Sherwood s8900 - transistor sourcing help
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special_20
Active Member
Jurb, I just seeing that bit of advice this morning. Alas I demounted affected components last night by desoldering leads... sorry. I don't think I did anymore insult to the board than necessary.
Updated picture set
I removed the board from the chassis and snipped the leads (figuring the good board is the reference board from here on out) allowing me to fully remove the board. The immediately affected components were desoldered and removed. Soaked burned area with Awesome Stuff cleaner and used a steel brush tip and an o-ring pick to scrape away carbon deposits. Most carbonization was surface adherent. The silk screen didn't like being rubbed that hard and took a bit of a beating (I tried to be gentle). Only one component appears to have ignited (a single vertically mounted resistor). The two adjacent transistors saw enough heat so as to test bad - no surprise there. Variable resistors probably saw enough smoke/contamination to be considered DOA. Several nearby resistors were toasty - one looked like a more modern replacement) The burn went pretty deep (estimate 1/2 way through board or more) but did not burn through. The solder pads on the back are all visually intact and seem in good shape. The silk screen on the back has BCE marks for transistors (which will be very helpful if I proceed here!).
I have not tested continuity. To test for continuity (and effective burn through) do I probe into the burn crater with a DMM probe (+) and test against some solder pads on back (-)? If I see continuity (not 0.00 or infinity) it would mean the burn went through the insulating layer to the copper; this would necessitate sealing the crater to re-establish an insulating layer? Is there a bigger concern for the health and integrity of the board beyond that?
Watthour, you suggested that I may need to epoxy this board. I assume that is to build back up bulk, level out surface, and regain an insulating layer. Any suggestions as to product? Would clear nail polish do the trick or do I need something higher/temp or more durable?
JURB
Super Member
OK, according to the pic where you have the components out, this is not all that bad. Not at all. I have had them where there was a hole clean through the board. You are kinda lucky I guess, the fire was put out in time. You can clean up whatever components check good with acetone, especially clean the little caps if any.
How does the bottom look ? How much foil is actually lifted ? There are ways to anchor that down, but usually if it wants to lift you want to scrape it and drape it at least, or better, use a small piece of wire. Solid wire, such as the cutoff end of a component after installation. (we save those actually)
Now the major task here is to rebuild that part of the circuit so it matches the other board.
Somehow you are going to have to trace the lines from the burnt resistor. It looks like in this case, one resistor fried and the rest is pretty much smoke damage. The aesthetic damage to the board I can see is minimal and can probably be left alone, and will not be all that visible once the componentry is in.
So far so good.
Now you got the one resistor that smoked. I bet it goes to a pin or whatever and out a wire to one of the outputs. I don't even remember where we were on checking those but at least one is most likely shorted. Does the other channel work ? If you've got all this stuff disconnected you should be able to fire it up and listen to one channel.
I hope you didn't cut the wires going to the board(s). They usually go to pins and the accepted method is to desolder the pins from the board, like if they're wire wrapped, leave them wrapped. If you cut them it is going to be a drag reconnecting them, but it is not impossible.
I am not sure how much more of this walkthrough we can do online. When I do stuff like this I get to flip the board back and forth and kinda reverse engineer it. It is not easy when not holding the thing in my hand. You might want to add top and bottom pics of both driver boards. I'll see what I can see. But I am sure you understand that the bottom line is to make it like the other channel.
Also, I don't believe the 2N3055 is quite hefty enough for the amp, and I thought those amps used Darlingtons. If the rails are anything above about 30 volts, the 3055 does not have enough headroom in that parameter IMO. There is an MJ2XXXX alot of people here use, I've used the MJ15024 with good results. They are like six bucks n change at Digikey. They are strong enough that this amp might simply not be ABLE to blow them. But let's not get ridiculous. I remember working on an HK Citation 12 and right in the scanned manual it is written in "2N3055". Not me buddy. the Vcbo of the 3055 is 70V and the rails in that amp are 42, which add up to 84, which is higher than 70. In my opinion, in a transformerless amp, you should be able to clip it into a heavy load at high line voltage and it should not fry out.
But at this stage, isolate/insulate all the loose wires etc. in there and have a listen to the working channel. Later, when you have the board repopulated, you can compare voltages, even with the outputs not installed. Might have to get a couple of about 100 ohm resistors. These will shunt E to B on the outputs and allow it to be in closed loop at which time you should have low DC offset and should be able to drive headphones for a short time. However DO NOT connect any speakers at that time. Many amps already have E-B shunts on the outputs, but since we cannot be sure of having the right print we cannot assume. (unless we can physically SEE the shunt resistors, plus they might be of too high a value for the non-output drive test)
As far as walkthroughs go, THIS IS RIDICULOUS ! But you know what ? I'm having a good time. What's more, I might not have to get my (_|_) to work anymore, maybe I can just phone it in.
