Dr nEon

FIXED - 13 year old DIY Calrec PQ1549 Oscillation Fault
« on: March 08, 2019, 08:34:47 AM »

Just fixed a curious fault which had developed on a mono calrec PQ1549 EQ which I built some years ago. Thought I'd post the fix , as it may help others narrow down the problem, if they experience a similar fault..

Symptoms:  On plugging in and engaging the eq, with all the gain controls flat, the sound produced was thin and piercing, as if the hi mid band was turned up to maximum on a very narrow Q.  I milked the front panel controls and found that neither of the hi mid or lo mid gain controls were having any effect.  The hi mid & lo mid frequency knobs were functional, and so I could sweep the frequency of the narrow Q sound.  At the highest freq point on the hi mid band, the EQ would go completely into self oscillation...squawk..ouch.. :o

I then realised that this fault was actually making the hi mid band boosted to max, and simultaneously making  the lo mid band subtracted to maximum cut.    The hi and lo bands were functioning properly.

This eq resides in a home studio, and isn't subject to heavy daily use, but this fault occurred last year,  during a heavy mixing session with all my analog gear switched on. ( It was an intensely hot summer, and a few bits of kit developed faults during this period).  I couldn't stop the session to execute repair, but I did try a quick subbing of ICs for new ones around the mid band areas ( I use NE5532 for lo and lo mid, and LM833 for hi mid and hi). This did not remedy the fault.

Finally I have time to look at this and yesterday , I subbed ICs again just to be sure, and the fault persisted.  looking at the schematic, I wasn't convinced, anyway, that it could be op amps  at fault, due to the nature of the practical layout -the way the physical IC's duties are split, and shared with parts of the circuit which were still working.

So looking around the area where Lo mid meets hi mid, for a fault that could influence both these bands... after making sure that DC voltages were good, and that no shorts had developed anywhere,   I decided to start checking the electrolytic and tantalum caps.   It made sense to me to start with  the 22uF electrolytic on the output of IC8A.   A quick dirty in-circuit check with a DMM capacitance range showed no reading at all on that cap.  The same quick check on all other caps showed sensible readings, so back at the now suspect cap on IC8A output, with my analogue ESR meter, the needle didn't deflect, i.e the cap's ESR had gone so high it's off the scale.

Popped out that suspect cap - a Panasonic FC 22uF 25v - fitted a new FC (of the same value)  and now the calrec is back in business , working fine, and sounding as lovely as ever..  (A master stroke of Jakob to realise the value of this fine circuit, and it's usefulness to us DIYers..)

This is a milestone for me... It's the first fault I've encountered in a piece of my DIY gear, which has occurred thru - let's say - natural wear and tear.. 

The bad cap has no physical deformations..Flat top, no bulging, and the base plug is intact.  Checking it out of circuit, the capacitance DMM still reads nothing.  An Ohmmeter check shows a degree of charging, but then hovers around 4 megohms.  ESR meter shows the same as in-circuit check did..way, way  off the scale.

Glad to have fixed it, but I just wonder what the collective opinion is on why this panasonic FC cap might have failed.?   Like y'all, I've replaced many many duff caps in old valve and solid state gear, as well as preventative recapping on some gear, mixer etc, but I confess, I haven't been expecting to need to replace a duff cap in a still relatively new piece of DIY gear. I'm pretty sensible with soldering times, and use a good soldering station, so I doubt I would have overheated it on installation.   Perhaps this is just a one-off manufacturing defect...

All thoughts welcome,

« Last Edit: March 08, 2019, 08:52:13 AM by Dr nEon »


Re: FIXED - 13 year old DIY Calrec PQ1549 Oscillation Fault
« Reply #1 on: March 08, 2019, 06:49:03 PM »
Good on you.  8) It's nice when we get to resolve issues. Well done.   At 13 years, capacitors would be due for a replacement.  A lot of preventative maintenance is recapping every x amount of years. Depending on the gear it varies. For example, An AMS/Neve VR desk gets  new caps every couple of years because the heat under the hood causes issues. But something like a trident 80B gets recaps every 5+ years as they do not run as hot.  But even if you do not have a recap schedule at 13  years it's plausible that it would need one as the caps may have been due anyway.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.


Re: FIXED - 13 year old DIY Calrec PQ1549 Oscillation Fault
« Reply #2 on: March 09, 2019, 03:24:46 AM »
Thanks, added to meta..

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

Dr nEon

Re: FIXED - 13 year old DIY Calrec PQ1549 Oscillation Fault
« Reply #3 on: March 09, 2019, 05:46:06 PM »
Cheers Jakob, and thanks Pucho... yep, makes sense,  and you're damn right about the VR. I've burned my fingers on a few, while mixin'...they should have called them Frying Faders, not flying faders!  ;D




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