My Dad's Vintage Hi-Fi Fisher Amplifier KX-100 / KX60 Tuner - Page 1
Forum section: electronics / projects

author: joeqsmith
author_rank: Super Contributor
timestamp: September 24, 2023, 09:28:41 pm
content: I had saved my dad's old Hi-Fi. Sure it has sat around ever since. Noticed my wife had decided to move the box to the garage, which is just one step closer to the dumpster. So I brought it back into my lab where it is safe for now. The amplifier is a Fisher KX-100.I remember going with my dad to buy it. I've spent countless hours listening to it. Like my wife's grandmothers old radio that her mom remembers listening to as a child, I just didn't have the heart to let it go to the dump.The amplifier has not been powered for several decades, so first things first, check to see if it has any damage. First thing I noticed was one of the finals appears to have started to leak. I remember being around 11 or so YR and one of the tubes failed. I had my mom take me to the local store (tubes were common then and they even had testers for customers to use). It turns out, this is the tube that appears to be failing. Note the others are still marked with Fisher.Opening the chassis up (zero concern about any sort of shock, as 30 years of sitting, everything should have discharged. lol). Started going through all the caps and sure enough, one was shorted. Outside of that, none of the smaller caps appear bad.I then started to reform all the electrolytics. Nothing appears to have a high leakage. I'll let her sit and cook.The case is made from cheap plywood. Similar to how I have made cases for some of my home projects. The wood had split where the grill slides in and it had a couple of deep scratches I wanted to smooth out. After repairs, mixed up some stain to match the original colors. I have no plans to use poly on it like I have with some of my projects. Thinking just some oil and I'll call it good.Maybe tomorrow I can turn on the chassis, less tubes, assuming all the caps check out after reforming.

author: rsjsouza
author_rank: Super Contributor
timestamp: September 24, 2023, 09:50:14 pm
content: Joe, great job keeping this relic out of the dumpster, especially with such sentimental value attached to it. I don't see any wax bombs, fortunately.I have my grandpa's Philips radio restored and it is a family heirloom.Have fun!

author: TimFox
author_rank: Super Contributor
timestamp: September 24, 2023, 10:09:57 pm
content: The cap in your first photo:Capacitors of that type and vintage often absorb moisture.I replaced each of the range caps in my Tektronix L,C meter, and they all tested bad out of circuit.I would replace each such cap with a modern polypropylene unit.

author: floobydust
author_rank: Super Contributor
timestamp: September 24, 2023, 10:29:24 pm
content: It looks like a fine amplifier.I would advise caution, I've never seen those electrolytics (cans and orange Cornell-Dubilier) at that age that has not failed. They don't have safety vents either.As long as the output tubes have proper bias, or else they will roast. I can't tell if the design is self-bias or not here, then the cathode cap is critical.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 25, 2023, 01:58:32 am
content: Typically I won't shotgun and most likely I will leave everything alone, only addressing the problems I identify. The small 150V 100uF orange, below 0.03*100*150 or about 450uA it's fine.https://www.mouser.com/datasheet/2/231/161587867495-2585920.pdfAt 159V (over the rating) it's around 220uA. At 130V, its below 60uA. BK RLC meter, ESR measures 0.65. Capacitance also checks good.The 200uF 250V cap measured about 40uA at 260V. ESR was under 1.5 ohms. Capacitance was a bit low but nothing of concern. The other 200uF was also alright but it's shared 40uF section isn't looking so good. During the cycle, the leakage was about double what it should have been below the spec'ed voltage, I cranked up the current to about 1.5mA. It made it to about 470V and broke down. No idea yet about that last can.I couldn't let the finish go and put a light coat of poly on. Well maybe one more tomorrow. lol. Scratches came out fine. Guessing those came from us kids.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: September 25, 2023, 11:05:07 pm
content: Carefull running without tubes some output transformers may arc over internally.Jeff

author: Johnny10
author_rank: Frequent Contributor
timestamp: September 25, 2023, 11:21:58 pm
content: I love restoring old tube amps. Right now I have 6 Stromberg Carlson Tube Amps from the late 60'sThey Drive a set of high Efficiency Klipsch Cornwall speakers.Without recapping they tend to last 3-6 months before they start having problems.Power Supply caps and signal caps tend to go. I believe from higher present day voltages and age of course.They sound as good or better after recapping.Also, I always up the voltage rating some of those 450 VDC caps they often see 550 VDC at start-up.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 25, 2023, 11:31:16 pm
content: Quote from: Jeff eelcr on September 25, 2023, 11:05:07 pmI won't bring it up until everything has been repaired. All the high voltage testing I am performing is with each part isolated and an external power supply. The small supply will only put out about 700uA. To test that 40uF leaky cap, I had to use my large supply. I've gone ahead and cut that cap open. I plan to insert the new caps inside the old can.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 25, 2023, 11:58:33 pm
content: Quote from: Johnny10 on September 25, 2023, 11:21:58 pmWhen I repaired my wife's grandmas old radio, I think I changed two caps. But had to change the design to use a different tube. Many years later, I found a tube for it a the Dayton show in a bushel basket. Bought two and tipped the guy. Changed the design back to stock. Maybe 10 years or so later I ended up having to pull some of the other caps. That has those old wax foil caps in it. Some now around 80 years old.The other can checks good so I plan to order some parts tonight. Maybe in a week or so I can fire it up.