How does the bottom look ? How much foil is actually lifted ? There are ways to anchor that down, but usually if it wants to lift you want to scrape it and drape it at least, or better, use a small piece of wire. Solid wire, such as the cutoff end of a component after installation. (we save those actually)
Now the major task here is to rebuild that part of the circuit so it matches the other board.
Somehow you are going to have to trace the lines from the burnt resistor. It looks like in this case, one resistor fried and the rest is pretty much smoke damage. The aesthetic damage to the board I can see is minimal and can probably be left alone, and will not be all that visible once the componentry is in.
So far so good.
Now you got the one resistor that smoked. I bet it goes to a pin or whatever and out a wire to one of the outputs. I don't even remember where we were on checking those but at least one is most likely shorted. Does the other channel work ? If you've got all this stuff disconnected you should be able to fire it up and listen to one channel.
I hope you didn't cut the wires going to the board(s). They usually go to pins and the accepted method is to desolder the pins from the board, like if they're wire wrapped, leave them wrapped. If you cut them it is going to be a drag reconnecting them, but it is not impossible.
I am not sure how much more of this walkthrough we can do online. When I do stuff like this I get to flip the board back and forth and kinda reverse engineer it. It is not easy when not holding the thing in my hand. You might want to add top and bottom pics of both driver boards. I'll see what I can see. But I am sure you understand that the bottom line is to make it like the other channel.
Also, I don't believe the 2N3055 is quite hefty enough for the amp, and I thought those amps used Darlingtons. If the rails are anything above about 30 volts, the 3055 does not have enough headroom in that parameter IMO. There is an MJ2XXXX alot of people here use, I've used the MJ15024 with good results. They are like six bucks n change at Digikey. They are strong enough that this amp might simply not be ABLE to blow them. But let's not get ridiculous. I remember working on an HK Citation 12 and right in the scanned manual it is written in "2N3055". Not me buddy. the Vcbo of the 3055 is 70V and the rails in that amp are 42, which add up to 84, which is higher than 70. In my opinion, in a transformerless amp, you should be able to clip it into a heavy load at high line voltage and it should not fry out.
But at this stage, isolate/insulate all the loose wires etc. in there and have a listen to the working channel. Later, when you have the board repopulated, you can compare voltages, even with the outputs not installed. Might have to get a couple of about 100 ohm resistors. These will shunt E to B on the outputs and allow it to be in closed loop at which time you should have low DC offset and should be able to drive headphones for a short time. However DO NOT connect any speakers at that time. Many amps already have E-B shunts on the outputs, but since we cannot be sure of having the right print we cannot assume. (unless we can physically SEE the shunt resistors, plus they might be of too high a value for the non-output drive test)
As far as walkthroughs go, THIS IS RIDICULOUS ! But you know what ? I'm having a good time. What's more, I might not have to get my (_|_) to work anymore, maybe I can just phone it in.
special_20
Active Member
Jerb,
The help is appreciated man! I can relate. My other hobby is fixing bicycles and it can be fun to help walk people through repairs.
Satus: Honestly, the back looks good. No lifted pads or burned traces. Silk screen on the back still has ECB markings for transistors (yes!). Damage wise I've got the following to replace in order to restore function: 3 resistors, 2 variable resistors, 2-3 transistors on board, and 1 output transistor is shorted. There is 1 suspect 4000uf cap upstream that I need to try to test. So, looks like I can likely salvage this bad boy and am going to soldier forward (figure I'll learn something from you guys
To that end, I'm open to suggestions on the outputs. 3055's are cheap. Jerb and Movinahead, anymore specific suggestions for the output transistors? It sounds like the originals were just enough. What should i be looking for to spec a replacement? This is outside of my experience as I've usually just gone with a modern equivalent.
The help is appreciated man! I can relate. My other hobby is fixing bicycles and it can be fun to help walk people through repairs.
