I’ve still got about 30 or so blank full panels of double sided FR4 material - out of a huge batch decades ago from Skycraft. Ought to last the rest of my life. But the big ones still in the works have gotten too complex to do that way. Couple years back I looked into doing another iteration through Sunstone - and jeezus it would have been almost two grand. JLC was $96 for a full batch of 15 2 ounce copper boards. I ordered 5 one ounce proofs to do the drilling and machining from last year, right before the shutdown to build the house.
I ordered from poly Paks and every other surplus place in the world. It’s all disappeared now. BUT we have ready access to ALL production OnSemi now - so that’s a pretty good trade. Back in the poly Paks days you couldn’t get access to Motorolas whole line without a tax ID. And I was just a teenager.
I ordered from poly Paks and every other surplus place in the world. It’s all disappeared now. BUT we have ready access to ALL production OnSemi now - so that’s a pretty good trade. Back in the poly Paks days you couldn’t get access to Motorolas whole line without a tax ID. And I was just a teenager.
Here in Canada, Motorola had distributors. I could just order anything. They also had free samples. Those were the days.
Yeah, surplus houses and stores were our savior. We had surplus houses in Toronto, so we made the trek in from Mississauga occasionally to go wild! We bought test equipment from "Crown Assets Disposal". So ex-military and government. Now it's on-line bidding. People drive prices up beyond what you can get stuff for in the surplus stores. We used to walk onto the army base and browse the store, that's where I got some test gear, and a car (really good car).
Yeah, surplus houses and stores were our savior. We had surplus houses in Toronto, so we made the trek in from Mississauga occasionally to go wild! We bought test equipment from "Crown Assets Disposal". So ex-military and government. Now it's on-line bidding. People drive prices up beyond what you can get stuff for in the surplus stores. We used to walk onto the army base and browse the store, that's where I got some test gear, and a car (really good car).
We had “distributors” here too - they just didn’t want to do business with you. Not like the Mouser and Digikey of today. They were just tiny little mail order houses that didn’t carry transistors rated over 80 volts. MCM finally broke the mold. Then I could fix amplifiers like this one, and every PA amp on the planet at the time. Original 2SD424’s! For $4!
You had Electronic Buyers Club. I used to buy from them. Great prices. We also had Future, which became Active.
Made the assumption that the transistor heatsinks came with screw and nut.
Now waiting for those.
Do I need insulating pads on the to-126 ?
Now waiting for those.
Do I need insulating pads on the to-126 ?
Depends on the TO-126. They make them with exposed leadframe and full over molded. And still call both a TO-126, even though technically they should be different.
Bingo. Unless you are expecting 400 volts on the tab. Data sheets for high voltage switching types in that package (typically for lighting ballasts) warn that additional insulation is needed for those uses.
Update: found q12 was dead, and also 4 of the .82 ohm resistors on one of the output boards. I went ahead and replaced q12 (first time crazy gluing a thermistor, it went smooth), replaced the diodes and also all 8 of the .82 ohm resistors on the output boards. Also replaced all diodes and q's on the input / driver board on the bad channel paying close attention to EBC pinout with the new replacement part #'s, in addition to checking continuity for any solder bridges that may have occurred. Excuse the bad tape job on Q1,Q2 as I didn't realize until after I put them in that they should be wrapped together, not separately. The foil is def not touching the legs though. I will clean that up.
DRUM ROLL...... DIM BULB TESTER NOW FULLY LIT! 😵 BUT my distortion light on the bad channel was no longer staying lit which I should take as a small win I guess?
I pulled the fuses out of the output boards and powered back up, DBT not fully lit, but dim. Then I put only 1 fuse in, also dim, switched the fuse to the other output board, DBT also dim. with both fuses in the outputs is where the DBT goes fully lit.
Aside from replacing only one 2sd424 (I know I know), I have not replaced all 2sb554 and 2sd424. could that be the issue now?
QUESTION: without the fuses in the output boards, should my DBT be lit ( albeit very dim), with only the input / driver board running (fuses on outputs pulled but still wired in input / driver board)?
I am at a stopping point, not knowing what to check next... Seeing that variacs can be had for not much money ( saw the post by @rayma on page 2, Thanks for that - https://www.acdcpowershack.com/prod...er-5-amp-500-watts-0-130vac-output-part-sc-5m) I am not opposed to picking it up, but I would be in the dark on steps to take to get this figured out.
DRUM ROLL...... DIM BULB TESTER NOW FULLY LIT! 😵 BUT my distortion light on the bad channel was no longer staying lit which I should take as a small win I guess?
