Hotrod Deluxe Reverb problem

[shaun]

Hi All,
working on a HRD III with a reverb problem, ie no reverb. I'm getting signal back from the tank to the amp, and the tank itself gives me a 60ohm DC resistance reading at the input and 214ohm at the output when disconnected, so it seems the tank is intact. I've reflowed the solder on the tank connections.

I recapped the amp and replaced the two 5w 470ohm resistors, raising them above the board. There wasn't too much heat damage as the amp has only had light use, but that was a good first step. It did not seem to have reached the channel-switching-problem stage yet. Channel-switching seems to be working fine, which seems to indicate that the low-voltage supply is working okay. The reverb is not switched at all, so I'm not sure that matters, except that the low-voltage supply seems to also power the reverb op-amp at +/-16v, according to the schematic (which is tiny and hard to read).

The reverb uses an op-amp to drive it rather than a tube - chip #BA4560. I haven't been able to get decent voltage readings off the chip, and I'm not sure why - they're kinda tiny and awkward - a pita, actually. I've checked the surrounding resistors (Fender used some super crappy resistors in these, so I replaced the 82k and 100k plate res on the PI), and I've checked for cold-solder joints, but I can't see anything amiss. So I'm wondering whether a BA4560 can go bad, or if anyone has had that experience.
Thanks for any guidance.

Hope all are well. Also, just as an aside, I discovered S. Luckey's website for the first time yesterday - how could I have missed it all these years? Most impressed and learning from it already. Thank you Mr. Luckey!

[tubeswell]

Firstly you have to check the +/- PS for the op-amp circuit is working properly.


Then if that's okay, look for clues in the circuit where there has evidently been too much current through the op-amp (like cooked resistors) - this should give you a pretty strong hint if the opamp is buggered.


But the most common problem with Fender Hot Rods/Blues De Ville (apart from Illinois caps) is the 5W cement resistors in the heater supply - which is a widely known flaw with the basic design of these amps and a reason not to buy one. And you're already onto that

[glass54]

Hi Shaun,
Continuing on from Tubeswell (he's right on the ball  ) check R78 and R79 as well as Zener Diodes CR13 and CR14. You have implied that you have already done some work and the amp appears to have limited heat damage.
Here is my procedure for this amp:
You will see a lot of heatstress/fractured joints/PCB discolourisation!! Check that C40 and C41 are not heat damaged ("crawling"plastic and top "swelling")
Clean up the pads on the 5W resistors and Zeners, then resolder but may need extra flux ie substantial oxidation of copper tracks/pads.
When happy, please confirm you have approx +/-16V DC as per schematic.
I am surprised that you have a BA5460 IC for U2 as I have only encountered TL072 ICs in my experience. Confirm  +16V DC at pin 8 and -16V DC at pin 4 of IC2. It seems like somebody has changed U2 IC in the past.
You may need an Oscilloscope, but injecting around 50 mV signal into U2B (or even a good guitar signal) straight into the REV Return will confirm operation of Rev Circuit if Op Amp is OK. (Reverb Pot near maximum of course ).
Trust this helps
Kind regards
Mirek
 

[shaun]

Thanks guys,
makes perfect sense. I checked the =/-16v, and it's good at the 5 watters and their caps, C40 and C41. It also looks good at the zeners, CR13 &14.

However, at the IC, BA4560, there seems to be a short between pins 3 and 4. I can see no reason why that would be other than the IC is toast. The +/-16v go to pins 4 and 8, and there is good continuity between the caps, C40 and C41, and the IC pins 4 and 8. However, there is also continuity between pins 3 and 4 of the IC, which means 16v is going to pin 3, the signal pin I believe.

I replaced the surrounding resistors - easy enough - just to ensure they're not throwing me for a loop.

It seems that BA5460 is obsolete, at least according to what I saw on Mouser, so now I'll try an equivalent and hope that does it. Unless anyone has another suggestion. Also, if anyone knows a good equivalent...

Thanks. I'll post what I find, and I'll keep my eye on further advice.
S

UPDATE: Can't find a BA4560 outside of China and don't want to wait a month for delivery. Can anyone tell me a good replacement/equivalent of a BA4560? I know next to nothing about op-amps, except they sometimes seem interchangeable.

