Yet another Crown DC300a

[Jerry_]

Happened across a dead DC300a, and I'm interested in doing some low cost poking around. When powered on, it instantly blows fuses, and a quick check shows that all of the transistor cases show continuity with the chassis ground. Visual inspection doesn't show anything obvious. I'm guessing that perhaps the insulator on one or more of the output transistors is no longer insulating properly, but I'm early in the investigation. The output and driver transistors are visible in the image below:

From what I've read here and elsewhere:

Output Transistors 3990 = 2N5631 ~ MJ21194G ~ (or 2N3773 with mods noted in the service manual)
Driver Transisitors 5402 = 2N5804 ~ ( I've also seen 2N6306 used)

To get started, I'll need to pull and check the existing transitors and most likely replace them which won't involve much cost, but it will involve some labor. I can't see testing and reinstalling the originals with new insulators as very practical, so sourcing replacements, attempting to match them and finally replacing them will be my first step. I have already removed and tested the big filter caps, and they are in pretty good shape.

I have enjoyed the Tom Waits thread and other DC300 threads, and I know this amp has a varied following with plenty of people in the "boat anchor" camp. I had the original DC300 previously, and I liked it. Also, a winter project for the bench is always a good thing. Any insight on preferred transitor type or sources is appreciated. Also, any insight regarding common failure points or issues that must be addressed on these is appreciated.

 

[ConradH]

If you have a curve tracer, check the devices for breakdown voltage. Some of the originals, at least in mine, were marginal. Remember that they're matched by Vbe, not Ib. Be sure the zeners and unobtainium weird preamp chip are good. There were a couple joints on a switch that were never soldered on mine! Do a good visual inspection of everything. IMO, SQ is better than some but not as good as the best.

 

[Blue Shadow]

. There were a couple joints on a switch that were never soldered on mine!

Found the internal fuse holder in a D-150a not soldered to the wiring, yet the amp worked just fine. Former Crown employee I talked to said, 'stuff happens' and it wasn't unheard of. But in both cases these were Crown amps and probably worked just fine. I know mine did.

 

[UncleBingo]

21194G has turned out to be a Godsend for me across a wide range of repairs.

 

[Jerry_]

Very helpful input. Thanks!

I've been looking at transistor matching, and I'm not clear on whether you match them in pairs or need to match all 8 (and how closely). I'm guessing all 8 based on the batch numbers inked on the transistor cases. Then, I have to wonder how many are required to get 8 matched.

 

[UncleBingo]

I haven't worked on a DC 300 but I've never matched output transistors in any high-current amp other than a VFet amp (and those were marked on the case).

 

[Jerry_]

After researching a bit more, it appears that those "smokin' deals" on 2N3773 transistors on eBay are NOT the way to go. Of all of the areas I wouldn't expect counterfeit product, one of them would have been transistors. Seems that 10 or 20 for $11 may be a good indicator that things may not be as they seem.

Regarding matching, I've read that some people do and some people don't. It is my understanding that mismatching won't show up in sound quality, but rather reliability issues when pushing the amp a bit as some transistors carry the load and others go lightly loaded. The internet, like the media and politics, is full of conflicting information. Sorting it all out and picking a path forward will be one of many challenges.

Here is one of many resources I found:

 

[Hunted11]

I would check to make sure you don't have a shorted cap in the power supply. Next check the rectifier.

 

[Jerry_]

Power supply caps are good. I might replace them for good measure, but they aren't shorted and they hold a charge. I checked the rectifier and that's good too. I've pulled 8 power output transistors so far. 6 of them check out good, but 2 are shorted. I've numbered them, so I know where each one came from. I did that to assist in looking for collateral damage.

 

[Hunted11]

I just wanted to suggest that as sometimes I overthink things and I get ahead of myself. Hopefully no one else was in there ahead of you and removed them all and just put them back anywhere.

 

[Jerry_]

I just wanted to suggest that as sometimes I overthink things and I get ahead of myself. Hopefully no one else was in there ahead of you and removed them all and just put them back anywhere.

Me too! I definitely appreciate the suggestions. They were very helpful, and allowed me to focus on some of the more accessible areas first. Looks like everything is untouched on the inside. I've got 15 transistors out now. 4 of them are shorted, and a few have wonky measurements.

The construction on this amp doesn't lend itself to service. I worked on a Fisher receiver, and the transistors were socketed. That really made it easy to pull, check and replace transistors. On this one, the transistors are all soldered in place, (which is probably a better for connectivity), but it's no fun removing them.

 

[Jerry_]

Ok, I'm getting ready to order transistors. For the outputs, it's pretty clear. I can use 2N3773 or MJ21194G. Both are available with the 2N3773 being slightly less expensive.

For the driver transistors, the originals are 2N5804 which aren't available. In the Tom Waits build thread, he used the MJ21194 for both the output transistors and the drivers. Could the same be done with the 2N3773? Why would Crown have used different driver and power output transistors if the same part would have worked for both? I've also read that nearly any BJT in the TO-3 form factor would work as drivers. Thoughts?

It should be said, that my intent is NOT to do a full Tom Waits refurb but to go a little further than just replacing the failed output transistors. I'd like to replace all 20 transistors on the output boards, replace the filter caps, convert the inputs to standard RCA jacks and do some cosmetic work on the power transformer, chassis and face plate. In addition to these goals, I'd like to address the "known" issues and likely failure points, but stay short of repopulating entire boards.

