Uher CR240 Troubleshooting Best Practice

[Uncle Bulgaria]

Ahoy,

With the success of Operation Switch Renewal, the CR240 can play tapes, to my great delight (one of those projects that has been hanging about since the Pleistocene). However, the right channel is popping and clicking with no music, and the left irregularly succumbs as well.

I'm just a dabbler, but keen to learn, so I'm throwing this one out to the floor. I would be grateful for any help the forum can offer from professional experience as to going about troubleshooting this problem. I'm not a circuit buff and my theory is hazy so I'm usually limited to fixing obviously blown components and checking out of spec. voltages. Top tips on how to work through this methodically and why would be much appreciated.

I have only added new belts, replaced the two 2200µF/10V power supply board (board no. 600 on diagram) capacitors with 35V ones, replaced the blown capacitor in the Z131 power supply and lubricated the motor so far.

Here is a circuit diagram (Dropbox Link). It's a little blurry, but legible. Also attached are some reference photographs.

[TonyDuell]

I have a CR210 and I had lots of problems caused by (a) bad contacts on the pushbutton switches (which have long 'sliders' with lots of contacts inside) and (b) bad contacts on the connectors between the plug-in PCBs and the backplane (motherboard, baseboard, whatever you want to call it).

[Uncle Bulgaria]

Thanks Tony. a) I have dealt with by making a new record switch (other thread in this forum). b) I haven't manually cleaned yet, apart from a bit of Servisol 10.

I wondered if there was some intermittent fault in the signal path. I've traced it back to before the volume control by ear, but as there doesn't seem to be a monitoring option I've not been able to tell if an external source has the same interference. Today I think I'll have a crack at understanding my oscilloscope...

[Uncle Bulgaria]

I've got it running after a fashion, but have a further problem. The left channel starts strongly and fades to near silence after several seconds. The right continues fine.

If I turn it off then immediately back on again, the same thing happens. I've tried to scope the signal but I'm not having much luck as some of the boards are inaccessible when enough are in for the path to be complete.

It seemed reasonable to assume a capacitor would cause this issue, but touching the very first point where the tape head meets the first circuit board gives me a buzz in the left channel that varies with volume, so it's going through the path all right.

I have removed and checked almost all the components of board 300 (Recording/Playback Amplifier).

The head has the same resistance for left and right.

Any ideas?

[Uncle Bulgaria]

Update: I've resoldered all the tape head wires. No change. Both channels play for a few seconds, then the left fades out leaving the right.

Touching pin 301 gives a variable buzz, the same with pin 314 (the equivalent output for the working right channel). However, touching the input pin for the right channel (309) the buzz is variable, but for L-in (pin 306) there is only a click.

Thus I'm almost certain the problem is in the upper half of the diagram of board 300 attached. If anyone could tell me what they think the likely culprit is and how this circuit works so I can see how you arrived at this conclusion, there would be much rejoicing!

The part of the circuit shown in the diagram is the right half of the photograph. the left side is to do with recording and doesn't have an effect on the problem.

Otherwise it's a trip to RS and wholesale channel replacement...

[TonyDuell]

I'd start by comparing the voltages on the transistor connections in the faulty channels with those on the equivalent transistors in the working channel.

[Uncle Bulgaria]

Thanks Tony. I wouldn't know what I was looking for though! The difficulty is that when the board is in position, the solder side is not accessible, so I'd have to solder on flying leads to all possible positions.

If I were going to that trouble I might as well swap the transistors around, which in fact is what I ended up doing last night. Now the left channel still fades, but the right (previously working channel) has disappeared entirely! It came back with some judicious wiggling of board 300, which annoyed me as I have gone over all the pin connections and resoldered them, despite no dry joints being evident under a loupe.

I suppose I'll have to go back over the whole board.

[ms660]

Extender board(s)?.....Etching skills?

Lawrence.

[Uncle Bulgaria]

OK Lawrence, that made me laugh. I know I'm being a bit wimpy - I was just hoping to take this away with me when I go sailing for a month or so next week, and felt I was at the end-game with getting it up and running with a successful switch and sound through the speaker!

I'm going to 'scope out the leads I can get to from the component side with the Dolby board out. No sound output as the Dolby board is a part of the connection to the speaker, but I might be able to see an injected wave and compare the channels. If not, I'm considering wiring up a couple of pins as I think there are actually only two or so fundamental connections for the signal path and DC, which would enable me to trace that section of 300 on the other side with it plugged into the motherboard at a distance.

At least my oscilloscope skills are improving!

[Uncle Bulgaria]

Interpretations please!

I've jury-rigged a headphone-out to three crocodile clips and used this to put a 1kHz sine wave into where the tape heads meet the motherboard (tape heads disconnected).

The sine wave is 0.5V.

At the collector of the first transistor in the path (T301 - labelled in the diagram as a BC 384 BS, but on the board both channels have a BC 550B) is the waveform shown, approx. 1.5V total amplitude.

At the emitter is the form shown; ~20mV.

Call me wet behind the ears, but shouldn't the wave in be amplified as the wave out?

[ms660]

The stage is overloaded, throttle back the input to a few mV then see what the 'scope displays, it's a small signal stage, 0.5 volts input is too high.

Lawrence.

[Uncle Bulgaria]

Of course. Foolish of me. I was looking at the datasheet and the differences between contacts measured in volts there got me thinking it required something more.

It's very noisy from the PC so hard to get a visible small amplitude trace to trigger, but the same contacts made result in more sine-like traces now. How does the signal path work? Is it Base T301 to Base T303 etc? Am I looking for the output on the Collector or Emitter. I'm looking at some descriptions of circuits using transistors-as-amplifiers but it's mostly over my head.

