Hey there.
This might be a silly question... and maybe even a silly question where the answer depends on so many factors its impossible to answer with the provided information but I'll give it a shot.

Is it a problem to use different capacitors of the same value? Slowly started recapping my Otari MTR-90 MKII and it got loads of 33uf 35v. caps on every audio board. Since I got a few Panasonic NG and FM 33uf at 100v around I don't need for other things and money is tight would I encounter any problems if I put those, then later replace the rest with FMs at for instance 35v or 50v.? Or even just try some Pana FRs since thats what my local electronics store got in stock?
Or am I likely to have audible difference between tracks or odd phase issues creeping in on drums or when I bounce stuff?

In the first place I'll just replace dodgy looking caps and then likely start off by replacing all caps of a certain value on all boards (plenty of 33uf and bipolar 10ufs). The machine sounds very good and got minimal issues, but since these caps are likely stock from '84 at the very least I prevent issues and might even improve the sound slightly.

No good at reading schematics, but trying to learn. I attached the schematics for the audio boards. Are the 33ufs responsible for providing power?

Sounds like you have more time than money. So, maybe you should do an experiment.

As an engineering prophet: These sound like power supply bypass caps and are unlikely to change the audio performance if replaced with anything reasonable near the current value.



-tINY

I have a MTR 80 in now. First thing is you need to measure the DC offset on the input and output pins on every audio path opamp. You will find polarity varies on the bipolar caps. I replace those with polarized caps after DC offset polarity is measured. 47uf/25V Panasonic FR series are a good swap, very small.

Replace the opamps with LME's or OPA's and you will be able to remove around 1/2 of those caps, no longer needed as the DC offset goes away. I use the largest caps I can fit for the power supply filters, 1000uf/25V Panasonic HD series.

For something of that age, new capacitors will be a huge benefit. If you are in for the long haul, make sure the supply of capacitors that you do have aren't shelf shot, as electrolytics age regardless of being installed or sitting on a shelf. FM and FR have the same ripple current and impedance, however FR has more than double the life cycle. The NG, maybe NHG? Those have a much lower ripple current and have 5 times shorter lifes span than the FR. I just bought a haul of 5000 capacitors last week and got my 47uf/25v FR series for $0.04/ each from Newark/Avnet, so a few dollars can give you peace of mind.

Thanks Really hoped for something like this.

Oh man, this hits a nerve cause that mod of yours been on my mind for a long time now but since I don't know how to measure DC offset and I've even been too chicken**** to attempt chip swapping on any of my DBX compressors yet this seems like bit of a mouthfull... also financially.
So might do the boring thing and just replace the 10uf bipolars with Nichocon Muse (?) which is the only well priced BP cap I found in that value that ain't from some obscure brand. Is it possible to measure DC offset with a run of the mill multimeter and the boards out of the machine?

Your head amp mod seems like something within my league though and will research it further.

So just 1000uf caps in the PSU? I expected some huge 10000 uf monsters like in my DDA, what a relief.

The mods to the DDA combined with the transformers in the PR99 seems to make the treble and mids a fair bit more pleasant so despite I don't doubt you're right, a major mod just seems like a lot of work and money compared to a straight swap. But so far I replaced the two 1uf electrolytics with Wima film caps... might make a differerence over 24 channels, who knows?

Oh, wasn't aware of the short life span of the NHG series... never mind those then. They also sounded like **** in my DDA, harsh sounding treble. Strange. But the preamps are modded now anyway and sounds lovely. Definitely in it for the long haul, it's my bread and butter and my whole lifestyle circling around this fascinating type of financial suicide.

FM, FC or FR it is then, thanks :-)

1000 uf are used on each of the 24 record/play cards, not the power supply. The head amp mod uses the Brown Dog HA 120xx conversion adaptor and a quality low noise opamp. I like the ADA 4898-1 here, .9 nv noise, 55V us slew rate and less than 1.5 ppm IMD. A few changes are also needed to the head amp circuit, I remove the 10 uf cap off the head input and replace that with a ferrite bead for rf rejection. Then it's direct coupled meaning the opamp input bias current reads the head impedance, not the passive large value resistors loading it. That lowers hiss so you never hear any until tape rolls.

Archiving guys love that stuff, "no electron left behind".