A few progress pics:

 

[ConradH]

IMO, you should download the three-part Crown Service School docs (digital docs subscriber forum, not sure?). PM me if you can't find it. Here's a page on transistor matching. Do it if you want reliability. Don't use anything other than known brand transistors from authorized vendors- the world is flooded with fakes.

 

[Jerry_]

IMO, you should download the three-part Crown Service School docs (digital docs subscriber forum, not sure?). PM me if you can't find it. Here's a page on transistor matching. Do it if you want reliability. Don't use anything other than known brand transistors from authorized vendors- the world if flooded with fakes.
View attachment 1358293

Thanks for the tip. I'm a subscriber, so I'll see if I can find the referenced document. Since I'm going through the trouble of replacing them, I definitely want reliability.

 

[Jerry_]

Parts arrived, and I'm going to start reassembling things. I did pick up a new bridge rectifier, as it was cheap insurance. Next, I was trying to figure out if I needed to replace the large electrolytic can capacitors. I want to keep my cost down, and I didn't want to replace them if they were good. Replacements would be around $100 or more, and I don't want to spend too much on a marginal amp.

I did a few quick tests. First, I applied about 30 volts DC, and both caps held the charge just fine. Then, after discharging them, I placed a meter on the cap to measure resistance. As expected, over time, resistance increased as it took on charge from the meter. This is a good indication of capacitor health. Finally, I wanted to get an idea of the actual capacitance. These caps are 13,700uf, and most handheld meters, including mine, won't go that high.

After poking around online, I decided to create a discharging rig to calculate the time constant and transpose to arrive at capacitance. My method was somewhat crude, but it seems to have worked out pretty well. In the pic, you can see my rig.

Here's what I did:

1. Set up the rig as pictured below.
2. Set up a DC voltage supply (30VDC)
3. Charge the cap up to 30 VDC
4. Remove the charge voltage, switch the resistor into the circuit and start the stopwatch (simultaneously).
5. Record time and voltage as the cap discharges with an eye for the theoretical 1t voltage of 11.1 volts in this case (30v x .37 = 11.1 v)
6. Extrapolate to the 5t period and continue measurement.

The time required to get down to the 1t period (37% of full charge or 11.1 VDC) was about 150 seconds. That would make the 5t period 750 seconds or 12.5 minutes. This calculated time worked out pretty well. I did the discharge study on both caps with the following results:

t (seconds) = R (ohms) x C (farads)

Since I'm trying to solve for C, we transpose to:

C = t/R or C = 150/ 10,120 or C = .014822 farads or C = 14,822 uf

Since caps are typically toleranced at around 20%, this quick and dirty study shows the caps, specified at 13,700 uf, are still good enough to put back in the circuit. Any thoughts on this process would be appreciated. Has anybody done something similar when lacking specific test equipment?

 

[chazix]

Has anybody done something similar when lacking specific test equipment?

Well, that's almost exactly what I do in such circumstances, FWIW. I just go for 1t, though. I didn't catch on to what you gained from measuring out to 5t.

I can't think of any reason that this method isn't suitable for measuring capacitance, as long as the discharge resistance is small enough to make the discharge current swamp out any internal leakage current.

I haven't worked out a really practical first-principles method for measuring ESR, but then, I don't let ESR worries keep me up at night.

Cheers,

chazix

 

[Jerry_]

FWIW. I just go for 1t, though. I didn't catch on to what you gained from measuring out to 5t.

Not much. I was just curious, and I figured I had the stop watch running so why not see if the theoretical discharge rate continues. Thanks for the response and additional insight!

 

[ivandezande]

Only thing I'd worry about is ESR, but in my DC300A Series II I just left them and felt they were probably fine. I did recap the main board however and felt that was very needed, and those main caps can get very warm from the transformer, at least they did on mine.
I honestly couldn't find a good replacement as far as size for them anyways, even if I did want to switch them out.

 

[Jerry_]

I found a seller on eBay that sells a main board refurb kit. Looks like caps, zeners, transistors, pots, etc. Kind of expensive for what you get, but would these items be worthwhile to address while I have everything apart? Here's a pic from eBay under the description "crown dc300a service kit. Pcb's: 7958 9555 9605 9655 9785 upto serial no: 81323"

 

[elnaldo]

After researching a bit more, it appears that those "smokin' deals" on 2N3773 transistors on eBay are NOT the way to go. Of all of the areas I wouldn't expect counterfeit product, one of them would have been transistors. Seems that 10 or 20 for $11 may be a good indicator that things may not be as they seem.

Regarding matching, I've read that some people do and some people don't. It is my understanding that mismatching won't show up in sound quality, but rather reliability issues when pushing the amp a bit as some transistors carry the load and others go lightly loaded. The internet, like the media and politics, is full of conflicting information. Sorting it all out and picking a path forward will be one of many challenges.

Here is one of many resources I found:

Hello, as already mentioned, the crown DC-300 (or 300A) service manual details the procedure used in the factory to match transistors, and the ranges they matched (tolerances), and which transistors to discard.

With modern replacements, if you go MJ15024 for example, I think they will be closely matched from factory. I did some measurements on a DC-300A before installing transistors, and they were almost "factory matched", very little difference between them.