Hi all
I had a working, but humming, NAD 7225pe before I managed to slip while probing and fry the output resistors on one channel. It was working fine after replacing all the caps apart from the offset on one channel. Anyway, I was always treating this as a learning experience.
I have now built a dim bulb tester and replaced the output transistors, Q409, Q411 and trimpot Q411. On powerup he display lights up but it only takes a few seconds before R437 smokes up and I flick the bulb tester off. The other channel may be ok but I turn it off so quickly it’s impossible to know. I am working through the components one by one on the suspect channel but everything is checking out ok. I did replace Q411 on both channels with KSC2690A-Y transistors which appeared to be a suitable substitute.
I guess as I was looking for some insight as to what could be causing to resistor to burn but also what strategy to take moving forward. I am considering buying a variac and also ways to power up just one channel? I am not entirely sure how to do that from the schematic. Any help appreciated.
Cheers
I had a working, but humming, NAD 7225pe before I managed to slip while probing and fry the output resistors on one channel. It was working fine after replacing all the caps apart from the offset on one channel. Anyway, I was always treating this as a learning experience.
I have now built a dim bulb tester and replaced the output transistors, Q409, Q411 and trimpot Q411. On powerup he display lights up but it only takes a few seconds before R437 smokes up and I flick the bulb tester off. The other channel may be ok but I turn it off so quickly it’s impossible to know. I am working through the components one by one on the suspect channel but everything is checking out ok. I did replace Q411 on both channels with KSC2690A-Y transistors which appeared to be a suitable substitute.
I guess as I was looking for some insight as to what could be causing to resistor to burn but also what strategy to take moving forward. I am considering buying a variac and also ways to power up just one channel? I am not entirely sure how to do that from the schematic. Any help appreciated.
Cheers
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There are two ways to keep r437 from burning. One is to put a smaller light bulb in the dim bulb tester, say a 60w or 40 w. You are using tungsten filiment bulbs aren't you? I have to buy mine at Salvation Army resale, the ones from the chain store are not a straight tungsten bulb.
The other thing to do is increase the wattage of R437 to something that won't burn. I've fixed a dozen or so class AB amps and I do not have a variac.
One problem with these low wattage NAD with 2n3055 is that you cannot buy homotaxial 2n3055 anymore. The originals had .15 emitter resistor built in, and a Ft of 200-400 khz. the new epitaxial transistors have no emitter resistance and the Ft is 2-6 mhz. So, if you put a new OT in, you have to cut the board emitter trace and insert a .22 or larger 3 W or larger emitter resistor. Then if Q415 oscillates ultrasonically, you have to cut the base trace and insert a 10 ohm 1w resistor between base & driver emitter.
This old NAD is a wimp & very cost reduced besides. No emitter resistors! Another great idea taken from Sinclair! Where are you? AUS? I've bought much more suitable Peavey PV-4c amp for parts or repair as low as $25 on ebay, +$35 freight. PV-8.5c, CS400, cs600, cs800, m-2600 are much more straightforward. doesn't gumtree operate online in commonwealth countries? Peavey MMA-875t monaural amp are ~$40 + freight working most any day, and similar model Crown MA-180 is about the same price. On the latter 2 models the heat sinks are dirty, the electrolytic caps are tired and dried up, the input boards are usually suitable only for telephone lines. But the RCA jack direct to the amp board usually works. Plus of the last 2 models, they have a speaker transformer. That protects your speaker automatically from a solder joint you made popping loose , producing DC on speaker, and burning the coil of a driver. Only M-2600 has not even a triac crowbar speaker protection.
The other thing to do is increase the wattage of R437 to something that won't burn. I've fixed a dozen or so class AB amps and I do not have a variac.
One problem with these low wattage NAD with 2n3055 is that you cannot buy homotaxial 2n3055 anymore. The originals had .15 emitter resistor built in, and a Ft of 200-400 khz. the new epitaxial transistors have no emitter resistance and the Ft is 2-6 mhz. So, if you put a new OT in, you have to cut the board emitter trace and insert a .22 or larger 3 W or larger emitter resistor. Then if Q415 oscillates ultrasonically, you have to cut the base trace and insert a 10 ohm 1w resistor between base & driver emitter.
