Sunn Solid state amp not repairable?

[TFM94]

Hey everyone,

quick backstory: A few months ago a bought a Sunn beta lead amp from a friend of mine. It was broken and didn't have any output, and the original owner didn't want to put any time or money into getting it fixed, so I got it quite cheap. Given the current prices of these amps (and how hard it is to find one) I thought there was a good budget for repair cost and it would still be a good deal. Plus, they're really fun amps for both guitar and bass.

I brought it to a local store that offers amp repair. After a week I got a notification that the amp cannot be repaired. When I picked it up I briefly talked to the repairman. He said that it's an unusual circuit and that he mostly works on tube amps and that he can't fix it. Fair enough. Just on my way out the store owner mentioned that they have a second repairman who has experience with solid state amps, so I agreed to have it checked by their second repairman. Again, after two weeks I got a notification that he can't fix it either. I didn't get any info why it can't be fixed, except that it might have something to do with the output section.

I don't want to criticize the repairmen and the shop here, I actually appreciate that they are honest about not being able to fix it, but it got me a bit worried about the amp. What can possibly make a solid state amp unrepairable? Is it the components and parts, or is it the circuit altogether? I've never worked on an amp myself, so it's hard for me to estimate the difficulty here.

I don't want to give up on this amp just yet, do you have any tips on how to proceed? What should I be looking for in a good repair shop to get it fixed, or at least to find out what the problem is?

 

[Jim C]

Maybe call the 2nd guy and ask why he can't repair it.
It is very possible the output transistors are no longer available.

 

[agedhorse]

Any qualified service tech should be able to fix that amp. There’s nothing unusual or remarkable about it.

Guys who mostly work on tube amps generally are not qualified to repair solid state amps because they often have no idea how they work.

 

[TFM94]

Any qualified service tech should be able to fix that amp. There’s nothing unusual or remarkable about it.

Guys who mostly work on tube amps generally are not qualified to repair solid state amps because they often have no idea how they work.

It's great to hear that it's generally fixable. There are a few more amp repair places in town, I will contact them and specifically ask about solid state amps.

 

[agedhorse]

I see that you are in Finland, I do not know what your options are there.

I should also mention that there are 2 unusual things about the preamp, the first being that it uses inverters for gain in the drive stages. An inverter is similar to an op-amp but is optimized for different purposes. The second thing is that the preamp selects channels based on the input jack being used. 1/4" (6.35mm) TS cables must be used. The amp won't work with TRS cables.

The power amp is very typical of that era.

If you find a Fender authorized service shop that is willing to work on it, I can share the service manual with that shop. The Sunn brand is still owned by Fender so I need to follow the proper procedure even though the brand is not current.

 

[Passinwind]

It's great to hear that it's generally fixable. There are a few more amp repair places in town, I will contact them and specifically ask about solid state amps.

I fixed plenty of those amps back when they were still in production. Very easy to work on, for the most part. Have you narrowed down which section or sections are broken?

Best of luck!

 

[TFM94]

If you find a Fender authorized service shop that is willing to work on it, I can share the service manual with that shop. The Sunn brand is still owned by Fender so I need to follow the proper procedure even though the brand is not current.

Thanks for sharing more information on this amp. There are a few more repair shops here in the south of Finland, but I don't think any of them are Fender authorized service shops.

I fixed plenty of those amps back when they were still in production. Very easy to work on, for the most part. Have you narrowed down which section or sections are broken?

I'm not sure in which section(s) the problem is located in. All I know is that the amp is dead quiet with either of the two inputs. The first repairman mumbled something about the output section, but he didn't seem to sure.

 

[Jeff Scott]

Thanks for sharing more information on this amp. There are a few more repair shops here in the south of Finland, but I don't think any of them are Fender authorized service shops.



I'm not sure in which section(s) the problem is located in. All I know is that the amp is dead quiet with either of the two inputs. The first repairman mumbled something about the output section, but he didn't seem to sure.

Have you tried using these connections to figure out whether the issue is with the preamp or the power amp? You would need another amp to help in this.

 

[jbybj]

I realize this is a long shot, but you’ve nothing to lose. Try cleaning all three send/return loops with some Deoxit. Exercise the jacks, hard patch the loops. It may be as simple as corrosion in the loop normals. If that’s the case, the reason the techs couldn’t fix it is because they didn’t find anything broken, electronically.

 

[TFM94]

Have you tried using these connections to figure out whether the issue is with the preamp or the power amp? You would need another amp to help in this.

View attachment 5105449 View attachment 5105450

I tried to run a preamp pedal into the fx return, but there was no sound. I have yet to try running the preamps into a separate power amp. I'll let you know how that works out once I get it back from the tech.

