Sansui QRX-7500 rebuild notes

[nabeelsayegh]

Annnnd.....it works!

Michael, I can't thank you enough. I am not going to even admit the amount of hours spent on trying to figure this one out. All 4 channels are firing correctly!.

Is it too early for a beer?!?

 

[michael_w]

You are quite welcome! I'm glad to hear things are working again. It seems you were led astray by the same mislabeling that I encountered (and like you I spent hours looking in dark corners....but the effort wasn't truly wasted because my troubleshooting skills have notably improved). I have been saved numerous times by other posts. Sometimes they are hard to find though.

If you are OCD you could test the performance of the circuit with the SQ matrix test download. I should have provided the link but I didn't think to and now I don't have it at the ready. Again, if my memory is correct, the trim pots on the other board can be slightly adjusted to improve the symmetry (if that has drifted). Just make sure you record the original positions because bad things will happen if you adjust too many pots too far (and this I know....). This suggestion is really a hack because it isn't covered in the service manual and I didn't go to the trouble of wiring up a spice simulation (simulating the hidden ICs would be a very tough task).

I'm guessing that you will leave the CD-4 decoder board for another day...it is actually even harder to troubleshoot.

And no worries...I do have beer on hand.

Enjoy, M.

 

[nabeelsayegh]

Agree on all points. No such thing as failures, just learning opportunities. I am pretty certain I have had enough of this beast and it does sound pretty good as is. I will see if I can find that SQ matrix download you mentioned....would be cool to see it in action. Is there such a test file for CD4 or am I going to have to rig a bunch of test gear to do that? If so, I may not have what is necessary to properly test that part out.

 

[michael_w]

Here is the link to the page with the CD-4 needle drop of Marvin Gaye's Mercy, Mercy Me http://pspatialaudio.com/CD4_classic.htm in flac format. Many PC sound cards support the flac format and you can just take the output from a 3mm headphone jack (converting to RCA) or RCA jacks if your sound card has them. Good quality RCA patch cables would help. If I recall correctly this needle drop is before the phono equalizer stage and, if it weren't for one big caveat, would be "plug and play" for the phono input on the QRX-7500 with the correct audio level for a phono input. The kicker is that most audio cards don't decode this flac file for the full range of frequencies that the CD-4 format requires (up to 45kHz or so). Your will need audio card with a minimum of 96kHz DAC conversion but preferably 192kHz DAC. These are certainly more common now but not standard issue by any means. In addition you will need to make certain that driver software for the audio card will output the full CD-4 signal. Only then will you be in a position to properly test the CD-4 board unless you happen to have a turntable with quad capabilities and a CD-4 test disc.

A digital oscilloscope with Fourier transform capabilities (or alternatively an audio power spectrum analyzer) is also something that you really need so you can monitor the 30kHz carrier signal. I used a fairly inexpensive Rigol oscilloscope which does this adequately but a good Tektronix scope (or something comparable) would be a bit better.

One thing that still amazes me is how clever the audio engineers were in that era because everything had to be done with imperfect recordings and analog circuitry running on the fly.

Testing the SQ board is far more straightforward.

 

[CrSens]

Annnnd.....it works!

Michael, I can't thank you enough. I am not going to even admit the amount of hours spent on trying to figure this one out. All 4 channels are firing correctly!.

Is it too early for a beer?!?

Funny thing, I went through the same ordeal when servicing my QRX-5001, for the life of me I couldn't figure out why the drop in level after changing TR 05. What is even worse, the board layout at that position and polarity of C09 indicated NPN transistor, and the schematic diagram also has it drawn as NPN, and since most installed transistors on that board seem to have painted over (or blackened) markings, it is hard to spot the mistake. Not in the habit of testing every single transistor I remove for pinout. On the side of schematic there is TR 05 listed as A726, and TR 06 as C1312, but transistors themselves are unmarked on drawing, and both drawn as NPN! Looking little down at the similar section with TR 07 and 08, TR 08 is drawn correctly as PNP (but again not marked). So, to make it maximally confusing, front left and rear right channel phase shifters are PNP (TR 05 and 08), and FL and RL are NPN (TR 06 and 07). On schematic in manual TR 05 is drawn (incorrectly) as NPN, and TR 08 correctly as PNP. And none are directly labeled on drawing. Cap polarity of C 09 (and C12) is wrongly marked on PCB, leading one to cursory assume all these 05-08 transistors are NPN..... Further schematic reading reveals that C and E are reversed on the PCB markings for TR 05 and 08. I wished I saw this thread before going through this myself.....

