Sears/Silvertone 69440 amp repair - Page 1
Forum section: electronics / repair

author: abrasumente
author_rank: Contributor
timestamp: September 10, 2025, 10:05:27 pm
content: Back with another repair, this time it's an old Sears/Silvertone 528.69440 amplifier!Yes, I know it's not worth fixing, I'm doing this for s&g and learning how electronics work.Anyway. Amp transformer has cooked primary windings. I found a transformer online that closely matches the VA rating of the old transformer and is also 70v center tapped on the secondary (amp needs two 35v feeds to rectify into 48.5 and 31v dc). So far I've found one of the two rectifiers are also cooked (readings both ways on diode and resistance testing out of circuit). It's my plan to replace all of the electrolytic caps that I can find as well. Thing is, I figure it's not the rectifier that killed the transformer. I suppose it could've been overdriven back in the day listening to Grateful Dead at max volume for 10 hours, but I figure the answer is probably in the board somewhere.What causes a rectifier/diode to short instead of just opening? What would you check next if you were on this fool's errand?edit: part of wiring diagram attached, x45 is the blown rectifier.

author: MathWizard
author_rank: Super Contributor
timestamp: September 12, 2025, 01:10:05 am
content: Was it broken when you got it or what happened ? You can check a lot of parts in the rest of the circuits, with a DMM while its unplugged. Especially if you remove the electrolytic caps, and maybe some others too. Just go around and check all the transistors and diodes at least.Do you have any power supply ? A lot of transistor circuits can work from lower voltages. I'd want to see the full schematic, or I'd make my own 1st, but there's probably stuff in there, that you could test from any 12V supply. I'd check all around with a DMM before I'd try that tho. And I can also set the max current on my PSU too. Otherwise, I'd use some small power resistor in series with the load, to limit max current. Or I'd measure the current w/ a DMM, and make sure it's reasonable.Please post the whole schematic.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 12, 2025, 02:13:06 pm
content: Good Morning Abrasumente, As we all know, Sears never manufactured any of the products they sold, solook over the insides of your amplifier and see if you can find a brand name. That might lead you to some documentation by the manufacturer, including repair manuals and circuit diagrams. I hope this helps some.

author: abrasumente
author_rank: Contributor
timestamp: September 12, 2025, 10:54:13 pm
content: As requested. I do have a power supply.

author: abrasumente
author_rank: Contributor
timestamp: September 13, 2025, 09:16:00 pm
content: Something else I would like to do is install fuses for component protection. I believe the best way to do this would be to figure max current draw on the secondary of each POWER transformer, and find an appropriate fuse for that voltage/current draw (and for my center tapped transformers, fuse "both" secondaries). I had a hell of a time trying to find a 70v center tapped secondary transformer, and would rather not have to buy another one.What are your thoughts on adding a polarized power cord and fusing the whole thing on the LINE side of the cord? It's got the original cord, it's not deteriorated, but would like to minimize nasty surprises without re-engineering the whole unit if possible.

author: MathWizard
author_rank: Super Contributor
timestamp: September 14, 2025, 01:12:06 am
content: Yeah IDK what sizes to use, but sounds good.So I guess it's not a guitar amp then. I would disconnect the PSU section from the rest of it, don't be afraid to desolder wires or even cut them or cut traces. Now that's overkill, but sometimes I'd do it. Now if there's old worn out caps still in there tho, they might not like the voltage getting too high if there's no load on the PSU. Do you have any old power resistors or something else that could handle a few watts from ~40V ? Of course there is there AMP itself, lots of people would never unhook the PSU.If you are recapping the big caps, then it would be easy to measure most all resistors in circuit, and check the BJT's.Then with the PSU section unhooked, it would be easy to apply 12V or 24V to the X38 and X41 side where it says 39V, and then do the same for the output side where it says 48V.Now you may need more voltage to get the biasing to work right, and you may need another base resistor from the Vcc to say the R124 that wanted 31V.So yeah if time is no issue and you want to get a feel how how each section works, try that. Even try 1 of the BJT circuits in a simulator, it's can be fun, and it can be way easier to see whats going on, and what voltages should be what, for any input voltage.Sim's can also be an easy way to make schematics or just place parts, etc, and not lose track of it all.

author: abrasumente
author_rank: Contributor
timestamp: September 14, 2025, 02:45:04 am
content: Oh I forgot about circuit simulators! I didn't know they were a thing until I signed up here, thanks for the reminder.No, not a guitar amp. It's a mid 60s radio console with a record player- cosmetically in fantastic shape but the folks we got it from couldn't remember the last time it worked. I believe them, seeing how the amp transformer looked like it let out all the magic smoke in a hurry. I briefly toyed with the idea of rewinding the primary side myself, but I'm just not knowledgeable enough to feel confident in the end result.I've got some odds and ends laying around from other projects I might be able to use. I've already cut some wires out of the way for individual testing and I'm not too worried about frying much of anything except the transistors, because I know exactly 0 about them or their ratings.I feel like one of the caps I'm probably going to have a hard time sourcing as its a multilayer.Worst case, for fusing the secondaries I'll just have to crank it and see what my meter says (IF I can get it working). If I'm doing it right the math says 2.5a fuses on the secondary side. I mean, this design was obviously good enough in the 60s and nobody died from changing the radio station, but I work around mains voltage daily and have a healthy respect for it. I do find it wild there AREN'T at least fuses in here. I knew better than to expect double insulation or grounding...LCR meter just got here. Last thing I was waiting on before really tearing into it.