author: andy3055
author_rank: Super Contributor
timestamp: September 26, 2023, 12:30:31 am
content: In my experience, you can power up minus the tubes because the output transformer primary will be open circuited without the output tube. But, you cannot power it up with the output tubes inserted if you don't connect the speakers. That will kill the output transformers.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 26, 2023, 01:09:35 am
content: Most likely I will not be testing with speakers but instead the network analyzer and resistive loads. New parts are on order.I couldn't resist laying down one more coat of poly. The plywood is looking good. I'll let it dry a few days and put the final finish on it.

author: andy3055
author_rank: Super Contributor
timestamp: September 26, 2023, 04:26:47 pm
content: That is OK. As long as you do not leave the output without a load, it will be safe.

author: TimFox
author_rank: Super Contributor
timestamp: September 26, 2023, 04:33:40 pm
content: The reason why you should not run a tube amplifier unloaded is that the load presented to the plates of the output tubes is then merely the magnetizing inductance of the primary, which is a very high impedance compared with the normal load seen with a terminated secondary.Starting from the normal DC bias current, when the tube is driven OFF, the result from that change in current into the high inductance can be a strong voltage spike.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 27, 2023, 02:19:48 am
content: There is a note in the operators manual warning about running it with a single speaker and adding the load resistor. The amp has internal loading resistors if you want to run it without speakers.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 30, 2023, 05:28:51 pm
content: My hearing isn't near good enough to tell if there is a problem with the amp or not but my test equipment is. Ran it though a series of tests on one channel and it seems decent enough I guess. Filters all appear to work correctly. Eventually I moved onto using the external load resistors and it appears the second channel can not achieve the same flatness. They state +/-1dB 20Hz to 20kHz. Sweeping 10Hz to 25kHz, the good channel is flat to about 1dB, where the bad channel droops about 4dB as you get beyond 10kHz (yeah, I can't hear that...)Swapping the entire set of tubes between the two channels had no effect. Amplifier has a tape recorder input that injects much further downstream. Like the tube swap, cuts the problem in half again. No effect. There are two more mylar caps used in the loudness contour circuit that I had not checked. You can change the state of that function which shorts them out. It has no effect.They have these hybrid 7-pin printed circuit filters that I assume were custom made. They are a coated package similar to an old disc cap. I wonder if these have changed over time.

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2023, 05:46:15 pm
content: Did you post your graph for the channel down by 4 dB at high frequencies?The lower one you posted is down by roughly 1 dB at high frequencies.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 30, 2023, 06:33:42 pm
content: They spec +/-1dB. +/-0.5, good enough. +/-2dB, not so good.I did not save the data for the bad channel but it has a similar shape. Once it gets past 10kHz, it runs out of steam. You can increase the treble to increase the level beyond 10kHz. Of course it raises the mid range as well and will still not meet the flatness spec. I was going to attach the HV probe to the network analyzer to track it down.If it is the hybrid, I doubt I will try to hunt one down or make a replacement.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 30, 2023, 08:14:06 pm
content: Comparing the two channels with the high filter off, loudness contour off. Doesn't really matter as I am just wanting to show the difference between the two channels. The bass and treble were both then set to the max, min and mid. Signal level was adjusted not to overload the network analyzer.First, you can see the problem is not the base. In all cases, both channels track well to about 1.5ish kHz. With the the treble set the minimum, both channels track well to 25kHz. As we increase the treble, the left channel starts to roll off. The more we increase the treble, the greater the difference.With the tape input, I can inject directly into this base/treble circuit, isolating everything before it. And again, also changing the loudness to short the two mylar caps you basically have a tube and a blocking cap and that hybrid. I had swapped the tubes with no effect.It's possible the hybrid filter is bad on the left channel, or the final stage is just rolling off.

