Audio preamp repair - one channel never goes above ~0v - Page 1
Forum section: electronics / beginners

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 02, 2020, 06:09:03 am
content: Hi,I'm trying to repair an audio preamp I have no experience with or schematics for (Conrad Johnson). The symptom was no sound in the left channel.Basically, it has a set of relays to select inputs, which then goes to a set of relays to set the volume (switches resistors in parallel, I assume).I fed a sine wave to the inputs, and discovered that on the right (working) channel, this was fed to the attenuation relay circuit as it should (-0.2V to +0.2V) - however, on the left (defunct) channel, it presented itself as -0.2V to 0V. The signal isn't clipped, it's just "squished" (wish I had a picture now).What could cause such behaviour? I've found what I think is a coupling capacitor,but although I'm unsure of its capacity, it seems far too small (markings: "VSH1 9650" on one side, and "12KO FC" on the other - looks like a silver mica or something). It measures correct on one side, incorrect on the other. But I assume this isn't the only place where the signal comes across.Sorry if I'm scant on details/pictures, at work so my options are limited.EDIT: I figured out the component - VSH are precision resistors from Vishay, 9650 is the date code. Could a faulty resistor account for such a weird behaviour?

author: The Soulman
author_rank: Super Contributor
timestamp: September 02, 2020, 07:43:20 am
content: Is this pre amp completely passive?If so, that (rf-filter?) capacitor might be the cause, remove it and try again.edit: a resistor behaving as a diode?

author: The Soulman
author_rank: Super Contributor
timestamp: September 02, 2020, 07:51:04 am
content: Do you have at-least a model number of the pre-amp?

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 02, 2020, 07:59:57 am
content: It's an ART tube preamp, mid 90s. So not passive, but the part of the circuit that disbehaves seems to be.

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2020, 03:14:34 pm
content: Have you looked at the signal levels when the tubes around the relay-switched networks are temporarily unplugged?

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 03, 2020, 09:16:13 am
content: Quote from: TimFox on September 02, 2020, 03:14:34 pmDo you mean the tubes in the amplification circuit? The unit has two boards; one low-voltage with input selection and attenuation, and one high-voltage with coupling and amplification circuitry.I've taken some further measurements, and it seems like there might be a partial short somewhere. I've assessed this by comparing the two units; however, they're not 100% identical, so it's all guesswork. Getting closer to figuring it out and mapping where the traces go, but any good tips could save me a lot of time (I have abour a half to one hour each day to work on this).I've attached pictures of top and bottom part of the main (working) unit. Is that a beauty or what?

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 03, 2020, 09:21:36 am
content: BTW, I've tried swapping tubes between the two units, but nothing changed.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 04, 2020, 07:17:12 pm
content: Quote from: TimFox on September 02, 2020, 03:14:34 pmOh my, I didn't really catch this properly.. lack of sleep can do the weirdest things!So, (accidentally) starting from the tube furthest away from the source signal, I removed one, powered up, measured the source signal in front of the first tube, then powered down, removed another tube etc.This is how it should have looked:This is how it was originally, before removing tubes:Now, I removed one tube at a time, starting from the one furthest away from the signal:So what should I make of this? It's as if each tube "drains" (no pun intended) the signal a bit for each step.The word "diode" got stuck in my head, so I tested all over the board, and vóila! One of them didn't seem right:I measured a 0.3v voltage loss in forward direction, and 0.6v in reverse. In circuit, but I double checked with the working unit, and it behaved properly.Now, I could of course try to replace the diode and cross my fingers, but I really want to understand how this works. Also, this is expensive equipment (and high voltage!) and I don't like unnecessary surprises :OEDITI'm having great issues with the attachments, managed to upload them finally, but inline viewing doesn't seem to work. Please bear with me..

author: Jwillis
author_rank: Super Contributor
timestamp: September 05, 2020, 05:13:20 am
content: Are all the heaters functioning on the tubes ? Do they all glow ? What tubes are they ? They should have a model written on them. If you know the pin outs on the tubes you can trace back where the problem occurs . A simple tetrode tube will have 2 pins for the heater , an anode , cathode and grid . Being 9 pin they may be pentodes . In any case if there are different tubes in the circuit they may NOT be interchangeable. So swapping tubes around may cause more confusion than help. Look up the pin outs of the tubes and check voltages at the socket . But be care full because the voltage at the anode can be very high . Depending on the amplifier they can be anywhere between 24V to several hundred volts DC.The Heaters should be around 6 to 12 volts depending on the tube installed .Check both AC and DC because they could be either. The grid voltage should change according to the signal or have an AC voltage . With the tube installed , The anode will have both a DC voltage and an AC voltage . The DC voltage being higher and the AC voltage being the amplified signal .

