Re: Tek 2235A re-cap


vdonisa
 

It's me again... after doing a little bit more research (looking at other 2200 series oscilloscopes with fans) I came to the following conclusions:

- the case is designed so that the fan should be installed as an "exhaust" (outblow) fan; the air flow would be from the intake located on the left side near the middle of the CRT's length and gradually heat up until it flows along the hottest heat sink (the one between the 3 damaged caps) then gets exhausted through the rear vent

- installed like this it will also help with the power resistor located on the mainboard, under the CRT, near the intake - i saw some slight coloration of the PCB around it so I believe it heats up handsomely

This being said I'm tempted to go for:

- Sunon HA60251V4-000U-999 fan (13.8dBa)
installed on
- Qualtek QLM-60-30-10 vibration dampening sleeve

The fan draws only 60mA from 12V so it should not be such a burden for the power supply. Also Tek is using (beefier) Sunon fans in other 2200 series models so the brand would not look out of place in the 2235A.

Cheers,
Valentin

--- In TekScopes@yahoogroups.com, "vdonisa" <vdonisa@...> wrote:

Hi Tom,

I already did, all electrolytic caps in the power supply were replaced with new, long life, 105C rated Panasonic and Rubycon. I was just musing that if that still doesn't work then I might go for 125 Celsius automotive rated ones :-)

But then I think the fan solution makes the most sense. As you mentioned, the 3 damaged caps were all having something in common - close proximity to the same heat sink (on one side or the other of it). A fan would blow right into that heat sink and help keep everything much cooler....

I just measured the distance between the holes and it appears that a 60mm fan (50mm hole distance) would fit just right. Mouser has in stock stuff like Ebm Papst 612FL - 12V, 0.4W, 11.2cfm, 16dBA - seems just right as a low noise solution. I'll wait though until you can get a look at the fan specs for the 2236A so I could know the proper voltage and max power it could draw. BTW I got my manuals from exactly the same source (together with a trigger knob) :-)

Thanks,
Valentin

--- In TekScopes@yahoogroups.com, "Tom Jobe" <tomjobe@> wrote:

Hi Valentin,
Do yourself a favor and change those three capacitors in the right rear corner. If your scope has cooked C970 on the front side of the heat sink, things have been getting very hot in there.
I don't have a copy of the 2236A service manual so I ordered one today from Qservice in Greece. I would like to see what component values are specified for the 2236A fan supply.
This very nice 2235A I have had some ordinary problems in the power supply which I fixed a year or two ago. It had one remaining problem with channel 2 which had not been fixed. It turns out that U180 was dead in the vertical amplifier, so I replaced it with the similar part out of a 2235 (which has a different part number). The channel 2 voltage gain is now too low, so I removed the U180 that came from the 2235 and ordered the proper U180 part number for the 2235A.
It will be a week or two before the U180 part and the 2236A service manual arrive. I will let you know what I find out about this fan mod, etc.
tom jobe...
PS If anyone has a 2236A service manual, could I get you to look up the values of a few components in the electrical parts list?



----- Original Message -----
From: vdonisa
To: TekScopes@yahoogroups.com
Sent: Monday, April 01, 2013 2:39 PM
Subject: [TekScopes] Re: Tek 2235A re-cap



Hi Tom,

Thanks for your answer!

I was thinking of a fan too (a low noise low RPM one that would just move the air a little bit without sounding like a vacuum cleaner like my HP 3325A lol). But then I was not sure where I could connect it without disturbing the power supply too much. Now that I know.... I do remember that connector, with an empty place for a capacitor nearby, I just didn't know what exactly it was for.

Otherwise if I can't find a decent fan solution, I already identified some 10u/100V that should be able to take some heat: Nichicon has them in their BT series (automotive 125 Celsius) also Rubycon in RX30 series.

Thanks again!