 

[agedhorse]

I realize this is a long shot, but you’ve nothing to lose. Try cleaning all three send/return loops with some Deoxit. Exercise the jacks, hard patch the loops. It may be as simple as corrosion in the loop normals. If that’s the case, the reason the techs couldn’t fix it is because they didn’t find anything broken, electronically.

Don’t go spraying anything into the amp without first identifying what the problem is.

The OP already ruled this out as the cause through troubleshooting with a preamp pedal.

 

[Michael Stanley 2112]

Hey everyone,

quick backstory: A few months ago a bought a Sunn beta lead amp from a friend of mine. It was broken and didn't have any output, and the original owner didn't want to put any time or money into getting it fixed, so I got it quite cheap. Given the current prices of these amps (and how hard it is to find one) I thought there was a good budget for repair cost and it would still be a good deal. Plus, they're really fun amps for both guitar and bass.

I brought it to a local store that offers amp repair. After a week I got a notification that the amp cannot be repaired. When I picked it up I briefly talked to the repairman. He said that it's an unusual circuit and that he mostly works on tube amps and that he can't fix it. Fair enough. Just on my way out the store owner mentioned that they have a second repairman who has experience with solid state amps, so I agreed to have it checked by their second repairman. Again, after two weeks I got a notification that he can't fix it either. I didn't get any info why it can't be fixed, except that it might have something to do with the output section.

I don't want to criticize the repairmen and the shop here, I actually appreciate that they are honest about not being able to fix it, but it got me a bit worried about the amp. What can possibly make a solid state amp unrepairable? Is it the components and parts, or is it the circuit altogether? I've never worked on an amp myself, so it's hard for me to estimate the difficulty here.

I don't want to give up on this amp just yet, do you have any tips on how to proceed? What should I be looking for in a good repair shop to get it fixed, or at least to find out what the problem is?

View attachment 5105210

The only time I ever had an amp declared "unfixable" by a guitar equipment tech, the amp in question was a very old Dan Electro (it was old in 1979).

It sat around for a couple of years. My uncle (Phd in Electrical Engineering) finally got around to having a look at it. His initial diagnosis (which turned out to be correct) was the output transformer had been blown.

We were unable to get information on Dan Electro amplifiers (it was 1979. I was a relatively new player and he had no clue about guitar equipment). I happened to mention my dilemma at my local instrument (Lou Rose Music in Edison, NJ) store. The son of the owner cracked a smile from ear-to-ear. (Warning I don't now believe, but I did then, this next statement). "My dad bought that company a few years ago. Leave it here, I'll have him take a look."

A week later: "It is the output transformer, but we don't have any of those parts left."

Now, I completely gave up on the amp and hurriedly rushed out and bought the first piece of junk I could afford (Stage 210).

About two months later, my uncle gives me my old amp in working condition as a birthday present.

I asked him: "Where did you ever find the part?"

"Do you know that thirteen inch Philco black and white television your mother asked me to fix about eight years ago?" (of course I knew it. That was my home-sick-from-school-laid-up-in-bed" television).

"Yes?"

"If she asks, tell her you haven't seen it in three years."

 

[ole Jason]

If you keep running into dead ends it may be worth a shot to reach out to Darkglass. They obviously wouldn't work on it directly but may have some leads on people around Finland with the skills to fix it.

 

[seamonkey]

Careful with plugging in speakers to a amp that looks dead, if it makes a thump turn it off.
Check out this vid, and the part 2

-40V dc out can be checked with a meter.
And a basic check of all the transistors can be done by pulling them out and checking with a meter.

This amp can be repaired, probably with less labor than a new amp of today.

In recent years, there has been a trend towards building electronic devices with components that are soldered directly to the PCB. This makes the devices more reliable and compact, but it also makes them more difficult to repair. In order to repair a device with soldered components, a technician must have the skills to desolder the components without damaging the PCB or the surrounding components. This can be a time-consuming and difficult process, and it is often more cost-effective to simply replace the PCB.

Western countries are building up the tech skills to repair complex devices.

There is a different culture of skill building in China. In China, there are many markets where young technicians and hobbyists can buy electronic components on the cheap. This allows them to repair their own devices, even if they are complex. In addition, there is a growing market for replacement modules for electronic devices. These modules are often high quality and can be purchased for a fraction of the cost of a new PCB.

We need to change our attitudes towards repairability. As consumers, we have the right to get our devices repaired at a fair and reasonable price. If we want to keep our devices out of landfills, we need to make sure that they are easy to repair. This means supporting companies that design and manufacture repairable devices, and it also means supporting technicians who have the skills to repair these devices.