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 05, 2020, 07:50:53 am
content: They're all 6922, dual triodes. I'll report back as soon as I get time to measure. Have to admit, I'm a bit hesitant to measure the high voltage section, but I guess I don't have much of an option now. 😳

author: Jwillis
author_rank: Super Contributor
timestamp: September 05, 2020, 11:44:09 pm
content: ART – Anniversary Reference Triode Preamplifierhttps://conradjohnson.com/owners-manuals/artman.pdfhttps://frank.pocnet.net/sheets/049/6/6922.pdfalso known as 6DJ8 and E88CC all very similar characteristics and Pin designations .Heater Pins 4 and 5 should be around 6.3 voltsPlates (Anodes) Pins 1 and 6 90 to 220 Volts provided dissipation does not exceed 0.8WCathode Pins 3 and 8Grid pins 2 and 7Separator Pin 9 may be not connected or grounded

author: Jwillis
author_rank: Super Contributor
timestamp: September 06, 2020, 05:05:29 pm
content: This type of amplifier is a cascade amplifier . Each tube starting from the input amplifies the signal in 5 stages . This gives a low output impedance . There are 10 triodes , 5 for each channel . The grid of the first takes an input and outputs a slightly more amplified signal from it's anode to the second tubes grid and so on to the third until you get the final amplified signal from the fifth tube. Between each tube will be a coupling capacitor . Capacitors block the high voltage DC at the Anodes and Pass the low voltage AC signal to the next tubes grid.Since finding a suitable diagram seems futile for the ART , it would be advisable to draw up a diagram by following each trace and component for the faulty channel. Work your way backwards from the final output to the input to see where the signal starts and stops . Best if you have a steady input signal like 1kHz from a generator but any distinguishable signal will work .

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 08, 2020, 08:55:08 pm
content: So, I figured out what was wrong. There was no plate voltage (in fact, it was -0.5v). No cathode voltage either, for that matter. Heaters were approx 45v-55v, seemed a bit high but ok (compared to the other unit).I found a defunct transistor, MPS U10 (https://alltransistors.com/pdfview.php?doc=mpsu10.pdf&dire=_update)Now, this is a transistor originally made for "high voltage video and luminance output stages in TV receivers". They're not in production anymore. When I try to find an equivalent, all I find seem to be geared towards switching power supplies and lamps. Not sure if they are ok? They're basically meant to be switched totally on or off, I guess?Also, on the MPS U10, base is the middle pin. All the ones I find (so far, I keep digging), have base on pin 1. Ireallywould prefer not to dance pins around if I can avoid it...

author: Jwillis
author_rank: Super Contributor
timestamp: September 09, 2020, 05:25:14 am
content: If your not opposed to new old stock , EBay has MPSU10 in the UK , Germany and Switzerland . Lots in the US.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 09, 2020, 06:21:21 am
content: I wouldn't mind buying NOS if I knew the seller was professional. However, every time I've tried, I've gotten stuff in bubble wrap, tape etc - i.e no ESD protection whatsoever, and who knows how they were treated before. Sometimes of course, I have to bite the bullet. But newer replacements are always preferred.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: September 09, 2020, 08:19:51 am
content: Found a replacement for the MPSU10 - CENU10. To quote the datasheet: "This device is an electrical equivalent to Motorola’s MPSU10" - nice! Apparently, they were used in all sorts of audio and video gear back in the day, for decades.Now, MPSU60 is the PNP counterpart, which also seems bust. No luck so far, but I keep on digging. Will post if I find something (there is a CEN-U60 as well, but can't seem to find it stocked at any of the major dealers).