This old NAD is a wimp & very cost reduced besides. No emitter resistors! Another great idea taken from Sinclair! Where are you? AUS? I've bought much more suitable Peavey PV-4c amp for parts or repair as low as $25 on ebay, +$35 freight. PV-8.5c, CS400, cs600, cs800, m-2600 are much more straightforward. doesn't gumtree operate online in commonwealth countries? Peavey MMA-875t monaural amp are ~$40 + freight working most any day, and similar model Crown MA-180 is about the same price. On the latter 2 models the heat sinks are dirty, the electrolytic caps are tired and dried up, the input boards are usually suitable only for telephone lines. But the RCA jack direct to the amp board usually works. Plus of the last 2 models, they have a speaker transformer. That protects your speaker automatically from a solder joint you made popping loose , producing DC on speaker, and burning the coil of a driver. Only M-2600 has not even a triac crowbar speaker protection.
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Thanks for the reply.
Yes I started with a 60W globe so in Australia this should equate to about 250ma of current. Getting incandescent bulbs is difficult.
Does the fact that R437 is burning up mean I have a shorted component elsewhere in that channel ? An LTSPice simulation suggests the current across R437 should be < 20ma but as a beginner I am assuming the simulation is correct.
The amp has sentimental value as it was my brothers who passed away prematurely last year from brain cancer so I would love to get it going 🙂
Any thoughts on how I can power up just one channel ?
Cheers
Yes I started with a 60W globe so in Australia this should equate to about 250ma of current. Getting incandescent bulbs is difficult.
Does the fact that R437 is burning up mean I have a shorted component elsewhere in that channel ? An LTSPice simulation suggests the current across R437 should be < 20ma but as a beginner I am assuming the simulation is correct.
The amp has sentimental value as it was my brothers who passed away prematurely last year from brain cancer so I would love to get it going 🙂
Any thoughts on how I can power up just one channel ?
Cheers
Yes, something else is shorted to burn R437. You need to limit the current to what will not burn it. a 25 w incandescent bulb? My local charity resale shop has a drawer full of old incandescent bulbs, I loaded up a month ago as there won't be any more. then with R437 not burning you can voltage probe around and see what else is wrong.
There should be a connector to the individual amp boards, so you can pull one. Make sure it doesn't short against anything.
You know about using only one hand probing with the power on, right? >24 v across your heart can stop it. Negative of DVM goes through alligator clip to center of rail caps (analog ground). No jewelry on hands or neck, 1 v through a ring can burn your flesh to charcoal. Wear safety glasses, parts blow up & solder splashes.
There should be a connector to the individual amp boards, so you can pull one. Make sure it doesn't short against anything.
You know about using only one hand probing with the power on, right? >24 v across your heart can stop it. Negative of DVM goes through alligator clip to center of rail caps (analog ground). No jewelry on hands or neck, 1 v through a ring can burn your flesh to charcoal. Wear safety glasses, parts blow up & solder splashes.
With a small receiver like this NAD when the tuner is off, as indianajo suggests, it would be better to use a smaller standard bulb, such as 40W, if you can. These were scarce even before they were obsolete but you may not be aware that Quartz-Halogen bulbs are also incandescent types and work as well as plain tungsten/vacuum types. I have a few of the 37W rated variety on hand and they were commonly used for decorative lighting, applying makeup, reading amps and so on.
Alternatively but also more expensive, the various bulbs used in refrigerators and ovens are plain tungsten types and with the right total rating, one or more in parallel can form a wide range of suitable DBTs.
I find the quartz-halogen bulb types and sizes are on open display right now, in my local convenience stores but not in large supermarkets or at electrical appliance stores. Maybe that's a marketing loophole here but it does mean I can still choose and replace the bulbs without fuss, where plastic construction, LED types would simply be a disaster. Having a convenient, low cost source of DBT spares is a nice bonus.