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2023, 08:35:04 pm
content: Can you put your probe to a point just ahead of the tone controls?An error that large does look like a problem in the tone control circuit.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 30, 2023, 09:23:53 pm
content: Easier to split the problem in half again. Its the final, driver or it's the tone control. To answer this, the two drivers were tied to the common working tone output. Doing this they track perfectly. So, yes it is the tone circuit. Not much to it, except that stupid hybrid RC network.... To split the problem in half again, I would use the good hybrid to drive the next stage. This will determine if the tubes (which were swapped), DC block and basically two resistors are fine.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 30, 2023, 09:27:58 pm
content: Adding a couple of resistors to T off to the two tubes, and then driving them off the suspected good hybrid. We can see the Left channel is still low. So, not the hybrid.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 30, 2023, 09:36:43 pm
content: Not that I wouldn't have swapped the tubes last time, but I have been known to make mistakes. Several a day actually. I went ahead and swapped them back to their original locations and the left still droops. There's not much left and I suspect we are seeing the RC of the block and volume control pot. It's the only thing left.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2023, 12:32:09 am
content: If you want to follow along, so far in attempt to track the low gain at the higher frequencies I have swapped all 5 tubes from L channel to R. No effect.Inject signal to tape input (1) to isolate the earlier stages. No effect. Problem is after this stage.Connect grid of L channel driver to R channel(2) to isolate the amplifier stage. Left channel now tracks Right. Problem is not driver or power output stage.Connect grid of V4 pin7 Left channel to grid of right channel to isolate the 7-pin hybrid. No effect. Hybrid is good.With switch set to short C21, both C19 and C21 are removed from the circuit. What is a bit odd is that the cathode voltage is low and for some odd reason the resistor used is also a different style than the other three used. Also oddly enough, the capacitor that was shorted C13, drives that hybrid.

author: KE5FX
author_rank: Super Contributor
timestamp: October 01, 2023, 01:09:15 am
content: I assume the soldering on that tube socket looks a lot better in person...And yeah, don't trust carbon-comp resistors. They drift, usually upwards.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2023, 03:26:41 pm
content: I would say ALL the solder joints look about the same. Wiring isn't that great. Some wires were hit with the iron. Most likely, I will leave all the construction as it was builtChecking the two 2.2k, one for the cathode bias, the other for the plate supply. Both are outside of 10%. However the other three cathode bias resistors are 5%, not 10%. The others are all well within tolerance. Certainly a problem but not sure if this is the cause of our lack of high frequency drive.***Checking the two foil DC blocks, they could be twins. ESR, capacitance, leakage all look very good. Also checked the volume control and balance between the two channels. The plate resistors for the two tubes are well within tolerance. Time to insert some new resistors....

author: KE5FX
author_rank: Super Contributor
timestamp: October 01, 2023, 05:16:24 pm
content: Yeah, I'd be a little concerned that a carbon-comp resistor that measures OK with a DMM might look different with several volts (or more) across it. Something similar might be happening in the hybrid circuit, for that matter.IMO, a serious restoration effort calls for replacing old carbon resistors right after swapping out the Black Beauties.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2023, 07:09:19 pm
content: Plan is to keep the amplifier in original condition as much as possible, right down to the solder joints and wires that were melted with the soldering gun when it was originally built. Just a mild cleaning, address some problems with the wood/finish, and replacing a few damaged parts.With the two bad resistors replaced, the base and treble set to max, the two channels track all the way to 8KHz. Even at 20kHz, there is only 1dB difference.This is with the amplifier back to the original wiring and injecting at the AUX ports (all sections in play).Not thinking I can hear 20kHz, let alone a 1dB difference. ..... Indeed, testing my hearing, 13.4kHz is about it.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2023, 10:24:44 pm
content: Next step, see if it will put out the rated power. They rate it for 50W, 25W per channel. I've been using 25W Ohmite resistor for loads. The Right channel starts to clip before is reaches the max power level.Once again, cutting the problem in half, I drive the tape input bypassing the front end. Still clips.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 01, 2023, 10:54:48 pm
content: Swapping the two sets of final output tubes, corrects the problem with the Right channel. Of course, the Left clips now. I had bought a used tube to replace the gassy one and that one isn't the problem. At least we have our answer. From what I remember the other three are original to the amplifier and would be over 60 years old now. They survived the 60s and 70s listing to hard rock.Going to be a shame to put Russian tubes in it....https://www.tubesandmore.com/products/vacuum-tube-7868-electro-harmonixComplete set on eBay.https://www.ebay.com/itm/122912628281

author: trobbins
author_rank: Frequent Contributor
timestamp: October 02, 2023, 01:55:01 am
content: Imho it would be worth your while inserting a 10 ohm 1% in the cathode of each 7868, to confirm you at least have balanced idle dc currents for each channel, and to allow you to monitor those tubes every 6 months or so for onset of failure.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 02, 2023, 02:09:03 am
content: High failure rate on the Russian tubes?

author: trobbins
author_rank: Frequent Contributor
timestamp: October 02, 2023, 02:23:19 am
content: It might not be a failure, but rather an imbalance in dc bias and/or ac gain. Not all tubes ares created equal, even from the same manufacturer. If you can measure the idle dc cathode currents, then there is some chance that a better dc balance can be achieved with the new tube, assuming the new tube is ok, by matching it with the other 2 tubes, and assuming it is not ac gain differences that are an issue (you have feedback around the output stage to minimise that aspect).

