Kenwood KR-7200 repair problem
dly66
Addicted Member
I thought most of those relays are sealed cases, not meant to be opened. Usually when they fail either the coil of the solenoid burns out and will read an open circuit or some part of the contact will break off. Over time one form of fatigue or another will eventually lead to failure.
One simple check of the relay is hearing for the click a few seconds after it powers up. Every one I have with a relay it is clearly audible if you are close to it. If you can get in a good position to see them when the amp powers up you should see the contacts shift sides when it clicks. That circuit is nearly always set up to give the amp circuits a few seconds to stabilize before engaging the relay. Also there is a little transistorized circuit that controls the relay. Basically after that initial delay if the DC level at the output of the amp circuit is below a certain level, I think normally 1.5-3 Vdc varies by manufacturer and model, the relay the relay clicks in. If the DC level is above the target on one channel the speakers won't connect at all, the relay will stay open. I had an auction Yamaha CR-800 I got that had 3 bad output transistors and the relay did not click until I replaced them and the other collateral damage done when they failed (just 2 fusible resistors here - credit designers). With the caramel crystals and brown coating when I cleaned the board I think someone spilled a cola into it when it was on and it ran all down the amp board. If you are getting output at the speakers that would indicate the solenoid is okay or if damaged it would be the side for the left channel. On the schematic that circuitry that includes Qe11-Qe15 looks to be the the relay control circuit.
Simple check in first paragraph. More complicated detailed relay check in next paragraph. On the KR-7400 they have the relay in the middle of the amp board. Looks like on the KR-7200 they have it on its own board labeled X13-1240-10 with some resistors and capacitors. On my 7400 they have the board pins that the wires from other boards attached to clearly labeled and used the old wire wrap technique. Test clips are often safer than probe tips since once slip connection wrong contact points can create a short in the wrong place and zap - flash, maybe burned tip, maybe fried something. Power back on and off. Had number of times bright flashes but did not blow things, a couple of bad experiences teaches you to be more careful. You might get away with probe tips on this one, especially if yours had the kind where there is only a small exposed cone on the tip. Regular longer tips be careful, lose grip on one slips the wrong way may not be pretty. Remember the power supply and amp circuits can deliver a good pop in this thing. Don't try to take resistance measurements or diode checks on energized circuits, just ac or dc voltage.
Going by the connection wire wrap pins indicated by the schematic for relay board, looks vertically mounted like the 7400 in tight quarters hopefully all pins not too hard to access. Looks like control coil power comes in on pins 11-14. Checking continuity with unit off the spec sheets for the modern units indicates coil on a 24DC model should be in the neighborhood of 650 ohms. Just a ballpark, actual values may be a bit different. With unit on and control circuit energizing it should read around 24 Vdc between those two pins, at least greater than 19 Vdc. Usually on this type relay the coil has a resistance or is open. If the resistance is many times what it should be coil on its way out. If open its dead. With power off check continuity between the pair of pins 5-2 and 18-1. Should be open with the power off. Since almost no amp has zero dc offset, with unit on put negative tip to case, I usually stick the tip in one of the holes in the lower side of the chassis away from anything electrical and let it rest there. With unit on should see same DC values at those pairs of pins, one voltage 5-2 and other channel 18-1. Should see same voltage at speaker terminals. If all that checks should be okay. If pin 1 or 2 is lower than its paired pin there may be a problem with contacts.
The KR-7400 used an Omron MY4-02-US-24DC relay fortunately Omron is still around I think the current version of the relay is the MY4-02-DC24. My local Grainger indicates they have one in stock at the local branch for cheaper than Mouser wants, about $10. Relays like this are not hard to check. If your relay clicks a few seconds of being powered up the coil should be okay. If you just sprayed the contacts with some kind of contact cleaner, like CRC's QD contact cleaner I would think the contacts are okay. If in doubt you can trigger the relay manually. If you have a DC power supply, a lot of 18V cordless drill chargers probably use a 24 Vdc adapter or something close. The spec sheet says 80% rated voltage should trigger, so something that puts out above 19.2 Vdc should trigger it. Some small wire, such as bell wire you can buy by the foot at Lowes or Home Depot (some of that has been used in past repairs) and electrical tape you can tie into pins 11 and 14 to power the relay. Check continuity between the paired pins manually triggering the relay.
