CR1020 sounds great, should VRs be replaced?

[pdxtim]

While adjusting one channel of CR1020 for bias the pressure from the screwdriver tip on VR 602 seemed to cause the voltmeter reading to jump around quite a bit. I was able to adjust to 10mV though and unit sounds good. Following advice here on AK I replaced the VRs on my Sansui G7700 even though sound was fine. Does the same apply to the CR1020, should VRs be replaced even though there’s no problem or should I leave well enough alone? Thanks for any advice.

 

[Nashou66]

While adjusting one channel of CR1020 for bias the pressure from the screwdriver tip on VR 602 seemed to cause the voltmeter reading to jump around quite a bit. I was able to adjust to 10mV though and unit sounds good. Following advice here on AK I replaced the VRs on my Sansui G7700 even though sound was fine. Does the same apply to the CR1020, should VRs be replaced even though there’s no problem or should I leave well enough alone? Thanks for any advice.

I replace all those open air VR's with Bourns trimpots .

Others will know which Bourns trimpot you should get exactly .

Athanasios

 

[mbz]

For the CA-1000, also has vertical boards, I use the 3296P series, eg,

3296P-502LF-ND

Mounts flush with circuit board with top adjuster for easy access. OEM is 4.7k, go with the 5k items, it wont cause you any stress. There are a few more options available (single turn,,,)

 

[pdxtim]

For the CA-1000, also has vertical boards, I use the 3296P series, eg,

3296P-502LF-ND

Mounts flush with circuit board with top adjuster for easy access. OEM is 4.7k, go with the 5k items, it wont cause you any stress. There are a few more options available (single turn,,,)

Thanks mbz. Is it a good idea to replace those VRs even if the receiver is running well?

 

[mbz]

Is it a good idea to replace those VRs even if the receiver is running well?

I not heard of issues with those VR's, ie, not especially problematic, unlike the open frame type used in early NADs...

The thing with the bias VR is, depending on the design of the bias circuit, if/when the VR fails open it could take out the output transistors and the odd resistor. Not nice but certainly not fatal, suitable modern day outputs are readily available.

The problem is that the CR-1020 is a nice piece so woth some basic TLC, but where do you stop? Recap power supply? I you replace the VR's then might as well replace the diff pair and electrolytics on power amp board. Any tantallums on power amp board would also be candidate.

If it was mine I'd do the work, I have the tools etc and it's no big deal. However for others I'd say if the VR's still operate smoothly/bias responds smoothly then they are ok.

 

[pdxtim]

In addition to replacing the VRs I’m considering replacing SC458 and SA844 since, from what I’ve read, they can be problematic. It seems the VRs and those transistors should be easy to access. After that I’d stop. What do you think? Thanks again for your help!

 

[mbz]

I’m considering replacing SC458 and SA844 since, from what I’ve read, they can be problematic

It's the outhouse shapped 2SC458LG that's problematic, see service bullitine below

Can't comment on 2SA844 being problematic or not...

 

[pdxtim]

It's the outhouse shapped 2SC458LG that's problematic, see service bullitine below

Can't comment on 2SA844 being problematic or not...

Thanks mbz, that’s great info. I thought SC458 should be replaced cause it’s on dlucy’s Top Ten worst transistor list. I did read a post by avionic though in which he said SC458 never gave him a problem in this application, so maybe I won’t mess with it. I appreciate your input!

 

[mbz]

I could be wrong but I think @avionic was talking about 2SC458LG's used for bias/temp compensation. "He" had not seen issues/failures when operating in this mode, almost certain he would replace LG's if used in audio path (I shouldn't speak for others so @)

The LG has 2 flat faces which makes better heatsink contact than the round faced TO-92 KSC1845 or...

 

[Bob Chieldon]

I not heard of issues with those VR's, ie, not especially problematic, unlike the open frame type used in early NADs...

The thing with the bias VR is, depending on the design of the bias circuit, if/when the VR fails open it could take out the output transistors and the odd resistor. Not nice but certainly not fatal, suitable modern day outputs are readily available.

The problem is that the CR-1020 is a nice piece so woth some basic TLC, but where do you stop? Recap power supply? I you replace the VR's then might as well replace the diff pair and electrolytics on power amp board. Any tantallums on power amp board would also be candidate.

If it was mine I'd do the work, I have the tools etc and it's no big deal. However for others I'd say if the VR's still operate smoothly/bias responds smoothly then they are ok.

Why would you say the differential pair needs replacing on this model, on a par in importance to a restoration as recapping, if I have read you right?

Are they known to degrade on this model?

I have 2 CR2020 which are fully restored apart from the original silicon.

I did try changing the differential pairs using 2sa970s i matched with a Peak DCA75 but ended with a worse offset than the originals,

 

[mbz]

Why would you say the differential pair needs replacing on this model, on a par in importance to a restoration as recapping, if I have read you right?

It was a general statement. What group of transistors (diff pair, VAS, drivers, outputs...) would you replace as part of preventive maintenance? In any case, why are we talking about replacing parts that are working ok... hopefully to get another 10 years of trouble free listening.

I'm happy that you disagree with the post.

Are they known to degrade on this model?

Not to my limited knowledge, quite the opposite, supposedly good transistors.

I did try changing the differential pairs using 2sa970s i matched with a Peak DCA75 but ended with a worse offset than the originals,

Possibly you have a problem elsewhere

 

[Bob Chieldon]

It was a general statement. What group of transistors (diff pair, VAS, drivers, outputs...) would you replace as part of preventive maintenance? In any case, why are we talking about replacing parts that are working ok... hopefully to get another 10 years of trouble free listening.

I'm happy that you disagree with the post.


Not to my limited knowledge, quite the opposite, supposedly good transistors.


Possibly you have a problem elsewhere

I wasn't agreeing or disagreeing, I didn't quite catch whether you meant replacement on a "only if faulty" or "matter of course as a full restoration" basis and wanted to be sure, it sounds like you meant the former.

The amp is otherwise perfectly behaved, makes the right power, measures the right sort of distortion, clips cleanly, etc. DC offset is good, around 14mV on the worse channel and 9mV on the better one, and so I have not pursued it but the better matched pair of 2SA970s were worse in respect of offset.

I posted some graphs from the DCA75 on a thread about it at the time.

B

Post in thread 'Mid restoration regrets?'

Jan 23, 2022

The KSA992s (below) I have seem better matched than original transistors

• Bob Chieldon

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