 

[michael_w]

Hi, I too have stumbled on discussion forums after the fact. I think well of the Sansui electronic fundamentals from that era but boy are their service manual circuit schematics and circuit board labeling prone to errors. Once a transistor is out the only thing I've found to get back on the right track is to check the DC offsets with the circuit energized. So far, at least with Sansui, physical access hasn't been too bad. With other components (e.g., a Kenwood receiver) I've had to solder tap wires onto the boards. I'm glad you got it figured out but I'm sure it took a longish time. What's that phase....everything is obvious once you know the answer.

 

[nabeelsayegh]

What's that phase....everything is obvious once you know the answer.

There is no question that this is true....i think the sister phrase is hind sight is 20/20. Situations like this remind me to photo document everything before I touch boards (especially helpful if you have a wire break off and its not obvious where it goes and/or if you pull a part like a cap or transistor and need to verify original polarity and/or pin orientation. One part at a time and never get a head of yourself....until the next time i screw up....lol.

 

[CrSens]

Hi, I too have stumbled on discussion forums after the fact. I think well of the Sansui electronic fundamentals from that era but boy are their service manual circuit schematics and circuit board labeling prone to errors. Once a transistor is out the only thing I've found to get back on the right track is to check the DC offsets with the circuit energized. So far, at least with Sansui, physical access hasn't been too bad. With other components (e.g., a Kenwood receiver) I've had to solder tap wires onto the boards. I'm glad you got it figured out but I'm sure it took a longish time. What's that phase....everything is obvious once you know the answer.

Hi, yes it took me about 10 hours all together, but the good part is it forced me to really fix up the quad boards, and there is quite a difference after changing all transistors and caps. I did not work on a CD-4 board, it works good, maybe better not mess with it. Next time I will be more careful when removing small signal transistors on Sansui PCB boards, and look for mistakes before rather then after.
In case anyone is interested, the QRX-5001 does output processed quad signals through the tape outputs, which makes it a very interesting little receiver. Possible to use it with external amps and home theater processors and receivers through their multichannel input, and still have genuine quadraphonic sound. I also got recently a QRX-6001, and in my first testing it seems that one does not output processed signals through tape outputs. I will test further, I just tested the fronts.

 

[KeithD]

Wow, AK is the best place on the internet! I had this exact same issue on F-2048 when I replaced all the transistors. I have a QRX-5500A I picked up recently. Changing the transistors on the F-2084 board dramatically reduced the background hiss on the rear channels, so I thought I'd get some more improvement by replacing the transistors on F-2048. Fell into the same hole described here. I confirmed all of the transistor identities on the board, but never thought to check the E-B markings. Fortunately, I only spend a few hours troubleshooting the issue before finding this thread. I was concerned about C09 being backwards given the board marking for that, but I DID take a picture of the board before starting any replacements, and that cap was clearly in reverse of the board marking, so I knew the marking had to be wrong.

Now, when I go back and look in detail at the board picture, I can see that TR05 has E and C switched from the board markings.



Similar situation with TR08, but harder to see in the picture I took.

Thanks again for this great thread. The F-2048 board was used in several QRX units (7500A, 7500, 6500, 5500A, 5500, 4500, 3500, 5001) as well as the QA-7000 integrated amp, and the QS-800A. I'd say this thread/information should be a "stickie" in the Sansui forum!

 

[CrSens]

Wow, AK is the best place on the internet! I had this exact same issue on F-2048 when I replaced all the transistors. I have a QRX-5500A I picked up recently. Changing the transistors on the F-2084 board dramatically reduced the background hiss on the rear channels, so I thought I'd get some more improvement by replacing the transistors on F-2048. Fell into the same hole described here. I confirmed all of the transistor identities on the board, but never thought to check the E-B markings. Fortunately, I only spend a few hours troubleshooting the issue before finding this thread. I was concerned about C09 being backwards given the board marking for that, but I DID take a picture of the board before starting any replacements, and that cap was clearly in reverse of the board marking, so I knew the marking at to be wrong.