Satus: Honestly, the back looks good. No lifted pads or burned traces. Silk screen on the back still has ECB markings for transistors (yes!). Damage wise I've got the following to replace in order to restore function: 3 resistors, 2 variable resistors, 2-3 transistors on board, and 1 output transistor is shorted. There is 1 suspect 4000uf cap upstream that I need to try to test. So, looks like I can likely salvage this bad boy and am going to soldier forward (figure I'll learn something from you guys
To that end, I'm open to suggestions on the outputs. 3055's are cheap. Jerb and Movinahead, anymore specific suggestions for the output transistors? It sounds like the originals were just enough. What should i be looking for to spec a replacement? This is outside of my experience as I've usually just gone with a modern equivalent.
JURB
Super Member
There are a few, and I thought I posted this. The Onsemi MJ15024 would be great for this providing the originals were not Darlingtons. Using non-Darlingtons in that circuit if it was designed for them could damage the drive circuit by the bases pulling too much current. And now that I mention it, look what happened. You can also use 2N3773s in there, IF THE ORIGINALS ARE NOT DARLINGTON. And if they were and the blown channels has the 2N3055s in it, now we know why.
So, this channel with the fried parts, is it connected to the 2N3055s or the other ones ?
So, this channel with the fried parts, is it connected to the 2N3055s or the other ones ?
JURB
Super Member
On a working channel, the voltage between E and B of each output will be a volt or so, if not, they will be more like a half a volt or so. These are the two pins at the bottom, might be hard to get to. Be very careful not to short to the big one in the middle, that is usually the end of a pair of outputs. You got a DVM, just find out that voltage on the working channel.
If it turns out to be like a volt of more, and of course that will NOT be the 3055s, that is how some idiot caused this who knows how long ago. If you DO indeed need Darlingtons, there are afew options. You can actually make them, but it is easier said than done because they will need heatsinking. Either that or to be so utterly overbuilt that money becomes an object.
QUESTION : Remember that one transistor with what looks like a mounting flange on it not being used ? Are there two of them on that board or only one ? If only one, I would guess it calls for Darlingtons, because that wold be the voltage amp and the drivers would have more heatsink on them if they had to drive regular transistors.
Think you can send up decent resolution pics of both sides of both boards ? That might help quite a bit. This is where we get into not blowing up the new parts.
If it turns out to be like a volt of more, and of course that will NOT be the 3055s, that is how some idiot caused this who knows how long ago. If you DO indeed need Darlingtons, there are afew options. You can actually make them, but it is easier said than done because they will need heatsinking. Either that or to be so utterly overbuilt that money becomes an object.
QUESTION : Remember that one transistor with what looks like a mounting flange on it not being used ? Are there two of them on that board or only one ? If only one, I would guess it calls for Darlingtons, because that wold be the voltage amp and the drivers would have more heatsink on them if they had to drive regular transistors.
Think you can send up decent resolution pics of both sides of both boards ? That might help quite a bit. This is where we get into not blowing up the new parts.
Moving Ahead
Well-Known Member
The unit doesn't use darlingtons in the outputs. (The A version does) . The originals are NPN bipolar single- S356. I could suggest MJ21194 from Om.
The drivers that jurb is questioning- are S409F npn. They look like a mini To-3. There should be 2 per board Q211 & Q212. If youre keeping the transistor sockets I can suggest 2N3440 or 2n3439 (maybe add a heat sink for looks). If the sockets are going maybe a To-220 package (check pin out !)
For Q210 pnp -S522 try NTE129 and for Q209 npn- S2104 try NTE 128.
Remember If you've still got a short somewhere these suggestions will burn up too.
In addition I can be highly unreliable so you should double check my suggestions.
Replace the 4000uF-50V caps - 35mm if I'm right. (Don't put 25mm in a 35mm clamp they look stupid)
Thanks jurb for getting him on the right road. The karma gods look favorably on that.
The drivers that jurb is questioning- are S409F npn. They look like a mini To-3. There should be 2 per board Q211 & Q212. If youre keeping the transistor sockets I can suggest 2N3440 or 2n3439 (maybe add a heat sink for looks). If the sockets are going maybe a To-220 package (check pin out !)
For Q210 pnp -S522 try NTE129 and for Q209 npn- S2104 try NTE 128.
Remember If you've still got a short somewhere these suggestions will burn up too.
In addition I can be highly unreliable so you should double check my suggestions.
Replace the 4000uF-50V caps - 35mm if I'm right. (Don't put 25mm in a 35mm clamp they look stupid)
Thanks jurb for getting him on the right road. The karma gods look favorably on that.