[glass54]

Shaun
The Schematic says TL072 for U2 so I would go with that. (Very common in a number of Fenders SS amps).
You could also try NJM4558 or BA4558 or equivalent. These are probably getting a little scarce as well.
But why change Fenders initial choice? If I was going to change to change away from a J-FET Op Amp, I would probably go with NE5532 (common, popular, well proven and well priced). I use these in Pedal replacement (and upgrades to BA4558 etc) better specs and readily available. In your application the NE5532 has a higher current drive capability.
If you want to play with various Op Amps I would fit a good quality 8 pin IC socket so you only have to unsolder/resolder once to avoid damaging the PCB with multiple resolders. The IC socket would give you a nice option to try different ICs.
Regards
Mirek

[PRR]

... BA4560, there seems to be a short between pins 3 and 4. I can see no reason why that would be other than the IC is toast. The +/-16v go to pins 4 and 8, and there is good continuity between the caps, C40 and C41, and the IC pins 4 and 8. However, there is also continuity between pins 3 and 4 of the IC, which means 16v is going to pin 3, the signal pin I believe.....

"Continuity" is not a short. Everything inside a chip is "connected", but not shorted. Get the OHMS of that.

[shaun]

Thanks for the feedback!
Mirek, I'm pretty sure the amp came with a BA4560 factory installed, so I'll replace with that - they're coming from China as we speak.

Thank you PRR. I understand what you're saying re continuity. I know very little about ICs, but they seem to be wired in a similar fashion to a tube such as a 12ax7 - two "triodes" and two "plates." So I couldn't understand why the power voltage would want to go to the "plate" and also to the signal input of the IC. Anyway, I'll check the OHMS, as you say. First I'll check the dictionary for what OHMS means. I was raised in NZ; there, it means On Her Majesty's Service. James Bond and all that.

I won't be the least surprised if the issue has nothing to do with the IC. I WILL be surprised if replacing it fixes the issue. All about the journey, I guess.

[thetragichero]

i wouldn't think of an op amp like a triode. they are (more or less) an ideal amplification stage like you'd see in a block diagram (they even use the same symbol, a triangle). there are two inputs, one inverting and one non-inverting, and one output. the inputs generally want to be at about half the supply voltage

[shaun]

Thanks thetragichero. Great info.That would mean that if the power supply is 16v, I should usually expect around 8v on the inputs. In my case,I have 16v on the input,which means a wiring problem or an IC issue. I wonder how often ICs go bad, or are bad from the factory. Probably one in a million.

[thetragichero]

your supply isn't 16v though is it? i thought it was 32v (+/-16v)?

[shaun]

Well, it reads +16v on pin 8, and -16v on pins 3 and 4. I think there should not be -16v on pin 3.
Hope that makes sense.

[Garrett335]

Howdy guys,
I'm having the exact issue with a Hot Rod Deluxe, I wanted to see what ever came from this.
This Deluxe I'm working on has the same BA4560 chip. The resistors and diodes mentioned have been replaced.
The reverb works faintly, if you hit the amp the reverb spring will make a sound, and you can hear faint reverb when cranked.
Voltages on the IC are as follows:
1. 8.93v
2. 8.6v
3. 8.74v
4. 16.9v
5. 0v
6. 0v
7.  3.5mV
8. 17.3v

Any help would be great! Thank you so much!
(Also I think the model I'm working on is Rev A schematic.)

[shaun]

If it's any help, I traded out the IC for a new one - it made no difference. Never figured it out - terrible amp tech that I am.

[AlNewman]

If it helps, my reverb works and the voltages are as follows:

1-23.4 mv
2-23.4 mv
3-21.3 mv
4-17.3 v
5-0.1 mv
6-0.1 mv
7-4.7 mv
8-16.97 v

Hope that haelps.

[Garrett335]

Thank you AINEWMAN!

Any idea what the first three legs of that IC do?
Your voltages on those pins are about three times the voltage I'm getting on mine, wonder if there are some damaged components leading up to that?

[pdf64]

Pin4 should be -16V.

[AlNewman]

Pin4 should be -16V.

You're right, my bad.  My measurement for pin 4 is in negative voltage. 

Thank you AINEWMAN!

Any idea what the first three legs of that IC do?
Your voltages on those pins are about three times the voltage I'm getting on mine, wonder if there are some damaged components leading up to that?

If you look at a schematic, it will show where the pins go.  I'm not familiar with the inner workings of of op amps, I'm sure somebody else here could help you.  There are data sheets online, maybe that could shed some light.  Look up your specific part number, and share a link of the datasheet, and maybe somebody will chime in.

The first 3 pin voltages I gave you are in millivolts, so my voltages are about 8.5v less than yours in a functional unit.  Since those are the only voltages that are drastically different than mine, I would start my search there.

[pdf64]

EDIT 
Pin
1 output A
2 inverting input A
3 non inverting input A
4 negative power supply
5 non inverting input B
6 inverting input B
7 output B
8 positive power supply

If inputs and outputs aren’t very close to 0V DC then something is wonky.

[rake]

https://www.amplifiedparts.com/products/op-amp-ba4560-dual-general-purpose-8-pin-dip