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-   -   Help diagnose Rotel RX-304 receiver - right FM channel faint (http://www.diyaudio.com/forums/solid-state/221083-help-diagnose-rotel-rx-304-receiver-right-fm-channel-faint.html)

bandolino 7th October 2012 05:32 AM

Help diagnose Rotel RX-304 receiver - right FM channel faint
 
Hello all,

I am looking for assistance with troubleshooting a Rotel RX-304 receiver in partial working condition. I have just completed replacing all the electrolytic capacitors. Before the recap, all functions worked ok. Now, after the recap, there is a problem with the FM. Output in the right channel is very faint. This problem only shows up in FM mode. Reception is fine, and muting works. Output to speakers and to headphones both show the same results.

I am an absolute beginner at amplifier repair. I have been reading up online, but usually find the information to be beyond my knowledge level. I use a digital multimeter with some proficiency, and I do not have an oscilloscope, a signal generator, or dummy load resistors. I do use a light bulb in series device (dim bulb tester) to first power on old amps safely. I do have the service manual, and have been poring over the schematics, but don't know enough to understand what to look for.

So far, I have checked the rail voltages, checked dc offset, done a simple test of the transistors with the unit powered off, and checked voltages on some transistors with the unit powered on. I also double checked all the wiring, and looked for loose solder or wires on the board as a result of my work.

The service manual for this receiver is available here:

B&W Group North America Service & Support - Service Manuals

The transistors tested in circuit with the power off seemed to test ok. A few of them tested non-open with reverse bias (I'm not sure if this is the correct way to say this). The transistors all appear to be NPN types, so with the black multimeter lead on Base, and the red lead on Emitter or Collector, most of the transistors tested open. The few that did not showed a voltage drop of about 1.8V on the Emitter, and 1.6V on the Collector. Also, I read somewhere that Emitter to Collector should be open, but every transistor in this receiver tested non-open in circuit, with all the results similar, around .8V Collector to Emitter, and 1.7V Emitter to Collector.

DC offset is about 34mV in the right channel, and 45mV in the left channel. I understand that this is a little high, but nothing to get too worked up about. There is a very noticeable jump in output in both channels and in DC offset when the amp is turned off. The momentary peak when the amp is turned off is around 9V in the right channel and 8V in the left channel. This decreases to about 40mV in 30 seconds, then increases up to about 400mV in 3-4 minutes. Finally, this drains down again very slowly (I think in 2 hours, it was down around 70mV). Is this an unusual pattern of DC offset for this amp?

To test the rail voltages and the transistors with the unit powered on, I did follow basic safety measures of wearing rubber gloves and being very careful about placing the multimeter leads. I left the black multimeter lead clipped to chassis ground. I did not touch the power supply filter capacitors. I turned off and unplugged the unit when I needed to move it around or plug anything into it. Other safety tips are much appreciated!

The rail voltages tested ok, with +rail of about +29VDC and -rail about -29VDC. I randomly tested the rectifier diodes, which also showed about 29VDC.

With the power on, FM stereo on and receiving a signal, output to headphones, I tested 4 different transistors: Q104 FM If amp, Q203 FM If amp, Q204 audio amp (in the AM/FM area of the board), and Q302 Stereo Auto-Control (in the pre-amp area of the board?). The schematic shows desired voltages for each transistor pin, and the voltages I observed were quite close to those shown on the schematic. Q302 has two sets of desired voltages listed on the schematic, one for mono, and one for stereo. With this set-up, it matched the stereo voltages.

Interestingly, the FM left channel, which had been working fine, stopped giving any output at all at some point during this testing. When I noticed this, I measured the voltages again, wondering if anything had changed, but all the transistors measured the same as before.

I also checked the voltages of Q302 in AUX mode with input from an iPod, and output to headphones. As before, the output worked fine. In this set-up, Q302 seemed close to the desired mono voltages. However, the Collector voltage, shown on the schematic as 1V on mono, measured at 0.09V.

When I was first wiring everything back together, I noticed a resistor with only one lead attached, connected to the anode of the FM stereo indicator LED, D001 in the schematic. This LED is very close to the function board, and the loose end of the resistor, which measures about 4.7kOhms, angles down in the direction of one pin of the FM function switch. That pin has a glob of solder on it with nothing connected to it. I figured this was an easy fix for the FM problem, and ran the receiver with this resistor connected to the FM switch pin. I wasn't sure if I should try this, because the wire to the anode of D001 connects through a 3.9kOhms resistor to +rail. Unfortunately (or fortunately?), nothing changed. That's when I moved on to further testing. Update to this loose resistor lead: while writing this, I decided to try this connection again, just to be sure. I lightly soldered the resistor lead to the FM switch pin (not sure which pin on schematic), and now the left FM channel works fine again. The right FM channel is still very faint.