I pulled the fuses out of the output boards and powered back up, DBT not fully lit, but dim. Then I put only 1 fuse in, also dim, switched the fuse to the other output board, DBT also dim. with both fuses in the outputs is where the DBT goes fully lit.
Aside from replacing only one 2sd424 (I know I know), I have not replaced all 2sb554 and 2sd424. could that be the issue now?
QUESTION: without the fuses in the output boards, should my DBT be lit ( albeit very dim), with only the input / driver board running (fuses on outputs pulled but still wired in input / driver board)?
I am at a stopping point, not knowing what to check next... Seeing that variacs can be had for not much money ( saw the post by @rayma on page 2, Thanks for that - https://www.acdcpowershack.com/prod...er-5-amp-500-watts-0-130vac-output-part-sc-5m) I am not opposed to picking it up, but I would be in the dark on steps to take to get this figured out.
Hi wiz1,
You have excessive current in the output stage. You have to find out why.
To trace the current you must apply lower voltage to the output stage to a point where it is not too high, but you can trace it. Two options. A variable, bipolar DC supply, or a variac. I feel like I'm talking to the wind sometimes.
You have excessive current in the output stage. You have to find out why.
To trace the current you must apply lower voltage to the output stage to a point where it is not too high, but you can trace it. Two options. A variable, bipolar DC supply, or a variac. I feel like I'm talking to the wind sometimes.
I am looking at 5amp variacs on Amazon now. They are around the same price (and look like the same design) as the link Rayma posted.
What test points would I use to check, and how many volts do you estimate I would need to push through it?
Going to look at some basic variac YT vids now.
The wind has passed am I am paying full attention!
What test points would I use to check, and how many volts do you estimate I would need to push through it?
Going to look at some basic variac YT vids now.
The wind has passed am I am paying full attention!
Hi wiz1,
Absolutely. I have an isolated model. Also an isolation transformer for use with other standard transformers. I was lucky to find a B&K 1655 "AC Power Supply". It has a meter and is nice to have on the bench. Note that most variacs are wired to be able to deliver much higher than normal line voltage. So monitor your output voltage. Maybe mark the dial around the no load 120 VAC point.
Mind you, a "dim bulb" tester is not isolated so the same cautions apply, same as it plugged in to the mains.
Increase the voltage slowly with all fuses installed in the amplifier do not exceed full line voltage. Watch the current draw and once it begins to increase more rapidly, measure the voltage drop across the emitter resistors. If you put the common of your meter on the speaker output (hot) you can measure emitter - base voltages to see if a transistor is being driven hard. Be careful probing. Doing this lets you measure voltages accurately and the measurements make sense. If you measure wrt common, you have whatever voltage on the speaker output adding or subtracting to your measurements and it may not be steady.
Do not measure the working channel. Don't tempt fate. Your faulty channel will not have normal voltages in all likelihood, and the working channel will not be normal at low voltages.
Absolutely. I have an isolated model. Also an isolation transformer for use with other standard transformers. I was lucky to find a B&K 1655 "AC Power Supply". It has a meter and is nice to have on the bench. Note that most variacs are wired to be able to deliver much higher than normal line voltage. So monitor your output voltage. Maybe mark the dial around the no load 120 VAC point.
Mind you, a "dim bulb" tester is not isolated so the same cautions apply, same as it plugged in to the mains.
Increase the voltage slowly with all fuses installed in the amplifier do not exceed full line voltage. Watch the current draw and once it begins to increase more rapidly, measure the voltage drop across the emitter resistors. If you put the common of your meter on the speaker output (hot) you can measure emitter - base voltages to see if a transistor is being driven hard. Be careful probing. Doing this lets you measure voltages accurately and the measurements make sense. If you measure wrt common, you have whatever voltage on the speaker output adding or subtracting to your measurements and it may not be steady.
Do not measure the working channel. Don't tempt fate. Your faulty channel will not have normal voltages in all likelihood, and the working channel will not be normal at low voltages.
I bought a cheap Harbor Freight DVM to monitor the output of the Variac. The voltage index around my knob is not very accurate. I added a female plug extension and cut the tips of the meter off and wired it in the plug. That cheap DVM is within a half a volt AC of my calibrated bench meter (surprisingly).
does the variac measure current draw also or would I need like a clamp on ammeter?
So, common from meter to hot (+) side speaker terminal on rear. The other probe goes to transistors on the output boards. measure voltage at emitter of each transistor (can I just clamp onto one leg of the emitter resistors for this instead of trying to probe the actual emitter?
measure voltages at all 8 transistors and record the voltages to see if any are out of wack...
Waste of time. Any place the 0.82 emitter resistor was blown, the associated output transistor is blown.
