Recapping 488 mki advice

[John Hostetter]

So my beloved 488 has a weird thing going on Channel 6. Before recording, everything monitors fine on this channel (levels through headphones, meters), and nothing is weird during recording (headphone monitoring, etc). However, when I play this channel back after recording (on either Cue or through the Mix) whatever is recorded is low (won't exceed -12 db on the meter) and muffled sounding. This ONLY occurs on channel 6.

This would mean that the problem would be on the R/P pcb right? Because this is the only part of the schematic that I can see that would only affect channel 6? I could be very wrong, which is why I am posting here.

I went ahead and took out the pcbs and am ready to replace the electrolytics on just the channel 6 portion of the R/P pcb and see if that will do anything.

Any thoughts or advice? I am an amateur who has taught himself to read schematics. I've built tons of guitar pedals and synth modules, but troubleshooting a Portastudio is new to me. I'm the first to admit I don't know what I'm talking about, so any advice is appreciated!

 

[SkywaveTDR]

This is why taking a board out of a unit is the wrong thing to do. You should try and emulate a Technician and he would not take the board out unless there was a sure reason he had to. I have seen the work on some guys changing caps on boards and my comment is they were better off just to send it in as they did not solve anything and now I have to work on it. This guy attached all the caps with silicon glue to the board. Why? He had a blown out Op Amp which he never addressed.
What you are doing is shooting in the dark and that usually takes very long time if not never fixes it. Record problems of only one channel often can be head dirt. If not that then bias should be checked with a scope to see if it is like the other channels- sometimes there are dirty relays or broken solder joints. Record function is one of the hardest to find as it combines two systems- the audio portion and then a bias portion mixed with each other- either one missing will stop record.

 

[John Hostetter]

Thanks for the response! It's good to hear that I've been looking in the wrong direction. Sure enough, I put it all back together and everything is exactly the same. I am reasonably competent when it comes to disassembly, reassembly and soldering, but you definitely saved me some time...as I was considering continuing the capacitor replacement.

I have been keeping up with cleaning the heads almost every use, so maybe I can rule out the dirty heads?

I have a scope so I'll give checking the bias a shot, although I'm not very familiar with the process. I do have a copy of the service manual, so we'll see if that gets me anywhere. If not, its going to the technician. I'm sure you'll all recommend I send it to the tech right away but I can't help myself! lol

Thanks for the input, Skywave!

 

[bgencarelle]

I am with Skywave, it is possible the heads are dirty and that you need a slightly better cleaning agent.

 

[SkywaveTDR]

I use denatured (Ethanol) to clean heads and other stuff- I buy it in 5 gal can if that tells you how much it is used. Iso is only used to clean PCB's and for cuts.
There can be other things going on such as record pot dirty or other things- some unit use relays.
I first confirm that play is correct as some record problems show up and then play is at fault.
That is why the MXT test tapes are used. Sometimes these can be a problem to figure out and resistance to sending it in to a Technician is fine but this is a pretty advanced unit for a beginner and experience with them is a plus to have on your side.