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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 26th Feb 2020, 2:03 pm   #1
kengolightly
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Default Tandberg TD20A logic problem

Have brought my Tandberg td20a out of warm dry storage and after quite a few problems with power supply to the logic board I am now at a standstill. Only record, stop and wind operate and have their led's lit. There are no movements on the meters when in record with a microphone source so unsure if it actually recording. Was recommended to check all the ic's in the logic board and I replaced them all, checked each one as I changed them but did not find any faults. I also obtained a new U3 SN74S471N which I tried and as soon as I powered up the deck it went into fast wind and nothing else. I put the original U3 back in and it reverted to the original condition. After reading the service manual it seems the U3 is the prom which controls all logic operations and may well control signals to the audio board. I am no expert and have a little knowledge of circuits. I have seen that there is a programme for the prom and would like any information as to how I can get this deck up and running. Seems a shame when it is such a good deck, Any help or pointers would be greatly appreciated.
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Old 26th Feb 2020, 6:03 pm   #2
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Default Re: Tandberg td20a logic problem

The 47S471 correctly is a PROM, more specifically 256 words of 8 bits each, i.e. 8 address (input) lines and 8 data (output) lines. Contrary to more common MOS EPROMs such as the 2716, unless programmed, the 47S471 outputs will have a low logic output at all addresses. Programming the chip using a PROM programmer is needed in order to program relevant bit cells to 1's, and cannot be reverted once done (the device is not erasable). (Technically, TTL PROMs like this tend to be of the 'fusible link' variety, i.e. programming physically destroys selected part of a metalization layer on the chip itself, by applying a high voltage during a short period of time to the pins in a specified configuration).

(P)ROMs were commonly used not just as memories per se, but as programmable logic chips, i.e. for a set of 8 digital input signals, the corresponding 8 output signals would be configured by programming the contents of the ROM. As an example, such logic conversion, not relevant in this case, just as an example, could be conversion from binary coded decimal (BCD) to binary code, e.g. if the address pins had the bit pattern 10011001 (99 hexadecimal), the bit cells at that address would have been programmed so that 01100011 (binary representation of 99) would have been output, and so on for all potential BCD codes. The chip in this case would implement conversion of BCD to binary.

Looking at the TD20A service manual (from http://sportsbil.com/tandberg/td20a-s.pdf - it's a big file, 100 megabytes!), on page 27, the general function of the PROM chip (U3) is explained, it takes a number of inputs essentially from the function control buttons (via chips U1 and U2 to create a memory of the last function selected), switches (speed and reel sizes) and microswitches (on the pinch roller engaging motor), and outputs control signals controlling the wind and rewind motors (on/off and motor voltage), the pinch roller engaging motor, and a mute signal for the playback amp (which goes via the EDIT/CUE button according to the description, i.e. the muting can be turned off manually). There's also a feedback line from one of the outputs to one of the inputs, via an RC filter. This is probably used so that the previous state of the machine can influence the next state, i.e. when going between modes such as WIND -> STOP where it might have a different braking behavior to WIND -> REWIND for instance.

Tandberg have had a bit of bad luck with the chosen chips in their logic controlled decks. The first generation (9000, 9100 and 9200 series) used chips from SGS' HLL (High Level Logic) chips (a version of DTL with higher noise margins (and supply voltage), often wired in let's say creative ways, making replacements with currently available chips tricky (although 4000-series CMOS can be used in some cases). The 10X/10XD hedged its bets on a completely different logic chip, the Teledyne 321, which is also unobtainium these days. The TD20A thankfully uses TTL chips, however, the bipolar PROM used in the TD20A while once a common logic component I would think has not been manufactured for many years, and at any rate, needs to be programmed with the original bit pattern in order to be useful.

A quick browse on the 'net came across this post in another forum, which unfortunately leads to a dead link, but it appears that the PROM contents has been read and saved by someone at least. https://www.tapeheads.net/showthread.php?t=42387

Failing to find a programmer for that specific type of logic chip, one course of action would be to replace the PROM with a slightly more modern MOS EPROM such as the 2764, which has a much larger memory size, and is not pin compatible, but can be wired to function in a similar way, by grounding all but 8 of the address lines. The 47S471 is a much faster part, but I doubt that speed would be an issue in this application. The 27 series EPROMs were very common in their day, and lots of microprocessor based equipment such as home computers used them, so relevant programmers are more readily available even today.

Hm, I'm wondering if a modern small CPU board such as an Arduino could be wired up as a logic replacement for the original chip. All that's needed is some form of programmable logic which maps 8 digital inputs to 8 digital outputs. We're getting far from vintage tape technology in this case, and one still needs to find the original PROM pattern.

