Re-capping values advice

[Doodski]

Have you removed the caps and accurately measured the dimensions? That needs to be done. You will need some isopropyl alcohol and a old toothbrush for cleaning. All that white glue/goo needs to be scraped off the PCB. Don't damage anything. Be gentle but firm and take your time. No rushing things.

For the isopropyl alcohol Walmart has 70% and 100% purity. If you buy 100% it is flammable and burns with a clear blue flame and that is dangerous. So maybe use 70%.

After the capacitors are removed then we can see what clearance we have to work with at the surface of the PCB so we don't contact any any other components. Be careful removing the caps and watch out acid fumes from any leakage the caps may have. Don't inhale that directly.

 

[coleslaw]

Thanks so much for your help everyone.
Based in Ireland. I can look into buying a multimeter or I can ask my electronics friend who I'm sure has one.
I already have the service manual but in the meantime I'll order the caps.
Is it difficult to learn to adjust the Bias and offset ? again I can chat with my friend for help but I'd love to be able to do it myself.

 

[Doodski]

Thanks so much for your help everyone.
Based in Ireland. I can look into buying a multimeter or I can ask my electronics friend who I'm sure has one.
I already have the service manual but in the meantime I'll order the caps.
Is it difficult to learn to adjust the Bias and offset ? again I can chat with my friend for help but I'd love to be able to do it myself.

Adjusting the Bias and Offset is not difficult. It does require a steady hand and proper procedure. We will cross that bridge when we get to it.

 

[coleslaw]

Have you removed the caps and accurately measured the dimensions? That needs to be done. You will need some isopropyl alcohol and a old toothbrush for cleaning. All that white glue/goo needs to be scraped off the PCB. Don't damage anything. Be gentle but firm and take your time. No rushing things.

For the isopropyl alcohol Walmart has 70% and 100% purity. If you buy 100% it is flammable and burns with a clear blue flame and that is dangerous. So maybe use 70%.

After the capacitors are removed then we can see what clearance we have to work with at the surface of the PCB so we don't contact any any other components. Be careful removing the caps and watch out acid fumes from any leakage the caps may have. Don't inhale that directly.

Yes I have the dimensions but have yet to remove the old ones which I will do. Presumably they need a week or 2 unplugged to discharge first before removing ? Will clean up the board as you describe.

Yikes Its after 4am here, time for bed .
Thanks again Gentlemen , I'll order some caps tomorrow. excited to be fixing up the amp.

 

[Doodski]

Before you order anything...

Scrape off the white glue/goo so we can see the components that are underneath the goo/glue.
It is important we inspect those components because if the capacitors leaked acid that will have corroded stuff and we need to ensure we can clean it and see that the integrity of the components is good or if a resister or whatever might need replacing too.

 

[Doodski]

I'd recommend and wait until @restorer-john sees this thread. He has a LOT of experience with this sort of thing and will probably save you time, money and headaches. He has discussed this topic before.

Two heads are always better than one. @restorer-john should also pop in here.

 

[restorer-john]

I would be carefully checking the secondary voltage (powered up) over those caps before you pull them out. They should be around +/-52V, but HK often shipped 220V gear into the UK and your voltage was 230-240V. That pushes the caps closer to their rated limit. It may not be the case, but in my experience, those Elna caps only bulge when they are run too close or above their rated WV. If that is the case, go with 71V/80V caps. I would have no problem with 10,000uF as modern caps are often considerably under value or close to rated value. Older caps were "generous" and often were 20% above their capacitance rating when measured.

Also, check each of the four bridge rectifiers used for the main rails. You can do that out of circuit by desoldering them and checking all the drops are as they should be (~0.6V) One diode out in a bridge and the amplifier would still work, but the caps would have a ton of ripple on them- causing them to heat up and expand. Not saying that has happened, but I have seen it before. Normally you get a ton of hum to alert you to that, but in this case you may not.

HK is using 4 non CT (centre tapped) secondaries and full wave rectifying each winding, then tying them together to form a +/- rail, one for each channel. Most unusual. See below:

 

[Doodski]

Summary to do list:
-Before removing the caps check the voltage at the caps for about +/-52V DC. (This can be dangerous and it's probably best to skip this step. Perhaps your electronic tech friend can help you here.)
-Get those caps out.
-Clean the PCB with isopropyl alcohol and a toothbrush.
-Remove goo/glue under the capacitors at the PCB.
-Inspect the peripheral components near the capacitors for corrosion and possible corroded thin lead legs.
-Check each of the four bridge rectifiers used for the main rails.
(Send us pics of the area on the PCB after cleaning so we can inspect for corroded stuff.)
(Send us pics of the bottom of the PCB so we can inspect for heat issues.)

