power transformer for Hiwatt Buldog Bass 20 cheapie amp

[52 Bill]

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...

If you do find a blown thermal fuse and can get the transformer to power up the amp, you can measure the supply voltages to find out for sure what the amp needs. If you should find that the main power supply is at +&-16 volts dc, then they didn't need to reduce the voltages for the opamps in the preamp.

Are there any dropping resistors in the power supply between the filter caps and the diodes?

[olddawg]

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...

I have equipment that I have used for years that I soldered an appropriate pig tail fuse in to replace an open thermal fuse. But I am a tech and I know it is there. It is not good practice to do it other than to troubleshoot.

[Jazz P Bass]

I really think you are up against the wall on this one.
The cost of a transformer, your time involved, no idea what the diodes are for, not sure what else in wrong with the amp.....
Buy this one, look inside & you will have all of your answers.
Hiwatt Bulldog 20 Guitar Amp - - BD20

[Jazz P Bass]

I truly believe that your are up against the wall on this one.
The cost of a new transformer, your time involved in this, not sure what the diodes are for, not sure if there are any other issues with the amp.....
http://www.zimbio.com/GuitarGear/art...ass+Guitar+Amp

[fredcapo]

JP. Yes I know. I do a lot of charity work and stuff just to learn. I think the most valuable part of the amp is the Hiwatt logo...

[fredcapo]

Don't want to leave this thread hanging. I opened the primary winding and jumpered the fusible and reinstalled. Removed the 1n4004's and installed some 15 v zeners. When i powered it up i saw a few volts across the zeners and assumed something was shorted, quickly powered down but they cooked - shorted. I think they were low current devices.

Anyway i removed the 2 op amps and the driver as well as the newly cooked zeners. I am seeing 23 volt rails essentially unloaded. Following the pcb traces it appears that the op amps as well as the tda2030 all get driven by the zenered rails.
So what value current and voltage Zener do you suggest I use to set the voltages?

[Jazz P Bass]

Can you post a picture of the board.?
Something isn't right.
You need more than zener diodes to make a supply.
Load resistors & caps for starters.
What is the opamp number.

[52 Bill]

Anyway i removed the 2 op amps and the driver as well as the newly cooked zeners. I am seeing 23 volt rails essentially unloaded. Following the pcb traces it appears that the op amps as well as the tda2030 all get driven by the zenered rails.
So what value current and voltage Zener do you suggest I use to set the voltages?

Why do you insist that there are Zeners in this amp? As I mentioned earlier, if the supply voltage is low enough the output chip and the opamps can all be run off the same power supply. Are you getting +&- 23 vdc unloaded?

Are there dropping resistors between the filter caps and the diodes? If there are none, then there never were any Zeners in the circuit.

[fredcapo]

It's very similar to the schematic that was posted in 4 above, except I didnt see the dropping resistors but need to look closer. The 23vdc voltage to the tda2030 is coming right off the caps but there were also a pair of 1n4001's straddling the electrolytics which did not make sense...I assumed they intended to install zeners like in the schematic to create rails for the op amps...let me print the schematic and trace it out... This thing looks too cool with its Hiwatt nameplate to not fix...duh

[Jazz P Bass]

Can you please supply the opamp part number that is on the part?
It may work on 23 volts.

[fredcapo]

They are 4558P's...so prob happy with less than 23vdc. When I get this working would like to use ceramic equivalent if available.

At the node where the rectified, filtered dc 23vdc exists, as well as the mystery diodes, the node connects directly to the tda2030a. Also off of the nodes, resistors feed the +/- Vcc of the op amps. Not sure how to calculate that drop.

On the pcb the diodes are named d1 and d2, while the similar schematic for the bulldog 30 calls them zd1 and zd2.

[fredcapo]

Here is the wrap up on this... The power supply is unregulated at +/- 22 which feeds the tda2030. The 1n4004 diodes that I thought should be zeners actually just supply the led. I replaced the 4558's with tl072's and they are fed +22 and gnd.

I put in a tda2050 in place of the tda2030. The unit now works, but shuts down in a number of seconds. Power down then up and comes on then shuts down again. Will post another thread to deal with that. Thanks.