One simple check of the relay is hearing for the click a few seconds after it powers up. Every one I have with a relay it is clearly audible if you are close to it. If you can get in a good position to see them when the amp powers up you should see the contacts shift sides when it clicks. That circuit is nearly always set up to give the amp circuits a few seconds to stabilize before engaging the relay. Also there is a little transistorized circuit that controls the relay. Basically after that initial delay if the DC level at the output of the amp circuit is below a certain level, I think normally 1.5-3 Vdc varies by manufacturer and model, the relay the relay clicks in. If the DC level is above the target on one channel the speakers won't connect at all, the relay will stay open. I had an auction Yamaha CR-800 I got that had 3 bad output transistors and the relay did not click until I replaced them and the other collateral damage done when they failed (just 2 fusible resistors here - credit designers). With the caramel crystals and brown coating when I cleaned the board I think someone spilled a cola into it when it was on and it ran all down the amp board. If you are getting output at the speakers that would indicate the solenoid is okay or if damaged it would be the side for the left channel. On the schematic that circuitry that includes Qe11-Qe15 looks to be the the relay control circuit.
Simple check in first paragraph. More complicated detailed relay check in next paragraph. On the KR-7400 they have the relay in the middle of the amp board. Looks like on the KR-7200 they have it on its own board labeled X13-1240-10 with some resistors and capacitors. On my 7400 they have the board pins that the wires from other boards attached to clearly labeled and used the old wire wrap technique. Test clips are often safer than probe tips since once slip connection wrong contact points can create a short in the wrong place and zap - flash, maybe burned tip, maybe fried something. Power back on and off. Had number of times bright flashes but did not blow things, a couple of bad experiences teaches you to be more careful. You might get away with probe tips on this one, especially if yours had the kind where there is only a small exposed cone on the tip. Regular longer tips be careful, lose grip on one slips the wrong way may not be pretty. Remember the power supply and amp circuits can deliver a good pop in this thing. Don't try to take resistance measurements or diode checks on energized circuits, just ac or dc voltage.
Going by the connection wire wrap pins indicated by the schematic for relay board, looks vertically mounted like the 7400 in tight quarters hopefully all pins not too hard to access. Looks like control coil power comes in on pins 11-14. Checking continuity with unit off the spec sheets for the modern units indicates coil on a 24DC model should be in the neighborhood of 650 ohms. Just a ballpark, actual values may be a bit different. With unit on and control circuit energizing it should read around 24 Vdc between those two pins, at least greater than 19 Vdc. Usually on this type relay the coil has a resistance or is open. If the resistance is many times what it should be coil on its way out. If open its dead. With power off check continuity between the pair of pins 5-2 and 18-1. Should be open with the power off. Since almost no amp has zero dc offset, with unit on put negative tip to case, I usually stick the tip in one of the holes in the lower side of the chassis away from anything electrical and let it rest there. With unit on should see same DC values at those pairs of pins, one voltage 5-2 and other channel 18-1. Should see same voltage at speaker terminals. If all that checks should be okay. If pin 1 or 2 is lower than its paired pin there may be a problem with contacts.
The KR-7400 used an Omron MY4-02-US-24DC relay fortunately Omron is still around I think the current version of the relay is the MY4-02-DC24. My local Grainger indicates they have one in stock at the local branch for cheaper than Mouser wants, about $10. Relays like this are not hard to check. If your relay clicks a few seconds of being powered up the coil should be okay. If you just sprayed the contacts with some kind of contact cleaner, like CRC's QD contact cleaner I would think the contacts are okay. If in doubt you can trigger the relay manually. If you have a DC power supply, a lot of 18V cordless drill chargers probably use a 24 Vdc adapter or something close. The spec sheet says 80% rated voltage should trigger, so something that puts out above 19.2 Vdc should trigger it. Some small wire, such as bell wire you can buy by the foot at Lowes or Home Depot (some of that has been used in past repairs) and electrical tape you can tie into pins 11 and 14 to power the relay. Check continuity between the paired pins manually triggering the relay.
50nstillhifi
Super Member
Cleaning relays can be "intense".. and I have read of a few posters having success although it was just as involved. I have not heard of issues afterwards but again the contacts are so small it can be an issue.. I am sure
There have been great gains made with using an indirect blast of compressed air, and it still comes in a can. I use a 100 psi refillable type compressor with a control nozzle, it gives me plenty of overhead when blowing out the larger areas of boards tainted with dust and other aged matter.
Deoxit is the known standard for particulate accumulations in switching.. but I'd rather spot some of the tougher challenges with WD.. it usually dissolves most issues. I then follow it up with a light lube oil.. most of these for small gearing (they have a needle like tip).