[ms660]

Originally Posted by Uncle Bulgaria

How does the signal path work? Is it Base T301 to Base T303 etc? Am I looking for the output on the Collector or Emitter. I'm looking at some descriptions of circuits using transistors-as-amplifiers but it's mostly over my head.

Ref to ground, the input signal from the head on playback is routed to the base of T301, T301 amplifies the signal and the output is taken from T301's collector, it's collector is DC coupled to the base of T303, T303 amplifies the signal further, again the output is taken from its collector, its collector is DC coupled to the base of T305, the output is taken from its emitter and is routed to the input of the Dolby noise reduction section.

Because the three transistors are DC coupled a fault that affects the DC conditions on one can change the DC conditions on the others.

Lawrence.

[Uncle Bulgaria]

The bee's knees, Lawrence!

I've now got a sine wave running at a low amplitude from my 'phone, which has removed the AC noise problem.

I can see the rise and decay of the L channel. The collectors are hard to access, and I got the pinout upside down to start with, so I have been looking at the emitters. There are very obvious waveforms on them, so even though it's the wrong leg perhaps they're of use?

The R channel has more jitter (though perhaps that's bad contact between the probe and the contact) but great increases in recognisable sine wave voltage at each stage, through to the output to the Dolby board. The L channel, when viewed at the emitter of T303 has a clear wave immediately on switching on, though with a flattened trough, which decays into the attached after a few seconds. Metering the same pin on T304 shows a clear sine wave of some tenths of a volt.

C303 is a new Vishay, which I believe is their common supply capacitor.

[ms660]

Are you sure those waveforms are on the emitters of T303 and T304? The reason I ask is that at 1kHz there should be almost zilch there, eg: flatline on the 'scope as both emitter resistors are by-passed to ground by a 100uF capacitor in the schematic, if they are the emitter waveforms then distortion aside the capacitors for whatever reason aren't doing their job, either they have lost their value or their connections to the emitters or ground are faulty.

Lawrence.

[60 oldjohn]

Originally Posted by Uncle Bulgaria

Update: I've resoldered all the tape head wires. No change. Both channels play for a few seconds, then the left fades out leaving the right.

Have you checked the wires for continuity ? Tape head wires have to move and are known to fracture.

John.

[Uncle Bulgaria]

John - I did check, as that was the problem I discovered with the 6.2V supply. It measured all OK, with continuity between the two ends of the wires that connect the PSU to the motherboard, but the 6.2V on the PSU end was a fluctuating 2.8V on the other! I was glad to catch that one.

However, these traces are being made with the signal coming almost directly into the motherboard, with the plug assembly that links the tape heads to the motherboard out of circuit and the problem's still happening.

Lawrence - I *think* so. Perhaps you can tell me otherwise. Here's board 300 with the relevant transistors and the accompanying pinout. As it's showing the legs I thought it must be from underneath. The photograph is labelled with the transistors. The flat sides are all on the left. The top of the board is seen when in place, so the upper legs of the transistors are easiest to get at with the probe.

[ms660]

Looking at the board layout and your photo....The Emitter is the one nearest the bottom of the board, the Collector is the one nearest the top of the board and the Base is the one in between. The Emitter is also marked on the board layout with the letter E, all the transistors (T301 to T306) are orientated the same way.

Here's the pinout details for the transistors that are fitted in that section:

https://www.alldatasheet.com/datashe...SC/BC550B.html

Lawrence.

[Uncle Bulgaria]

Ah! So the pinout is actually looking from the top, but showing the legs below? No wonder I was confused. I was engaging in mental gymnastics to align what I thought was the bottom view with the top I was seeing on the board. I didn't notice the significance or existence of the board 'E'.

Always undone by the amateur errors...

So, the traces above are actually useful in that they show there's a problem with what's happening at T303.

Am I right in thinking that a supply problem is unlikely as C303 deals with both channels?

I have removed and measured R: 301, 304, 307, 308, 312, 316 and the variable trimmer resistor R324.

Is the emitter a little like the cathode on a triode, biased similarly? In the case of T301 by R304, and T303 by R312. Both bypassed, as you mentioned, by 100µF capacitors. (Sidenote: I don't understand the significance of the assembly on the right, connected to C306/307 by pins 305 & 303 - if there's nothing on the emitter side it can't be something to do with feedback? Do the diodes mean it's about setting DC conditions for the transistors?)

[ms660]

Originally Posted by Uncle Bulgaria

Am I right in thinking that a supply problem is unlikely as C303 deals with both channels?

Is the emitter a little like the cathode on a triode, biased similarly? In the case of T301 by R304, and T303 by R312. Both bypassed, as you mentioned, by 100µF capacitors. (Sidenote: I don't understand the significance of the assembly on the right, connected to C306/307 by pins 305 & 303 - if there's nothing on the emitter side it can't be something to do with feedback? Do the diodes mean it's about setting DC conditions for the transistors?)

C303 decouples the supply to the first two transistors on both channels, a 'scope's display at that point should be flat, eg: supply line is clean, the voltage at that point should be somewhere around the supply voltage at connection 315 in the schematic, which so far as I can make out is given as 6.2 volts.

Transistor bias and emitter resistor etc see here:

https://en.wikipedia.org/wiki/Bipola...sistor_biasing

I might be wrong but so far as I can make out the diodes to the right (D301, D303, D305) are activated by the Tape Select switch (Chrome, Ferrichrome etc) via transistor T3 and connects R325 across R320 to alter the feedback characteristic of the amplifier to suit the type of tape selected.

Lawrence.