I have not yet tested either IC101 FM If amp, or IC301 MPX amp. The schematic shows desired voltages for each pin of those, as well.

I did not test the power amplifier IC, which in this receiver is a SanKen SI-1125HD, a large, 16-pin hunk of plastic. I found some photos of its internals on a Polish forum, which show that it contains 6 actual TO-92 transistors (and 4 transistors of another sort?) on a board with other parts (Wzmacniacz Sony TA-313 Brak zasilania). I am assuming that the FM problem does not originate in the power amp IC, since modes other than FM work fine. Feel free to challenge my assumptions, however, as that's why I am here!

Well, after my long explanation, can anyone offer advice for what to do next? I assume I need to test IC101 and/or IC301. Does anyone have an idea what's up with the resistor going from D001 to the FM function switch? I couldn't find that anywhere on the schematic or the wiring diagrams in the service manual. Many thanks for any help offered!

sregor 8th October 2012 11:07 AM

IMO you are way over complicating problem. Any thing wrong before the outputs of IC 301 (pins 4 and 5) will affect both channels, so all that probing in IF and FM are telling you nothing, and could cause other problems. Also, anything after the selector switches will cause the problems in other inputs, so you have output of IC 301, two transistor buffers for each channel, associated circuitry in between, and the selector switch itself. Takes about 30 seconds to locate problem with a scope. An ac millivoltmeter would also take you to the problem. You may get enough signal to read in on the AC scale of some DMM's.
If you recapped it, did you do the 2 small electolytics 319 and 320 near ic 301? My best guess would be broken connection in the ribbon cable going to the selector switches, but who knows. Good luck.

Also - the output modules are either unobtainable or way over priced for NOS so be careful with them.

bandolino 10th October 2012 05:52 AM

Thank you for your helpful reply, Steve. I am so much a beginner that I couldn't figure out what role each IC plays, and where they lie in the signal path. I don't really understand the path the signal follows, the path the supply power follows, and how they are related, exactly. So, thank you for clarifying things.

I did replace C319 and C320.

So, focusing on the area between IC 301 and the selector board, I checked the values of the electrolytic caps and the resistors between pins 4 & 5 and L301 (the relay just before the selector board). They all had correct values. I didn't check the relay itself, as I haven't yet learned how to do that. I also didn't check the ceramic and mylar capacitors.

I then turned on the receiver, and tested DC voltages on IC 301 pins 4 & 5, and on Q303 and Q304. Here's the results (in volts DC):

IC 301
pin 4: 12.67 (left channel output)
pin 5: 11.99 (right channel output)

Q303 (left channel)
B: 1.65
C: 8.33
E: 0.99

Q304 (right channel)
B: 2.45
C: 1.84
E: 1.77

Expected values for Q303 and Q304
B: 1.8
C: 7.6
E: 1.2

Q303 matches the expected values pretty well, and Q304 is way off. Does this mean that there is a problem with Q304, or could these values just be a reflection of a problem elsewhere?

I have checked the ribbon cable connections, and they all have continuity. I actually replaced all the ribbon cables going to the selector and tone control boards, because the stock ones were flimsy and broke soon after I took the boards out.

I don't really understand what you mean about finding the problem with an AC milivoltmeter. Would you like to expand upon that explanation?

Thanks again for any help.

Ian Finch 10th October 2012 07:03 AM

You may do well to buy a book for a meaningful reply to that big question.
BARNES & NOBLE | How to Test Almost Anything Electronic by Delton Horn, McGraw-Hill Professional Publishing | Paperback.
This is not simply explained in a few clauses of forum advice. In brief, an oscilloscope is an AC voltmeter with a graphical readout, right? A DMM can also measure AC but its readout is numeric, so the info is not so detailed or specific to whether there are other components to the measurement like DC voltage or interfering AC, High frequency Audio or even RF that will be invisible to a DMM. At a pinch, an experienced person can use a DMM to fault-find AC circuits, as sregor suggests. Otherwise, the average person can't discern what they are measuring.