Now, when I go back and look in detail at the board picture, I can see that TR05 has E and C switched from the board markings.

View attachment 2899692

Similar situation with TR08, but harder to see in the picture I took.

Thanks again for this great thread. The F-2048 board was used in several QRX units (7500A, 7500, 6500, 5500A, 5500, 4500, 3500, 5001) as well as the QA-7000 integrated amp, and the QS-800A. I'd say this thread/information should be a "stickie" in the Sansui forum!

To make the situation more humorous, the QRX-6500 F-2048 board which I recently fixed up (older design) does not have the PNP transistors in TR05 and TR08 locations, they are NPN.....and the board is configured as such. It also decodes SQ differently than the board in 5001, seems like smaller bleed resistors are used (I did not check), there is less output in the back channels on mono or near mono sources, but consequently better sound with high separation SQ sources. I also find the power amp of QRX-6500 extremely good sounding (2000X design?).

 

[KeithD]

To make the situation more humorous, the QRX-6500 F-2048 board which I recently fixed up (older design) does not have the PNP transistors in TR05 and TR08 locations, they are NPN.....and the board is configured as such. It also decodes SQ differently than the board in 5001, seems like smaller bleed resistors are used (I did not check), there is less output in the back channels on mono or near mono sources, but consequently better sound with high separation SQ sources. I also find the power amp of QRX-6500 extremely good sounding (2000X desigh?).

Yes, I'm pretty sure it's a carryover from the original QR-6500, which was Sansui's first TOTL quad receiver. I have one of those, and it's incredibly well made. I think they just put the updated quad boards in the QR-6500, updated the knobs and faceplate printing a bit and then called it a QRX-6500.

I've seen pictures of other F-2048 boards. The one in my QRX-5500A has Tantalum capacitors for many of the small value caps. On some of the boards I've seen there are e-caps in those positions. Interesting there are boards with other transistors at TR05 and 08. Have you seen any boards that correctly match the schematic? They seem to have used the same drawing in all the SM that show the F-2048.

Agree the separation is quite good - better than I have in the QR-6500, but not the same as in my QRX-8001 (but close on the "surround" setting.) It's the "Hall" setting that is disappointing in all the early units. Only the F-2087/88 boards get this setting right for a true "concert hall" effect.

 

[CrSens]

Yes, I'm pretty sure it's a carryover from the original QR-6500, which was Sansui's first TOTL quad receiver. I have one of those, and it's incredibly well made. I think they just put the updated quad boards in the QR-6500, updated the knobs and faceplate printing a bit and then called it a QRX-6500.

I've seen pictures of other F-2048 boards. The one in my QRX-5500A has Tantalum capacitors for many of the small value caps. On some of the boards I've seen there are e-caps in those positions. Interesting there are boards with other transistors at TR05 and 08. Have you seen any boards that correctly match the schematic? They seem to have used the same drawing in all the SM that show the F-2048.

Agree the separation is quite good - better than I have in the QR-6500, but not the same as in my QRX-8001 (but close on the "surround" setting.) It's the "Hall" setting that is disappointing in all the early units. Only the F-2087/88 boards get this setting right for a true "concert hall" effect.

I have to say I have not seen any quad boards which completely match the SM diagrams, but I did not pay that much attention after the QRX-5001 multiple error situation (errors in SM, errors in PCB markings, actual component install ignores both PCB markings and SM). I simply pay attention to the board itself and take note when desoldering. I think overall all Sansui quad units are very interesting and great in in their own way. I did not touch the CD-4 boards in 5001 and 6001, but they both work very well and decode CD-4 flawlessly (admittedly after some careful adjustments of separation dials and understanding of how carrier gain control should be adjusted). Sansui quad completely changed my listening environment, can't go back to 2 channels now.