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 02, 2023, 02:57:01 am
content: Restoring vintage tube equipment is a labor of love. It is not a project you will complete in a pair of months, let alone a pair of weeks.But when you are done, it provides a satisfaction that is difficult to convey with words.I cannot offer any further technical advice to what has already been provided. But I sincerely wish you can complete your project.Patience is your greatest asset here.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 02, 2023, 04:16:11 am
content: I can't imagine anyone pulling apart their tube amps every six months to check bias currents. I would guess this particular amp was in use for 40-50 years and outside of mom taking me to get a new tube for it once, it worked fine. If the Russian tubes are so poor as not to be trusted, I may try to hunt down some NOS parts.This particular amplifier is slightly different than the schematic I posted. That was for some revision of the X-100-B. I wasn't able to locate the correct one but may end up marking up that schematic to match.Quote from: schmitt trigger on October 02, 2023, 02:57:01 amThanks. The amplifier is basically working now outside of having a weak tube. Sounds like the problem is going to be sourcing a good set.I've cleaned up the mechanical bits as much as I plan to. No polishing the parts. It's old and I want it to look and smell its age.The wooden case is a different story. Waiting for the coating to cure, more wet sanding, more coats.... I thought I would have time this weekend to finish it up but ended up adding another coat. A few more days of dry time and I will have another look at it.I think I posted about restoring my dad's old Garrard turntable. Now that thing took some time. I was very lucky in that case as someone had given me the exact turntable when I was a kid and for some reason I had saved it all these years. It was missing parts but was in good enough shape it made a very good donor. It supports LPs, 78s and 45s. You can auto change all record sizes. There is an adapter for the 45s. As a kid, we would stack 10 or so 45s on it. It will automatically place the needle and pick it up and does this for the record type selected. It's all mechanical and sorting that out took me a few days.Once the amplifier is done, I still have the tuner to go through.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 02, 2023, 05:07:13 am
content: Many vintage amps have voltage sense terminals, or jack sockets, to maintenance check the cathode currents. The other common method is to use an insert bias checking adaptor that either comes with a meter or uses a separate meter. Others extend a custom maintenance socket to make it relatively easy to check during operation.Not everyone has the $ to afford what is being asked for NOS brand name matched-set valves, and not everyone has a pleasant experience installing a new set of matched valves, and not every amp survives an output stage valve going bad without collatoral damage. Also quite a few find that a higher mains voltage, as often encountered nowadays, has caused them grief. Just saying

author: joeqsmith
author_rank: Super Contributor
timestamp: October 02, 2023, 12:54:22 pm
content: Fisher saw no reason to install monitoring terminals. After 60+ years, I have no plans to start making any changes to the amplifier beyond some basic repairs.At some point I will look at the output spectrum, maybe do a two tone test or something. For now, single tone time domain is good enough.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 03, 2023, 09:11:15 am
content: Looking at the 2nd & 3rd harmonics of the amplifier, I ended up joining another forum to see if anyone could verify the levels I am measuring. I ran the same test on both channels with the gain set to about 25% to keep the weak channel well within it's linear range. The levels match for both channels.Also have a lead on a set of output tubes. A member here offered to send me a few unknown ones as well, which I want to again say thank you. The last time I purchased a new tube was in the 90s to repair my old Drake ham radio. Guessing that audio amps are far more common and with it being 30 years later, tubes are getting very hard to come by.https://audiokarma.org/forums/index.php?threads/question-on-fisher-kx-100-2nd-and-3rd-harmonic-levels.1041163/

author: joeqsmith
author_rank: Super Contributor
timestamp: October 03, 2023, 11:49:55 pm
content: Replacing the weak tube, both channels clip symmetrically at the same level. 5 ohm loads, so say 20W. Maybe with a better match it would run 24W. That, or the manual may be suggesting that with higher THD and only running one channel at a time you can reach 24.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2023, 12:22:50 am
content: Testing it with a single channel, just starts to clip at 27W. At 29.5W, it's clipping pretty bad. Power output stage looks good.Time for the debates of contact cleaners.... Pots had some drag and noise. I put some lite oil on a toothpick and applied a VERY small amount on each rotating shaft and let it sit overnight with the shafts pointed upward. Audio guys are cringing. lol. I then flushed the wipers with ProClean a few times while I worked the pots. They have a nice feel to them now and the noise has greatly improved. Running it tonight, when adjusting the power levels, it seems clean. ProClean is not a contact cleaner. I use it for PCB work. I like it because it doesn't leave any residue.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2023, 12:50:39 am
content: Quote from: joeqsmith on October 02, 2023, 04:16:11 amI grew up in Britain where the 45s had the same small spindle hole as 33s and 78s. I remember the auto-change feature where you could stack records on the spindle where they would be supported on a small metal pivot and held steady by a control arm placed on top. When each record finished playing, the tone arm would move to the side, the pivot would retract and drop the next record down on the the platter, and the tone arm would move back and drop down to start playing.But I recall 45s in the USA had a big wide hole in the middle. How did the auto-change feature work with these? Did you have to clip in plastic adapters with the small spindle hole?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2023, 02:35:17 am
content: You are correct, they had a much larger hole. There were three different spindles the the records would fit onto. A short and tall thin one for manual and auto loading LPs. Then the large diameter tall one for the 45's. There were a sets of tangs that would move inward and outward to drop the record. These mechanical tangs were controlled though the inside of that spindle down inside the turntable.If you check out this site, the spindle looks like what they show for the Garrard RC-80 and RC-90 (top right side of page).https://midimagic.sgc-hosting.com/changerx.htmI don't have a single 45 anymore or I would make a demo video for you of it running. It is quite the mechanical wonder seeing it work. Searching YT, I found one very similar to my dads. It appears to be from the 70's but the changer seems similar.I believe this is the one my dad had.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2023, 03:09:06 am
content: The first video you linked with the auto-change on the LPs worked exactly the same with 45 singles without having the middle knocked out. We never had the large center hole or fat spindle in the UK, and so I always wondered why 45s had that bit in the middle that you could in theory remove. As in the second video, no one would ever actually remove it as it would "damage" the record. There was just one spindle that would equally work with 33s, 45s and 78s. I never thought about the spindle being interchangeable. The cartridge would also flip over with a lever, one side for 78s and the other for singles and LPs.Those turntables/record players honestly seemed like mechanical marvels.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2023, 03:12:59 am
content: I attached a few photos of it for you with the three spindles and a spare needle.In this ebay ad, there are some photos of the mechanical wonders. I think I spent a day or so just studying the thing before I attempted to work on it.https://www.ebay.com/itm/295657399428