Your admission to replacing all the ribbon cables going to the selector should ring a bell though. Are you sure you have continuity across your work, input circuit to output circuit? I can't understand how the cables broke unless you damaged them in disassembly, but it seems evident that whatever problem you have, is caused by your own handywork. Make certain that what you reconnected is, in fact, connected correctly and not also shorting to another conductor such as the chassis or center to shield braid on the L&R signal leads. Too long with a hot iron can cause such a short. In the particular case of the L&R audio from the tuner, check not just end to end of any replacement work but right to the circuit it's supposed to connect to, when the selector is in the appropriate position.

The application of a little logic and the schematic should lead you to success if the problem is just restricted to FM stereo. Maybe check polarity on some of the local caps you replaced too.

sregor 10th October 2012 11:25 AM

Quote:

Originally Posted by bandolino (Post 3196032)

<snip>

I then turned on the receiver, and tested DC voltages on IC 301 pins 4 & 5, and on Q303 and Q304. Here's the results (in volts DC):

IC 301
pin 4: 12.67 (left channel output)
pin 5: 11.99 (right channel output)

Q303 (left channel)
B: 1.65
C: 8.33
E: 0.99

Q304 (right channel)
B: 2.45
C: 1.84
E: 1.77

Expected values for Q303 and Q304
B: 1.8
C: 7.6
E: 1.2

Q303 matches the expected values pretty well, and Q304 is way off. Does this mean that there is a problem with Q304, or could these values just be a reflection of a problem elsewhere?

I have checked the ribbon cable connections, and they all have continuity. I actually replaced all the ribbon cables going to the selector and tone control boards, because the stock ones were flimsy and broke soon after I took the boards out.

I don't really understand what you mean about finding the problem with an AC milivoltmeter. Would you like to expand upon that explanation?

Thanks again for any help.

AC millivoltmeter - sometimes called a VTVM used to measure AC voltages and will correctly measure signal voltages at audio frequencies. Many meters and DMM do not measure high frequencies, and most are hard to use at low (1V or less voltages. Meter on AC can be used to see signal as opposed to DC or operating point voltages. (AC signal should be around half a volt from the outputs (4 and 5) of IC 301, to the selector switch and shouldn't change.)

However, something is going wrong with Q304 - although it could be the transistor, it looks like something is getting too much voltage on the base. I noticed an error in the schematic - on the AM/FM version (page 11 ) they show the negative side of C309 and C310 elctrolytics as on the IC side, in the AM/FM/LW schematic (page 13) they show it on the transistor side - it should be on the transistor side. If one of those is in backwards, would probably cause your problem. A better solution is to use a film type cap for the .47uF caps - it looks like there is enough room and polarity becomes a non issue. Too much voltage on the base causes the transistor to saturate and no signal can get through. ATry unsoldering either end of c310 and see if the voltages go to normal.

Mr. Finch's advice should be taken to heart. A basic understanding of what you are measuring, what it should be and how the circuit works are key to troubleshooting (and not just electronics but technology in general) Good luck.

bandolino 22nd October 2012 07:27 PM

Right FM channel fixed - C310 polarity was reversed
 
I apologize to all for the long delay. Steve, your advice was right on target. I had replaced C310 incorrectly, with the negative lead on the IC side. I took it out and reversed it, and the right FM channel is now working correctly. The left channel is also working fine. Thank you so much! I don't know how long I would have poked around in there without noticing this.

For future reference if anyone else works on this receiver, there are a few capacitor polarity errors both in the schematics, as Steve noticed, and on the PCB itself.

On the PCB, C611 and C612 are marked with the positive lead on the IC601 side, but the correct position is with the negative lead on the IC601 side. Looking at the board from above, C611 and C612 are pretty much in line with three large pins positioned in between IC601 and IC301. The negative leads should be pointing away from the pins, like so:

-(C612)+ _____ (Pin18) _____ (PinE3) _____ (Pin19) _____ +(C611)-

The other polarity error I noticed on the PCB is with C301, which is near IC301. The negative lead should be on the IC301 side, but on the PCB, it is marked away from IC301.

Ian and Steve, I appreciate the advice from both of you to learn more about the basics of how circuits work and what to measure to find out what I need to know. Obviously, I am ignorant of much of this basic information. I will check out the book you suggest, Ian. I will also check my work a bit more carefully before posting for help here again. Thanks to both of you for your patience with me.

Also, Ian, it did occur to me early on that my work replacing the ribbon cable might be the source of the problem, and I did check continuity both of the cable connections and further out into the circuit. I didn't find any problems there, which is part of why I was stuck for a solution and asked here.

Good luck to all!


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