 

[KeithD]

Yes, I agree. Once I heard Sansui "Synthesized" quad on the QR-4500 I bought several years ago, my listening experience transformed, and that was the early version of the technology! Sadly, quad never really caught on for music listening, much to the company's financial detriment. I find most music genres are enhanced by the synthesized surround mode, but not all. I think classical is much better and more natural in the "Hall" mode of the last version of Vario-matrix found in the QRX-9001/8001/7001/6001 and QSD-1 and 2 models. It's not truly a "hall" effect in the other versions of Sansui quad. Additionally, some music is so well recorded that stereo with the right amp/speaker combo can create a very appealing soundstage. I picked up some Magnepan SMGa speakers a few weeks ago, and they really do create a sound field that is one step closer to a live performance.

 

[CrSens]

Yes, I agree. Once I heard Sansui "Synthesized" quad on the QR-4500 I bought several years ago, my listening experience transformed, and that was the early version of the technology! Sadly, quad never really caught on for music listening, much to the company's financial detriment. I find most music genres are enhanced by the synthesized surround mode, but not all. I think classical is much better and more natural in the "Hall" mode of the last version of Vario-matrix found in the QRX-9001/8001/7001/6001 and QSD-1 and 2 models. It's not truly a "hall" effect in the other versions of Sansui quad. Additionally, some music is so well recorded that stereo with the right amp/speaker combo can create a very appealing soundstage. I picked up some Magnepan SMGa speakers a few weeks ago, and they really do create a sound field that is one step closer to a live performance.

Try the SQ mode on with stereo records (not just quad encoded), a lot of records have out of phase extreme left and right information recorded, it simply folds back into rear channels in SQ decoder and greatly enhances ambiance and adds appearance of depth perspective to front channels. I prefer it to vario-martix in most cases. Good front speakers are required for best effect, and rear speakers should be adjusted to output same volume at listening position as fronts when amp is in stereo "plus back" mode.

 

[GradoFTE+1]

Hi everyone:

Happy to see this thread, as I have obtained an original QRX-7500 from a friend (actually, he had it stored for many years, and then I stored it for many years), which I have pulled out of its sleep and am now interested in restoring.

I have followed the comments here carefully but have a number of questions that I seek help on. First and foremost, I have several decades of electronic repair experience, so this query focuses more on the unknowns than "how to solder" . I >have< downloaded the various documents I could find, have opened up the unit to do a general visual perusal of the internals, and have checked the continuity on the various fuses that were visible/accessible. Some guidance requested please, considering:

1) I have not attempted to power the unit up and do a general assessment. Would this be recommended FIRST?

2) I am really interested only in two-channel operation (don't know if this makes a difference in how I should approach this)

3) Since the early technology is really a brain-salad surgery mix of boards and wires, are there any recommendations on how I should approach this in terms of "which board first/second/etc.?" My ongoing HK PM-655 recap (underway) is a cakewalk compared to this....

4) Are there any cautions of what NOT to do?

5) Given that I'm looking for operational capability, and less modification/update, I'm assuming the best bet is: "If it ain't broke, don't fix it (re: transistor replacement)?

6) Michael_w above clearly gave a VERY detailed and technically thorough process of what he did (THANK YOU!!!); in terms of the capacitors, would using "standard" electrolytic caps as opposed to the suggested types degrade performance? I'm half deaf at this point in my life and wondering if the cost/effort would be worth the effort? (Yes, I understand the value of high quality components, I used to build and peddle replacement custom Klipsch crossover networks)

7) Are there ANY tutorials/vids ANYWHERE that would help carry me through this process?

8) Am I nuts to undertake this?

Hoping to receive some helpful advice, considering the previous responses, I believe I've come to the right place. THANKS EVERYONE!!

Chris

PS - HiFi audio out of Brooklyn, New York sells cap kits. Can anyone comment on whether they are reputable? Thanks!

 

[KeithD]

Hi everyone:

Happy to see this thread, as I have obtained an original QRX-7500 from a friend (actually, he had it stored for many years, and then I stored it for many years), which I have pulled out of its sleep and am now interested in restoring.

