Wrong resistors, amp damage

[ricci]

In this amp,
https://music-electronics-forum.com/...etch?id=813344

I replaced all the electrolytics and all the ceramic resistors but due to the incorrect description of the seller, R32 and R33 I bought them from 0.33 ohm instead of 330 ohm, consequently the 1A F1 fuse blown which I promptly replaced but I did not yet have resistors correct, so I didn't test the amp to see if the 0.33ohm resistors had done any other damage besides blowing the fuse. After receiving and soldering the 330 ohm resistors, I foolishly forget to plug in the speaker and when I turned on the amp I heard a vibrating noise coming from the transformer and of course no sound. At this point I don't know if the vibration and no sound fault was generated by the wrong 0.33 ohm resistors or by not having connected the speaker before turning on the amp. In your opinion, was the fault caused by the too low value of R32 and R33 ?

Thanks,regards !

[The Dude]

It's a solid state amp, so running it without a load is fine- actually preferred until it's verified that there is no DC voltage at the output. I would measure the resistance from the zener diode side of the resistors to ground to see if something shorted. My bet is that either or both CR9 & 10 might have shorted.

[ricci]

Do you say that the zener are short-circuited even if I removed the wrong resistors and put the correct resistors?

[The Dude]

You powered the unit up with lower value resistors, which would have caused the zeners to try to "zene" with a higher supply voltage- possibly too high. I didn't say the zeners ARE shorted. I speculated that they could be. No reason to guess. Just measure the resistance across the zeners.

[DrGonz78]

I think the dude is saying that those diodes took a bit of stress when the amp failed. They may work fine still but either way they might have been stressed in circuit. Check them with your diode checker and make sure they are not shorted or something. Some people might put in new diodes for good measure as a result.

edit: the dude was posting and I simulposted.

[Enzo]

Once damaged by the wrong resistors, putting in right resistors will not heal the zeners. TEST for shorts.

[ricci]

It's a solid state amp, so running it without a load is fine- actually preferred until it's verified that there is no DC voltage at the output. I would measure the resistance from the zener diode side of the resistors to ground to see if something shorted. My bet is that either or both CR9 & 10 might have shorted.

You were right .. measuring the two zener the buzzer of the multimeter sounds both in direct connection and in reverse and indicates 0, I assume this is a symptom of a short circuit. I replaced them and now my amp is back to life. Thank you all

[J M Fahey]

You were lucky

You could easily have blown all integrated circuits in the preamp, all 5 of them.

Glad main fuse blew quickly and avoided more damage.

[ricci]

You were lucky

Yep, fortunately :-) but i had in mind to rejuvenate this amp and i also bought enough TL072 ;-)

I would take this opportunity again to ask one thing:

i had replaced R32 and R33 with two 10W because hot area but could I get some audible benefit if I also replace R84, R85 and R88 ? I have a feeling this amp has lost some volume over time..
Anyway, where they are the pcb is clean because maybe they don't dissipate much heat, unlike R32, R33, CR9 and CR10 where the pcb is blackened by the much dissipated heat.

[pdf64]

What do they measure?
Probably would need to continuously overdrive the power amp hard to stress them much.
If they’re still within tolerance and look ok then they probably can’t be affecting the audio power output unduly.

A reduction in power output might be due to the main power supply ecaps aging.

Blackened pcb is bad news though, it can become conductive. The best thing is to cut out charred board, eg with a dremel type power tool.

[ricci]

What do they measure?

0.47 ohm 5W

A reduction in power output might be due to the main power supply ecaps aging.

The two 2200uF capacitors are the only ones that I have not replaced because the ones I bought seem to me of dubious quality, a pair of brand JAMICON 50V and another SAMWHA 63V but I just put them aside. Your opinion ?

Blackened pcb is bad news though, it can become conductive. The best thing is to cut out charred board, eg with a dremel type power tool.

The board is not charred, it is the area around the two zener but now I have cleaned it up and it is much better.

[ricci]

Nobody can tell me if I can find improvements in replacing the original filter capacitors, still working after 35 years, with equivalents of Samwha or Jamicon? From what I have read, it seems that Samwha often create problems in TVs from Samsung, LG and other brans.

[pdf64]

I’ve no experience of those brands. I’m doubtful of making such snap judgments based solely on brand. What are the detailed specs, eg max operating temperature, operational life?

What causes you to suspect the caps you’ve bought of being poor quality?
Did you buy them from a manufacturer approved distributor / reseller?

Perhaps consider buying a cheapo component tester that includes ESR, so the old and new caps can be assessed.

[Enzo]

Hi ricci
Remember always, this is just a guitar amp, not a high precision piece of lab gear. Tonal nuance due to brand of filter cap will be infinitely small. And as to TV sets? Totally different technology. Your amp runs at audio, the TV set runs at ultra high frequencies, must have exquisitely tune circuits.

[J M Fahey]

FWIW I use tons of Samwha caps in Guitar amps for some 8 yeas now (before I used TRec), they work without problems, just respect voltage ratings.

I had only 1 problem with Samwha and admittedly by not following my own rules: once I made a +/-62V rail amp and used 4700uFx63V cps ... about 1 Month later one of them blew its rubber seal

I replaced them with same rating 4700uFx63V EPCOS because thatīs all I had on hand and he was in a hurry (what else?) asking Bass player to bring it back next week for replacement. (He lives in another City some 400 km away)

He never ever came for that (no Juan, it works fine!!!!), so I know they are working.

But if you derate them by 20% or more: they will last.

My daily bread and butter supply filtering is 2 x 2200uFx50V Samwha, with +/-40 - 42V rails: smooth as silk, used at least a couple thousand of them, or more.