Reviving a portable Dolby 372

[MarkCkraM]

Hello from Oz,

I am hoping to find some help to revive a two channel portable Dolby 372. At the moment it operates mostly great but in play with NR on the right channel is making a scratchy static noise. It doesn't seem to happen all the time but begins and intensifies after use in playback.
I have gently wriggled out the 4 boards that are mounted on pins and given the female side a spray of Deoxit. It seemed to work quietly briefly but the noise in the right channel returned.
It would great to use it, but I think it either needs a service or maybe it's a simple thing I could adjust.

Are there any service folks here who could help?

Any advice will be greatly appreciated.
Thanks
Mark

 

[dshoaf]

Hi Mark,

I don't know if I know what a Dolby 372 is. How about some pictures with the skins off? How about a link to a schematic?

There are some excellent 'arm chair' bench techs here who can likely help out if you give them the info they need.

Cheers,

David

 

[MarkCkraM]

Hi David,

Thank you for your interest. For now here is some information from the user manual.

I will post some interior images tomorrow.

Thank you
Mark

 

[MarkCkraM]

Hi Everyone,

Here are some photos of the circuit boards.

I look forward to any information to try resolving the noise in the right channel in "play" "NR on" mode.

Thank you
Mark

 

[MarkCkraM]

Here is the last image

Cheers

 

[KentTeffeteller]

The Dolby 372 is a portable, two channel Dolby A professional noise reduction unit, intended for use with professional tape machines a la Nagra, StellaVox, ReVox half track high speed models, Otari MX 5050 B II, etc. Not a consumer model or for cassette use.

 

[MarkCkraM]

Thanks Kent,
I am using it on a IV-S and Stellavox mixer. Oddly, after opening it up to take the cards out, which I hadn't done with the larger ones, the noise seems to have disappeared. I did notice there is a bolt with spacers to secure the cards in place and the lower card had slipped out, maybe causing a bad connection? Since it is portable it may be prone to knocking etc.
Maybe this was the issue?
Thanks for your response.

 

[dshoaf]

Ok, now I'm more oriented to what you have courtesy of Kent.

Given the new news, then, I'd suggest you clean all the board connections and switch connections with DeOxit. There's a long thread on it here.

Then, reassess the situation. I'll bet the symptoms change or go away completely.

Cheers,

David

 

[MarkCkraM]

Thanks Dave, Bookmarked for eternity

 

[LexDM3]

A couple of suggestions:

1) Re-clean with DeOxit. Spray both the connector pins and sockets and then plug the boards in and out at least 5 times. While DeOxit is fabulous stuff, whatever you are cleaning usually needs to be mechanically worked a bit to get truly clean. The fact that it worked OK for a bit after you sprayed one side of the connector might support this theory.

2) It looks like the two channel boards are identical. If so, mark them and then try reversing them. If the noise goes go the other channel, the problem is in that board. If not, it's elsewhere.

My bet is that a thorough cleaning with DeOxit will do the trick. Maybe not but I hope so.

 

[MarkCkraM]

Thanks LexMD3,

I have invested in cans of both Red and Gold Deoxit products. The noise propped up again briefly this morning but then nothing 10 hours later. I think an extra thorough clean is what it needs.

So according to the link posted before I would begin with red deoxit formula (say 4 cleans and 4 plug in and outs) followed by gold deoxit formula for a clean sustained contact.

Thanks for your advice, I really appreciate it.

Mark

 

[KentTeffeteller]

Agreed, I think you have some dirty contacts here. And likely cleaned good, you're likely OK from there.

 

[MarkCkraM]

Thanks to the responses here, I am happy to say after a thorough Deoxit the scratchy static noise has ceased and the dynamic range has broadened. Thank you for the valuable advice.

 

[MarkCkraM]

Hello,
The noise returned and I proceeded with as many deoxit cleansing cycles as possible with no success. I also found that an oily film began to build up along the circuit boards so I flushed the interior out with air and arduously dabbed the oily residue out. I bought some Stabilant 22 but decided to hold off using that. Next stop is to get a full replacement of all electrolytic capacitors, as recommended by Dolby Laboratories, I think that by now this is the only way to resolve the reoccurring problem. Thankfully, a highly qualified engineer is willing to do this.
My question is, how sparing should deoxit be used? and
What are the best type of replacement electrolytic capacitors for a 1980s Dolby Type A money could buy? The ones on it look Japanese.
Thanks in advance
Ckram

 

[huiyugan]

Hello Mark,

Recently I got a Dolby 372, would you like to send me the manual about it? I cannot find any information from internet due to it's a vintage device. Thanks.

BR,

 

[Okke 70s]

It might also be a bad transistor. The quick-test is to carefully touch them (or use cold spray) while it's playing. If it makes noise when you touch a specific transistor, you know it's bad.

Replacing it would be simple, but you might have to recalibrate the unit.

 

[KentTeffeteller]

And you need a MRL test tape to realign and set the levels correctly. Dolby sounds awful if the levels are not matched.

 

[Rkatit]

Hi David,

Thank you for your interest. For now here is some information from the user manual.

I will post some interior images tomorrow.

Thank you
Mark

Hi Mark, I to am trying to revive a Dolby 372, any chance that you have the schematics for the metering and DC-DC converter. Many thanks Andy.