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2023, 03:17:23 am
content: Quote from: joeqsmith on October 04, 2023, 02:35:17 amThis is what a single looked like when manufactured in the UK. Coincidentally, this exact record was in my father's record collection. I played the heck out of it as a child.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2023, 03:23:51 am
content: I have never seen a 45 like that.I'm guessing that the large hole was to support the auto loaders found on jukeboxes. Pure guess. Did you have these industrial record machines where you lived during the 50-70s?****Diamond needle + dog hairs.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2023, 03:42:38 am
content: Wow. Those Garrard Type As are "modern" (early sixties). Even in the fifties they made the then equivalent of a "boom box", which was a record player built into a carrying case with a locking lid and a handle that you could pick up and take with you to parties. (You still had to plug them in, though.) But even those one-box wonders sometimes had automatic play and record change features--because parties, of course.Quote from: joeqsmith on October 04, 2023, 03:23:51 amYes, there were jukeboxes, but I don't have any knowledge of the mechanicals. The working mechanism was all hidden from view. At a guess, they might have worked in a similar way to more modern CD multichangers.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2023, 12:52:57 pm
content: I had a portable record player as a kid that my sister gave me that was inside a suitcase with cloth covering. It had the amp and speaker but no auto changer.We had a jukebox during this time that was open so you could see the mechanics in operation. The records were stored vertically, and played vertically. The need came in from the side. I had seen and used records by that time and remember thinking, how does that record play like that. It was pure magic.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 05, 2023, 12:23:51 am
content: They spec hum and noise to be 88dB with the volume set to its minimum. They don't specify any other details about the setup but the low end is where the noise is.Tried sweeping from 10Hz to 500Hz and we can clearly see the 60Hz signal just below 55dB. Plugging in a set of headphone, terminating the two inputs to ground, turning off all the crap in the room and turning up the volume way past what I would ever use, I can hear that 60Hz. It's faint. Case is not yet buttoned up.I also had a look at how much coupling there was between the two channels by terminating one input to ground and measuring its output while driving the other input. It's similar to looking at the harmonics, there's no spec for it and without something to compare it against hard to say. Coupling increases with frequency and at 20kHz, its about 20dB down. For all I know these harmonic levels and coupling may give the amp some unique coveted sound...