If you pull all those, if the lamp on the dim bulb tester goes out, you have gotten some shorts out. Since you cannot buy matching transistors (same production date, same factory, same part #) you will have to buy 8 new transistors. I use MJ21194/93 typically, in lots big enough to hold a couple of outliers back in the matching jig. Whoops, the price in USA went up 25% today. Or MJ21195/96, whatever is in stock that day.
You can measure the current through any O.T. by measuring the voltage at the top and bottom of the emitter resistor. I=V/R. This is while on the DBT. Do not use 2 hands to measure any voltage across a component while amp is powered up, >25 v across your 2 hands can stop your heart. Measure the 2 voltages reference the analog ground, write them down, then subtract. Connect the black of the DVM to the analog ground with an alligator clip lead. The analog ground is the connection of C05 and C04, or if on that channel, C03 and C02.
Your drivers are probably blown. You did not say what measurement you took to declare Q12 blown. Vce must be >1 on a dim bulb tester. The other 3 drivers are likely damaged.
With all the O.T. on the bad channel removed, and the drivers removed since one was bad, power up on DBT and see if Q9 and Q10 predrivers have > 1 v Vce. Lamp should be dim. Not, they are bad too. I tend to use MJE15031/32 for drivers, if the 2SA1011 2SC2344 (or whatever they used that day) are TO220 parts. P18 says this amp has +-81v rails, which is pretty high voltage for class AB.
The new To3 transistors likely have smaller holes than the old ones. Buy new screws # nuts. I use #4. Don't forget new heat sink grease unless you replace the mica washers with silicon pads.
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Hi wiz1,
Yes, my variac has the ability to measure voltage and current. Just now I brought up a Bryston 4B NRB I just repaired. If anyone thinks I would risk a new set of 8 - MJ21195 and MJ21196, plus drivers, to a dim bulb, not a chance! Besides, when you have problems, you need to be able to control the voltages in order to control current and be able to troubleshoot. A dim bulb drops variable voltage with current draw. So everything changes as the current draw goes up and down. Wonderful!
I have very good AC and DC current probes also. Very handy for some amplifiers that want you to pull a fuse and place an ammeter in series. Pretty great for chasing current in equipment as well. These are very expensive and I don't recommend anyone buy one unless they use it a lot. That said, if you ever run into an HP 428B with probe (the probe and meter are calibrated together) cheap enough, buy it! That is a DC clamp-on meter with a FS range of 1 mA, up to 10 amperes. Pretty amazing equipment.
Yes, my variac has the ability to measure voltage and current. Just now I brought up a Bryston 4B NRB I just repaired. If anyone thinks I would risk a new set of 8 - MJ21195 and MJ21196, plus drivers, to a dim bulb, not a chance! Besides, when you have problems, you need to be able to control the voltages in order to control current and be able to troubleshoot. A dim bulb drops variable voltage with current draw. So everything changes as the current draw goes up and down. Wonderful!
I have very good AC and DC current probes also. Very handy for some amplifiers that want you to pull a fuse and place an ammeter in series. Pretty great for chasing current in equipment as well. These are very expensive and I don't recommend anyone buy one unless they use it a lot. That said, if you ever run into an HP 428B with probe (the probe and meter are calibrated together) cheap enough, buy it! That is a DC clamp-on meter with a FS range of 1 mA, up to 10 amperes. Pretty amazing equipment.
Variac should be here later this week. I will be digesting what was written above and ask clarifying questions before proceeding. I appreciate everyone's help!
Q12 was shorted out when I checked with continuity. I pulled it and confirmed it was bad on the mk- 168 transistor tester.
Q12 was shorted out when I checked with continuity. I pulled it and confirmed it was bad on the mk- 168 transistor tester.
Looks like Mouser has the best prices at the moment.
Would you happen to have handy the Mouser pn#s for the mica pads and the #4 screws and nuts?
I do have a jar of new grease.
The MJ21195/6 and all other TO-204 transistors use #6 mounting hardware. Just like the TO-3 transistors. The original hardware will be fine.
What does vary is the thickness of the case. New transistors have thin flanges, old Motorola (2N3055 example) have thick cases. Japanese 2SD424 and similar also have thick flanges. If anything, the hardware will protrude more which isn't a problem.
36-4662-ND is the Digikey number for a TO-204/TO-3 mica insulator. I use tons of them. Just used 8 of these tonight.
What does vary is the thickness of the case. New transistors have thin flanges, old Motorola (2N3055 example) have thick cases. Japanese 2SD424 and similar also have thick flanges. If anything, the hardware will protrude more which isn't a problem.
36-4662-ND is the Digikey number for a TO-204/TO-3 mica insulator. I use tons of them. Just used 8 of these tonight.