps it's shorted somewhere it shouldn't, and still gives a resistance reading because of it. 

I wouldn't think magnetization would cause such a slope. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

mjrippe

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #8 on: April 23, 2020, 09:53:40 PM »
Yeah, that is really steep.  Doug is probably right, don't buy a degausser.  Try the LF sine wave thing and if that does not help it is probably toast.

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #9 on: April 29, 2020, 08:37:51 AM »
Yeah, that is really steep.  Doug is probably right, don't buy a degausser.  Try the LF sine wave thing and if that does not help it is probably toast.

I tried the sine wave thing, and sadly it's had no effect.

I had a look in my transformer box and I've found a UTC A-18 transformer. According to the UTC catalogue, its specs are: 15k:80K +15dbm

The Sowter input transformer for the Altec 436 specs are: 15k:80k 37dBu

Apart from the discrepancy in headroom, they seem to be the same. Could I get away with using the A18 as the input to the RS124? Would it just mean I couldn't feed it as much level?


EmRR

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #10 on: April 29, 2020, 11:16:36 AM »
Look up those level references
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #11 on: April 29, 2020, 11:37:04 AM »
Look up those level references

I've been re-reading this link:
http://audiojudgement.com/how-to-calculate-decibels/

My head is spinning a bit, but I think I understand the difference between dbm and dbu, but not how one would relate to the other. Where would you recommend I start?


EmRR

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #12 on: April 29, 2020, 12:00:09 PM »
They generally aren’t far apart.

Consider +40 dbm is 10 watts.  Look at your own typical send levels. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #13 on: April 29, 2020, 12:11:38 PM »
They generally aren’t far apart.

Consider +40 dbm is 10 watts.  Look at your own typical send levels.

Thanks Doug. I don't fully understand it, but if +15dbm is just less than 5 watts, I'm sure I don't put out that much when I send from my interface, so I should be fine.

I'll do some more reading I think.

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #14 on: April 29, 2020, 12:48:15 PM »
I had a look in my transformer box and I've found a UTC A-18 transformer. According to the UTC catalogue, its specs are: 15k:80K +15dbm
The Sowter input transformer for the Altec 436 specs are: 15k:80k 37dBu
+30  dBu for part # 1231.   Sowter's numbers  appear a little off.
Apart from the discrepancy in headroom, they seem to be the same. Could I get away with using the A18 as the input to the RS124?
you could use the A-18.
the A-18 is rated at +15,   it is the input transformer on the Scully 280;
A-10 on LA-2a ,  both devices handle +4 dBu.
actually any 1:1 would function,  there is a plenty of input gain in the 436/124.
« Last Edit: April 29, 2020, 01:03:20 PM by gridcurrent »

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #15 on: April 29, 2020, 12:51:20 PM »
Thanks a lot - that's much appreciate.

Now I've just got to remember which project I bought an A18 in the first place? Glad I did now anyway!

EmRR

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #16 on: April 29, 2020, 02:57:37 PM »
Thanks a lot - that's much appreciate.

Now I've just got to remember which project I bought an A18 in the first place? Glad I did now anyway!

Probably for a UA limiter interstage. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #17 on: April 29, 2020, 06:25:23 PM »
So I tried the A18 in the RS124 circuit, and I'm still getting the high pass effect out of the compressor.

I took the A18 out of circuit and just played some music through it from my interface and I'm still getting the high pass effect.

As an experiment, I then took at A20 I had lying about and played some music through that and again I was getting that tinny hi pass effect.

Could this be an impedance mis-match with my interface? I've never had a problem before.

I don't know enough about transformers to know what this could be though.

I suppose the good news is that perhaps my Peerless 4651 isn't faulty after all.

I'm using an RME Fireface 400 as an interface, which has an output impedance of 75r and an input impedance of 10k.

Any thoughts would be much appreciated...

volker

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #18 on: April 29, 2020, 08:00:27 PM »
10k is too small of a load for that transformer. You could try it backwards as a stepdown. Other than that, have you rechecked all components? Coupling capacitors, grid resistors etc. Have you ruled out the output transformer and associated wiring?

letterbeacon

Re: Problem with Peerless 4651A - Altec/ RS124 input TX
« Reply #19 on: April 29, 2020, 08:07:19 PM »
Thanks for your reply.

Yes I’ve double checked all the components and even rebuilt it from P2P onto turret board and still no luck.

Is there a way I can build a dummy load for transformers so it sees a higher impedance? I think I’d like to try and do a frequency sweep with it out of circuit so I can totally rule out it being a bad transformer.


 

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