Problem with Peerless 4651A - Altec/ RS124 input TX

letterbeacon · Apr 23, 2020

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 · Apr 23, 2020

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?

letterbeacon · Apr 23, 2020

EmRR said:
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 · Apr 23, 2020

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.

letterbeacon · Apr 23, 2020

Is this big hi pass effect the symptom of a broken transformer, something can’t be fixed?

Is this a common occurance?

mjrippe · Apr 23, 2020

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 · Apr 23, 2020

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 · Apr 23, 2020

letterbeacon said:
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.

mjrippe · Apr 23, 2020

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 · Apr 29, 2020

mjrippe said:
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 · Apr 29, 2020

Look up those level references

letterbeacon · Apr 29, 2020

EmRR said:
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 · Apr 29, 2020

They generally aren’t far apart.

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

letterbeacon · Apr 29, 2020

EmRR said:
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.

gridcurrent · Apr 29, 2020

letterbeacon said:
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.

letterbeacon said:
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.

letterbeacon · Apr 29, 2020

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 · Apr 29, 2020

letterbeacon said:
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.

letterbeacon · Apr 29, 2020

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 · Apr 29, 2020

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 · Apr 29, 2020

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.

Problem with Peerless 4651A - Altec/ RS124 input TX

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