Here are some additional thoughts on the topic:

• The trend towards soldered components is likely to continue, as it allows manufacturers to produce more reliable and compact devices.
• However, the lack of repairability of these devices is a growing concern.
• We need to support companies that design and manufacture repairable devices.
• We also need to support technicians who have the skills to repair these devices.
• By changing our attitudes towards repairability, we can help to keep our devices out of landfills and extend their lifespan.
• We also create strong local economies with local repair shops.

 

[micguy]

It is very possible the output transistors are no longer available.

Yes, you might have to accept more recent models of transistors, which are probably more robust than what originally came with the amp. It's almost certain you can find parts that will work as well, or (more likely) better than what it came with new. You just need to find a tech who's skills go beyond changing tubes, shorted power supply capacitors, and resistors that are black from overheating.

 

[Jim C]

Yes, you might have to accept more recent models of transistors, which are probably more robust than what originally came with the amp. It's almost certain you can find parts that will work as well, or (more likely) better than what it came with new. You just need to find a tech who's skills go beyond changing tubes, shorted power supply capacitors, and resistors that are black from overheating.

My mistake. I have read posts about older power transistors are NLA and figured Sunn would have the same issues.
This has kept me away from the Boogie M series.

 

[agedhorse]

Careful with plugging in speakers to a amp that looks dead, if it makes a thump turn it off.
Check out this vid, and the part 2

-40V dc out can be checked with a meter.
And a basic check of all the transistors can be done by pulling them out and checking with a meter.

This amp can be repaired, probably with less labor than a new amp of today.

In recent years, there has been a trend towards building electronic devices with components that are soldered directly to the PCB. This makes the devices more reliable and compact, but it also makes them more difficult to repair. In order to repair a device with soldered components, a technician must have the skills to desolder the components without damaging the PCB or the surrounding components. This can be a time-consuming and difficult process, and it is often more cost-effective to simply replace the PCB.

Western countries are building up the tech skills to repair complex devices.

There is a different culture of skill building in China. In China, there are many markets where young technicians and hobbyists can buy electronic components on the cheap. This allows them to repair their own devices, even if they are complex. In addition, there is a growing market for replacement modules for electronic devices. These modules are often high quality and can be purchased for a fraction of the cost of a new PCB.

We need to change our attitudes towards repairability. As consumers, we have the right to get our devices repaired at a fair and reasonable price. If we want to keep our devices out of landfills, we need to make sure that they are easy to repair. This means supporting companies that design and manufacture repairable devices, and it also means supporting technicians who have the skills to repair these devices.

Here are some additional thoughts on the topic:

• The trend towards soldered components is likely to continue, as it allows manufacturers to produce more reliable and compact devices.
• However, the lack of repairability of these devices is a growing concern.
• We need to support companies that design and manufacture repairable devices.
• We also need to support technicians who have the skills to repair these devices.
• By changing our attitudes towards repairability, we can help to keep our devices out of landfills and extend their lifespan.
• We also create strong local economies with local repair shops.

Wait a minute, parts are always soldered directly to the PCB's, except in point to point and turret/tag strip construction.

We have already more or less stopped supporting companies that manufacture reasonably repairable equipment, and we have also stopped supporting qualified, skilled technicians/shops. That horse has left the barn, in large part due to the cost of providing real customer service. Who is to blame? The average consumer.

My mistake. I have read posts about older power transistors are NLA and figured Sunn would have the same issues.
This has kept me away from the Boogie M series.

The MOSFET transistors used in the Mesa Boogie amps are in fact discontinued except for repackaged die in small quantities, or wafer level production lots on tens of thousands of pieces. It's the particular lateral MOSFET semiconductor process that's becoming obsolete.

 

[johnk_10]

i've repaired a lot of Sunn Beta and Coliseum amps over the years and IME, they're pretty easy to fix.

 

[fdeck]

I think repairability is a complex issue. Miniaturization per se reduces environmental impact from refinement of raw materials, and condemns less stuff to the landfill. This is true, even if the miniaturized assemblies aren't repairable at the component level, or become obsolete. The overall impact depends on the actual reliability of the assembly.

Miniaturization also helps cope with the labor market issues, by allowing repair to be taken care of in a centralized fashion. The most un-repairable amp is the one full of replaceable parts, that's sitting in the basement (the home landfill) because there's no local tech, and it's too big to ship economically.

 

[InkCow]

That horse has left the barn, in large part due to the cost of providing real customer service. Who is to blame? The average consumer.

I think this is mostly true (especially the point about the costs of support), but I think there’s also a bit of being a victim of its own success. This is purely anecdotal, but it feels like consumer electronics, which are produced in orders of magnitude larger scales than, say, 30 years ago, seem more likely to be discarded due to obsolescence than failure.