JURB
Super Member
That addon, two hole thing probably was added by Sherwood. I remember those and a few other types, that one has a slit and you can press it on and off by hand, so do the type used in some older scopes n **** with the fins all radial. they were all used to enhance the free air power dissipation of the transistor but were not very efficient in doing so. to be efficient you need to be at the bottom of the case, where the wires come out. But this was better than nothing.
If you are right and it was not Darlington then there should be four of those heat, well whatevers on those boards. In fact one of the boards has plainly an ECG or NTE transistor on it. And that might be the channel that works.
I need more pics, both sides of both boards.
If you are right and it was not Darlington then there should be four of those heat, well whatevers on those boards. In fact one of the boards has plainly an ECG or NTE transistor on it. And that might be the channel that works.
I need more pics, both sides of both boards.
special_20
Active Member
I'll see what I can do tonight for more pics The2n3055's are associated with the bad channel. The good Channel is associated RCA S356. This S356 part number matches the schematic - I'm assuming the working channel is all original. High degree of certainty these aren't darlington outputs. Also sounds like there are several options for replacement. In doing some reading on DIYAUDIO.com, a few posters raised specter of oscillation when using up-spec'd replacements for 2n3055 transistors. Is this a concern here?
Per the manual, they are saying you should have +/- 1/2 volt across the output transistors. Alas, I currently cannot test as I broke a polyester cap when demounting the 3000uf capactitor for testing (soldered between cap and case). So I'll need to refresh parts before I can proceed at all.
I did put an ESR meter on the big can caps (and a few of the little ones). The ESR looks high for all - hovering at or above 0.1 for all which is the break line on my chart for capacitors >500 uf. Given the large values of these caps, that ESR value should be even lower.... So I'll be adding those to the growing list of parts
I'll continue looking for evidence of a short on the board as it is still a possibility. I'm working on the assumption that this unit failed originally and that Q209/210 and Q213 were taken out. Someone replaced the affected parts, but didn't get to the root cause as they burned again... ESR is high on caps. A short elsewhere isn't impossible. Could it possibly be DC offset and bias drifting into Class A territory and frying everything?
Related question. In other sections of the manual, the trim pots are labeled "Balance" and "Bias". Is "Balance" the same as DC offset? Instructions for adjustment are not the clearest... Might need to borrow a oscilloscope when it comes time to set/measure as their technique involves Intermodulation Distortion Analyzer (which I don't have). Do you two gents by chance have more direct ways of measuring and setting Balance and Offset?
Per the manual, they are saying you should have +/- 1/2 volt across the output transistors. Alas, I currently cannot test as I broke a polyester cap when demounting the 3000uf capactitor for testing (soldered between cap and case). So I'll need to refresh parts before I can proceed at all.
I did put an ESR meter on the big can caps (and a few of the little ones). The ESR looks high for all - hovering at or above 0.1 for all which is the break line on my chart for capacitors >500 uf. Given the large values of these caps, that ESR value should be even lower.... So I'll be adding those to the growing list of parts
I'll continue looking for evidence of a short on the board as it is still a possibility. I'm working on the assumption that this unit failed originally and that Q209/210 and Q213 were taken out. Someone replaced the affected parts, but didn't get to the root cause as they burned again... ESR is high on caps. A short elsewhere isn't impossible. Could it possibly be DC offset and bias drifting into Class A territory and frying everything?
Related question. In other sections of the manual, the trim pots are labeled "Balance" and "Bias". Is "Balance" the same as DC offset? Instructions for adjustment are not the clearest... Might need to borrow a oscilloscope when it comes time to set/measure as their technique involves Intermodulation Distortion Analyzer (which I don't have). Do you two gents by chance have more direct ways of measuring and setting Balance and Offset?
special_20
Active Member
oh, and thanks again for your time and attention. It's reassuring to have some guides
Binkman
Lunatic Member
Assure that any solder chassis grounds are replaced by screw terminals. Solder rots on plated metal. will provide a good ground reference, too. i.e. nickel plate with solder over 30 years?
As long as traces are good by visual? I wouldn't be worried as in traces. poor solder joints perhaps but these are good trace and repairable thickness.
As long as traces are good by visual? I wouldn't be worried as in traces. poor solder joints perhaps but these are good trace and repairable thickness.
Moving Ahead
Well-Known Member
The balance-center voltage and bias can both be set with a DMM. Both these trim pots can be upgraded.
No pot for offset. The DC is stripped prior to speakers with C281. I'd probably go for 4700uF. 50+volts, 35mm.