@restorer-john advice:
-Go with a higher working voltage like 71V/80V caps.
-10,000 micro Farad caps are optional. (@Doodski is not responsible )

Notes:
-Order capacitors after this list is completed.
-If you need help (and you will) with each step then ask as you go.
-The capacitors will store a charge as @coleslaw mentioned. Use a resister to discharge them or let the amp sit for a couple hours and discharge them with a wire or something but make sure you don't electrocute yourself. Some capacitors bite pretty hard when you take it across the hand or forearm but these are not huge caps so...

 

[amirm]

Summary to do list:
-Before removing the caps check the voltage at the caps for about +/-52V DC.

That is good advice but if his hands are not steady, or has the meter in the wrong range (e.g. ohm), he could do serious damage to the unit. I think given his level of skill, I would not have him do anything while the unit is on.

 

[trl]

Or any suggestions are most welcome

I would choose the one that has the highest Current ripple @ 100...120 Hz and the highest operating life (in hours, at 40...50 C, if available on the datasheet).

 

[somebodyelse]

That is good advice but if his hands are not steady, or has the meter in the wrong range (e.g. ohm), he could do serious damage to the unit. I think given his level of skill, I would not have him do anything while the unit is on.

If I get stuck or don't feel confident I have an electronics friend who solders a lot and can help me out.

This is a case where the electronics friend should at least supervise.

 

[coleslaw]

The amp has not been plugged in now for about 2 weeks, presumably the caps would be fully discharged by now ?

 

[Vini darko]

The amp has not been plugged in now for about 2 weeks, presumably the caps would be fully discharged by now ?

Not necessarily. Its best to be safe and dicarge them. @restorer-john will have good tips for doing it safely. I use a 10k 5W sand cast resistor wich I hold by the body (not entirely safe).
Caps can hold significant charge for months.
Edit : looking at the schematic above It looks like the caps may remain charged

 

[PierreV]

That is good advice but if his hands are not steady, or has the meter in the wrong range (e.g. ohm), he could do serious damage to the unit. I think given his level of skill, I would not have him do anything while the unit is on.

This 100%.

One shouldn't tell someone who is about to buy his first multimeter to work on live stuff! The advice given here by our resident techies is immensely valuable but what seems routine for them often isn't for the average joe.

I would describe my level as low intermediate: I can fix and understand some things (not enough) and have a couple of thousands of euros in equipment (rework station, good mm, soldering irons, heating bed, oscilloscope, bino microscope and power supply) but I wouldn't go peeking in a live amplifier with the standard probes that came with my mm. Way too easy to make a short, be in the wrong mode, suddenly move the device etc...

If I have to measure something potentially tricky such as a relatively high voltage on relatively large caps, I have even resorted to soldering a couple of wires to the measurement points and use banana plugs on the other side.

Oh, and by all means, do wear safety goggles, you've only got one pair of eyes!

 

[coleslaw]

Was going to just use a screwdriver touching both terminals . But if that's a bad idea ?Can a multimeter be used ?

 

[PierreV]

Was going to just use a screwdriver touching both terminals . But if that's a bad idea ?Can a multimeter be used ?

Decently sized screwdriver is OK, may lead to a small "bang" - wire with resistor as specced above would be smoother. I wouldn't use the multimeter for discharging purposes.

 

[Speedskater]

If in fact, each channel has it's own power supply system. Do one channel at a time.

 

[coleslaw]

Chaps a bit of a mix up with information. I opened the unit up to a bit of a surprise. I had loaned this amp to a friend for many many years and it appears he had the caps changed himself Apologies for the mix up

So to reiterate the original factory caps were 8200uf 59v 85° C (although the manual recommends 8200uf 63v)

The caps now in place are rated at 6800uF 100v 105° C

So should these still be changed ?

As an aside I wonder if the manuals recommendation of a higher voltage is due to what restorer-john mentioned about HK importing US gear to the UK voltage specs.

Thanks again everyone,

 

[SIY]

105 degrees and higher voltage are both good things for reliability.

 

[Doodski]

Chaps a bit of a mix up with information. I opened the unit up to a bit of a surprise. I had loaned this amp to a friend for many many years and it appears he had the caps changed himself Apologies for the mix up

So to reiterate the original factory caps were 8200uf 59v 85° C (although the manual recommends 8200uf 63v)

The caps now in place are rated at 6800uF 100v 105° C

So should these still be changed ?

Are the caps in the unit right now bursting out the tops of the caps? If so then replace them. It would help immensely to get a pic of the actual unit in chat here. If replacing them go for the 8200 micro Farad rating and up the voltage and temperature if you can find a cap that will fit. The caps will also have a hours rating. Try to get one with a high hours rating. So up the voltage rating, up the temp rating and up the hours rating.

After you find some caps link me up to the <.pdf> of the caps or send me the web link here. Then I can go over the numbers and type and make sure all is good.