Ensoniq DP/4+ REPAIR (cap leakage, corrosion)

[58Glammer]

yes you kink, bend the legs.

clean off the existing solder. then make one soft pillow on one pad and slide the kinked leg on. Make sure it is properly bedded.

I do it like this …

 

[Obsoletemachines]

hi, i make mine a bit shorter than yours.. remember parasitic capacitance.
for 3.3uf i use SMD caps
Also to remove caps easily and not do damage to things like the 18khz anti alias filters use Kapton tape and a heat gun. Be careful with heat levels because the PCB on the DP4+ is fragile and too much heat will cause flex.

 

[suchenderxxx]

Hi,

im repairing a DP4+. I recapped the unit. Changed an defective Opamp. Replaced the potentiometers.

Sofar so good. But i have a problem on Channel 1.

If i play a sine Wave i get distortion. If i play a Kickdrum i get a high frequencie noise which sounds modulated like the kickdrum.

i put an sine wave generator on the outputs of 1+2 and this one sound same on both channels. So the problem area must be the Input?

I tried to measure on different Opamps. But i dont know how to measure the noise and to find the problem.
Because on my Oscilloscope i can see the Kickdrum or the Sinewave but both channels look identical.

Now i want to measure and to find the problem in the circuit but what is the best approach know?

 

[suchenderxxx]

So, there we are… with the analog board. That‘s my 4th unit so far … still not tested, but cleaned.. I followed Obsolete Machine‘s tip to use Nichicon FGs here in the audiopath as I had some here.. Renewed the pots as some of them were pretty nasty covered with electrolyte and corroded.
Also renewed some of the diodes and a 4,7 pF cap that was burned. I redid all areas where ecos sat, overworked the black traced and reflowed all holes to the other side of the PCB. Also took off the Sony ICs as the legs were corroded as hell.. I had to clean them legs and get off the old solder and spilled electrolyte. Hopefully the ICs can take a lot of heat amd still work. It was a pain to get them out without harming the solderjoints. A lot of passion needed here. Finally I put them onto sockets.
This time I just used through hole caps.
The problem I already see here using the through hole versions is troubleshooting. Much harder to get to the little parts. And if you find a fault, you might take off some of the elcos again.
Digital board is also finished and already working (tested in another unit) I will post a extra reply to that.

Where did you get the Trimpotis? I searched for them but i have not found them.

 

[Script]

Look for 'single-turn' trim pots.

 

[58Glammer]

Where did you get the Trimpotis? I searched for them but i have not found them.

Got them from RS ..

 

[58Glammer]

If i play a sine Wave i get distortion. If i play a Kickdrum i get a high frequencie noise which sounds modulated like the kickdrum. …

Try to adjust pot1 „Dist. Adj. Ch 1“ I just use a headphone, put the input gain of the channel very low and the output on max. Then I turn the pot for Ch 1 and leave it where I hear no more distortion.

Btw. I use music on the input for that adjustment.

 

[Script]

Just finished repairing a DP4 classic. But different PCB and all thru-hole. So I put it in a separate thread.

https://groupdiy.com/threads/ensoniq-dp4-classic-rev-c-repair.85147/

 

[Obsoletemachines]

Where did you get the Trimpotis? I searched for them but i have not found them.

Bourns still available from Mouser.
Did you burn them.
use Kapton tape to avoid this.

 

[Obsoletemachines]

you need a THD analyzer to do this properly.

Try to adjust pot1 „Dist. Adj. Ch 1“ I just use a headphone, put the input gain of the channel very low and the output on max. Then I turn the pot for Ch 1 and leave it where I hear no more distortion.

Btw. I use music on the input for that adjustment.

 

[suchenderxxx]

Bourns still available from Mouser.
Did you burn them.
use Kapton tape to avoid this.

Yes burnt one of them, maybe my audio Problems on channel one are from this Poti?
It is the DC Offset Poti for channel 1.

 

[suchenderxxx]

you need a THD analyzer to do this properly.

so i would need an THD Analyzer, are there any affordable options?

 

[suchenderxxx]

Try to adjust pot1 „Dist. Adj. Ch 1“ I just use a headphone, put the input gain of the channel very low and the output on max. Then I turn the pot for Ch 1 and leave it where I hear no more distortion.

Btw. I use music on the input for that adjustment.

I will first change the burnt trimpoti for DC Offset on channel 1. then i will try adjusting the Distortion on this channel.
Thank you.

 

[Obsoletemachines]

Yes burnt one of them, maybe my audio Problems on channel one are from this Poti?
It is the DC Offset Poti for channel 1.

nah that pot makes a minute difference and does not account for 'bad dc offset' which is clipping s/h chop signal

 

[58Glammer]

you need a THD analyzer to do this properly.

I know .. works quite well with REW.. Next time I will compare the methods.

 

[suchenderxxx]

I know .. works quite well with REW.. Next time I will compare the methods.

what is REW?

 

[58Glammer]

what is REW?

A very nice program where you can e.g. measure THD .. also generates different Waveforms, you can scope signals.. ect You need to check it out

 

[Script]

Very little progress on my wrecked Unit A (for lack of time and priority).

Finally had time to compare DC on all op amps, AD and DA converters. Was comparing between wreck Unit A and a working Unit B.

All pins deviate between units by -2mV referenced to ground. This might be a symptom or not (some leakage somewhere). Also +/-15V rails are slightly off by those -2mV.

More importantly, though, voltages on some pins of the converters deviate substantially. Working CH 2 measures mostly the same as on my working Unit B.

So all leaked areas need checkup again for connectivity. Looking also at C238 and periphery. Thanks to Obsoletemachines for the hint above.

Fingers crossed the converters themselves are still alive.

 

[Script]

Set out replacing all not-yet swapped out dual op amps on the audio board today. And as I thought, it made no difference.

Ch 1 dead
Ch 2 fully working
Ch 3 dead
Ch 4 working but distorted.
Outputs all work fine.

Noticed that a resistor R296 on input 4 was broken. Don't have 9k31 so I hacked in parallel (16k || 22k). The two resistors rest sandwiched on their sides towering up. A bit tricky but doable.--
Makes me think there might be more broken components across the board.

 

[Script]

Briefly checked signals before and after the styro caps. They look the same on scope across all channels. Was quite sure the problem isn't there.

Also swapped some HC4053 around, including U4 and U5 -- unit behaves the same, no difference.

So next step is to check the AD converters. Idea is to place them in working Unit B. Got one out without damage to the board...