My latest success was a CT 650 Pioneer, belts were worn and gearing dry. Using the prior stated techniques.. its now "quiet as mices"..
dly66
Addicted Member
The relay can be checked. If it clicks and moves the contacts and there is continuity (low resistance) between the pins that should be, it should be good. So the relay operation can be validated. Or it looks like you could use test leads, with the alligator clips, and jumper pins 5 to 2 and 18 to 1 to effectively bypassing the relay if I am reading the schematic right. Just don't send too much power through them it won't take much to fry them and their higher resistance an issue as more current tries to go through them. I would just test output with the relay bypassed using headphones plugged into the headphone jack, if you have the 1/8 to 1/4 adapter handy. I have a cheap pair I use pretty regularly. And I do have a better pair. Looks like most Best Buys have the adapters for $5 so should not be hard to get one. That will keep the power very low. You can also check DC offset at the speaker terminals. If no left at headphones but good right, likely not the relay. I tested my Yamaha amp board before I put new outputs in by basically using it as a headphone amp. Let the driver transistors drive the headphones. That trick may not work with all amp designs.
If you wanted to you could make test leads to tap into circuits with. Feeding a headphone amp is one option, powered computer speakers another. I have seen suggested put a small non-polarized capacitor in the positive wire to help isolate the front end (block dc). I used a spare small film cap I had. You could tap it into the connection between the preamp board and the amp board for each channel and see if you get good signal there. I had to use that technique to figure out where the Scott was going wrong. Find where the sound goes dead. Good technique between boards, would be leery probing within the amp circuit. Can verify that preamp (and thus its pots and switches) is okay and narrow it down to the amp board.
If you wanted to you could make test leads to tap into circuits with. Feeding a headphone amp is one option, powered computer speakers another. I have seen suggested put a small non-polarized capacitor in the positive wire to help isolate the front end (block dc). I used a spare small film cap I had. You could tap it into the connection between the preamp board and the amp board for each channel and see if you get good signal there. I had to use that technique to figure out where the Scott was going wrong. Find where the sound goes dead. Good technique between boards, would be leery probing within the amp circuit. Can verify that preamp (and thus its pots and switches) is okay and narrow it down to the amp board.
kintave
New Member
I replaced the NTE transistor (Qe7) and its complement in the left channel with the On transistors that arrived from Digikey today. I checked the voltage in and out of the relay, and it's fine. Left channel voltage reads 11 mV, and right channel reads 35 mV. What was I reading there? Is that the DC offset? Bias (as measured across the emitter resistors of the power transistors) is 20 mV in both channels, so I don't know what accounts for the difference between them.
There is output from the left channel, but it's very quiet. I was hoping that was just due to a mismatch between Qe7 and Qe9, but apparently not.
I'll make that signal tester. Fortunately, I have an old cable sitting around. I'll give everything another go with Deoxit while I'm at it.
There is output from the left channel, but it's very quiet. I was hoping that was just due to a mismatch between Qe7 and Qe9, but apparently not.
I'll make that signal tester. Fortunately, I have an old cable sitting around. I'll give everything another go with Deoxit while I'm at it.
dly66
Addicted Member
That is the dc offset. It is almost never the same between channels. As far as dc offset 11 and 35 mV are good values for most amps. The Scott I zeroed out but after a number of hours of use it is up to 31 and around 60 mV on the other channel. A number of old models the service manual spec indicated as long as it was below 200 mV it was good. Above that they say look for a bad component. Not too many have a dc offset adjustment.
If the relay is clicking in and making contact the dc offset should be the same on each side of it. If the speaker side of the relay was different would indicate a problem in the relay or control circuit. Most relay control circuits I remember click the relay off if the DC offset reaches the 1.5-3 V dc range, which should not damage the woofers. Full power supply voltage which can happen when an output transistor dies shorted out can quickly fry most woofers, the more powerful the amp the higher the voltage the woofer sees. Was testing an AV amp did not check dc offset first and the 4" woofer in the test speaker hooked to that channel died a quick death. Won't make that mistake again. It was sending 35 Vdc out not even turned on, just plugged in. So in power terms P = V^2 / R = 35*35/8 = 150 watts continuous. Small woofer was toasted before I could even react. Fortunately Parts Express had a very suitable replacement, just took them 3 months to get it back in stock. The Sansui 3000/3000A in the late 60s was known for problems and toasting woofers, as was their 5000 design with the early driver board design. Sansui had stability and reliability problems with their early high power solid state designs. Some designs there is not much protecting the woofers if an output shorts out.