Alternatively but also more expensive, the various bulbs used in refrigerators and ovens are plain tungsten types and with the right total rating, one or more in parallel can form a wide range of suitable DBTs.
I find the quartz-halogen bulb types and sizes are on open display right now, in my local convenience stores but not in large supermarkets or at electrical appliance stores. Maybe that's a marketing loophole here but it does mean I can still choose and replace the bulbs without fuss, where plastic construction, LED types would simply be a disaster. Having a convenient, low cost source of DBT spares is a nice bonus.
Thanks for the advice. Yes I am very careful when working on the amp. I have an isolation transformer, mains safety switch and now a bulb tester. . Always keep one hand in pocket etc. If there are any other safety tips I am always very keen to hear them. I am cursing myself for slipping and shorting the amplifier after I had successfully recapped it....Not careful enough obviously 🙂
Q405 a small signal BC556B controls the current of the KSC2690A-Y. If you had to replace the BD139 the odds are this has been damaged when the BD139 failed. Compare the base voltage on the KSC2690A-Y with that on the BD139 in the other channel.
If you need to replace the BC556B Jaycar sell BC556 so you could buy a few and select one with hFE around 300.
They also sell parrot clip test leads which can be hooked securely around resistor/transistor leads and removed with the power off before and after.
I use the red parrot clip for that and put the ordinary black probe in the speaker black terminal. In that way one can look for smoke with a hand on the power switch and then move to the light globe and the meter.
When measuring voltage differences in equipment it is better to measure each separately and subtract the results to deduce the readings.
They also sell parrot clip test leads which can be hooked securely around resistor/transistor leads and removed with the power off before and after.
I use the red parrot clip for that and put the ordinary black probe in the speaker black terminal. In that way one can look for smoke with a hand on the power switch and then move to the light globe and the meter.
When measuring voltage differences in equipment it is better to measure each separately and subtract the results to deduce the readings.
Thanks so much for your advice. I did grab some transistors from Jaycar yesterday. They appear to be ok in circuit but I will replace them anyway on the suspect channel. I also got a 25w globe to see if I can avoid the smoke....
Read this thread My Transistors, original or copy? before using semiconductors from ebay.
I usually order semiconductors from authorized distributors of On, ST, or other major semiconductor manufacturers.
There are some trusted hobbiests selling surplus on e-bay. There are also many silk screen artists that can counterfeit any label part number you desire on any kind of trash. As most purchasers do not run a full incoming inspection test on received parts, the star rating covers mostly the prompt delivery and visual appearance of the parts.
I usually order semiconductors from authorized distributors of On, ST, or other major semiconductor manufacturers.
There are some trusted hobbiests selling surplus on e-bay. There are also many silk screen artists that can counterfeit any label part number you desire on any kind of trash. As most purchasers do not run a full incoming inspection test on received parts, the star rating covers mostly the prompt delivery and visual appearance of the parts.
I have never used ebay to buy semiconductors, the KSA1220AY is available from Element 14 at a cost of $1.01 which is less than the cost of packaging and posting.
I almost exclusively purchase from Mouser & Digikey. I only purchase off ebay if I find a local seller I have purchased from before(i.e. not direct from China basically).
Cheers
Cheers
Is thermal runaway something that happens immediately without a load ? i.e. now I have the amp back together (new type of 2n3055/MP6955, new 2sd669/649) can I try to power it up with a variac and dim bulb tester to check it doesnr smoke up anything ? Or should I retrofit the 0.22ohm 5W emmitter resisters before doing so ?
Cheers
Cheers
You need to check the output transistor emitter resistors after an output transistor failure. They frequently fail. I don't know what you can get away with but having an OT resistor open will not establish idle current on output transistors, at the minimum. You could substitute a 1 W half ohm emitter resistor temporarily if you were waiting for the right part to arrive.