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2023, 12:46:36 am
content: Worked on the low freq noise tonight. It still doesn't meet their 88dB spec but it's close.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2023, 03:23:39 am
content: On the audio channel I linked, some of these guys will polish the entire chassis, tube shields, knobs.... My dad's will continue to just look like a nice old amp.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 07, 2023, 02:51:35 pm
content: Buffed out the case and inserted the grill. Using natural resources, even though it's just particle board and plywood, it's a pain to work with. Wood doesn't take same the stain throughout the surface. You can use some treatments but blah. Then grain structure reflects light differently. In this case, 90% of the stain on top is original. Blending stains is somewhat like trying to match paints. I am not very good at it.The strings that hold the box lid up are also 60+ years old.I've been listening to the amplifier for a few days now. It's getting a good workout and not showing any new problems yet.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2023, 03:12:45 pm
content: The amplifier still has not been buttoned up. When running it for several hours, I noticed that the power transformer gets fairly warm to the touch. I have nothing to gauge it on. Our AC mains is running 122-123V. Seems when I was growing up with my analog meter, the mains was around 110. How accurate that old meter was, or my memory, hard to say. They spec'ed the amplifier to run from 120-105, or 112.5 +/-7%ish. Seems tight. They claim 160W draw, guessing full volume. They fuse it at 3.2A, slo-blo. Still has the original fuse.Looking at the input/output with a scope, it doesn't appear the transformer is starting to saturate with the higher voltage. Tried to run the AC at two voltages to see if it had any effect on the temperature. I am measuring at the top of the transformer where there is a small grove cut into the laminates. Pretty much center. I have no idea what the winding to laminates theta is. In both tests, the amplifier was allowed to run for 3 hours. Same load, volume.... Data was taken using a Kill-o-watt meter.At 106.3V, power factor was 0.84, 132 VA, 112W, 1.24A, reached 51.3 deg C or a 29.5 rise.At 122.7V, current draw was 1.46A and it reached 53.7 or 31.9 deg rise.Guessing that it's normal for it to run this hot.Over the last week, guessing I have about 25 hours of runtime on it now listening to music and outside of the transformer temps, I'm not seeing or hearing any problems. Started looking at headphones. A friend has one of these amplifiers:https://www.schiit.com/products/asgardFriend suggests reading the FAQ. You have the artsy-fartsy, touchy-feely people asking questions to the engineering group. It's pretty funny. I like that 0.002% THD vs my dads amp at 0.5%. Dad's old amplifier has a headphone output which is basically tied to the 4ohm output through a 330 ohm resistor to limit the power (11.6^2/330+8 or 398mW). My modern 70's/80's headphones, you have to crank up the volume to hear them and then you are getting into where the amplifier starts to clip. I used to have a set of KOSS HV/1LC which were much higher impedance. I had tacked on a couple of resistors in parallel with the 330s to get me a bit over a Watt when driving the old time 8ohm headphones.Wow, used asking price for the Koss, I think is more than I paid for them new.https://www.ebay.com/itm/185916149315Plan to borrow a few different sets from friends to see if they sound any better than my vintage tech. While searching for headphones, came across this site which uses a more analytical approach to doing reviews. I bet they get some flack from audio fanboys...https://www.soundguys.com/how-we-test/

author: IanB
author_rank: Super Contributor
timestamp: October 13, 2023, 03:21:05 pm
content: Headphones are interesting. They seem to fall into two camps: monitoring, or listening.Headphones designed for sound engineers can be extremely accurate and flat in their response, so you hear exactly what is there. Sound engineers need this, but the result can be clinical and perhaps lacking when listening for pleasure.Headphones designed for listening may have deliberate sound shaping, designed to produce a better listening experience at the cost of technical accuracy.Each has a place, but it is good to know what your requirements are before buying.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2023, 03:41:59 pm
content: My wife has never heard the amplifier and has never used anything besides ear buds for a headset. I had her try out out the amplifier with a new set up Skull ear buds. I then had her try my 70s headphones. Got an interesting look on her face as she was hearing details in the music that she wasn't used to. We will see what she thinks about the ones I borrow.I was thinking to buy a set from Audio Technica. I like the M70x, not for the 40kHz range, which I can't come close to hearing anymore, but the pads look comfortable. Then a friend showed me their Beyerdynamic. Guessing it is like having soft kitty fur surrounding your ears.https://www.audio-technica.com/en-us/headphones/best-for/studiohttps://www.sweetwater.com/store/detail/DT700ProX--beyerdynamic-dt-700-pro-by-closed-back-studio-mixing-headphones

author: joeqsmith
author_rank: Super Contributor
timestamp: October 17, 2023, 02:43:42 am
content: When I was a kid, I had for some reason scribble the tube socket on the inside of the lower case cover. Rather than cleaning it off, I left it. Started out with 5 coats of poly on the case but by the time I buffed it out, I am sure I lost a layer.Also shown is my buck transformer I used to test the low line.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 19, 2023, 01:03:17 am
content: Almost made it two weeks of moderate use but then it started to make an odd noise that almost sounded like audio from the local station for maybe a quarter second, then it was gone. I kept it running and this pulsed audio would come and go at random. This eventually turned into a 60Hz hum, which of course would also go away. This went on for a few hours and eventually it didn't recover. Nothing odd with the voltage levels, plate wasn't glowing and the filaments appeared normal. Let if cool, pulled it back apart to have a look and it was back to working fine.Problem was only on one channel so I swapped the two tube sets and let it run. Sure enough the problem eventually returned but on the other channel. Eventually tracked to the output stage. Two of the output tubes were still originals to the amplifier and sure enough, another is starting to go. They do not appear to be gassy but noticed a small flake inside one of the tubes. Attempting to hunt down a few more NOS parts.Shown with one of my vintage Pioneer headphones. To me, these sound very good but we will see what the wife has to say when I do my headphone shootout for non audio buffs....

