Hartke HA3500 C401 Venting- Help Please

[protojim]

Enzo, or neone that can help me please.

I have a hartke HA3500 from Dec 1993 I just recieved.

The fuse is blown & the B+ cap is short (C401)

as I didn't see/measure any other shorts in the power supply, I initially replaced cap w/ 105 degree cap of same value/voltage and the fuse.

worked fine for about 20min (master volume on 3, preamps volumes at about 4 , with an 8 ohm load).
Then it vented the cap (I turned off before it shorted).

In both of these times I played for a few mins then stopped and was about to resume maybe even hit a note when pow there goes the cap again..

I tested the bridge rectifier D401 didn't appear shorted (fwd, or rev biased) but the measurements were slightly different from the neg and pos portion of fwd biasing.
.48v and .51v

I have a new toroidial xformer , so I made the swap, and replaced c401 again.. I checked the bias between tp301 and 302 and was 2.8mv with an 8ohm load. (I left it alone). On fri night, sounded good played a couple tunes and was done, but I just powered up today (Mon), and same thing after about 20min if that. The cap vented yet again.

This Ha3500 does not have 2sd2155's, or 2sb1429's but rather is using KTC5200's, and KTA1493's which appear to be a similar/compatible? part.

I am at a loss- the rectifier didn't seem bad, the xformer is/was new, what else could be taxing c401 to make it vent?

could it be a faulty bridge, or are the power transistors the wrong ones entirely, or is there something else I am overlooking?

[protojim]

that was actually tp1 and tp2 for the bias check not tp301 and 302 heh...

also noticed on foil side of pwr. amp assembly where CN501 pins G,G,and out are- there appears to be a place where maybe they arced?
there is a couple spots that look like solder on the runs.. one on the 2 pins labeled g really close to one on the run that goes to out.

I would assume if this arced it would grnd the output... could this cause my problem. I did not measure a short between the areas in question but I cleaned them up (removed some solder) and covered with hot glue...

[protojim]

It appears I have installed a wrong fuse that was sold to me as fast blo- but it's not
I have the right type on the way now.
anyway-I was able to shut power off before cap completely shorted.

[Jazz P Bass]

Four things come to mind that can overheat an E Cap.
Incorrect voltage
Incorrect polarity
Too high of a ripple going in to it.
AC voltage.
Assuming you are covered on the first two, that leaves ripple & AC.
If the bridge is bad (or going bad) that would explain both.

[protojim]

Well that could make sense- because fri I took it to my buddies house when I played, and I bet with everything we had goin (loaded that 15amp circuit), his voltage was nowhere as stiff as my outlet as I only had this unit plugged in.

I thought that polarity, and overvoltage would be the only things to cause this- thats why I wasn't too keen on messing with the pwr amp section, and opted for a xformer swap that I had setting here.
My experince with bridges is they either work or don't (are shorted) that's why I was hesitant about the readings I took- didn't test shorted.

And I don't have an 8amp 400 volt bridge on hand (PBU804, or GBU804)

I will now go scavenge for this part. Does neone have an extra they can send/sell me, or refer me to get one or two without getting gouged

Bummin' in Toledo, OH

P.S. It appears I have an extra x-former that will be for sale! which style do you need I have the older original rectangular one, or the newer style torroidal with 2 different mounting plates (Square 4"x4", or rectangular 3"x5")

[Jazz P Bass]

I would attempt to measure the bridge rectifier.
Compare the B+ & the B- precap voltages.
Vdc & Vac.
This amps sounds like it needs to be on a well equipped test bench.
A signwave generator, a scope, two meters, a dummy load, ....

[protojim]

There's not really a whole lot to the B+ B- power supply - 7 parts including the xformer.
I'll attach a schematic.

The power supply seems pretty straight forward- I've changed 2- 4700uf 100V electrolytics(C401, and 402), and a new xformer and replaced the fuse.
I did not change the bridge (didn't test short) and 3 film caps. (c346 .1MF 200v, c320L and c321L both are .1MF 100V on my board but the schematic shows .22MF 100V, and are labled c322, and c323).

I do have a scope (older dual trace techtronics), an HP signal generator, can make a dummy load, and have a fluke 189 meter. I may need a little guidance it's been awhile

http://music-electronics-forum.com/a...628-ha3500.pdf

[Jazz P Bass]

So monitor the B+ & B_ pre capacitor.
What I would look for is the ripple voltage.
Measure it.
If the B+ is worse than the B- coming out of the rectifier, than the BR is bad.
You can use the Fluke set on Vac to measure the ripple.
Or you can use a a scope for a visual indication.
The reason I am leaning this way is because it takes a while to fail.
Another thought: are the caps you have from the same supplier, same batch?
Might have all bad caps.
Unlikely, yes.

[protojim]

Ok depopulated the power supply back to the rectifier. I will hook up and monitor in the a.m.
I did buy these caps from same manufacturer/lot but they all measure good.
The original B+ cap (Samwha 85 degree celcius) was short and the unit had a blown fuse when it got to me.
I replaced with Nichicon 105 degree K(M) of same value/voltage. (man when these vent they make a freakin mess spewing waxxy substance all about)

[Jazz P Bass]

As an aside, I have seen C436 (0.1/200v) short out.
What it would do if it was "trying" to short I do not know.

[protojim]

C436 tested good.
I even had a blow dryer on high heat blowing directly on it. It increased in capacitance from 102n to 125n

If that cap were to fail it could be a catastrophic failure in the secondary winding as it would directly short the winding.
and seeing how there's no real protection in the secondary- I would wonder if the mains fuse would blow in time to save the xformer?

what do you think of this idea-

I am considering installing two 2 1/4 amp fuses - one on each ac wire coming from the secondary.
The secondary is rated @ 4.55 amps- I believe this would be sufficient to prevent the failure mentioned above from occuring.

[protojim]

Got parts in-

I relplaced PBU804, and C436 .1 200v with a .1 250v film (what I had) , and finally got the right fast blo 8a fuse.

I had the amp on and pluged into 4 ohm load - played on it after a few min idle for tube to warm up(10 to 15 min tops ) tube preamp @ 4 - master @ 3.5 to 4. Sounded good of course - heh

noted that the filter caps (C401, C402) were warm, heatsink for rectifier was cold, amp section heat sink seemed cold

I measured/observerd -88.23 VDC and .030 VAC on B- And +88.05 VDC and .045 VAC on B+

I was wondering if it were normal for a difference of 15 millivolts of ripple on B+ v.s. B-?

seems like it wouldn't matter.... dunno I guess I'm paranoid of C401 venting again-

[Jazz P Bass]

Warm is good.
Hot is bad.