I have followed the comments here carefully but have a number of questions that I seek help on. First and foremost, I have several decades of electronic repair experience, so this query focuses more on the unknowns than "how to solder" . I >have< downloaded the various documents I could find, have opened up the unit to do a general visual perusal of the internals, and have checked the continuity on the various fuses that were visible/accessible. Some guidance requested please, considering:

1) I have not attempted to power the unit up and do a general assessment. Would this be recommended FIRST?

2) I am really interested only in two-channel operation (don't know if this makes a difference in how I should approach this)

3) Since the early technology is really a brain-salad surgery mix of boards and wires, are there any recommendations on how I should approach this in terms of "which board first/second/etc.?" My ongoing HK PM-655 recap (underway) is a cakewalk compared to this....

4) Are there any cautions of what NOT to do?

5) Given that I'm looking for operational capability, and less modification/update, I'm assuming the best bet is: "If it ain't broke, don't fix it (re: transistor replacement)?

6) Michael_w above clearly gave a VERY detailed and technically thorough process of what he did (THANK YOU!!!); in terms of the capacitors, would using "standard" electrolytic caps as opposed to the suggested types degrade performance? I'm half deaf at this point in my life and wondering if the cost/effort would be worth the effort? (Yes, I understand the value of high quality components, I used to build and peddle replacement custom Klipsch crossover networks)

7) Are there ANY tutorials/vids ANYWHERE that would help carry me through this process?

8) Am I nuts to undertake this?

Hoping to receive some helpful advice, considering the previous responses, I believe I've come to the right place. THANKS EVERYONE!!

Chris

PS - HiFi audio out of Brooklyn, New York sells cap kits. Can anyone comment on whether they are reputable? Thanks!

The QRX-7500 is a very nice quadraphonic receiver, though I don't own one. I will jump to number 8 and give you my opinion. If you are only interested in 2 channel stereo and will not use the quad features at all, then this is not the receiver for you, especially if it needs work. I really don't think there is much point to using quad equipment only in two channel mode. There is a heck of a lot of great stereo equipment out there (and you may have some) that is designed for stereo and hence is far less complicated than a quad unit. If the QRX-7500 needs service and you are not experienced with quad units, it will take many, many hours to get things working, and you might not be successful. (I'm speaking from experience when I first got into quad.)

If you are going to power it up, you should do so on a dim bulb tester. That will at least prevent you from burning something up if there is a fault.

 

[GradoFTE+1]

The QRX-7500 is a very nice quadraphonic receiver, though I don't own one. I will jump to number 8 and give you my opinion. If you are only interested in 2 channel stereo and will not use the quad features at all, then this is not the receiver for you, especially if it needs work. I really don't think there is much point to using quad equipment only in two channel mode. There is a heck of a lot of great stereo equipment out there (and you may have some) that is designed for stereo and hence is far less complicated than a quad unit. If the QRX-7500 needs service and you are not experienced with quad units, it will take many, many hours to get things working, and you might not be successful. (I'm speaking from experience when I first got into quad.)

If you are going to power it up, you should do so on a dim bulb tester. That will at least prevent you from burning something up if there is a fault.

Thank you, Keith, valid points. I have not decided what to do yet but I appreciate the response!

 

[michael_w]

Hi Chris, I've actually rebuilt two 7500s now (with a third just for repair on hold until I can clear a few other projects off the bench...).

My two cents...for what it is worth.

1. If the 7500 you have was working fine when it was last used then it is unlikely to be a major problem just powering it up. I've not had anything solid-state self destruct that way. If you have concerns I would first check the fuses and then visually inspect the circuit boards for anything suggestive of degradation or catastrophic failure (e.g. burn marks). Following that, use a Variac (or other autotransformer) to slowly raise the supply voltage while monitoring the supply current. Sometimes the oxide layer in caps degrades if not regularly energized and this method can restore them to proper operation.

After power on, listen to hear that the protection relay is energized. Next check the main power rails using an oscilloscope to look at the dc voltages and ripple. As a first pass I typically just use modern high impedance headphones and the FM tuner. However you can always first check to see if there is output from the four pre-amp channels with a1 kHz sine wave using the AUX function and an oscilloscope probe.