author: joeqsmith
author_rank: Super Contributor
timestamp: October 21, 2023, 08:46:54 pm
content: With the amplifier sorted out, started work on the FM receiver. Actually, I've been listening to the amplifier while I work. The string style tuner was locked up. Took most of it apart and cleaned/lubricated all the moving parts. Then some minor cleaning of the remainder of the chassis. The glass face was really a mess after 60+ years, had a layer of crud on the backside and plenty finger crud on the front. Note the old selenium bridge rectifier.First problem I noticed was one of the lamps has failed. These have a custom house number 150082-7. Looking at them under a microscope, there is some marking but I can't make it out. The filament is crush inside some bent over copper rather than welded like a modern bulb. Note how they have painted all the lamps with white paint on the backside block the light. The paint is flaking off.All in all, it looks in decent shape. Not finding much wrong with it.

author: TimFox
author_rank: Super Contributor
timestamp: October 21, 2023, 09:04:27 pm
content: In tube units of that vintage, the plastic-coated paper capacitors (many visible in the underchassis view) are suspect.Especially, screen by-pass capacitors may have absorbed moisture, and thereby reduce the screen voltage.They can be replaced with modern polypropylene axial- or radial-lead units, whatever fits better.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 21, 2023, 09:18:41 pm
content: Like the amplifier, I have no plans to do a cross the board part replacement. Just replacing the parts that are bad and will keep everything as original as possible. Lots of old carbon resistors in the tuner. Everything looks good and so I powered it up. Was curious if the beam tube would still work. It's been running about an hour connected to the amp.

author: TimFox
author_rank: Super Contributor
timestamp: October 21, 2023, 10:43:00 pm
content: As a conservative approach, you can verify the DC voltages at points bypassed by paper capacitors to see which need replacing.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 21, 2023, 11:44:17 pm
content: I've been pulling one lead and running them up to about 10% higher than their rated voltage and checking for leakage. Plus measuring their capacitance and ESR. Same technique I did with the amp. In some cases I could test them in-circuit.Thinking about the worse thing I own for dead caps, my first DSO. I would guess I have replaced 10 or so Tants since I bought it. I bought a bag of 20 or so to have on-hand. I hate to think what it would require for me to change them all. This tuner and amplifier have another 20 years on that scope.Looking at the case, the wood is in decent shape. A few minor scratches. I sanded them out and will stain it tomorrow.It's been running since I powered it up. Put the face plate on finger tight until I sort out the bulbs to show what it looks like. None of that AM rubbish.***Vintage I-POD being used for the local radio station, commercial free.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 22, 2023, 12:29:30 am
content: The cabinet has a good sized chip missing on the outside edge. A bit more difficult to match, so cut out a sliver from the inside top to use for raw materials to repair it.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 22, 2023, 03:26:55 pm
content: With the glue for the grafted slivers now set, time for some sanding.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 22, 2023, 03:30:57 pm
content: Was able to sand most of the scratches out. There are some deep battle scars that I left behind.

author: rsjsouza
author_rank: Super Contributor
timestamp: October 22, 2023, 05:21:26 pm
content: Caracter...

author: joeqsmith
author_rank: Super Contributor
timestamp: October 22, 2023, 06:34:28 pm
content: Indeed.No luck finding replacement bulbs. My motorcycle trailer uses something similar. They are a bit shorter and fatter but will fit. These are made for 12V and running them a bit lower voltage, you get about the same intensity. With them having such a short filament, the coupling to the glass creates a hot spot. They also draw about double the current even at these lower voltages. It looks like a few places are offering LED replacements. Based on their dimensions, looks like they will fit. May try a few and see how they look.Electronics is simple. Finding lightbulbs and fixing damaged wood, that's where the time goes.

author: richnormand
author_rank: Unknown
timestamp: October 22, 2023, 09:44:29 pm
content: In a similar vintage receiver I made a simple diode/resistor/led replacement.Used two warm white LED in a frosted 3D printed translucent "bulb" tube to diffuse the light.Replaced all the set so the are the same colour.Works wonderfully at much less current draw that the originals.In reading the whole thread I would like to mention that it is worthwhile to check grid voltages to make sure a leaky coupling cap does not shorten the tube life.Fast and easy to do if you have a schematics with the proper voltage on it.These will go forever with the proper TLC....

author: joeqsmith
author_rank: Super Contributor
timestamp: October 23, 2023, 12:10:09 pm
content: Quote from: richnormand on October 22, 2023, 09:44:29 pmI'll let you know how the aftermarket LED replacements work out. Would much rather use the OEM lamps but have had no luck finding one.As I posted early on, I checked the caps for leakage with a HV supply running them above their rated voltage. I also checked their capacitance and ESR. This was done before power was ever applied. When it was ready to be powered, I checked all the voltage levels. The schematic provides pretty much all the grid, plate and supply rails. After, I started checking the AC performance. Then slowly splitting each identified problem in half till I found the root cause.Once everything has been repaired, I do a long test to see if things are going to be stable. Pretty much the same technique I would use to bring up any old equipment of this era.I did think about making a test jig to sweep the power output tubes after the one started to go intermittent.