Something is upset on the left channel killing the output. You are getting the idle current flowing across the resistor so in the past I have found just one bad component or break in the circuit such as a bad solder joint can reduce it to being faint. I would expect that the STV-3 diodes are probably okay if you can adjust bias since the current through the trimmer comes through the STV-3.
Trying to tap in to the circuits I would first aim for where the pre-amp feeds into the amplifier circuits. If both channels good where they come into the amp board its somewhere in the left amp channel. Reduces the section you are looking for the problem in. If left is dead at the input to the amp circuit we are looking for a problem upstream of the amp board. Tapping in to the circuits can help narrow greatly where the problem has to be and reduce the guessing. Whatever is being tied into the circuit you want a fairly high input impedance. Most line level inputs are about 50k ohms if memory is correct should be okay. Putting small bipolar cap in the +ve wire blocks dc from the front of whatever is being tied into the amp circuits so if dc is high won't fry the front end of your tester.
Looking at the schematic of the amp board "L in" is pin 1 and ground pin 2, and "R in" pin 22 and ground pin 21 which is also tied to pin 2 ie the ground is common.
If the relay is clicking in and making contact the dc offset should be the same on each side of it. If the speaker side of the relay was different would indicate a problem in the relay or control circuit. Most relay control circuits I remember click the relay off if the DC offset reaches the 1.5-3 V dc range, which should not damage the woofers. Full power supply voltage which can happen when an output transistor dies shorted out can quickly fry most woofers, the more powerful the amp the higher the voltage the woofer sees. Was testing an AV amp did not check dc offset first and the 4" woofer in the test speaker hooked to that channel died a quick death. Won't make that mistake again. It was sending 35 Vdc out not even turned on, just plugged in. So in power terms P = V^2 / R = 35*35/8 = 150 watts continuous. Small woofer was toasted before I could even react. Fortunately Parts Express had a very suitable replacement, just took them 3 months to get it back in stock. The Sansui 3000/3000A in the late 60s was known for problems and toasting woofers, as was their 5000 design with the early driver board design. Sansui had stability and reliability problems with their early high power solid state designs. Some designs there is not much protecting the woofers if an output shorts out.
Something is upset on the left channel killing the output. You are getting the idle current flowing across the resistor so in the past I have found just one bad component or break in the circuit such as a bad solder joint can reduce it to being faint. I would expect that the STV-3 diodes are probably okay if you can adjust bias since the current through the trimmer comes through the STV-3.
Trying to tap in to the circuits I would first aim for where the pre-amp feeds into the amplifier circuits. If both channels good where they come into the amp board its somewhere in the left amp channel. Reduces the section you are looking for the problem in. If left is dead at the input to the amp circuit we are looking for a problem upstream of the amp board. Tapping in to the circuits can help narrow greatly where the problem has to be and reduce the guessing. Whatever is being tied into the circuit you want a fairly high input impedance. Most line level inputs are about 50k ohms if memory is correct should be okay. Putting small bipolar cap in the +ve wire blocks dc from the front of whatever is being tied into the amp circuits so if dc is high won't fry the front end of your tester.
Looking at the schematic of the amp board "L in" is pin 1 and ground pin 2, and "R in" pin 22 and ground pin 21 which is also tied to pin 2 ie the ground is common.
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kintave
New Member
Alright, I was out of town for a while, but I finally got a chance to build a probe tester and work backwards until I could see where the signal dies.
It's strong coming out of the preamp, but it dies somewhere on the filter board. I've probed around, and I'm about 80% sure it's one of the resistors. That board is the single most annoying one to remove, but I'll take it out tomorrow and see if the resistors are to spec, and if any are cracked.
I never would have found that if I hadn't built that probe.
Here's hoping I can finally fix this tomorrow...
It's strong coming out of the preamp, but it dies somewhere on the filter board. I've probed around, and I'm about 80% sure it's one of the resistors. That board is the single most annoying one to remove, but I'll take it out tomorrow and see if the resistors are to spec, and if any are cracked.
I never would have found that if I hadn't built that probe.
Here's hoping I can finally fix this tomorrow...
dly66
Addicted Member
I just put a post out for suggestions for transistor replacements for my old Scott. Looked at the top of the solid state saw this just below it was wondering if you were still around. I don't think I could have fixed the Scott, or at least there would have been a number of wild goose chases, until I rigged up mine. Also helped me figure out a situation with my 2000A so now everything works with it.
Happy hunting on the filter board. I know recapping the KR-7400 that guy was not an easy one to do. Nearly silent in terms of background noise and sounds a lot better after the recap.