Dont forget to order two of these or something similar -C170/C171
http://au.element14.com/murata/de2f3ky103ma3bm02/cap-cer-f-10nf-250v-rad/dp/1515845
Have a look at the diodes and try and get some markings -power supply and X12 on the main amp board
No pot for offset. The DC is stripped prior to speakers with C281. I'd probably go for 4700uF. 50+volts, 35mm.
Dont forget to order two of these or something similar -C170/C171
http://au.element14.com/murata/de2f3ky103ma3bm02/cap-cer-f-10nf-250v-rad/dp/1515845
Have a look at the diodes and try and get some markings -power supply and X12 on the main amp board
special_20
Active Member
Alright gents, I think I might bail or at least delay on this repair. I don't say this often, but I think I'm in over my head (and am leary of spending >100 bucks on a "roll of the dice" repair). This thing is old. It uses a lot of obsolete caps that are rarities and/or expensive. I don't have a good manual. The non-output transistors don't seem to show up in any cross-reference and I'm uncertain of speccing adequate replacements. I think it's time to fold...
Thanks for your help and I'll reactivate this thread if I get an itch to repair it. Alternatively, PM me if anyone need a parts unit!?
Thanks for your help and I'll reactivate this thread if I get an itch to repair it. Alternatively, PM me if anyone need a parts unit!?
Moving Ahead
Well-Known Member
I'd love to get my hands on that thing. I'd be a pleasure to get her singing again.
special_20
Active Member
back on the horse and thanks for all the kind offers of assistance! Turns out I just needed some free time focus and get a good grasp on what needed to be done. That came by way of an unexpected cold last week. Too sick to go into work; not too sick to track down parts and solder
So, I cleaned up the burned board, recapped the power supply, replaced both filter caps (filter cap on bad channel had blown), replaced the output transistors and rebuilt the burned board with new resistors and transistors. I'll get some picks up shortly.
I brought it up on a dim bulb tester: no shorts. I can hear FM on some headphones. Seems like she's coming back to life
Now the next big questions.
How does one set DC bias with a volt meter?
I assume DC offset is adjusted with the "DC Balance' pots. What are target DC offset readings? I took the following measurements across the 0.47 ohm emittor resistors:
Left Channel: 36.3mv / 0.18ma (stock original channel)
Right Channel: 13.5mv / 0.07ma (repaired updated channel)
I've read De-oxit is a no no on the pots. What do you guys suggest to clean the control pots on this unit?
Thanks all!
So, I cleaned up the burned board, recapped the power supply, replaced both filter caps (filter cap on bad channel had blown), replaced the output transistors and rebuilt the burned board with new resistors and transistors. I'll get some picks up shortly.
I brought it up on a dim bulb tester: no shorts. I can hear FM on some headphones. Seems like she's coming back to life
Now the next big questions.
How does one set DC bias with a volt meter?
I assume DC offset is adjusted with the "DC Balance' pots. What are target DC offset readings? I took the following measurements across the 0.47 ohm emittor resistors:
Left Channel: 36.3mv / 0.18ma (stock original channel)
Right Channel: 13.5mv / 0.07ma (repaired updated channel)
I've read De-oxit is a no no on the pots. What do you guys suggest to clean the control pots on this unit?
Thanks all!
JURB
Super Member
Hold on, what do you mean "across" the emitter resistors ? From one lead to the other ? If so you did measure bias.
With no load or with a cap coupled amp the balance control will not affect that reading. Balance is set measuring to ground. If the amp is cap coupled you want about half the supply voltage (DC) going to that cap. Slightly less is OK, but not too much. If the amp is cap coupled you can also set the bias with a load, though I still recommend setting all that stuff unloaded.
"Left Channel: 36.3mv / 0.18ma (stock original channel)"
Something went wrong there, I get 77 mA with those numbers. That is a little under 2½ watts per transistor at idle, which is not bad. Most likely 0.18 mA would make that thing sound "thin" at lower volumes if you didn't get some real bona fide crossover distortion.
But the thing now, since we know the bias is not too high, which is the most important thing at this stage, is to get some power out of that thing. On the DBT is fine, just that it will light up with load and the volume going up. Once you are happy with that, it is time to work without a net. (the DBT that is) If all your transistors etc. are good, you should have clean power to a real load. Deal with the bias later. This is a test just to see if it is really fixed. Things can fool you. Sometimes they will run headphones perfectly but not speakers.
Just keep your eyes, nose and ears peeled. Feel the heatsinks, even feel the transistors on the board. there is not going to be over about 50 volts on them I think, so just don't ground yourself. Or to be on the safe side, run it a bit and turn it off and check for heat.
