Thanks very much, Ian, the electrolytics must be my next project. But I must say re-assembling the machine was very difficult - spring attachment, solenoid re-assembly, Record LINE and MIC knob re-assembly - (that WAS difficult and very strange) Presumably the electrolytics in the Cct around those 10 transistors? Or should it be ALL electrolytics on the Pre-Amp PCB? I haven't touched the Power PCB at all. Whilst the machine is up and running I can take a breather and collect the electrolytics. What do you suggest? Meanwhile I have an old Acer Aspire 9300 (17.1/4" screen) laptop giving me Beep - bip-bip. on Startup - which error message is interpreted as VGA problem. Out with the paint-stripper Hot Air gun and attack those solder connections on the MoBo!! Michael
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Perhaps you should re-read posts 10-20 where capacitor substitutions were discussed. There are many brands and grades to choose from and dealers tend to specialise in only a few. I quoted Panasonic there as the industry standard and most common which I use because of wide availability in appropriate grades, size options and fewer problems.
I shall return it to the bench for tests. It could be those wretched in-line solenoid operated switches - though I have injected Contact Cleaner into them. I must study the Cct Diag. to find a test point which will mimic signal from the heads. But before I do that, though, it occurs to me there is another test I can do - for this machine plays in reverse - so I must play a tape backwards . . . .
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Michael
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Michael
As we suggested earlier, transistors are not the only components in audio circuits and apart from mechanical switching faults, capacitors are more likely at fault after so many years. I assume from your comment, you have already had to deal with switch problems.
Unfortunately, it is not enough to use contact cleaner when the contacts are worn - they may not even be touching. Use a meter to verify that the relevant switches actually do something.
Unfortunately, it is not enough to use contact cleaner when the contacts are worn - they may not even be touching. Use a meter to verify that the relevant switches actually do something.
Thanks Ian. No, I have not had to deal with switch problems - the subject came up as a possible cause and I was told 'how to clean them' - which I did (possibly!) - b ut your caveat is acknowledged! As I now have the machine back on the bench, on its side with the contact side of the Pre-Amp board exposed I shall turn power off, use a DMM and do some simple switch tests. Thank you for your guidance, Ian, it is much appreciated. Let you know the results.
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Michael
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Michael
I have been checking those in-line switches without much success - simply because I am not sure of their configuration. For the most part each switch is in two linear rows and each row is in groups of three contacts. Presumably, if they are in threes, this is Make pin 1&2 = Break pin 2&3 and in the other direction: Break pin 1&2 = Make pin 2&3 etc. Of course with so many components soldered to those pins I can get false readings. Do you agree?
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Michael
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Michael
Yes, switches can be confusing but nowhere near as confusing as logic control or microprocessors, so be thankful that you can visually place the switches and deduce the connections from the few options possible. You will be able to identify the signal path and mark it on a copy of the schematic and follow it that way if you can't otherwise identify what you are looking at. Don't forget, you have a working channel for comparison - use it to determine by measurement, whether you do or don't have obvious switching or related problems.
I have just seen your post, Ian, Thank you! Again you are correct - I do have one working channel from which to compare! That's a good idea which I shall follow up - marking a copy of the schematic with the signal path. From the Layout Plan on p.34 of the Service Manual Parts List I have extrapolated the configuration and numbering of all the double in-line switches 1 - 5. All are slide switches with make/break - break/make contacts. In the non-operated position all the make contacts show as 'made' and in the operated position all the newly 'made' contacts also show as 'made'. So there are no switching problems. Which brings me inevitably to Electrolytic caps.
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Michael
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This is the time to do some signal tracing with your generator before ripping parts out in the hope of shotgunning the problem. As suggested before, you follow the schematic back from the output of the circuit containing the forward/reverse switching where the problem appears to be. Use a low level signal to avoid blowing your amplifier and speakers away when you finally make the correct contact and as mentioned, compare the channels to check what is correct.
Thanks for your Post, Ian. I agree - now is indeed the time. One good thing is that the Heads are connected to the Pre Amp PCB by two 7-pin plocketry. The two Male 7-pin connector blocks are mounted either side of that long 15+15 contact REV - FWD solenoid operated SW1. Not sure yet which 7-pin connector is from FWD or which is from REV, but being pins they give a good point for injecting a tone (I think?) So I must now see if my Sig.Gens work!
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Michael
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Michael
Michael, it seems you may not follow what is meant by signal tracing. The idea is to to use a probe from the generator's output and apply that manually to each node of the signal processing circuit(s), backwards along the signal path, from output to input and thus identifying where the signal stops being heard at the output. Ref.post #11.
Obviously, the recorder switching configuration must be set to amplify with that circuit under test in line, in order to hear/see and compare the results.
Injecting a signal to the preamplifier or head amplifiers external connections will tell you little that you can't already hear (or rather, not hear) simply by using the recorder in normal operation . You trace through the circuit from within, at the PCB level - assuming the circuit is where signal gets lost and it's not due to an external switch malfunction or other connected device. Study the schematic carefully and map out the signal path from cap. to transistor, resistor etc, so you have a reference - a street map to follow and refine as you go by trial and error.