Of course, it might not be the PROM that is at fault at all. I'd try reading pages 26 and 27 in the service manual and trying to see if the logic signals on the inputs and outputs of the PROM seem reasonable and work from there.

Last edited by ricard; 26th Feb 2020 at 6:30 pm. Reason: 27 series EPROM and Arduino info added.
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Old 26th Feb 2020, 6:48 pm   #3
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Default Re: Tandberg td20a logic problem

Hello Ken,

A very nice looking R2R and pretty complex control wise with all the digital logic. I assume the new U3 you brought was just the unprogrammed device, which would probably explain why you then only got fast wind to work.

As Ricard explains you would need the original Tandberg PROM code and PROM burner hardware to be able to programme the new U3.

Seems a bit drastic to change all the logic chips on the logic board without first diagnosing that they were in fact defective, depending upon your soldering skills etc could do more harm than good.

You say that you do not have much knowledge of circuits but at the same time you say you tested the logic chips as you replaced them but found no faults, what sort of tests did you do ?

It is not impossible for a PROM to fail but generally not common, so the fault (s) may not be due to the PROM itself.

This potentially could be very complex to fault find even for an expereinced person. I agree with Ricard that first step would be to check the control logic signals arriving at U3 and try and diagnose from there, it probably will not be easy.
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Old 26th Feb 2020, 7:12 pm   #4
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Default Re: Tandberg td20a logic problem

Assuming microphone is OK, DC supplies are OK and Record amp transistors are all good then main thing that would stop the record signal being seen on the VU meters is the control logic signal into the Audio Board transistors Q111 (left channel) and Q211 (right channel) being logic high at the transistor base connections.
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Old 27th Feb 2020, 6:31 pm   #5
kengolightly
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Default Re: Tandberg TD20A logic problem

Thank you both for your help. Ricard I am studying the service manual and trying to make sense of it.
DMcMahon my soldering skills were not necessary as the ic's on the logic board pull out and push in. Both transistors were ok.
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Old 28th Feb 2020, 10:20 am   #6
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Default Re: Tandberg TD20A logic problem

You are very fortunate that the logic chips are socketed, it would have been a very onerous and risky task if they had been soldered in, especially if plated through holes/double sided artwork involved.

So if the record amp transistors have the correct logic low base levels in Record mode then it looks like the record issue is not related to a logic problem, in that case you would have to monitor through the record amp (s) analogue circuity to locate the issue.

If both left and right record VU meters are not showing anything when attempting to set-up for record then maybe a common reason.
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Old 28th Feb 2020, 12:58 pm   #7
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Default Re: Tandberg TD20A logic problem

I have managed to get the prom programme and am in the process of finding how to write it to the new prom. When I manage this and have the logic functioning as it should then I can address further problems. I f you have any advice on prom programmers it would be greatly appreciated.
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Old 28th Feb 2020, 9:20 pm   #8
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Default Re: Tandberg TD20A logic problem

My PROM burner/programmer experience is just a distant memory now so I will leave it to someone more up to date with it to answer.
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Old 2nd Mar 2020, 4:39 pm   #9
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Default Re: Tandberg TD20A logic problem

Have been given a link to a Canadian guy who can supply me with a new written PROM. I have sent him the zip file for him to programme it with. If anybody else needs this file which is for 3.5/7.5 and another for 7.5/15 please private message me and I will send you a copy.
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Old 22nd Mar 2020, 1:34 pm   #10
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Default Re: Tandberg TD20A logic problem

Have received programmed pro from Stephan in Canada and when fitted on power up deck just goes into rewind. Nothing else works. Putting back the original prom I have record,stop,and rewind as before but no play, fast forward or free. Both speeds are working and reel size and in record I have now meters moving with the microphone input. Stephan suggested I send both proms back to him to see if the new one was programmed correctly. Any thoughts as to checks to make before this.
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Old 1st Aug 2020, 3:49 pm   #11
kengolightly
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Default Tandberg td20a

Having come back to try and get this working again I now have a new U3 prom which when I now power up no logics work. With the old prom on power up it just went into wind on its own.
I have 5v and 24v as it should be.

When I turn on I have the record led lit and the stop led lit which is wrong.

The standby should be lit and the play. The U3prom would make no difference to this I am thinking and it is the same with the prom removed.

Looking at the schematic I see that pin 6 on U13 is feeding zero v to J401-1 and J404-4 is 5v. on the record. On the Stop U11 pin 1 is zero v and J404-4 is 5v.

The standby has U8 pin 6 has 5v at J401-2 and J404-4 5v
The play U8 pin 3 has 5v at J404-3 and J404-4 5v.

Any pointers as to this would be greatly appreciated.
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