You can also use a DVM in diode test mode to check all diodes and any transistors. With transistors it really keys on the base, the other two legs amount to either diodes going to base or diodes going away. Have found blown transistors that way before a number of times.
Happy hunting on the filter board. I know recapping the KR-7400 that guy was not an easy one to do. Nearly silent in terms of background noise and sounds a lot better after the recap.
You can also use a DVM in diode test mode to check all diodes and any transistors. With transistors it really keys on the base, the other two legs amount to either diodes going to base or diodes going away. Have found blown transistors that way before a number of times.
dly66
Addicted Member
Just downloaded a copy of the service manual so I can help more. Did they use a lot of the 8 leg op amps in those or did they start that with my generation? On the 7400 the tone controls were all op-amps, no transistors.
Are the signals good at the L and R inputs to the filter board? If not the problem may be between the boards. That is what I ran into with the Scott, ended up being fried components soldered between outer tabs of the input selector.
Are the signals good at the L and R inputs to the filter board? If not the problem may be between the boards. That is what I ran into with the Scott, ended up being fried components soldered between outer tabs of the input selector.
kintave
New Member
Yeah, still here! And seriously, thank you again for all your help. I wouldn't have gotten this far without it.
Good news, I finally got the left channel working. It turns out something had gotten stuck in one of the filter switches (I don't know how), so I didn't even need to replace anything, although I cleaned off some excess rosin anyway. Cleaned all the switches with Deoxit and everything is back to normal. Fingers crossed it'll stay that way.
I'd previously checked all the transistors on the tone controls - no op amps, just transistors - and they seem fine. I think it was just that one connection problem on the filter board.
Unfortunately, there's still a pretty noticeable hiss. I'm having a hard time tracing where the hiss comes in. It's noticeable especially on headphones, but hard to hear with the probe. I've replaced pretty much every single electrolytic capacitor at this point, except C301 and C302, the big 6800 uF caps at ground. Could those be responsible for the hiss? They're the only ones I haven't replaced because everything is annoyingly connected to them by wire wrap directly onto the leads. In order to replace them I'll have to cut off the leads, figure out the best way to conect them to new caps, and figure out how to hold the new caps in place (since the current ones are held in by metal straps).
Don't want to celebrate too early, but I think I'm almost there. Just want to make sure everything stays working (in case there's still a loose connection somewhere), and then I just need to deal with that lingering hiss.
Good news, I finally got the left channel working. It turns out something had gotten stuck in one of the filter switches (I don't know how), so I didn't even need to replace anything, although I cleaned off some excess rosin anyway. Cleaned all the switches with Deoxit and everything is back to normal. Fingers crossed it'll stay that way.
I'd previously checked all the transistors on the tone controls - no op amps, just transistors - and they seem fine. I think it was just that one connection problem on the filter board.
Unfortunately, there's still a pretty noticeable hiss. I'm having a hard time tracing where the hiss comes in. It's noticeable especially on headphones, but hard to hear with the probe. I've replaced pretty much every single electrolytic capacitor at this point, except C301 and C302, the big 6800 uF caps at ground. Could those be responsible for the hiss? They're the only ones I haven't replaced because everything is annoyingly connected to them by wire wrap directly onto the leads. In order to replace them I'll have to cut off the leads, figure out the best way to conect them to new caps, and figure out how to hold the new caps in place (since the current ones are held in by metal straps).
Don't want to celebrate too early, but I think I'm almost there. Just want to make sure everything stays working (in case there's still a loose connection somewhere), and then I just need to deal with that lingering hiss.
dly66
Addicted Member
High background noise can also be transistors. My Sherwood used 2SC693F transistors in the tone control section had a background roar that went up and down with the volume. Replaced the four of them with new KSC1845 transistors dramatically quieted it down. Let me look at the schematic and see if it has any known to get noisy. The 2SC693 not on the hit lists, probably not that common but found a thread where someone mentioned replacing them in the tone section of a Sansui 1000X and saying it made a big difference.
Is the noise there with no source on (ie AUX with no input)? Does it go up and down with the volume control? If so probably in the preamp. If not likely amp section.
When you said the filter board I looked at the schematic and it looked like the only thing on it were two switches and a few small caps.
I guess with the next generation KR-x400 Kenwood went big with the op-amps.
Is the noise there with no source on (ie AUX with no input)? Does it go up and down with the volume control? If so probably in the preamp. If not likely amp section.
When you said the filter board I looked at the schematic and it looked like the only thing on it were two switches and a few small caps.
