Spare Boards Needed

For some reason that escapes me, some DAP owners were able to convince themselves that the Expander boards were not needed in their units. Accordingly, there are many DAPs out there that are missing Expander boards. Owners of those boxes would like to put them back into original condition.

Now is the time to clean out your junk box and get rid of those boards (Expander and others) that are collecting dust in your shop. If you want to donate them to the cause, I’ll pass them along for free. If you want to sell any boards for DAPs, I’ll consider purchasing them at a reasonable cost and reselling them at the same price.

Please send me an email to let me know what you have. My email address is admin at dap310.com.

Parts

Recently I acquired a new stock of parts for the DAP 310. My inventory now includes most parts EXCEPT the meter bezels or the FETs. If you have a DAP that needs repair, let me know. Most repairs are $200 or less. You’re also welcome to send me an email if you’re having a problem with a repair. admin at dap310 dot com.

Trimmer Pots

The trimmer potentiometers on the 310 are not very robust. They are not designed to be turned many, many times. If a pot is in an audio circuit, it’s easy to hear when it is going bad. When it is in a control circuit, like the FET BIAS pot in a DAP, it can be difficult to diagnose as a source of a problem.

Bourns 3386H-100Recently, a 310 that I refurbished would periodically slam the low frequency meter downscale. The problem was very intermittent. As I was going through the final alignment, the FET BIAS control was very touchy. At that point, I decided to replace the pot. When I did, both problems disappeared. It was immediately obvious that the pot had failed.

Most of the pots in the 310 are Bourns 3386H-1-202 or equivalent. If you’re replacing caps and bringing a DAP back to life for long-term use, you might want to go ahead and replace the pots. They’re very easy to replace thanks to the single-sided printed circuit boards.

Electroyltic Capacitors

It’s somewhat axiomatic that replacing all electrolytic capacitors in a DAP should be the first step in a refurb. They’re about 43 years old, and they deteriorate over time. Axial capacitors are still available, but they’re getting expensive, and the selection is not what it used to be.

It’s possible to eliminate three electrolytic capacitors on the Equalizer board. The output of each of the three channels is capacitively coupled to the next stage, which is a 2K pot followed by another cap. While there is a possibility that there could be a little DC voltage on the output of one of the 301 op amps, it’s not likely.

That being said, it has been recommended that you can remove C20 on Z1 (shown as 6uF), and you can. However, there will probably be an offset voltage on the output of Z1 if you do. That offset voltage will vary with the 301 in the circuit. While the offset is very low and not of much consequence, it will put DC on the 2K pot in the Compressor card that follows. DC on a pot can cause some nasty scratching noises. You decide.

DAP EQ Board-1920

Damaged Barrier Strip

The ground screw of the input/output barrier strip on a 310 that I’m refurbishing had a seized thread that caused someone to twist the head off. After a little research, I found that the Cinch 5-140-Y barrier strip is an exact replacement. If you need to order one, be aware that many of the pictures that sellers use for these parts are incorrect. The “Y” terminal is a solder tail that feeds through back panel into the case.

Cinch 5-140-Y

Cinch 5-140-Y

The original on this one was made by Kulka, but the Cinch fits perfectly. Cinch may have been used by the factory in some models.

Remove the existing strip by removing four screws. Let the strip hang away from the back of the DAP. Gently remove the heat shrink tubing around each wire. Thread each wire into a short piece of 1/8″ heat shrink tubing before sweating it back onto the proper Y solder tab. Shrink the tubing so that none of the wiring can short against the metal case. Re-install the strip. Good as new.

The DAP 310

The Dorrough Discriminate Audio Processor Model 310 revolutionized radio processing. After its release in 1973, about 9,000 units were sold. AM and FM radio enjoyed a new fidelity that it had never seen before.

The DAP inspired a lot of radio engineers and technicians to test their favorite modifications. The crossover points were changed. The peak limiter was disabled. The release times on the compressor cards were decreased. The list goes on. A lot of people learned more about audio from working with their DAPs than they did from any other single piece of equipment. Along the way, Mike and Kay Dorrough gave generously of their time and their spare parts keeping owners and operators happy.

Not all of the replacement parts necessary to keep DAPs alive are still available. However, unless the printed circuit boards or meters are damaged, almost all DAPs can be brought back to life and returned to service. Internet broadcasters have started buying up DAPs and having them restored. They work extremely well for that application, and they sound great.

It would be impossible to identify all of the modifications that have been made to DAPs over the years. Some of them are Dorrough-sanctioned, many of them are not. The fact of the matter is, if a DAP is restored to its original condition with very few modifications, it can be a wonderful sounding piece of equipment.

There have been lots of copycats and claimed improvements by competitors over the years, but there has been only one DAP. There is no question that Mike Dorrough is the father of multiband audio processing. This website is dedicated to this piece of history that thankfully will just not go away. Hopefully you will find content here that will help you maintain your DAP or at least to reminisce about the good old days.

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