Several DAP310 Cards Available

Over the years, I’ve managed to accumulate a few more cards than I need for repair inventory purposes. Specifically, I have several Equalizer cards for sale. They are fully restored, tested, and guaranteed. While I have some other cards as well (to be offered at a later date), they are not yet refurbished.

Trying to be fair, I’ve set a price of $30 each plus shipping, which is about what I charge to refurbish one. If you need one, please send me an email at admin at First come, first served.

Update on Parts Availability

While I wasn’t looking, all major manufacturers have discontinued the 8-pin DIP plastic version of the ubiquitous LM301 op amp. When such a popular part is discontinued, there’s usually a years-long supply in multiple warehouses. I have had little luck finding any 301s in stock at an affordable price. While I don’t need any at the moment, they are used extensively in the DAP 310. Sure, there are many chips that will work as a replacement, but a lot of my customers ask that their units be maintained as close to stock as possible.

For some reason, I’m seeing an increasing number of LM741 failures. I cannot imagine 741s being discontinued, but I also never imagined the 301 being discontinued. It’s possible to get an SMD version and solder it to an adapter board that plugs into the 8-pin socket, but that solution adds time and money to a refurb. I just bought a lifetime supply of 741s.

The potentiometers on DAPs (Bourns 3386H-1-202LF, primarily) are now over $1.00 each in quantity. While that doesn’t seem like much, I typically don’t charge extra for minor parts when I refurb a DAP. I have a pretty good supply of those pots that will last a while, but it looks like I may have to charge regular labor plus parts for a refurb. Everything is getting more expensive.

A lot of people over the years have removed Expander cards from their 310s, and they’ve apparently discarded them. I regularly get requests for them, and I don’t have them. I’m seriously considering producing one. If you’re interested in getting some, please let me know.

Repairs Available

If you have a DAP310 for repair or refurbish, please let me know. I’m now accepting repairs. Contact me by email, and I’ll provide you with my contact information. The same caveats apply: Meters, meter bezels, FETs, and several other parts are generally not available. Some DAPs have been butchered beyond repair.

Repair costs are still very reasonable. Most of the (available) parts are in stock. A real concern is that shipping costs are increasing significantly. If you have one you want repaired or restored, don’t wait. I have the time now, but that could change. I also repair and restore Leslie 122, 147, 251, and other model tube-type amplifiers and some tube-type guitar amplifiers.

My direct email address is admin at this domain.

Dorrough 610 Processors

While this site is dedicated to the Dorrough 310, of late, having seen fewer 310s come in for repair, I’ve had the opportunity to play with several 610s. The two most recent I’ve seen were actually scheduled for disposal. Thankfully they have been fully restored and will be returned to the customer soon. Between the two, a lot of repairs were required:

  • Shorted tantalum capacitors
  • Updated several tantalums to electrolytics
  • Two dead LEDs in a meter
  • Two bad ribbon cables
  • Bad op amp
  • Two bad pots on the front panel
  • Bad RFI filter on IEC connector
  • Cleaned all switches
  • Flaky circuit board interconnects
  • Correlated schematics to circuit boards
  • Removed modifications

At this point, I have been able to restore every 610 to normal operation. The plastics on the front have deteriorated and gotten brittle. I’ve been able to stabilize most of the mounting studs, but some are still problematic.

If you have a 610 you want me to restore, send me an email. I may be able to help. These units are very different from every other piece of broadcast equipment I’ve repaired, but they are pieces of history. They command high prices on eBay, Reverb, and Worthpoint.

DAP610 Assistance

This site is for the DAP310, but as you probably know, Mike and Kay Dorrough created a magnificent follow up to the 310 called a 610. A regular customer asked me to look at one for him. I’ve repaired a couple of them, but this one is exhibiting very strange problems. If you have repaired one of these, please send me an email. I’d like to pick your brain a little bit. UPDATE: See my update in the comments section of this post.

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


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.