author: richnormand
author_rank: Unknown
timestamp: October 23, 2023, 02:39:39 pm
content: Perfect.Sorry to have missed that these were the coupling caps in your previous post.Should last another lifetime now.Still have an old Heathkit receiver from my dad in use everyday in my shop!

author: tautech
author_rank: Super Contributor
timestamp: October 25, 2023, 11:06:32 am
content: Quote from: joeqsmith on October 23, 2023, 12:10:09 pmThat style lamp is known as Festoon lamps/bulbs and were also used in automotive where if they were placed horizontal road vibrations made for a short life.The best style of filament carrier wire were gold flashed where they were folded/crushed onto the filament.Available in different lengths and voltage/wattage configurations.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 25, 2023, 12:32:12 pm
content: I wasn't able to find anything close. The ones for the trailer have a much shorter filament and draw about 600mA @ 6.8V. The glass diameter is a bit thick and there is a clearance issue with a mounting screw. End caps are also a larger diameter.See attached for data on the stock bulb sizes.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: October 25, 2023, 01:15:01 pm
content: Hi Joe,you are looking for 6V-bulbs, right? Will 3W do?https://www.elting.de/Soffitte-6x36-mm-3WBest regards

author: themadhippy
author_rank: Super Contributor
timestamp: October 25, 2023, 02:43:04 pm
content: another source for those lamps is old illuminated door bell switches ,a nighttime wander around your neighbourhood may turn up a replacement

author: joeqsmith
author_rank: Super Contributor
timestamp: October 25, 2023, 04:10:36 pm
content: Quote from: Roehrenonkel on October 25, 2023, 01:15:01 pmNotice how short the filament is. I suspect they used such a long filament in the original lamp to avoid making a hot spot where it couples to the glass.This is one problem I see with the trailer lamps I have. I could add a diffuser to try and get it to give a more uniform coupling.

author: KE5FX
author_rank: Super Contributor
timestamp: October 25, 2023, 06:54:08 pm
content: Quote from: themadhippy on October 25, 2023, 02:43:04 pmUser name checks out

author: joeqsmith
author_rank: Super Contributor
timestamp: October 26, 2023, 02:26:01 am
content: Borrowed a few headphones. Four of us tried to evaluate them independently. Both my wife and I tried various types of music. Everyone thought the Sennheiser HD555 sounded the best. We all thought the vintage Pioneers performed poorly. Of course, they all sound much better than the cheap ear buds I normally use.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 28, 2023, 02:25:10 pm
content: Waiting on some new LEDs which I hope will get it closer to the look of an incandescent. In the mean time, I finished up the case. Shown after 5 bushed coats of poly and buffing. You can see the deep scratches in the one side of the cabinet. From what I remember, my dad had made a slide out cabinet for his HI-FI. This way you could have easy access to the turntable and let the amp and turner run in free air. I think it had rubbed over the years of use. I could have erased the history but instead chose to preserve it.

author: themadhippy
author_rank: Super Contributor
timestamp: October 28, 2023, 03:57:21 pm
content: QuoteT'is common when using filter meant for tungsten ,something to do with the bits of the colour spectrum that are missing from the leds. There are filters designed for led sources but not really a cost effective option considering the amount you need.If your in the mood to experiment you could get a filter swatch book from the likes of rosco or lee,the samples should be enough to do a couple lamps.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 28, 2023, 04:00:59 pm
content: HI-FI next to my Dad's poker chips. Project is pretty much done now except for the bulbs. The grill on the tuner is actually from the amplifier. I inserted the chrome plug where my Dad had drilled it out for a rotary switch that allowed him to select his speakers. The switch was long gone.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 31, 2023, 12:50:19 am
content: New LEDs are better but still far off from the original incandescent bulbs. Looks ok with the lights dimmed in the room. Calling it good enough.

author: rsjsouza
author_rank: Super Contributor
timestamp: October 31, 2023, 10:52:41 am
content: They indeed look better than the prior iteration. Hopefully they last the next 50 years...

author: joeqsmith
author_rank: Super Contributor
timestamp: April 26, 2025, 01:46:25 pm
content: While going through my mom's papers, I found all of the documentation for my Dad's old Hi-Fi.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 26, 2025, 01:51:10 pm
content: These articles were included as well.My sister visited a while back. We both had played a lot of music on this system. She got to listen to it again after all these years. It was well worth the effort, just to see the smiles it brought.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 27, 2025, 11:18:16 am
content: That's incredible, Joe! If you haven't done so and whenever you have time, make sure this material has decent scans on the internet.Memories coming from experiencing the old equipment has the same effect on us as looking at a photograph album. I love doing that whenever I am visiting my parents home.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 28, 2025, 02:05:30 pm
content: I agree. The old Hi-Fi, like my toy trains that I have been playing with, have little monetary value. Their value to me is the memories they bring.