I'm new to audio repair, but not to electronics in general and built a headphone amp once.
Anyway, i decided I wanted to learn to repair amplifiers so I picked up a "for parts or repair" Primare I20.
It would power up and the display and pre-amp section seemed fine, but had no output.
Once I had it home, I started testing the voltage at the test points on the board. My guess was a power supply issue as this is a dual mono amp and other than the pre-amp, nothing else is shared across both channels.
The -14v supply was low (high since it's a negative supply?) at almost 0v. I also notice the +6.1v supply was low at about 1v. at a 7v test point right before the zener.
Checking the voltage at the input of the negative voltage regulator, I had close to 0v. As a comparison, the positive regulator had about 36v at its input. Voltage before R2 was -40v. I removed C8 as I assumed it was shorted. After doing that the input voltage jumped to -36v, but the output was still low at about -7v.
I removed C34 and that fixed the output voltage issue.
Moving on the the +7v supply, in similar fashion I removed C10 and that fixed the problem.
So I have 3 caps to replace C8 and C10 are both tantalum caps, and C34 is an aluminum electrolytic.
I have 2 questions:
Anyway, i decided I wanted to learn to repair amplifiers so I picked up a "for parts or repair" Primare I20.
It would power up and the display and pre-amp section seemed fine, but had no output.
Once I had it home, I started testing the voltage at the test points on the board. My guess was a power supply issue as this is a dual mono amp and other than the pre-amp, nothing else is shared across both channels.
The -14v supply was low (high since it's a negative supply?) at almost 0v. I also notice the +6.1v supply was low at about 1v. at a 7v test point right before the zener.
Checking the voltage at the input of the negative voltage regulator, I had close to 0v. As a comparison, the positive regulator had about 36v at its input. Voltage before R2 was -40v. I removed C8 as I assumed it was shorted. After doing that the input voltage jumped to -36v, but the output was still low at about -7v.
I removed C34 and that fixed the output voltage issue.
Moving on the the +7v supply, in similar fashion I removed C10 and that fixed the problem.
So I have 3 caps to replace C8 and C10 are both tantalum caps, and C34 is an aluminum electrolytic.
I have 2 questions:
- Any brand recommendations for capacitors. I think I got a pretty good deal on the amp ($180) and the cost of better caps isn't going to break my budget.
- Is there anything else I should be checking? From the schematic, it doesn't look like there is much around these caps to check. The resistors seem fine. I also believe the diodes are fine and they seem to be there to protect the regulators from reverse voltage. I'll order some regulators since they are both relatively cheap and common and will likely get used on some other project if not needed here.
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470nF is pretty small and so a film cap might be a good/better option. If the leads don't fit directly then drill the PCB into the print to accommodate them. That said modern tants are better than those of decades ago and so as long as they meet the voltage and size needed you can fit any decent commercial grade parts.
I would look at replacing ALL similar tants in the unit as old ones are trouble waiting to happen.
I would look at replacing ALL similar tants in the unit as old ones are trouble waiting to happen.
Interesting, I never even thought of SMD in this instance.
Surface mount SMD is generally a lot more reliable than the old leaded caps of years ago but it seems there are still possible issues judging by some of the documentation 'out there'.
This from Kemet:
https://sh.kemet.com/Lists/Technica...ap Failure Analysis Review by Javaid Qazi.pdf
Surface mount SMD is generally a lot more reliable than the old leaded caps of years ago but it seems there are still possible issues judging by some of the documentation 'out there'.
This from Kemet:
https://sh.kemet.com/Lists/Technica...ap Failure Analysis Review by Javaid Qazi.pdf
Interesting read. More importantly, it may me reañize that I probably have ceramic caps in the amp. This is base on the cap construction diagrams in the article.
I assumed they were tantalum because that's what the datasheet for the regulator recommends, but based on the construction they are likely ceramic.
I assumed they were tantalum because that's what the datasheet for the regulator recommends, but based on the construction they are likely ceramic.
The data sheet I have doesn't specify a type for the one at the reg input. It is shown as 0.1uF and these are typically to ensure stability of the regulator.
The output caps are specified as optional and either tant or electrolytic. You will find in practice that they are not critical as long as 'something' is there. The output caps are more important for applications where transient current demands are high, something that isn't the case with a preamp. If the circuit were supplying a load of say 50 milliamps and the load could randomly spike to 800 milliamps for a couple of milliseconds then a tant would be the best choice.
The output caps are specified as optional and either tant or electrolytic. You will find in practice that they are not critical as long as 'something' is there. The output caps are more important for applications where transient current demands are high, something that isn't the case with a preamp. If the circuit were supplying a load of say 50 milliamps and the load could randomly spike to 800 milliamps for a couple of milliseconds then a tant would be the best choice.
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AVX TPS series are very good. Be sure to choose the highest voltage rating possible that still fits 100% physically. Also replace all of the old tantalum caps in the PSU as the brand/type used has proven to be unreliable.
edit: I see you think they are ceramic. That is an easy one to spot as tantalums have the + indicated with a colored band. If they are really ceramic Primare has chosen either a very bad quality type or a too low rated type. Then choose X7R types of a reputable brand with 100V rating for C7/C8. For C10/C14 one can choose 100V or 50V rated types.
edit: I see you think they are ceramic. That is an easy one to spot as tantalums have the + indicated with a colored band. If they are really ceramic Primare has chosen either a very bad quality type or a too low rated type. Then choose X7R types of a reputable brand with 100V rating for C7/C8. For C10/C14 one can choose 100V or 50V rated types.
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Thanks for the recs. I ordered some 100v Kemets X7R from Mouser.
The caps in the amp now don't have any markings at all and aren't polarized. The have the metal sides/contacts that go the whole height of the capacitor whereas the tantalums look to just have the contact near the bottom.
The data sheet I was looking at was for the TI regulators. The installed regulators are National Semiconductor, but i couldn't find a datasheet for those.
The caps in the amp now don't have any markings at all and aren't polarized. The have the metal sides/contacts that go the whole height of the capacitor whereas the tantalums look to just have the contact near the bottom.
The data sheet I was looking at was for the TI regulators. The installed regulators are National Semiconductor, but i couldn't find a datasheet for those.
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Caps arrive and installed and the amp works again!
What a great feeling it is to bring something back to life for < $1 in components. (+ $7 shipping).
Thanks for the recs on the caps. I picked up about 10 each since they are so cheap as well as some other things I needed to spread out the shipping.
What a great feeling it is to bring something back to life for < $1 in components. (+ $7 shipping).
Thanks for the recs on the caps. I picked up about 10 each since they are so cheap as well as some other things I needed to spread out the shipping.
pleased to hear it is all back up and running.