Valentin

--- In TekScopes@yahoogroups.com, "Tom Jobe" <tomjobe@> wrote:
>
> Your observation that heat might be what kills those capacitors in the right rear corner of the mainboard, is probably worth considering.
> The 2235A has a much improved aluminum heat sink for Q946, Q947 and Q9070 which might further aggravate the situation for those capacitors, while helping the transistors and the MOSFET attached to the heat sink.
> Something you might consider is to use the onboard power supply for the 2236A case fan. The 2235A has the complete fan circuit less the capacitor C965. All of this is just in front of that aluminum heat sink, and the place for C965 is marked "2236A only". The back panel on the 2235A has a hole pattern to accept a standard 12 volt fan (50mm square bolt pattern on a 60mm diameter fan?), and the pins to connect the fan's power are just ahead of the heat sink and marked "W9965".
> If you look at the power supply schematic for a 2232 or a 2236A you will see the fan circuit and component numbers. I may have a copy of the fan schematic around here somewhere, but I think there is a free copy of the 2232 manual in several places. You don't even need the schematic if you decide to do this mod, just connect the fan with the polarity that moves the air in whichever direction you prefer.
> I have added this fan circuit to several 22xx scopes. Many of the older 22xx scopes had some kind of an undocumented fan circuit on the mainboard but you had to populate it. The 2215A and 2235 scopes would be examples that had the undocumented fan circuit.
> tom jobe...
>
>
>
> ----- Original Message -----
> From: vdonisa
> To: TekScopes@yahoogroups.com
> Sent: Saturday, March 30, 2013 6:19 PM
> Subject: [TekScopes] Re: Tek 2235A re-cap
>
>
>
> This is interesting, maybe the gentleman that replaced C942 & C943 with 4.7u/250V knew something, anyway his repair has lasted. In my case C925 was OK and looked like being the original one.
>
> Maybe I should find some 10u/250v although by looking at the schematic I can't understand how they would get so much voltage. Unless there's some weird transient action there at power turn on/off.
>
> Other than this it might be the heat in that compartment, I probably should pull out the infrared thermometer and check what's going on.
>
> Thanks!
>
> --- In TekScopes@yahoogroups.com, "Tom Jobe" <tomjobe@> wrote:
> >
> > For whatever reason, the three capacitors in the right rear corner of the mainboard fail on the 22xx scopes with your kind of power supply (C925, C942, C943) but they are easy to change without removing anything. Also in the same corner of the mainboard the SCR Q935 often fails on 22xx scopes, but your 2235A probably has the upgraded MCR72-4 SCR just like this one has, so you may have no worries there. I was just looking inside of this 2235A power supply to see how it differs from an ordinary 2235 and it is all quite similar, but upgraded in several ways.
> > tom jobe...
> >
> >
> > ----- Original Message -----
> > From: vdonisa
> > To: TekScopes@yahoogroups.com
> > Sent: Saturday, March 30, 2013 8:57 AM
> > Subject: [TekScopes] Tek 2235A re-cap
> >
> >
> >
> > Hi All,
> >
> > I just finished replacing a bunch of old aluminum capacitors in a recently received 2235A and I'm posting here to share my experience and ask a few questions. This is my first post here so please bear with me :-)
> >
> > The scope was in working status so it was about replacing any easily accessible capacitor in sight :-) So far I replaced only those located on the main board that can be accessed without removing the daughter boards. This means complete power supply cap replacement plus a few other cap spread elsewhere on the main board.
> >
> > What I noticed:
> >
> > - there was a previous repair work on C942 ad C943 (both 10u/100V), they were replaced with 4.7u/250V of types not normally found in a Tek; this looked like an old repair (not recent)
> >
> > - C970 (4700u/10V, +5V supply filter) was bulged and cracked at the base rubber vent and sitting in a weird position (because of the bulge lol)
> >
> > - all other caps were looking OK
> >
> > For replacements I used mainly Panasonic EEU-FR and EEU-ED series except C906 (Panasonic EET-ED) and the 10u/100V ones where I used Rubycon YXF.
> >
> > Now for the questions:
> >
> > - does this look like a decent choice of caps or could I have done better? (I'm looking at it as a "restoration of memorabilia" for myself; I'm interested in doing good work, cost is no objection)
> >
> > - there's a bunch of small electrolytics on the daughter boards, they seem to be mostly 22u/10V of Nichicon VX type, should I have a second session and replace these too? Are these prone to fail?
> >
> > - looking on the schematic C942 ad C943 don't seem to be stressed too much, any known reason why they failed and needed to be replaced (long time ago)? Should I over spec them (higher voltage than the original 100V)?
> >
> > - any good source for C906 270u/450V with 4 pins? I couldn't find any so I used a standard 10mm pin spaced snap-in, fortunately the PCB layout helped me and I was able to drill 2 extra holes and fit it there?
> >
> > - anything else that you gentlemen know to be frequently failing and should be replaced?
> >
> > Thanks!
> >
>

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