I guess with the next generation KR-x400 Kenwood went big with the op-amps.
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dly66
Addicted Member
A number of the old transistors, from the late 60s up into the mid 70s were known for not aging well. Some act up, a number just become noisy. My first restoration was a Pioneer SX-535 and it was loaded with transistors on the dirty 11 list. It had 16 total of 5 of the types on the list. When I started it sounded like a receiver with a major head cold.
The old blocky Hitachi 2SC458LG was probably the worst. Which is funny it was supposed to be a low noise transistor. I have a future project Pioneer that probably has at least 20 of those in it.
The old blocky Hitachi 2SC458LG was probably the worst. Which is funny it was supposed to be a low noise transistor. I have a future project Pioneer that probably has at least 20 of those in it.
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dly66
Addicted Member
Was checking and in some other brand vintage models the Hitachi 2SC1345, there should be four on the tone amp board X11-0007-11, have been noise problems. The transistors used in the tone controls tend to be the most noise prone in the late 60s early 70s units. If its them the noise should go up and down with the volume control. The volume control comes after that board.
Four more should be in the mic amp section. Checking on replacements.
If like the Sherwood it should be a steady background hiss or roar when those types get noisy.
Four more should be in the mic amp section. Checking on replacements.
If like the Sherwood it should be a steady background hiss or roar when those types get noisy.
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dly66
Addicted Member
Looks like the BC550C should work for the 2SC1345. I have a pair of those replacements in the amp driver boards of my old Scott. They are a higher gain transistor and low noise. The ZTX694B is liked as a replacement by Echowars he is a veteran on this site. Either one will probably work there. Even the KSC1845 since it is the tone section may be okay there. Some have used that one. The KSC1845 is a popular replacement transistor covers many old types. So does its KSA992 complement.
Be aware of pinouts to match make sure replacement is in right. The 2SC1345 should be ECB, BC550C is CBE, ZTX694B is EBC, and the KSC1845 is ECB if I am reading the datasheets correctly. Usually amounts to whether you bend the middle pin so it offsets forward, or backward, and how much to rotate. Cheap transistor checker comes in handy here for pinouts and gain matching, and seeing if the replacement is in the gain league as the original.
If the DC offset out of an amp channel is high the 2SA620 used for the differential pairs can also give trouble. The KSA992 is an often suggested option, BC560C may be another. Have used the KSA992 types a number of times, they are the replacement differential pairs in my Sherwood S-7200. The original 2SA666 would make a rat-a-tat noise on one channel after about 10 minutes being on. Often a sign of differential pair problem is the DC offset at the speakers terminals. That seems to get controlled primarily by the differential pairs. When it gets toward 200 mV or more there is definitely a problem. Ideally it should be in the single digits or tens of mV. That is why a new pair of BC550/560 transistors are at the front of the Scott amp driver board. Could not get DC offset with the trimmer on one channel to go much below 200 mV. After putting them in I could adjust to zero. This is a recording of the noisy 2SA666 pair I put out there on youtube:
Be aware of pinouts to match make sure replacement is in right. The 2SC1345 should be ECB, BC550C is CBE, ZTX694B is EBC, and the KSC1845 is ECB if I am reading the datasheets correctly. Usually amounts to whether you bend the middle pin so it offsets forward, or backward, and how much to rotate. Cheap transistor checker comes in handy here for pinouts and gain matching, and seeing if the replacement is in the gain league as the original.
If the DC offset out of an amp channel is high the 2SA620 used for the differential pairs can also give trouble. The KSA992 is an often suggested option, BC560C may be another. Have used the KSA992 types a number of times, they are the replacement differential pairs in my Sherwood S-7200. The original 2SA666 would make a rat-a-tat noise on one channel after about 10 minutes being on. Often a sign of differential pair problem is the DC offset at the speakers terminals. That seems to get controlled primarily by the differential pairs. When it gets toward 200 mV or more there is definitely a problem. Ideally it should be in the single digits or tens of mV. That is why a new pair of BC550/560 transistors are at the front of the Scott amp driver board. Could not get DC offset with the trimmer on one channel to go much below 200 mV. After putting them in I could adjust to zero. This is a recording of the noisy 2SA666 pair I put out there on youtube:
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dly66
Addicted Member
I make use of a cheap tester from Amazon seems to work well. If you blow it up (I did once - did not make sure a big filter cap was discharged) they are not very expensive:
https://www.amazon.com/gp/product/B00OOQC2E8/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B00OOQC2E8/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1