If everything checks out, you can more on to the amplifier outputs. I first check the dc offsets. If the bias circuit is badly off the receiver will use excessive power even when in idle. A lot of people use dim bulb testers (I have one) but I usually default to power resistors (at 8 or 10 ohms) rated from 50 or 250 watts while monitoring things with an oscilloscope probe. With a sine wave everything should look nice, smooth and symmetrical until a channel starts clipping at or above the rated power. I actually hook up speakers only after a rebuild as a final test. I also check things with a Leader distortion meter.

2. If you are only interested in stereo operation then I pretty much agree with the previous comment. There are options for simulated quad sound from a stereo signal so you don't need a quad disc or four track tape player. I am mainly interested in the repair/restoration so I only keep a few items for bench test comparisons.

3 & 4. Rebuild: Most boards are straightforward but it is easy to make a mistake. If any of the decoder boards are not working right then it could be a bit of a pain to get things right. Usually rebuilds start with the supply board(s) and then move along the audio signal path. The two tone boards are stacked and hard wired. You can get to many things in place but removing them has its virtues (take photos, just in case). There are a lot of parts if one chooses to do a full rebuild. I would not recommend a full rebuild unless one has a good desoldering tool. Make sure you use the lowest temperature setting that works and be gentle. Damaging circuit board pads is something to avoid (although I still have occasional problems here and there).

5. This is tricky. It may well be that you are off the hook and everything is still in working order. However there are many shades of gray because some parts just don't age well. Hiss and hum are often subtle attributes; modern high efficiency headphones can reveal unwanted attributes that were of no concern some 50 years ago. I do note that every FM tuner I've looked at has drifted and so alignments always improve things (in some cases dramatically). The ferrite inductor cores are frustratingly fragile and easy to break (and nearly impossible to replace). Most people are advised to leave sleeping dogs lie. Nothing in the 7500 FM front end is particularly at risk. I built a simple jig to hold the air gap capacitor pulley so that I could pull out the board for recapping. Nothing of note happens with the 7500 but the late 60s to early 70s Fisher or mid to late 70s Kenwood tuners have quirks that can leave one in tears. I have quite a few stories written down which I haven't gotten around to posting.

6. Discussions on caps are all over the map. My hearing acuity is still good but I've certainly lost the higher frequencies (anything at or above 8 kHz). Modern caps are much more uniform and usually bench test with lower ESR and losses. At some level I think replacing them is a matter of taste. I do find that electrolytic caps at 1uF or less typically age poorly (in terms of ESR and losses). Replacing these with film caps makes sense if their is room on the board (and be prepared to drill out slightly large holes for the legs if needed). Caps exposed to excessive heat typically lose capacitance (not a problem in the 7500) while others often drift to higher values (they still work but are out of spec). Some modern manufacturers are notorious for poor quality control (many LED bulbs die well before their expected lifetime is reached). I minimize my component costs by buying in bulk and shopping around for odd lots that are heavily discounted (always from reputable vendors...counterfeit capacitors and transistors are a bane).

7. No videos that I know of for the 7500.

8. It all depends on what your final goal is.

Online cap kits: I've never used them but, for a single rebuild, they are probably cost competitive (and some come with detailed instructions). Buyer feedback is usually pretty high. From my perspective they don't look to be 100% complete. My costs are typically 1/3 to 1/2 the list price.

That's all for now.

 

[CrSens]

Most of the problems will be in quad processing boards (noisy transistors), and driver boards should be recapped. If you are only using 2 ch mode noisy quad boards will not be an issue. The rest is just the usual deoxit treatment to get the thing working as a stereo receiver.

 

[GradoFTE+1]

Hi Chris, I've actually rebuilt two 7500s now (with a third just for repair on hold until I can clear a few other projects off the bench...).

My two cents...for what it is worth.

1. If the 7500 you have was working fine when it was last used then it is unlikely to be a major problem just powering it up. I've not had anything solid-state self destruct that way. If you have concerns I would first check the fuses and then visually inspect the circuit boards for anything suggestive of degradation or catastrophic failure (e.g. burn marks). Following that, use a Variac (or other autotransformer) to slowly raise the supply voltage while monitoring the supply current. Sometimes the oxide layer in caps degrades if not regularly energized and this method can restore them to proper operation.

