Deltalab Effectron repair needed

[pvision]

Looking for someone in the UK who actually knows these things and who can diagnose & repair. Mainly ADM 1024 models. They all have the same fault - audio passes OK but no delay output

If anyone has succeeded in finding the root cause of this problem, please chime in!

I have one on the bench which passes delay signal occasionally but it's accompanied by random whooshing & banging. The output pot on these has the dry signal fed to a centre tap, inphase delay to the RH tap and antiphase delay to the LH tap. I get the dry signal with the pot in the centre position so I know the audio path is good. Signal goes walkabout somewhere in the digital section

 

[JohnRoberts]

Delta modulation uses a simple 1 bit delay technology. Troubleshoot the A/D convertor for the 1 bit data stream at its output, likewise does the output of the RAM chips show the same 1 bit data stream. The deltamod DAC is a simple up/down integrator.

Most likely suspect is faulty memory chips or solder joints for address lines.

JR

 

[Ricardus]

Ah the classic Effectron. We had one in the rack in the 80s when I was mixing a Zepplin tribute band.

 

[pvision]

JR

Appreciate the advice but I'd be better off finding someone to do this than me sitting with my scope trying to make sense of it - I have tried more than once

In reality it's above my pay grade

 

[JohnRoberts]

Back when those were being manufactured RAM memory chips were expensive, and there are probably several RAM chips wired in series. It might be as simple as swapping a few out to find the bad one(s)....or not.

JR

 

[pvision]

My feeling is that the problem lies not in the pure digital part, but in an analog switch CD4016, etc. When the thing works, the test signal is accompanied by whooshing & banging noises that sound very un-digital

I can't understand why they might be intermittent - most faults are permanent. I've tried tapping components & flexing the board but haven't had any joy. I don't have any freezer spray so should invest in some

They're not easy things to grok from the schematic or from the board, but I appreciate all suggestions

 

[JohnRoberts]

The delta modulation a/d probably uses multiple CD4000 series CMOS logic. Simply the A/D conversions involves an integrator that creates an up/down triangle wave. This integrator output (triangle wave) gets compared to the input audio signal. A shift register captures the digital state of the comparator. If the triangle wave is still below the audio level the shift register latches in a digital 1 (telling it to keep ramping up) and shifts it in for one clock period. After the next clock cycle it looks again. If the triangle wave (integrator output) has caught up to exceed the audio level the comparator changes state to digital 0, and starts shifting digital 0 into the memory, telling the integrator to start ramping down. So in practice this triangle wave chases the audio signal.

Some advanced delta modulation schemes use a variable rate up/down so if the stays high or low for multiple clock periods the rate of change increases. If the clock frequency is high enough the sound quality is respectable, but this requires a lot of memory chips to deliver significant time delay. At lower clock frequency to squeak out more delay the sound quality gets raspy.

With a scope you can troubleshoot the A/D by looking for the comparator and triangle wave. The comparator output will look like a stream of digital ones and zeros, modulated by the input audio.

The playback decoding is simpler. The digital stream of ones and zeros gets fed into a simple up/down integrator.

JR

 

[auddoc]

I used to do warranty repairs/ service for DeltaLab, Lexicon, Eventide, Publison, etc., back in the day. I will digest the described symptom and see what comes to mind that I can contribute.

 

[NOON]

I've fixed a few of these that had an assortment of strange symptoms. Found a few bad electrolytics so went through and replaced them all and everything settled down. Considering they're decades old and have likely sat in a hot rack their entire lives it's not surprising.