Help with DC on volume pot

[mkusan]

Hello all, i really need guidance on this one. I have Rotel integrated amplifier from 80s. Symptoms (related or not I don't know) are: popping with membrane excursion upwards from both channels while pressing low filter button, same happens while turning bass knob, this symptoms are more pronounced in the first 2-3 minutes after power on. Second symptom is scratchiness in left channel while turning volume pot up or down, even if I move balance all way to the left I can still hear it, related to this (i think ) needle of VU-meter of the right channel declines itself from starting position on power up without music playing. Third symptom if I touch the chassis while no music is playing I can hear that in speakers. All of the above happens on all inputs independently is something is connected or not. I tracked all of the above symptoms to preamplifier, if I plug mp3 player in main amp in everything is fine, no popping no noise.

What I have done so far:

the unit was recapped last year (electrolytic caps only)
changed volume pot to sealed tkd
changed all transistors in tone board
changed C501,C502 ceramic 300pF to film caps
changed C545,C546 tantalums 3,3uf to electros
checked and re-soldered suspicious solder joints


My shotgun approach so far has not yielded any results and I am out of ideas and would appreciate any help. I took some voltage readings, default values in brackets E-C-B everywhere:

B+ (39,6) 41.1
B- (39,6) 41.1
voltage regulator Q904 (35.3-5?-35.9) 34-53,4-34,6 schematics shows collector voltage 5V, but I believe that is a typo, please confirm

Q506 (11.8-26.7-12.3) 11.1-26-11.6
Q505 (11.8-26.7-12.3) 11-26-11.4
Q504 (2.5-16.6-3.1) 2.8-13.9-3.4
Q503 (2.5-16.6-3.1) 2.9-13.2-3.5
Q502 (0.2-3.1-0.8) 0.2-3.4-0.8
Q501 (0.2-3.1-0.8) 0.2-3.5-0.8
Q508 (3.1-9.3-3.7) 2.8-10-3.4
Q507 (3.1-9.3-3.7) 2.8-10-3.4

I am running out of ideas and would appreciate all the help that i can get.
If better resolution schema is needed I will gladly upload it somewhere.

TIA

[Mooly]

Try and put the file for the circuit in a zipped folder and post that. Too blurry to work from as it is

[mkusan]

Originally Posted by Mooly

Try and put the file for the circuit in a zipped folder and post that. Too blurry to work from as it is

Hi Mooly, here it is.

[amptech]

it would seem by changing part's at will you might have made things worse...have you kept hold of the old parts?

[amptech]

mkusan, I would say these old units suffer from poor solder joints need a reflow..did you spray clean all front controls to remove built up of crud?

[Mooly]

Thanks for the circuit,

things to check with NO signal applied. And that is a typo on Q904.

As the amp has been recapped and worked on you need to make several checks.

There are three distinct stages in the preamp, a two transistor gain stage, a single emitter followe and a single stage incorporating the tone controls.

You need to measure from COLD and quickly the voltages on Q503/4 collectors.
The voltage on Q505/6 emitters.
The voltage on Q507/8 collectors.

Write down the results.

Now let the amp get fully hot and repeat the measurements. There should be virtually NO difference between cold and hot readings.

If pressing the low filter button causes a pop then that suggests DC is present across the switch terminals. It may well pop if there is a signal present.
So measure the voltage ACROSS the switch in the off position. It should be zero.

Be very careful making measurements as any short could damage power amp/speakers. Might be best to isolate power amp inputs while working on it.

[MikeBettinger]

I would suggest a grounding issue to the filter amp pcb. The schematic does not show where the ground to the board or to the volume control originates, only the connections E4 and E7. As I see it the only way to develop a voltage across the low filter switch is if the resistors from the coupling caps to ground (R529- R535) are not keeping the filter sides of the cap properly referenced to ground and the leakage across the caps (C517) could build up and cause a pop. R561 not grounded well could cause the same issue on the bass control. Electrolytics leak even when new. I remember equipment of this vintage supplying ground to the PC boards through the mounting screws and odd ball problems like this creaping in over time. Just a thought. Mike

[mkusan]

Originally Posted by Mooly

You need to measure from COLD and quickly the voltages on Q503/4 collectors. COLD 12.8/13.5 WARM 13.2/13.8
The voltage on Q505/6 emitters. COLD 10.5/10.8 WARM 11/11
The voltage on Q507/8 collectors. COLD 9.4/10.1 WARM 10/10.1

Write down the results.

If pressing the low filter button causes a pop then that suggests DC is present across the switch terminals. It may well pop if there is a signal present.
So measure the voltage ACROSS the switch in the off position. It should be zero.

Switch has two rows with three pins in each row. Across each pins the voltage is zero.

@amptech

I kept old transistors, tantalums and ceramics.

@MikeBettinger
I have to pickup my child now I will check those resistors and get back to you.

I think certain time line of my (wrongdoings) is needed here. I recapped amplifier in February this year, since then it worked flawlessly until two weeks ago when old problem of popping while cold while pressing low filter turned up again, cold weather maybe

Then I opened the amp again and changed transistors (since I presumed they are most thermally sensitive of all components). Then those tantalums and then ceramic disk. I may be wrong but changing C501,C502 ceramic 300pF to film caps made the popping less intensive.

thank you all for helping me.

[mkusan]

Originally Posted by MikeBettinger

I would suggest a grounding issue to the filter amp pcb. The schematic does not show where the ground to the board or to the volume control originates, only the connections E4 and E7. As I see it the only way to develop a voltage across the low filter switch is if the resistors from the coupling caps to ground (R529- R535) are not keeping the filter sides of the cap properly referenced to ground and the leakage across the caps (C517) could build up and cause a pop. Mike

I took some measuring in circuit, I did not have the time for lifting one leg as my wife is baking cookies and that leaves only me to guard the kid but maybe this data will be of some importance. I took a picture of resistors in question and the fourth one (R550 4.7K) from the bottom looks like it has seen better days. Please comment as I don't have much experience with resistors, that hole on the third one (R546 330K) looks suspicious to me also, but I don't know.

R546(330K) 0.0162M
R550(4.7K) 4.7K suspicious one
R530(100K) 76K
R534(22K) 21K
R536(330) 77K
R540(10K) 9.88K
R542(22K) 21K
R529(100K) 76K
R533(22K) 21K
R535(330K) 77K
R539(10K) 9.91K

[Mooly]

Thanks for the readings... they seem OK and don't show much all that much drift. Also I'm sure the resistors will be OK. Unless you just desolder one end with braid and at the sametime use a croc clip as a heat shunt on the leg you can end up making things worse.

The resistors are the very last suspects. They all look normal tbh, the "hole" is just the ceramic or whatever coating... not a problem.

Look at the circuit and the "top" channel.
Resistor R531 can either bypass the filter consisting of C519 and C521 when the switch is "off" or place those components in circuit when on. So measure the DC voltage across those two series caps from end to end. That is from the junction of C517/R531 and C521/R539. If that is not zero volts DC then you will get a thump as the switch is operated. All those points are referenced to ground via R529 and R539 so they should be truly at 0.000 vdc. Make sure there is no signal applied.