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Post subject: Hot Rod Deluxe Issue
Posted: Tue Mar 18, 2014 2:59 pm
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Hi, I wondered if anyone had any help or advice. I've got a gig on Saturday and my Hot Rod Deluxe has decided that it just won't work correctly. The model is a 1996 version, totally original apart from a Celestion Vintage 30.

There is some significant distortion to the output to the speaker and the level of the output fluctuates with no specific pattern. I've tried to isolate the issue and I plugged the Preamp output into a different amp, then plugging the output of the second amp into the Hot Rod's output speaker. Everything was as normal I would've said. So are we looking at a power amp issue?

I've changed the power amp valves, this makes no difference. I wondered if it could be the filter caps? I measured at each test point.

C31 = 455V
C33 = 456V
C35 = 415V
C36 = 364V

These are the filter caps in the circuit I believe for the HT part of the amp. Do these readings seem high or is approx. 25V higher a normal tolerance?

If I'm totally on the wrong track does anyone have any further suggestions? If so it would be very much appreciated.

Thanks.

Tom.


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Post subject: Re: Hot Rod Deluxe Issue
Posted: Tue Mar 18, 2014 3:23 pm
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Hi chalkaero,

So that I am clear, you plugged the "Preamp Out" from the HRDlx into another amp's "Power Amp In", and the sound was normal? You then used another amp with only the speaker from the HRDlx, and the speaker sounds normal?

When you changed output tubes, did you check the bias? Were your voltage readings without the output tubes installed?

If the bias is OK, then possibly, your PI tube has a problem. Looking at the back of the amp, the third tube from the right is the PI. Try substituting a known good 12AX7, 5751 or 12AT7 in the PI position.

It's possible that you've lost a screen grid resistor on one of the output tubes, indicating a bad tube in that position.

If the PI is OK, and the screen grid resistors are OK, you might well have a filter cap issue, so you would need to check the filters caps values. Also look to see if any of the caps are leaking, evidenced by a grayish crusty looking substance coming from the positive lead of the cap.

Some things to check anyway.

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Hot Rod Deluxe Issue
Posted: Tue Mar 18, 2014 3:24 pm
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Location: Province de Québec, Canada
Could be many thing.

Check speaker wire / connexions/ wire
Check output power tubes , check and clean tubes socket 's contacts with contact cleaner .Unplug and plug tubes may clean contacts if you do 10 times .

Check for broken solder on printed circuit ( power Supply )
Check for a burn resistor .
Check if heater circuit is ok ; 6.3 volts ac average
Check Phase Inverter circuit : all resistors ( 12AX7 close to the output tubes )

Your voltage readings are not the issue .


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Post subject: Re: Hot Rod Deluxe Issue
Posted: Tue Mar 18, 2014 3:29 pm
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IMO filter caps are good but may have a broken solder between these caps and circuit .


I write to check Power Supply .


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Post subject: Re: Hot Rod Deluxe Issue
Posted: Wed Mar 19, 2014 8:50 am
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shimmilou wrote:
Hi chalkaero,

So that I am clear, you plugged the "Preamp Out" from the HRDlx into another amp's "Power Amp In", and the sound was normal? You then used another amp with only the speaker from the HRDlx, and the speaker sounds normal?

When you changed output tubes, did you check the bias? Were your voltage readings without the output tubes installed?

If the bias is OK, then possibly, your PI tube has a problem. Looking at the back of the amp, the third tube from the right is the PI. Try substituting a known good 12AX7, 5751 or 12AT7 in the PI position.

It's possible that you've lost a screen grid resistor on one of the output tubes, indicating a bad tube in that position.

If the PI is OK, and the screen grid resistors are OK, you might well have a filter cap issue, so you would need to check the filters caps values. Also look to see if any of the caps are leaking, evidenced by a grayish crusty looking substance coming from the positive lead of the cap.

Some things to check anyway.


Hi, thanks for you both for these helpful responses. Firstly, yes that is how I set up my test, using the Hot Rod's speaker and it's peramp section but another amp's power section. Voltage readings were taken with the tubes in, I also tried different pre amp tubes and a different phase inverter tube.

Bias was checked and set in the middle of the quoted tolerances.

I'll check up on the resistor idea when I get home but I thought reporting this may help anyone who might be able to assist me.

Thanks once again for your help and advice.

Tom


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Post subject: Re: Hot Rod Deluxe Issue
Posted: Sat Mar 22, 2014 5:05 am
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Here's an update on this problem.

After some head scratching, I did notice that the diode next to bias one was not the correct type, though seen as it's been like since I bought it 3 1/12 years ago I can't see how this would be the problem. I've replaced it anyway but no change.

Next, I've checked all the resistors on the power amp/phase inverter board and found that R57 was showing an open circuit. I replaced this resistor and then tried the following.

I also checked the voltages on the test points. Next to pin 1 of the phase inverter tube, the test point quoting 11.5V shows over 200 volts, as does pin 6 of the same tube. Something else that concerns me is that on the Blue and Brown leads which go to the output transformer I have around 450 volts rather than the quoted 65V.

I must admit that I'm extremely confused by this. Since I've replaced the resistor I mentioned, there is an extremely loud noise, a distorted, oscillating feedback, coming through the speaker even with no signal travelling through it.

One other thing is that the voltage at test point 26 is about 25V rather than 37.5V.

I've checked the power supply and all the voltages seem fine.

Has anyone got any ideas?


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Post subject: Re: Hot Rod Deluxe Issue
Posted: Sat Mar 22, 2014 6:11 am
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chalkaero wrote:
Here's an update on this problem.

After some head scratching, I did notice that the diode next to bias one was not the correct type, though seen as it's been like since I bought it 3 1/12 years ago I can't see how this would be the problem. I've replaced it anyway but no change.
If amp was working since 3 1/3 years the type must be the good one , what do you mean by type ? The number ? , different number could be same .

Next, I've checked all the resistors on the power amp/phase inverter board and found that R57 was showing an open circuit. I replaced this resistor and then tried the following.

Sometime you must disconnect resistors to read the value . Did you do that ?

I also checked the voltages on the test points. Next to pin 1 of the phase inverter tube, the test point quoting 11.5V shows over 200 volts, as does pin 6 of the same tube. Something else that concerns me is that on the Blue and Brown leads which go to the output transformer I have around 450 volts rather than the quoted 65V.

From ground, blue or brown wire may read 450 volts, everything is fine there

I must admit that I'm extremely confused by this. Since I've replaced the resistor I mentioned, there is an extremely loud noise, a distorted, oscillating feedback, coming through the speaker even with no signal travelling through it.

If you move the printed circuit and you did not replace all wiring between the two printed circuit and wiring from power transformer you'll get some oscillations, wiring dress is very important in this amp

One other thing is that the voltage at test point 26 is about 25V rather than 37.5V.

I've checked the power supply and all the voltages seem fine.

Has anyone got any ideas? Nothing more I can say to help you ( see my first answer ) , see a qualified tech



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Post subject: Re: Hot Rod Deluxe Issue
Posted: Sat Mar 22, 2014 10:27 am
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chalkaero, on the schematic, look at the voltages shown inside the rectangular blocks which are DC, and disregard the voltages shown in ovals which are AC. The AC voltages are references used with a signal injected into the input jack.

You should have high voltage (around 450 VDC) on the blue, brown and red wires which are the primary wires for the output transformer (OT). The blue and brown wires feed voltage to the Plates of the output tubes.

You should also have high voltage on pins 1 and 6 of each preamp tube and the PI tube.

Check all test points (rectangles) and note the readings.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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