author: abrasumente
author_rank: Contributor
timestamp: September 18, 2025, 09:58:04 pm
content: Only other thing I've found is one of the big caps reads low. I can't find a direct replacement (500uf 50vdc radial with leads) so I have one that matches capacitance and voltage coming- but uses the can casing as the negative side. Don't really see a problem with this as the negative lead on the old cap went directly to the amp chassis ground.Got my replacement transformer in, and it is H U G E. Easily twice the size of the old one. I'll have to fab something to mount and ground it, but luckily the inside of the cabinet is pretty spacious.

author: TimFox
author_rank: Super Contributor
timestamp: September 18, 2025, 10:35:28 pm
content: In axial-lead polarized aluminum electrolytic capacitors, the "can" is always the negative electrode.It may be covered by an insulating layer.

author: eetechTom
author_rank: Contributor
timestamp: September 19, 2025, 05:03:00 pm
content: It's very common for diodes to short when they fail. Make sure you use the tried and true diode test to check for shorts on the output transistors. Those being X39, X40, X42, X43. Also measure the resistance of the 0.39 ohm emitter resistors. Good luck!

author: abrasumente
author_rank: Contributor
timestamp: September 21, 2025, 06:42:13 pm
content: Progress!I successfully replaced both rectifiers (was able to source a pack of four so figured why not). Emitter resistors were fine. Replaced that 50v/500mfd cap, built a new platform for the new transformer to mount on and drilled a few holes in the plate and cabinet to try and induce convection cooling. Made quadruple sure my connections and solder joints were solid, and caaaaaaarefully gave it the beans. No smoke came out, so i put it back in the cabinet, hooked everything back up, and slowly applied power til I got up to mains voltage and.... GOT SOUND! But only on the right channel ☹️Far as I can tell this next issue is not in the amp itself. My output transistors have stable voltage. Ive gotta figure out what color the left input signal wire is, probe it with the scope and see if there's any action on it. It could be in the balance knob too, all four knobs are real scratchy and could use some deoxit.Oh, and the turntable gear is broken. I was pretty bummed about that. Gonna try to find a period correct replacement once I get it playing audio properly.This thing definitely has that "old" sound. I ran some Tool through it and it sounds like yesterday... I wonder what the specific properties are that make it sound like that (aside from zero signal processing software lol).edit: More interesting is when plugging in a set of headphones the issue is replicated, sound only out of the right speaker. BUT even when running the balance control all the way down to the left side, once you start to turn the volume up the right side plays anyway (although at a slightly lower volume). While I figure that's just the nature of solid state tech, I figure the circuit wouldn't "bleed" until volume got turned up pretty high. Here, I get to about 25% on the loudness control and the right channel starts playing. Looks like I may have to discuss with the boss (wife) about using the kitchen table so I can pull the whole thing out....

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 22, 2025, 08:17:14 pm
content: Good Morning Abrasumente, Congrats on your partial success, and if you find what the problem with the left channel is, let us know, please. Too bad you don't have a dedicated work bench. You oweThe Bossa favor.

author: abrasumente
author_rank: Contributor
timestamp: September 25, 2025, 11:35:21 pm
content: Quote from: BILLPOD on September 22, 2025, 08:17:14 pmMaybe one day I'll have a larger work area... I have a small spot on my computer desk where I keep my iron, esd mat etc as i usually work on smaller stuff, but I really need a full dedicated bench. I'll definitely owe her some flowers by the time I'm done!Work's been wild this week, but I should have enough free time this weekend to pull the receiver and probe it. I had a matched pair of sk3004 pnp germanium transistors come in this week should I need them. Both the amp and receiver have several of them.

author: eetechTom
author_rank: Contributor
timestamp: September 26, 2025, 03:46:31 pm
content: Typically I only check the emitter resistors if an output transistor fails, but it doesn't cost anything to check them so why not. Always do a close inspection for cracked/sketchy solder joints on older gear like this. The bigger the component pin/leg being soldered, the more likely the joint can become stressed and fail. Same for components that run hot.

author: abrasumente
author_rank: Contributor
timestamp: September 27, 2025, 07:55:15 pm
content: Pulled the receiver this morning and I am happy to report the left channel output wire was hanging on by a thread. Re-soldered that connection and put it all back together. Both channels play! Nothing better than a simple fix.Now I have to restring the tuning dial wire and find a new turntable... thanks for the suggestions everyone!

author: abrasumente
author_rank: Contributor
timestamp: October 16, 2025, 10:50:58 pm
content: Welp, ran "okay" for a few weeks, but when playing the other day I had a pretty sudden drop in volume. Gonna pull the amp again, but I think it's the move to replace the rest of the power caps at minimum, then recheck everything.They're 500mfd/50vdc, except for a multicap that I'll probably have to find separate caps for (1000/300/250 mfd). I did replace one already with a NOS from 19xx, but I don't know that this is the long term move. So far I've only had luck finding a few axials close to that value, and I can't stomach spotting $9 a piece with a 20 count minimum order. Even going up to 100 vdc doesn't give me much room to work with. Is there an upper limit with respect to capacitor voltage? Going to a 500mfd/150vdc gives me a nice cap with 2000 hours and screw terminals. I have plenty of room in the cabinet for mounting.edit: my question is more in relation to things like esr. Its got germanium transistors in it and I'd rather not fry them since they're also unobtanium. Maybe some type of current limiting is in order?