With the bias not set, at idle, one channel may run warmer than the other, but after cranking up a couple of your favorite tracks, when they warm up more they should be about the same temperature. In fact switch it to mono to make sure the sound is the same out of each channel.
If everything works out, get the bias etc. set and you are done unless you feel like throwing in more caps. Caps will not affect bias and balance unless they're leaky. High ESR has no effect on DC. (well unless it is a main filter in which case you got a bunch of hum anyway)
With no load or with a cap coupled amp the balance control will not affect that reading. Balance is set measuring to ground. If the amp is cap coupled you want about half the supply voltage (DC) going to that cap. Slightly less is OK, but not too much. If the amp is cap coupled you can also set the bias with a load, though I still recommend setting all that stuff unloaded.
"Left Channel: 36.3mv / 0.18ma (stock original channel)"
Something went wrong there, I get 77 mA with those numbers. That is a little under 2½ watts per transistor at idle, which is not bad. Most likely 0.18 mA would make that thing sound "thin" at lower volumes if you didn't get some real bona fide crossover distortion.
But the thing now, since we know the bias is not too high, which is the most important thing at this stage, is to get some power out of that thing. On the DBT is fine, just that it will light up with load and the volume going up. Once you are happy with that, it is time to work without a net. (the DBT that is) If all your transistors etc. are good, you should have clean power to a real load. Deal with the bias later. This is a test just to see if it is really fixed. Things can fool you. Sometimes they will run headphones perfectly but not speakers.
Just keep your eyes, nose and ears peeled. Feel the heatsinks, even feel the transistors on the board. there is not going to be over about 50 volts on them I think, so just don't ground yourself. Or to be on the safe side, run it a bit and turn it off and check for heat.
With the bias not set, at idle, one channel may run warmer than the other, but after cranking up a couple of your favorite tracks, when they warm up more they should be about the same temperature. In fact switch it to mono to make sure the sound is the same out of each channel.
If everything works out, get the bias etc. set and you are done unless you feel like throwing in more caps. Caps will not affect bias and balance unless they're leaky. High ESR has no effect on DC. (well unless it is a main filter in which case you got a bunch of hum anyway)
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Moving Ahead
Well-Known Member
In other words you'll see a large capacitor - 3000uF / 80v. with a DMM you should get a 70v reading from positive to ground.
Next you'll see two large caps 4000uF / 50 v. with a DMM you need to get 33v reading from positive to ground. Adjust the 500K trimpot on left and right channel to achieve 33v.
Bias:
The 500 ohm trimpot will adjust your bias. Don't adjust too fast and in large increments. Take it easy. wait 5min before adjust. My intuition would say about 25mV+3mV. The adjustment is done with no signal and no volume.
As Jurb says
At normal listening levels it shouldn't get more than lukewarm. Crank it for a bit and the heat sink will get hot but never too hot to touch.
There are no trim pots for DC offset. It is a cap coupled unit. With your workshop speakers attached (must have a load attached) use a DMM and measure DC volts at the speaker terminals. You should see close to zero. If its say 20mv you may need to replace the 2x 4000 uF capacitors. These are what strip DC prior to your speakers.
Jurb tells you a lot and theres a lot to take in. Take it easy and you'll get through it all.
Also make sure you use mini grabbers on your DMM.
special_20
Active Member
all right, big step backwards last week. I got into hooking up speakers. No joy; just a hum out of the bad channel.
Good channel still works. I recapped the power supply and that seems to be working. The Pre-amp section seems to be working as I get sound out of the good channel on using FM and Aux.
On the bad channel I went through and double checked my outputs: emiter resistor is still good; output transistors aren't shorted (test good with DMM); I get a good ESR value reading on the output capacitor. I can only conclude that I still have a pre-driver issue.
Any tips for trouble shooting my pre-amp board?
Anything else I should be checking (i.e. would bias, balance or offset being completely out of whack cause this or is this a clear lack of signal?)
Good channel still works. I recapped the power supply and that seems to be working. The Pre-amp section seems to be working as I get sound out of the good channel on using FM and Aux.
On the bad channel I went through and double checked my outputs: emiter resistor is still good; output transistors aren't shorted (test good with DMM); I get a good ESR value reading on the output capacitor. I can only conclude that I still have a pre-driver issue.
Any tips for trouble shooting my pre-amp board?
Anything else I should be checking (i.e. would bias, balance or offset being completely out of whack cause this or is this a clear lack of signal?)