power transformer for Hiwatt Buldog Bass 20 cheapie amp

[fredcapo]

I have a cheapo "Hiwatt" Bulldog 20 bass amp with an open primary on the PT...go figure. No schematic. PT is marked MT-15US. I want to fix it for the kid. How do i figure what secondary voltage PT will work? It is centertapped.
Typical op amp circuit. Not talking a lot of power here. Thanks

[52 Bill]

How do i figure what secondary voltage PT will work?

I will assume that it has an output power amp chip, is this right? If it is, look up the data sheet for the chip and see what the chip maximum voltage specs are.

Also check the main filter caps voltage ratings. They will be rated a little higher than the main supply voltages. This should lead you to a B+ voltage number that you can work backwards from to find the transformer's approximate secondary voltage rating.

[Jazz P Bass]

The first look see should be at the power supply.
Single or dual polarity.
Look at the power supply capacitors.
What voltage are they rated at?
How are the opamps powered?
Linear regulator?
Plus & minus voltage?
And try to identify the output section.
Single IC probably.
What is the number on it?

[mozwell]

i know its the wrong model, but it might be close
i may guess at a 15-0-15 secondary for the PT, but check what your main power supply caps are rated for & work back.... as the others have already said

[fredcapo]

Thanks. Since there is a 15 in the part number, that may be a hint. I will look at circuitry when back in the shop.

Who owns Hiwatt these days? I found a website that is not up but has a support email that I clicked and requested a schematic. Good luck, right.

[teemuk]

Who owns Hiwatt these days?

Two different companies own the trademark in different locations around the globe.

Fernandes Guitars owns the trademark at least in Japan, USA, Germany and France.

Music Ground "owns" the trademark in UK. Fernandes actually tried to apply for the trademark in UK in the early 1990's, however, the Patent Office in the UK reported that the word “Hiwatt” was "descriptive of and non-distinctive for goods powered by high wattage, and the mark could not be a registered trade mark unless it had become distinctive through use." Basically, in such state the trademark can be open prey for anyone. For instance, Audio Brothers (a former OEM vendor for Fernandes) also sold Hiwatt amps individually, which led to Fernandes terminating the manufacturing contract with them. Audio Bros still kept building Hiwatt amps up to the late 1990's´.

Both companies (Music Ground and Fernandes Guitars) basically employ various third party companies to manufacture their amps. Today Fernandes, for example, buys their designs from the large Asian OEM vendors such as Woosung Chorus Industries of Korea (which probably makes amps for hundreds of smaller brands and also under their Megatone brand). Formerly Fernandes has bought their amp designs from at least two other different vendors including the aforementioned Audio Bros and Leech Manufacturing.

Music Ground's Hiwatt amps are also contract manufactured. The company is reputedly from UK but unknown to me.

Good luck, right.

Since you obviously have a Woosung Chorus -made amp I recommend contacting the source directly. At least there's nothing to lose.

[fredcapo]

Thanks for the info.

On the hiwatt, The electrolytics off the diodes are 2200 @ 35 on each rail and internally they are using 50vdc electrolytics.

I have a similar quality cheapo amp that has a 15-0-15 PT that is running 2200@25 and 35 internally. And the PT looks bigger and heavier on the hiwatt.

So not sure what that implies. I did not notice if there are zeners in place...that would tell all...

[fredcapo]

Something wrong...

Re this bulldog 20, the output device is a TDA2030 rated at 18 W at +\-16. So whe secondary would I need to get 16 volts?

The funky thing is where I expect there to be Zener diodes across the electrolytics as per the bulldog 30 schematic, there are 1N4004's ...

[52 Bill]

...the output device is a TDA2030 rated at 18 W at +\-16

So it is marked TDA2030A? Maximum voltage for the A version is + & - 22 volts dc. A 32 vct (16 vac per half) transformer will get you approximately + & - 22.5 volts dc. I'd try and find a 28-30 vct to be safe.

The funky thing is where I expect there to be Zener diodes across the electrolytics as per the bulldog 30 schematic, there are 1N4004's ...

Has someone else been in there before? I've seen idiots try and replace Zeners with any old silicon diode before.

[fredcapo]

I dont know the history of this amp. It looks new and the soldering on the rectifiers looks factory. The orientation of the banding matches the silkscreen, so they are reverse biased like zeners...so they were invisible...correct?

So the supply was essentially not regulated... The primary winding is failed so perhaps there is a link...

I am rusty on the rectification stuff. If the 30 volt ac secondary winding is center tapped we get appx + and - 30 vdc when filtered?

[Jazz P Bass]

Basic rule: Vac x 1.414

[52 Bill]

I dont know the history of this amp. It looks new and the soldering on the rectifiers looks factory. The orientation of the banding matches the silkscreen, so they are reverse biased like zeners...so they were invisible...correct?

So the supply was essentially not regulated... The primary winding is failed so perhaps there is a link...

Yes they would have no effect on the voltage in the low voltage section. There may be problems with the preamp ICs. If the primary winding is open, I'd guess that there is a thermal fuse in there.

I am rusty on the rectification stuff. If the 30 volt ac secondary winding is center tapped we get appx + and - 30 vdc when filtered?

No, it's kinda like having two separate power supplies one for the positive voltage and one for the negative voltage. Take one half of the secondary which is 15 volts ac and multiply it by 1.414 to get the dc voltage.

[fredcapo]

OK. I now remember that 1.414 from my days at the former RCA Institutes on 31St NYC (as per my photo). So some things learned in school are useful.

But the zeners on the bulldog 30 schematic are 15 volt if I recall... So what zeners would you suggest?

OK I looked at the schematic for the bulldog 30 and they supply the B+ and B- to the TDA2030 then Zener it down to +\- VCC at 15 for the op amps. So if there are no zeners in my amp the op amps are likely fried.

Even though this will be a freebie fix at least I am (re)learning something.

[olddawg]

Have you tried to locate and replace the thermal fuse in the PT primary with a pigtail fuse? Try cutting the bobbin paper with a razor blade and see if one is there. I wouldn't send it out fixed that way, but it would allow you to trouble shoot and make measurements. It's a pretty common thing.

[fredcapo]

I did not know that it was internally fused, so I will try that in the shop tomorrow. Looks like a Philmore transformer that came up in google images.

Even though a relatively simple amp, without the schematic and without putting more time than it's worth it's hard to figure what's going on esp why there are 1N4004's across the electrolytics...