Obviously, the recorder switching configuration must be set to amplify with that circuit under test in line, in order to hear/see and compare the results.
Injecting a signal to the preamplifier or head amplifiers external connections will tell you little that you can't already hear (or rather, not hear) simply by using the recorder in normal operation . You trace through the circuit from within, at the PCB level - assuming the circuit is where signal gets lost and it's not due to an external switch malfunction or other connected device. Study the schematic carefully and map out the signal path from cap. to transistor, resistor etc, so you have a reference - a street map to follow and refine as you go by trial and error.
Sig Gen OK. I find there are two more sets of plocketry for the heads - this time they are 4-pin and these are to the (A) FWD and (B) REV PB Heads. I have removed the plugs in order not to compromise the head integrity. (Don't want to cook them inadvertently!) Then I soldered a short tag to the solder side of the PCB at pins 1(R) and 4(L) thus enabling easy crocodile connection from the Sig Gen. Next I injected a tone at pin 1(R) and, using a battery powered mini amp, followed this tone through from TR7 - which is the first Transistor in the chain from the Heads - to TR's 9&10. The tone gets progressively louder, but diminishes at TR11 and is low level at TR12 - with a bleed through at the same level at TR12b (L). I only changed the Transistors from TR1 - TR10. I then injected tone into the same point for (L) channel. The results were the same - a fainter tone at TR12 than found in the (R) channel test but duplicated at TRb (R). Maybe C's 37,39 & 40 all being electrolytics are suspect? C 41 and 42 are also electrolytic but are not involved in this test. Maybe TR's 11 & 12 are also suspect? Whatever the reason I must take some voltage readings with the DMM. All TR voltages are given on the Cct. Diag. I'll post this for now . . . .
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Michael
Thank you for this, Ian. Yes, I'm clueless as to the correct 'modus operandi' for signal tracing and don't even know what a 'node' is or how to recognize one. Please would you advise me on this? For it seems unless I follow the recognized method I will ... just get no-where! But I learn fast (I hope). I've just downloaded the Data for TR 11 (an FET 2SK68) and TR12 (2SC2130) in order to get the pin-outs identified for measurement with my DMM (Thandar TM351)
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Michael
Nodes become self-evident. 2 or more components or branches of a circuit join - maybe at one point or spread along wire leads or PCB traces over some distance but when they are electrically the same point, the junction(s) in common all fall under the term "node". Usually, the black dots at the junctions of components make it dead obvious.
Thanks again, Ian. Very concise! - and therefore very obvious.
I'm gazing at the solder-side of the Pre-Amp PCB and there are no markings to guide me so I have a printout of that board in a form of X-Ray view (as in the Service Manual) and have marked on it with fine black marker the TR#'s and, with a Tip-ex pen, all the TR pin-out pin 3 (Base) in the same way as on the Component side of the PCB. This is to help identify where I am on my 'map' of Nodes and Junction points. So I am attempting to inject a tone at the Node before the Phones and monitor it at the Output. I shall do that for both (L) and (R) in an attempt to find out the cause for the (R) channel not working. Now the Switches have been tested OK there can only be a faulty Cap, an un-changed TR or maybe a Diode. Interestingly, I notice that 'Line Out' bypasses the output stage with its TR12 and the VU meters so I should first inject at the TR11 FET Drain and monitor at the Line Out. Here goes!
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Michael
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'Seems like you are getting the picture here. VR4 & 6 affect the output levels of the monitor (preamp) amplifier so I assume these are accessible controls and not just little preset pots. Otherwise, they may need checking, that the wipers make contact properly. but don't lose the setting position of presets. They are often precision settings in recording equipment.
I would replace those several 16 and 25 volt/10 uF capacitors in that area without further ado, using 25V rating for all. If you wait until you have cranked through every part to make a full list and then pay unnecessary airfreight when you can buy local as required, you will still be dragging out the restoration well beyond Christmas. CPC, Farnell, RS, Cricklewood Electronics and others stock those small, common values in the UK.
I would replace those several 16 and 25 volt/10 uF capacitors in that area without further ado, using 25V rating for all. If you wait until you have cranked through every part to make a full list and then pay unnecessary airfreight when you can buy local as required, you will still be dragging out the restoration well beyond Christmas. CPC, Farnell, RS, Cricklewood Electronics and others stock those small, common values in the UK.
Thanks Ian for this. VR4 and VR3 are presets, I think - for they have a 'slot-head' logo by them as controlling the input to the monitor (preamp) for FWD and REV through SW4-2. I must inject a tone at the base of TR8 and test at the terminals of VR3. Also re-check the SW4-2 wipers to see if that tone is getting through on the (R) channel.
I take your point re: the caps. I know there's something strange going on around the output stage which I'm puzzled about. Maybe it IS a cap problem there.
All this means I must dismantle the machine again in order to gain access to the components. Once dismantled I have not found a way to apply power for tests though. A physical problem with the two Record Level knobs - their construction and a 'damping' black plastic insert - make for difficult dis-assembly and even more difficult re-assembly.
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Michael
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