After power on, listen to hear that the protection relay is energized. Next check the main power rails using an oscilloscope to look at the dc voltages and ripple. As a first pass I typically just use modern high impedance headphones and the FM tuner. However you can always first check to see if there is output from the four pre-amp channels with a1 kHz sine wave using the AUX function and an oscilloscope probe.

If everything checks out, you can more on to the amplifier outputs. I first check the dc offsets. If the bias circuit is badly off the receiver will use excessive power even when in idle. A lot of people use dim bulb testers (I have one) but I usually default to power resistors (at 8 or 10 ohms) rated from 50 or 250 watts while monitoring things with an oscilloscope probe. With a sine wave everything should look nice, smooth and symmetrical until a channel starts clipping at or above the rated power. I actually hook up speakers only after a rebuild as a final test. I also check things with a Leader distortion meter.

2. If you are only interested in stereo operation then I pretty much agree with the previous comment. There are options for simulated quad sound from a stereo signal so you don't need a quad disc or four track tape player. I am mainly interested in the repair/restoration so I only keep a few items for bench test comparisons.

3 & 4. Rebuild: Most boards are straightforward but it is easy to make a mistake. If any of the decoder boards are not working right then it could be a bit of a pain to get things right. Usually rebuilds start with the supply board(s) and then move along the audio signal path. The two tone boards are stacked and hard wired. You can get to many things in place but removing them has its virtues (take photos, just in case). There are a lot of parts if one chooses to do a full rebuild. I would not recommend a full rebuild unless one has a good desoldering tool. Make sure you use the lowest temperature setting that works and be gentle. Damaging circuit board pads is something to avoid (although I still have occasional problems here and there).

5. This is tricky. It may well be that you are off the hook and everything is still in working order. However there are many shades of gray because some parts just don't age well. Hiss and hum are often subtle attributes; modern high efficiency headphones can reveal unwanted attributes that were of no concern some 50 years ago. I do note that every FM tuner I've looked at has drifted and so alignments always improve things (in some cases dramatically). The ferrite inductor cores are frustratingly fragile and easy to break (and nearly impossible to replace). Most people are advised to leave sleeping dogs lie. Nothing in the 7500 FM front end is particularly at risk. I built a simple jig to hold the air gap capacitor pulley so that I could pull out the board for recapping. Nothing of note happens with the 7500 but the late 60s to early 70s Fisher or mid to late 70s Kenwood tuners have quirks that can leave one in tears. I have quite a few stories written down which I haven't gotten around to posting.

6. Discussions on caps are all over the map. My hearing acuity is still good but I've certainly lost the higher frequencies (anything at or above 8 kHz). Modern caps are much more uniform and usually bench test with lower ESR and losses. At some level I think replacing them is a matter of taste. I do find that electrolytic caps at 1uF or less typically age poorly (in terms of ESR and losses). Replacing these with film caps makes sense if their is room on the board (and be prepared to drill out slightly large holes for the legs if needed). Caps exposed to excessive heat typically lose capacitance (not a problem in the 7500) while others often drift to higher values (they still work but are out of spec). Some modern manufacturers are notorious for poor quality control (many LED bulbs die well before their expected lifetime is reached). I minimize my component costs by buying in bulk and shopping around for odd lots that are heavily discounted (always from reputable vendors...counterfeit capacitors and transistors are a bane).

7. No videos that I know of for the 7500.

8. It all depends on what your final goal is.

Online cap kits: I've never used them but, for a single rebuild, they are probably cost competitive (and some come with detailed instructions). Buyer feedback is usually pretty high. From my perspective they don't look to be 100% complete. My costs are typically 1/3 to 1/2 the list price.

That's all for now.

Hi Michael - apology for the late reply as I absolutely appreciate your thorough response. I have not yet done anything and I suppose the "next best step" would be to bring it up on a variac, as you suggested. Internal inspection shows all fuses intact, and there is nothing that suggests visible issues. Maybe I should just verify operation, clean the pots, fix light issues where appropriate, and enjoy. Or maybe you'd be interested in rebuilding it for me . Thanks again!